3 - Motors and loads

3 - Motors and loads
3
36
chapter
Motors
and loads
Introduction to motor technology
Information on loads and motor electrical behaviour
3. Motors and loads
Summary
1
2
3.1
Three phase asynchronous motors
38
3.2
Single-phase motors
42
3.3
Synchronous motors
43
3.4
Direct current motors commonly named DC motors
45
3.5
Operating asynchronous motors
47
3.6
Electric motor comparison
50
3.7
Types of loads
51
3
4
5
6
7
3.8
Valves and electric jacks
56
8
9
10
11
12
M
37
3. Motors and loads
3.1
Three phase asynchronous motors
This section describes the physical and electrical aspects of motors.The operating
principle of the most common types of motors is explained in detail.
The powering, starting and speed control of the motors are explained in brief. For
fuller information, see the relevant section.
3.1
Three phase asynchronous motors
The first part deals with 3-phase asynchronous motors, the one most
usually used for driving machines. These motors have a number of
advantages that make them the obvious choice for many uses: they are
standardised, rugged, easy to operate and maintain and cost-effective.
b Operating principle
The operating principle of an asynchronous motor involves creating an
induced current in a conductor when the latter cuts off the lines of force in
a magnetic field, hence the name “induction motor”. The combined action
of the induced current and the magnetic field exerts a driving force on the
motor rotor.
A Fig. 1
An induced current is generated in a
short-circuited shading ring
Let’s take a shading ring ABCD in a magnetic field B, rotating round an
axis xy (C Fig. 1).
If, for instance, we turn the magnetic field clockwise, the shading ring
undergoes a variable flux and an induced electromotive force is produced
which generates an induced current (Faraday’s law).
According to Lenz’s law, the direction of the current is such that its
electromagnetic action counters the cause that generated it. Each conductor
is therefore subject to a Lorentz force F in the opposite direction to its own
movement in relation to the induction field.
An easy way to define the direction of force F for each conductor is to use
the rule of three fingers of the right hand (action of the field on a current,
(C Fig. 2).
The thumb is set in the direction of the inductor field. The index gives the
direction of the force.
A Fig. 2
Rule of three fingers of the right hand to
find the direction of the force
The middle finger is set in the direction of the induced current. The shading
ring is therefore subject to a torque which causes it to rotate in the same
direction as the inductor field, called a rotating field. The shading ring rotates
and the resulting electromotive torque balances the load torque.
b Generating the rotating field
Three windings, offset geometrically by 120, are each powered by one of
the phases in a 3-phase AC power supply (C Fig. 3).
The windings are crossed by AC currents with the same electrical phase
shift, each of which produces an alternating sine-wave magnetic field.
This field, which always follows the same axis, is at its peak when the
current in the winding is at its peak.
A Fig. 3
Principle of the 3-phase asynchronous
motor
The field generated by each winding is the result of two fields rotating in
opposite directions, each of which has a constant value of half that of the
peak field. At any instant t1 in the period (C Fig. 4), the fields produced
by each winding can be represented as follows:
- field H1 decreases. Both fields in it tend to move away from the OH1 axis,
- field H2 increases. Both fields in it tend to move towards the OH2 axis,
- field H3 increases. Both fields in it tend to move towards the OH3 axis.
The flux corresponding to phase 3 is negative. The field therefore moves
in the opposite direction to the coil.
A Fig. 4
38
Fields generated by the three phases
3. Motors and loads
3.1
Three phase asynchronous motors
If we overlay the 3 diagrams, we can see that:
- the three anticlockwise fields are offset by 120° and cancel each other
out,
- the three clockwise fields are overlaid and combine to form the
rotating field with a constant amplitude of 3Hmax/2. This is a field with
one pair of poles,
- this field completes a revolution during a power supply period. Its
speed depends on the mains frequency (f) and the number of pairs of
poles (p). This is called “synchronous speed”.
b Slip
A driving torque can only exist if there is an induced current in the shading
ring. It is determined by the current in the ring and can only exist if there is
a flux variation in the ring. Therefore, there must be a difference in speed in
the shading ring and the rotating field. This is why an electric motor operating
to the principle described above is called an “asynchronous motor”.
The difference between the synchronous speed (Ns) and the shading
ring speed (N) is called “slip” (s) and is expressed as a percentage of the
synchronous speed.
s = [(Ns - N) / Ns] x 100.
In operation, the rotor current frequency is obtained by multiplying the power
supply frequency by the slip. When the motor is started, the rotor current
frequency is at its maximum and equal to that of the stator current.
The stator current frequency gradually decreases as the motor gathers speed.
The slip in the steady state varies according to the motor load. Depending
on the mains voltage, it will be less if the load is low and will increase if
the motor is supplied at a voltage below the rated one.
b Synchronous speed
The synchronous speed of 3-phase asynchronous motors is proportional
to the power supply frequency and inversely proportional to the number
of pairs in the stator.
Number
of poles
Speed of rotation in rpm
50 Hz
60 Hz
100 Hz
2
3000
3600
6000
4
1500
1800
3000
6
1000
1200
2000
8
750
900
1500
10
600
720
1200
12
500
600
1000
16
375
540
750
A Fig. 5
Synchronous speeds based on number
of poles and current frequency
Example: Ns = 60 f/p.
Where:
Ns: synchronous speed in rpm
f: frequency in Hz
p: number of pairs of poles.
The table (C Fig. 5) gives the speeds of the rotating field, or synchronous
speeds, depending on the number of poles, for industrial frequencies of
50Hz and 60Hz and a frequency of 100Hz.
In practice, it is not always possible to increase the speed of an asynchronous
motor by powering it at a frequency higher that it was designed for, even
when the voltage is right. Its mechanical and electrical capacities must be
ascertained first.
As already mentioned, on account of the slip, the rotation speeds of loaded
asynchronous motors are slightly lower than the synchronous speeds given
in the table.
v Structure
A 3-phase asynchronous squirrel cage motor consists of two main parts:
an inductor or stator and an armature or rotor.
v Stator
This is the immobile part of the motor. A body in cast iron or a light alloy
houses a ring of thin silicon steel plates (around 0.5mm thick). The plates
are insulated from each other by oxidation or an insulating varnish.
The “lamination” of the magnetic circuit reduces losses by hysteresis and
eddy currents.
39
3
3. Motors and loads
3.1
Three phase asynchronous motors
The plates have notches for the stator windings that will produce the rotating
field to fit into (three windings for a 3-phase motor). Each winding is made
up of several coils. The way the coils are joined together determines the
number of pairs of poles on the motor and hence the speed of rotation.
v Rotor
This is the mobile part of the motor. Like the magnetic circuit of the stator,
it consists of stacked plates insulated from each other and forming a
cylinder keyed to the motor shaft.
The technology used for this element divides asynchronous motors into
two families: squirrel cage rotor and wound slip ring motors.
b Types of rotor
v Squirrel cage rotors
There are several types of squirrel cage rotor, all of them designed as
shown in figure 6.
From the least common to the most common:
A Fig. 6
Exploded view of a squirrel cage rotor
• Resistant rotor
The resistant rotor is mainly found as a single cage (see the definition of
single-cage motors below). The cage is closed by two resistant rings
(special alloy, reduced section, stainless steel rings, etc.).
These motors have a substantial slip at the rated torque. The starting
torque is high and the starting current low (C Fig. 7).
Their efficiency is low due to losses in the rotor.
These motors are designed for uses requiring a slip to adapt the speed
according to the torque, such as:
- several motors mechanically linked to spread the load, such as a
rolling mill train or a hoist gantry,
- winders powered by Alquist (see note) motors designed for this
purpose,
- uses requiring a high starting torque with a limited current inrush
(hoisting tackle or conveyors).
Their speed can be controlled by changing the voltage alone, though this
function is being replaced by frequency converters. Most of the motors
are self-cooling but some resistant cage motors are motor cooled (drive
separate from the fan).
A Fig. 7
Torque/speed curves of cage rotor
types (at nominal voltage)
Note: these force cooled asynchronous high-slip motors are used with a speed
controller and their stalling current is close to their rated current; they have a very
steep torque/speed ratio. With a variable power supply, this ratio can be adapted
to adjust the motor torque to the requisite traction.
• Single cage rotor
In the notches or grooves round the rotor (on the outside of the cylinder
made up of stacked plates), there are conductors linked at each end by a
metal ring. The driving torque generated by the rotating field is exerted on
these conductors. For the torque to be regular, the conductors are slightly
tilted in relation to the motor axis. The general effect is of a squirrel cage,
whence the name.
The squirrel cage is usually entirely moulded (only very large motors have
conductors inserted into the notches). The aluminium is pressure-injected
and the cooling ribs, cast at the same time, ensure the short-circuiting of
the stator conductors.
These motors have a fairly low starting torque and the current absorbed
when they are switched on is much higher than the rated current (C Fig. 7).
40
3. Motors and loads
3.1
Three phase asynchronous motors
On the other hand, they have a low slip at the rated torque. They are
mainly used at high power to boost the efficiency of installations with
pumps and fans. Used in combination with frequency converters for
speed control, they are the perfect solution to problems of starting torque
and current.
• Double cage rotor
This has two concentric cages, one outside, of small section and fairly
high resistance, and one inside, of high section and lower resistance.
- On first starting, the rotor current frequency is high and the resulting
skin effect causes the entire rotor current to circulate round the edge
of the rotor and thus in a small section of the conductors. The torque
produced by the resistant outer cage is high and the inrush is low
(C Fig. 7).
- At the end of starting, the frequency drops in the rotor, making it
easier for the flux to cross the inner cage. The motor behaves pretty
much as though it were made from a single non-resistant cage. In the
steady state, the speed is only slightly less than with a single-cage
motor.
• Deep-notch rotor
This is the standard rotor.
Its conductors are moulded into the trapezoid notches with the short side
on the outside of the rotor.
It works in a similar way to the double-cage rotor: the strength of the rotor
current varies inversely with its frequency.
Thus:
- on first starting, the torque is high and the inrush low,
- in the steady state, the speed is pretty much the same as with a
single-cage rotor.
v Wound rotor (slip ring rotor)
This has windings in the notches round the edge of the rotor identical to
those of the stator (C Fig. 8).
The rotor is usually 3-phase. One end of each winding is connected to a
common point (star connection). The free ends can be connected to a
centrifugal coupler or to three insulated copper rings built into the rotor.
A Fig. 8
Exploded view of a slip ring rotor motor
These rings are rubbed by graphite brushes connected to the starting
device.
Depending on the value of the resistors in the rotor circuit, this type of
motor can develop a starting torque of up to 2.5 times the rated torque.
The starting current is virtually proportional to the torque developed on
the motor shaft.
This solution is giving way to electronic systems combined with a standard
squirrel cage motor. These make it easier to solve maintenance problems
(replacement of worn motor brushes, maintenance of adjustment resistors),
reduce power dissipation in the resistors and radically improve the installation’s
efficiency.
41
3
3. Motors and loads
3.2
3.2
Single-phase motors
Single-phase motors
The single-phase motor, though less used in industry than the 3-phase, is fairly
widely used in low-power devices and in buildings with 230V single-phase mains
voltage.
b Squirrel cage single-phase motors
For the same power, these are bulkier than 3-phase motors.
Their efficiency and power factor are much lower than a 3-phase motor
and vary considerably with the motor size and the manufacturer.
In Europe, the single-phase motor is little used in industry but commonly
used in the USA up to about ten kW.
Though not very widely used, a squirrel cage single-phase motor can be
powered via a frequency converter, but very few manufacturers offer this
kind of product.
v Structure
Like the 3-phase motor, the single-phase motor consists of two parts: the
stator and the rotor.
• Stator
This has an even number of poles and its coils are connected to the
mains supply.
• Rotor
Usually a squirrel cage.
v Operating principle
Let’s take a stator with two windings connected to the mains supply L1
and N (C Fig. 9).
The single-phase alternating current generates a single alternating field H
in the rotor – a superposition of the fields H1 and H2 with the same value
and rotating in opposite directions.
At standstill, the stator being powered, these fields have the same slip in
relation to the rotor and hence generate two equal and opposing torques.
The motor cannot start.
A Fig. 9
Operating principle of a single-phase
asynchronous motor
A mechanical pulse on the rotor causes unequal slips. One of the torques
decreases while the other increases. The resulting torque starts the motor
in the direction it was run in.
To overcome this problem at the starting stage, another coil offset by 90°
is inserted in the stator.
This auxiliary phase is powered by a phase shift device (capacitor or
inductor); once the motor has started, the auxiliary phase can be stopped
by a centrifugal contact.
Another solution involves the use of short circuit phase-shift rings, built in
the stator which make the field slip and allow the motor to start. This kind of
motor is only found in low-power devices (no more than 100W) (C Fig. 10).
A 3-phase motor (up to 4kw) can also be used in a single phase arrangement: the
starting capacitor is fitted in series or parallel with the idle winder. This system can
only be considered as a stopgap because the performance of the motors is
seriously reduced. Manufacturers leaflets give information regarding wiring,
capacitors values and derating.
A Fig. 10
42
Single phase short circuit phase-shift
rings
3. Motors and loads
3.2
3.3
Single-phase motors
Synchronous motors
b Universal single-phase motors
Though little used in industry, this is most widely-made motor in the
world. It is used in domestic appliances and portable tools.
Its structure is similar to that of a series wound direct current motor (C Fig. 11).
As the unit is powered by alternating current, the flux in the machine is
inverted at the same time as the voltage, so the torque is always in the
same direction.
It has a wound stator and a rotor with windings connected to rings. It is
switched by brushes and a collector.
A Fig. 11
Universal single phase motor
It powers up to 1000W and its no-load rotation speed is around 10,000
rpm. These motors are designed for inside use.
Their efficiency is rather poor.
3.3
Synchronous motors
b Magnetic rotor synchronous motors
v Structure
Like the asynchronous motor, the synchronous motor consists of a stator and
a rotor separated by an air gap. It is different in that the flux in the air gap
is not due to an element in the stator current but is created by permanent
magnets or by the inductor current from an outside source of direct current
powering a winding in the rotor.
• Stator
The stator consists of a body and a magnetic circuit usually made of silicon
steel plates and a 3-phase coil, similar to that of an asynchronous motor,
powered by a 3-phase alternating current to produce a rotating field.
• Rotor
The rotor has permanent magnets or magnetising coils through which runs
a direct current creating intercalated north-south poles. Unlike
asynchronous machines, the rotor spins at the speed of the rotating field
with no slip.
There are thus two distinct types of synchronous motor: magnetic motors
and coil rotor motors.
A Fig. 12
Cross section of a 4 pole permanent
magnet motor
- In the former, the rotor is fitted with permanent magnets (C Fig. 12),
usually in rare earth to produce a high field in a small space.
The stator has 3-phase windings.
These motors support high overload currents for quick acceleration.
They are always fitted with a speed controller. Motor-speed controller
units are designed for specific markets such as robots or machine
tools where smaller motors, acceleration and bandwidth are
mandatory.
- The other synchronous machines have a wound rotor (C Fig. 13). The
rotor is connected rings although other arrangements can be found as
rotating diodes for example. These machine are reversible and can work
as generators (alternators) or motors. For a long while, they were mainly
used as alternators – as motors they were practically only ever used when
it was necessary to drive loads at a set speed in spite of the fairly high
variations in their load torque.
The development of direct frequency converters (of cycloconverter type)
or indirect converters switching naturally due to the ability of synchronous
machines to provide reactive power has made it possible to produce
variable-speed electrical drives that are powerful, reliable and very competitive
compared to rival solutions when power exceeds one megawatt.
A Fig. 13
Synchronous wound rotor motor
43
3
3. Motors and loads
3.3
Synchronous motors
Though industry does sometimes use asynchronous motors in the 150kW to
5MW power range, it is at over 5MW that electrical drives using synchronous
motors have found their place, mostly in combination with speed controllers.
v Operating characteristics
The driving torque of a synchronous machine is proportional to the
voltage at its terminals whereas that of an asynchronous machine is
proportional to the square of the voltage.
Unlike an asynchronous motor, it can work with a power factor equal to
the unit or very close to it.
Compared to an asynchronous motor, a synchronous one has a number
of advantages with regard to its powering by a mains supply with
constant voltage and frequency:
- the motor speed is constant, whatever the load,
- it can provide reactive power and help improve the power factor of an
installation,
- it can support fairly big drops in voltage (around 50%) without stalling
due to its overexcitation capacity.
However, a synchronous motor powered directly by a mains supply with
constant voltage and frequency does have two disadvantages:
- it is dificult to start; if it has no speed controller, it has to be no-load
started, either directly for small motors or by a starting motor which
drives it at a nearly synchronous speed before switching to direct
mains supply,
- it can stall if the load torque exceeds its maximum electromagnetic
torque and, when it does, the entire starting process must be run
again.
b Other types of synchronous motors
To conclude this overview of industrial motors, we can mention linear
motors, synchronised asynchronous motors and stepper motors.
v Linear motors
Their structure is the same as that of rotary synchronous motors: they
consist of a stator (plate) and a rotor (forcer) developed in line. In general,
the plate moves on a slide along the forcer.
As this type of motor dispenses with any kind of intermediate kinematics
to transform movement, there is no play or mechanical wear in this drive.
v Synchronised asynchronous motors
Type
Permanent
magnet
bipolar
Caracteristics
2 phases, 4 wires 4 phases, 8 wires 2 phases 14 wires
No. of steps/rev. 8
Variable
reluctance
unipolar
24
Operating
stages
Hybrid
Bipolar
12
These are induction motors. At the starting stage, the motor works in
asynchronous mode and changes to synchronous mode when it is almost
at synchronous speed.
If the mechanical load is too great, it can no longer run in synchronous
mode and switches back to asynchronous mode.
This feature is the result of a specific rotor structure and is usually for lowpower motors.
v Stepper motors
Step 1
The stepper motor runs according to the electrical pulses that power its
coils. Depending on the electricity supply, it can be:
- unipolar if the coils are always powered in the same direction by a
single voltage;
- bipolar if the coils are powered first in one direction then in the other.
They create alternating north and south poles.
Intermediate
state
Stepper motors can be variable reluctance, magnetic or both (C Fig. 14).
The minimum angle of rotation between two electrical pulse changes is called
a step. A motor is characterised by the number of steps per revolution
(i.e. 360°). The common values are 48, 100 or 200 steps per revolution.
Step 2
A Fig. 14
44
Type of stepper motors
3. Motors and loads
3.3
3.4
Synchronous motors
Direct current motors commonly named DC
motors
The motor rotates discontinuously. To improve the resolution, the number
of steps can be increased electronically (micro-stepping). This solution is
described in greater detail in the section on electronic speed control.
Varying the current in the coils by graduation (C Fig. 15) results in a field
which slides from one step to the next and effectively shortens the step.
Some circuits for micro-steps multiply by 500 the number of steps in a
motor, changing, e.g. from 200 to 100,000 steps.
Electronics can be used to control the chronology of the pulses and count
them. Stepper motors and their control circuits regulate the speed and
amplitude of axis rotation with great precision.
A Fig. 15
Current steps in motor coils to shorten
its step
They thus behave in a similar way to a synchronous motor when the shaft
is in constant rotation, i.e. specific limits of frequency, torque and inertia
in the driven load (C Fig. 16).
When these limits are exceeded, the motor stalls and comes to a standstill.
Precise angular positioning is possible without a measuring loop. These
motors, usually rated less than a kW, are for small low-voltage equipment.
In industry, they are used for positioning purposes such as stop setting
for cutting to length, valve control, optical or measuring devices, press or
machine tool loading/unloading, etc.
A Fig. 16
3.4
Maximum torque depending on step
frequency
The simplicity of this solution makes it particularly cost-effective (no feedback
loop). Magnetic stepper motors also have the advantage of a standstill
torque when there is no power. However, the initial position of the mobile
part must be known and integrated by the electronics to ensure efficient
control.
Direct current motors commonly named DC motors
Separate excitation, DC motors (C Fig. 17) are still used for variable
speed drive, though they are seriously rivalled by asynchronous motors
fitted with frequency converters.
Very easy to miniaturise, they are ideal for low-power and low-voltage
machines. They also lend themselves very well to speed control up to several
megawatts with inexpensive and simple high-performance electronic
technologies (variation range commonly of 1 to 100).
They also have features for precise torque adjustment in motor or generator
application. Their rated rotation speed, independent of the mains frequency,
is easy to adapt for all uses at the manufacturing stage.
A Fig. 17
DC motor
On the other hand, they are not as rugged as asynchronous motors and
their parts and upkeep are much more expensive as they require regular
maintenance of the collectors and brushes.
b Structure
A DC motor consists of the following components:
v Inductor or stator
This is a part of the immobile magnetic circuit with a coil wound on it to
produce a magnetic field, this winding can be replaced by permanent
magnets specially in the low power range. The resulting electromagnet
has a cylindrical cavity between its poles.
v Armature or rotor
This is a cylinder of magnetic plates insulated from each other and
perpendicular to the cylinder axis. The armature is mobile, rotates on its
axis and is separated from the inductor by an air gap. The conductors are
distributed regularly around it.
v Collector and brushes
The collector is built into the armature. The brushes are immobile and rub
against the collector to power the armature conductors.
45
3
3. Motors and loads
3.4
Direct current motors commonly named DC
motors
b Operating principle
When the inductor is powered, it creates a magnetic field (excitation flux)
in the air gap, directed by the radii of the armature. The magnetic field
“enters” the armature on the north pole side of the inductor and “leaves”
it on the south pole side.
When the armature is powered, its conductors located below one inductor
pole (on the same side as the brushes) are crossed by currents in the same
direction and so are subjected to a Lorentz law force. The conductors below
the other pole are subjected to a force of the same strength and in the
opposite direction. Both forces create a torque which rotates the motor
armature (C Fig. 18).
When the motor armature is powered by a direct or rectified voltage U
and the rotor is rotating, a counter-electromotive force E is produced. Its
value is E = U – RI.
A Fig. 18
Production of torque in a DC motor
RI represents the drop in ohm voltage in the armature. The counterelectromotive force E is related to the speed and excitation by E = k ω φ
where:
- k is a constant of the motor itself,
- ω is the angular speed,
- φ, is the flux.
This relationship shows that, at constant excitation, the counterelectromotive force E, proportional to ω, is an image of the speed.
The torque is related to the inductor flux and the current in the armature by:
T=kφI
When the flux is reduced, the torque decreases.
There are two ways to increase the speed:
- increasing the counter-electromotive force E and thus the supply
voltage: this is called “constant torque” operation,
- decreasing the excitation flux and hence the excitation current, and
maintain a constant supply voltage: this is called “reduced flux” or
constant power operation. This operation requires the torque to
decrease as the speed increases (C Fig. 19).
A Fig. 19
Torque/speed curves of a separate
excitation motor
Furthermore, for high constant power ratios, this operation requires
motors to be specially adapted (mechanically and electrically) to
overcome switching problems.
Operation of such devices (direct current motors) is reversible:
- if the load counters the rotation movement (resistant load), the device
produces a torque and operates as a motor,
- if the load makes the device run (driving load) or counters slowdown
(standstill phase of a load with a certain inertia), the device produces
electrical power and works as a generator.
b Types of direct current wound motors (C Fig. 20)
• a and c parallel excitation motor (separate or shunt)
The coils, armature and inductor are connected in parallel or powered by
two different sources of voltage to adapt to the features of the machine
(e.g.: armature voltage of 400V and inductor voltage of 180V). Rotation is
reversed by inverting one of the windings, usually by inverting the armature
voltage because of the much lower time constants. Most bi-directional
controllers for DC motors work this way.
A Fig. 20
46
Diagrams of direct current motor types
• b series excitation motor
This has a similar structure to the shunt excitation motor. The inductor coil
is connected in series with the armature coil, hence the name. Rotation is
reversed by inverting the polarities of the armature or the inductor. This motor
is mainly used for traction, in particular in trolleys powered by accumulator
batteries. In locomotive traction, the older TGVs were driven by this sort
of motor; the later ones use asynchronous motors.
3. Motors and loads
3.4
3.5
Direct current motors commonly named DC
motors
Operating asynchronous motors
• series parallel motor (compound)
This technology combines the benefits of the series and parallel excitation
motors. It has two windings. One is parallel to the armature (shunt winding)
or is a separate excitation winding. It is crossed by a current that is weak
compared to the working current. The other is in series. The motor has an
added flux under the combined effect of the ampere-turns of both windings.
Otherwise, it has a subtracted flux, but this system is rarely used because
it causes operating instability at high loads.
3.5
Operating asynchronous motors
3
b Squirrel cage motors
v Consequences of variation in voltage
• Effects on the current
Voltage increase has two effect. During the starting phase the inrush current
will be higher than nominal and when the machine will be running, the
absorbed current increases steeply and the machine is likely to overheat,
even when operating at low load. This increase is due to the saturation of
the machine.
• Effect on speed
When the voltage varies, the synchronous speed is not altered but, when a
motor is loaded, an increase in voltage causes the slip to decrease slightly.
In practical terms, this property cannot be used due to the saturation of
the motor, the current increases steeply and the machine is likely to overheat.
Likewise, if the supply voltage decreases, the slip increases and the absorbed
current increases to provide the torque, which may also cause overheating.
Furthermore, as the maximum torque decreases with the square of the
voltage, there is a likelihood of stalling if the voltage drops steeply.
v Consequences of a variation in frequency
• Effect on the torque
As in any electrical machine, the torque of an asynchronous motor is of
the type: T = K I φ.
(K = constant factor dependent on the machine) .
In the equivalent diagram as shown (C Fig. 21), the coil L produces the flux
and Io is the magnetising current. Note that the equivalent schema of an
asynchronous motor is the same as that of a transformer and both devices
are characterised by the same equation.
A Fig. 21
Equivalent diagram of an asynchronous
motor
In an initial approximation, forgetting the resistance and considering the
magnetising inductance only (i.e. for frequencies of a few Hertz) the Io current
is expressed as: Io = U / 2π L f and the flux expressed as:
φ = k Io.
The machine torque is therefore expressed as:
T = K k Io I. Io and I are the rated currents the motor is sized for.
To keep within the limits, Io must be maintained at its rated value, which
can only be the case if the U/f ratio remains constant.
Consequently, the torque and rated currents can be obtained as long as
the supply voltage U can be adjusted to the frequency.
When this is not possible, the frequency can still be increased, but the Io
current decreases and so does the working torque since it is not possible
to exceed the machine’s rated current continuously without running the
risk of overheating it.
To operate with a constant torque at any speed the U/F ratio must be
kept constant. This is what a frequency converter does.
47
3. Motors and loads
3.5
Operating asynchronous motors
• Effect on speed
The rotation speed of an asynchronous motor is proportional to the frequency
of the supply voltage. This property is often used to operate specially
designed machines at high speed, e.g. with a power supply at 400Hz
(grinders, laboratory or surgical devices, etc.). Speed can also be varied
by adjusting the frequency, for example from 6 to 50Hz (conveyor rollers,
hoisting equipments, etc.).
v Speed control in 3-phase asynchronous motors
For a long time, there were not many ways of controlling the speed of
asynchronous motors. Squirrel cage motors mostly had to be used at
their rated RPM.
Set speeds could practically only be obtained by motors with pole changing
or separate stator windings, which are still widely in use.
With frequency converters i.e. AC drives, squirrel cage motors are now
often speed-controlled, so can be used for purposes hitherto confined to
direct current motors.
v Pole-changing motors
As we have already seen, the speed of a squirrel cage motor depends on
the mains supply frequency and the number of pairs of poles. So a motor
with two or more speeds can be made by combining windings in the
stator to correspond to different numbers of poles.
This type of motor can only have1/2 speed ratios (4 and 8 poles, 6 and 12
poles, etc.). It has six terminals (C Fig.22).
For one of these speeds, the mains supply is connected to the three
corresponding terminals. For the other, these terminals are connected to
each other and the mains is connected to the remaining three.
Mostly, for both high and low speed, the motor is started direct on line
involving no special device (direct starting).
In some cases, if the operating conditions require it and the motor allows it,
the starting device automatically moves into low speed before changing
to high speed or before stopping.
Depending on the currents absorbed by the Low Speed (LS) or High
Speed (HS) changes, both speeds can be protected by a single thermal
relay or by two relays (one for each speed).
Such motors usually have low efficiency and a fairly low power factor.
v Separate stator winding motors
A Fig. 22
Types of Dahlander connections
These motors, with two electrically separate stator windings, can produce two
speeds in any ratio. However, their electrical characteristics are often affected
by the fact that the low speed windings have to support the mechanical
and electrical stress of high speed operation. So motors in low speed
mode sometimes absorb more current than they do in high speed mode.
Three or four speed motors can be made by changing the poles on one or
both of the stator windings. This solution requires additional connectors on
the coils.
b Slip-ring motors
v Rotor resistance
The resistor externally inserted into the rotor circuit in this kind of motor
defines:
- its starting torque,
- its speed.
A resistor permanently connected to the terminals of a slip-ring motor
lowers its speed and the higher its value, the more the speed drops. This
is a simple solution for speed variation.
48
3. Motors and loads
3.5
Operating asynchronous motors
v Slip-ring speed control
Slip-ring rotor resistors can be short-circuited in several steps to adjust
speed discontinuously or accelerate gradually and fully start the motor.
They have to support the entire duration of operation, especially when
they are intended for speed control. This implies they can be bulky and
costly.
This very simple process is used less and less because it has two major
drawbacks:
- at low speed, a great deal of power from the mains supply is dissipated
and lost in the resistors,
- the speed obtained is not independent of the load but varies with the
load torque the machine exerts on the motor shaft (C Fig. 23).
For any one resistor, the slip is proportional to the torque. For instance,
the drop in speed caused by a resistor can be 50% at full load and only
25% at half load, whereas at no load, the speed hardly changes and is
closed to the synchronous speed minus the slip.
If the machine is constantly monitored by an operator, this one can change
the resistor value as required to set the speed in a certain area for fairly high
loas, but adjustment is practically impossible at no load condition.
To reach a point of “low speed at low torque”, it inserts a very high resistance
and then the slightest variation in the load torque changes the speed from
zero to nearly 100%. This is too unstable.
Adjustment can also be impossible for machines with specific variation of
the load torque relevant to the speed.
A Fig. 23
Torque speed characteristics of a slip
ring motor
Example of slip ring operation. For a variable load exerting a load torque
of 0.8 Cn, different speeds can be obtained as represented by the sign •
in the diagram (C Fig. 23).
For the same torque, the speed decreases as the rotor resistance increases.
b Other speed control systems
v Variable voltage regulator
This device is only used in low-powered asynchronous motors. It requires
a resistant squirrel cage motor.
The speed is controlled by increasing the motor slip once the voltage
drops.
Its use was fairly widespread in cooling systems, pumps and compressors,
uses for which its torque availability gives satisfactory results. It is gradually
giving way to more cost-effective frequency converters.
v Other electromechanical systems
The other electromechanical speed control systems mentioned below are
less used now that electronic speed controllers are in common use.
• AC squirrel cage motors (Schrage)
These are special motors where the speed is controlled by varying the
position of the brushes on the collector in relation to the neutral.
• Eddy current drives
This consists of a drum connected directly to an asynchronous motor
running at constant speed and a rotor with a coil feeded with direct
current (C Fig.24).
The movement is transmitted to the output shaft by electromagnetic
coupling. The slip of the unit can be adjusted by adjusting coil excitation.
A built-in tacho-generator is used to control velocity with precision.
A ventilation system is used to evacuate the losses due to the sleep. This
was a principle widely used in hoisting apparatus, cranes in particular.
A Fig. 24
Cross section of an eddy current drive
Its structure makes it a robust system with no wearing parts that can be
used for occasional purposes and up to a power of 100kW.
49
3
3. Motors and loads
3.5
3.6
Operating asynchronous motors
Electric motor comparison
• Ward Leonard motor generator set
This device, once very widespread, is the forerunner of DC motor speed
controllers. It has a motor and a DC generator which feeds a DC motor
(C Fig.25).
A Fig. 25
Ward Leonard arrangement
The speed is controlled by regulating the excitation of the generator.
A very small current is used to control powers of several hundred kW in
all the torque and speed quadrants. This type of controller was used in
rolling mills and pithead lifts.
This was the most efficient speed control system before it was made
obsolete by the semiconductor.
v Mechanical and hydraulic speed controllers
Mechanical and hydraulic speed controllers are still in use.
Many mechanical speed control systems have been designed (pulleys/belts,
bearings, cones, etc.). The drawbacks of these controllers are that they
require careful maintenance and do not lend themselves easily to servocontrol.
They are now seriously rivalled by frequency converters.
Hydraulic speed controllers are still widely used for specific purposes.
They have substantial power weight ratios and a capacity to develop
continuous high torques at zero speed. In industry, they are mostly used
in power-assisted systems.
As this type of speed controller is not relevant to this guide, we shall not
describe it in detail.
3.6
Electric motor comparison
The table (C Fig. 26) gives a brief summary of all the types of electric
motor available, their main feature and fields of use.
We should point out the place held by 3-phase squirrel cage motors where
the description “standard” is all the more relevant since the development
of electronic speed control devices has fitted them perfectly to fit closely
to the application.
A Fig. 26
50
Comparison of electric motors
3. Motors and loads
3.7
3.7
Types of loads
Types of loads
We can classify the loads in two families:
- the active loads which put moving a mobile or a fluid or which change its
state like the gas state in the liquid state,
- the passive loads which do not get a driving force like lighting or the
heating.
b Active loads
This term covers all systems designed to set a mobile object or a fluid in
motion.
The movement of a mobile object involves changing its speed or position,
which implies applying a torque to overcome its resistance to movement
so as to accelerate the inertia of the load. The speed of movement is
directly related to on the torque applied.
v Operating quadrants
A Fig. 27
The four possible situations for a
machine in a torque-speed diagram
The figure 27 illustrates the four possible situations in the torque-speed
diagram of a machine.
Note that when a machine works as a generator it must have a driving force.
This state is used in particular for braking. The kinetic energy in the shaft
is either transferred to the power system or dissipated in a resistor or, for
low power, in machine losses.
v Types of operation
• Constant torque operation
Operation is said to be constant torque when the charge’s characteristics
in the steady state are such that the torque required is more or less the
same whatever the speed (C Fig.28).
A Fig. 28
Constant torque operation curve
This is the operating mode of machines like conveyors, crushers or hoists.
For this kind of use, the starter device must be able to provide a high
starting torque (1.5 times or more the nominal rate) to overcome static
friction and accelerate the machine (inertia).
• Operation with torque increasing with speed
The characteristics of the charge imply that the torque required increases
with the speed. This particularly applies to helical positive displacement
pumps where the torque increases linearly with the speed (C Fig.29a) or
centrifugal machines (pumps and fans) where the torque varies with the
speed squared (C Fig.29b).
The power of displacement pumps varies with the speed squared.
The power of centrifugal machines varies with the speed cubed.
A starter for this type of use will have a lower starting torque (1.2 times
the motor’s nominal torque is usually enough).
a
b
A Fig. 29 a/b Variable torque operation curve
51
3
3. Motors and loads
3.7
Types of loads
• Operation with torque decreasing with speed
For some machines, the torque required decreases as the speed increases.
This particularly applies to constant-power operation when the motor provides
a torque that is inversely proportional to the angular speed (C Fig.30).
This is so, for example, with a winder, where the angular speed needs to
drop as the diameter of the winder increases with the build-up of material.
It also applies to spindle motors on machine tools.
The constant-power operating range limited by its very nature: at low speed
by the available current from the speed controller and at high speed by the
torque the motor can provide. The driving torque on asynchronous motors
and the switching capacity of DC motors should therefore be checked
carefully.
A Fig. 30
Decreasing torque operation
The table (C Fig.31) gives a list of common machines with their torque
law depending on speed.
Type of machine
Torque law depending on speed
Conveyor
Constant
Rotary press
Constant
Helical displacement pump
Torque increasing linearly with speed
Metering pump
Constant
Centrifugal pump
Torque increasing with the speed squared
Fans and blowers
Torque increasing with the speed squared
Screw compressor
Constant
Scroll compressor
Constant
Piston compressor
Constant
Cement kiln
Constant
Extruding machine
Constant or decreasing linearly with speed
Mechanical press
Constant
Winders, unwinders
Constant or decreasing linearly with speed
Pulpers
Constant
Sectional machine
Constant
Crusher
Constant
Mixer
Torque increasing linearly with speed
Kneader, calender
Constant or decreasing linearly with speed
Centrifuge
Torque increasing with the speed squared
Machine tool spindle
Constant or decreasing linearly with speed
Hoist
Constant
A Fig. 31
Torque characteristic per machine
When a machine starts, it often happens that the motor has to overcome
a transitory torque, such as in a crusher when it starts with a full hopper.
There can also be dry friction which disappears when a machine is running
or a machine starting from a cold stage may needs a higher torque than
in normal operation when warm.
b Passive loads
There are two types of passive charge used in industry:
- heating,
- lighting.
52
3. Motors and loads
3.7
Types of loads
v Heating
Heating is a costly item for industrial premises. To keep these costs
down, heat loss must be reduced; this is a factor which depends on
building design and is beyond the scope of this guide.
Every building is a specific case and we cannot allow ourselves to give
vague or irrelevant answers.
That said, proper management of the building can provide both comfort and
considerable savings. For further information, please see the Schneider
Electric Electrical Installation Guide or the Cahier Technique 206 available
from the Schneider Electric website.
If necessary, the best solution may be found by asking the advice of the
electrical equipment supplier’s experts.
v Lighting
• Incandescent lighting
Incandescent lighting (trademarked by Thomas Edison in 1879) was an
absolute revolution and, for many years afterwards, lighting was based on
devices with a filament heated to a high temperature to radiate visible light.
This type of lighting is still the most widely used but has two major
disadvantages:
- extremely low efficiency, since most of the electricity is lost in heat
consumption,
- the lighting device has a lifetime of a few thousand hours and has to
be regularly changed. Improvements have increased this lifetime (by
the use of rare gases, such as krypton, or halogen).
Some countries (Scandinavian ones in particular) plan to ban this type of
lighting eventually.
• Fluorescent lighting
This family includes fluorescent tubes and fluocompact lamps.
The technology used is usually “low-pressure mercury”.
Fluorescent tubes
These were introduced in 1938. In these tubes, an electric discharge makes
electrons collide with mercury vapour, which excites the mercury atoms and
results in ultraviolet radiation.
The fluorescent matter lining the inside of the tube transforms the radiation
into visible light.
Fluorescent tubes dissipate less heat and last longer than incandescent lamps
but require the use of two devices: one to start them and one called a
ballast to control the current of the arc once they are switched on.
The ballast is usually a current limiting reactor connected in series with
the arc.
Fluocompact lamps (C Fig.32)
These work to the same principle as a fluorescent tube. The starter and
ballast functions are performed by an electronic circuit in the lamp, which
enables the tubes to be smaller and to be folded.
A Fig. 32
Fluo compact lamps
Fluocompact lamps were developed as an alternative to incandescent
lamps: they save a significant amount of power (15W instead of 75W for
the same brightness) and last much longer (8000 hours on average and
up to 20,000 for some).
53
3
3. Motors and loads
3.7
Types of loads
Discharge lamps (C Fig.33)
Light is produced by an electric discharge created by two electrodes
within a gas in a quartz bulb. Such lamps all require a ballast, usually a
current limiting reactor, to control the current in the arc.
The emission range depends on the gas composition and is improved by
increasing the pressure. Several technologies have been developed for
different functions.
A Fig. 33
Discharge lamps
Low-pressure sodium vapour lamps
These have the best lighting capacity but they have a very poor colour
rendition because they radiate a monochrome orange light.
Uses: motorway lighting, tunnels.
High-pressure sodium vapour lamps
These emit a white light tinged with orange.
Uses: urban lighting, monuments.
v High-pressure mercury vapour lamps
The discharge is produced in a quartz or ceramic bulb at pressures
exceeding 100kPa. The lamps are known as fluorescent bulbs and are
characterised by the bluish white light they emit.
Uses: car parks, supermarkets, warehouses.
• Metal halide lamps
This is the most recent technology. The lamps emit a colour with a wide
spectrum.
The tube is in ceramic to enhance lighting capacity and colour stability.
Uses: stadiums, shops, spotlighting.
• LED (Light Emitting Diodes))
This is one of the most promising technologies. LEDs emit light by means
of an electric current through a semiconductor.
LEDs are used for many purposes but the recent development of blue or
white diodes with a high lighting capacity opens up new avenues, in particular
for signage (traffic lights, safety displays or emergency lighting) and motor
vehicle lighting.
A LED has an average current of 20mA, with a voltage drop of 1.7 to 4.6
depending on the colour. Such properties are suited to very low voltage
power supply, for batteries in particular.
Mains power requires the use of a transformer, which is economically
perfectly feasible.
The advantage of LEDs is their low power consumption which results in a
very low operating temperature and an almost unlimited lifetime. In the
near future, it will be possible to incorporate such a lighting into buildings
at the construction stage.
However, a basic diode has a very low lighting capacity. Powerful lighting
therefore requires a great many units to be connected in a series.
As LEDs have no thermal inertia, they can be used for innovating purposes
such as simultaneous transmission of light and data. To do this, the power
supply is modulated with high frequency. The human eye cannot detect this
modulation but a receiver with the right interface can detect the signals
and use them.
v Powering incandescent lamps
• Constraints of direct powering
The resistance of the filament varies widely due to the very high temperatures
(up to 2500°C) it can reach during operation.
When cold, resistance is low, resulting in a power inrush current for a few
to several dozen milliseconds when the lamp is switched on and which
can be 10 to 15 times that of the nominal current.
54
3. Motors and loads
3.7
Types of loads
This constraint applies equally to ordinary and halogen lamps. It requires
reducing the maximum number of lamps that can be powered by the
same device such as a remote control, modular contactor or relay on
ready-made circuits.
300
200
100
t
0
(s)
-100
• Light dimming
This can be achieved by varying the RMS voltage powering the lamp.
-200
Voltage is usually adjusted by a triac used to vary the triggering angle in
the mains voltage cycle.
-300
The waveform of the voltage applied to the lamp is illustrated (C Fig. 34).
0
A Fig. 34
0,01
Gradual powering of the lamp also reduces, or even eliminates, the power
surge when it is switched on.
Note that light dimming:
- alters the colour temperature,
- shortens the life of halogen lamps when low voltage is maintained for
long periods. The filament is not regenerated so efficiently at low
temperature.
0,02
Current waveform
Some halogen lamps are powered at low voltage through a transformer.
Magnetisation in a transformer can produce power surges 50 to 75 times
greater than the nominal current for a few milliseconds.
Suppliers also offer static converters which do away with this disadvantage.
• Powering fluorescent lamps and discharge lamps
Fluorescent tubes and discharge lamps require control of arc intensity.
This function is performed by a ballast device inside the bulb itself.
The magnetic ballast (i.e. limiting current reactor (C Fig.35) is commonly
used in domestic appliances.
A Fig. 35
Magnetic ballast
A magnetic ballast works in conjunction with the starter device. It has two
functions: to heat the electrodes in the tube and to generate a power surge
to trigger the tube.
(V)
The power surge is induced by triggering a contact (controlled by a bimetal
switch) which breaks the current in the magnetic ballast.
600
400
When the starter is working (for about 1 sec.), the current absorbed by the
light is about twice the nominal current.
200
0
t (s)
As the current absorbed by the tube and ballast together is mainly inductive,
the power factor is very low (0.4-0.5 on average). In fixtures with a large
number of tubes, a capacitor must be used to improve the power factor.
-200
-400
-600
0
0,02
0,04
This capacitor is usually applied to each light appliance.
0,06
Capacitors are sized to ensure that the overall power factor exceeds 0.85.
(A)
In the most common type, the parallel capacitor, the average active power
is 1µF for 10W for all types of lamp.
300
200
100
The parallel capacitor layout creates stress when the lamp is switched on.
0
t (s)
-100
-200
-300
0
A Fig. 36
0,02
0,04
Voltage and current waveforms
0,06
As the capacitor is initially discharged, switching on creates causes a
power surge (C Fig.36).
There is also a power surge due to oscillation in the power inductor/capacitor
circuit.
The electronic ballast (C Fig. 37), first introduced in the 1980s, does away
with these disadvantages.
The electronic ballast works by powering the lamp arc by an electronic
device generating a rectangular alternating voltage.
There are low frequency or hybrid devices, with frequency ranging from
50 to 500Hz, and high frequency devices with frequency ranging from
20 to 60kHz. The arc is powered by high frequency voltage which completely
eliminates flickering and strobe effects.
A Fig. 37
Electronic ballast package
55
3
3. Motors and loads
3.7
3.8
Types of loads
Valves and electric jacks
The electronic ballast is totally silent. When a discharge lamp is heating up,
it supplies it with increasing voltage while maintaining a virtually constant
current. At continuous rating, it regulates the voltage applied to the lamp
independently of fluctuations in the mains voltage.
A Fig. 38
As the arc is powered in optimal voltage conditions, 5-10% of power is
saved and the lifetime of the lamp is increased. Furthermore, the output
of an electronic ballast can exceed 93%, whereas that of a magnetic
device is on average only 85%. The power factor is high (> 0.9).
Electronic ballast schematics
An electronic ballast does however have some constraints with regard to
the layout used (C Fig. 38), since a diode bridge combined with capacitors
leads to a power surge when the device is switched on. In operation, the
absorbed current is high in third harmonic (C Fig. 39), resulting in a poor
power factor of around 55%.
(A)
0,6
0,4
0,2
t
0
(s)
-0,2
-0,4
-0,6
0
A Fig. 39
3.8
0,02
The third harmonic overloads in the neutral conductor. For more information,
see Cahier Technique 202: The singularities of the third harmonic.
Electronic ballasts usually have capacitors between the power conductors
and the earth. These anti-interference capacitors induce a constant leakage
current of about 0.5-1mA per ballast.
This limits the number of ballasts that can be powered when a residual
current device (RCD) is installed (see the Cahier Technique 114 Residual
current device in LV).
Current waveform of an electronic
ballast
Valves and electric jacks
b Forward
To complete the view of industrial loads that can be linked to automation
systems, we should include a brief description of some commonly used
devices: electrically-controlled screwjacks and valves.
Processes require loads to be positioned and moved. This function is ensured
by pneumatic and hydraulic screwjacks, but can also be controlled by
electromechanical ones. These can be built into motor starter units or linked
to regulating devices for, e.g. positioning control. The following pages give
short description of these positioning devices.
There is a very large market in valves to control fluid flow. These are used to:
- arrest fluid flow (stop valves),
- change the fluid circuit (3-channel valves),
- blend products (mixer valves),
- control flow (regulation valves).
Fluids can be liquids or gases (ventilation or chemical industry).
b Electric scewjacks
Linearly driven applications require heavy-duty electric screwjacks that
are powerful, fast, long-lived and reliable.
Manufacturers offer wide ranges of electric screwjacks for practically all
requirements.
v Structure of an electric screwjack
Electric screwjacks (C Fig.40) comprise a control shaft or driving member,
a guide unit and an electric motor.
The photo shows an electric screwjack for linear movement.
The movement of the driving member can be linear, for travel, or rotational.
For linear movement, a screw nut system makes the driving member travel
in a line.
Two of the most common systems are the ball screw and the acme screw.
The acme screw is made of rolled steel and the nut is made of plastic.
A Fig. 40
56
Electric screwjacks
3. Motors and loads
3.8
Valves and electric jacks
This is a fairly cost-effective design with useful properties: plastic and
metal can work together well without catching.
The acme screw works quietly, so it is suitable for offices, hospitals, etc.
Another of its assets is its high friction coefficient. This design is particularly
well suited to screwjacks used in applications where they must be self-locking,
i.e. with no recoil against the mass of the load. For instance, when a screwjack
is used to adjust the height of a table, one with an acme screw enables the
table to withstand heavy loads without altering its vertical position. This means
that no brake or other locking mechanism is required to maintain the load
in place when it is idle.
The ball screw system is used for high performance purposes (C Fig.41).
The ball screws in the screwjack are made of steel and have a row of ball
bearings in a closed system between the nut and the screw.
A Fig. 41
High performance electric screwjack
This design gives a very low friction coefficient between the nut and the
screw due to the rolling contact between the ball bearings, nut and
tracks.
Wear is low compared to an acme screw, so the ball screw has a lifetime
10 times longer in identical operating conditions. This lifetime also implies
that a ball screw can withstand heavy loads and long operating cycles.
Its low friction coefficient makes the ball screw especially efficient because
it does not overheat.
The ball screw is therefore highly suited for situations requiring lengthy
operation at high speed.
A screwjack with a ball screw system has very little play, so its precision
is significantly better in applications where position and precision are
crucial.
v Product family
Electric screwjacks can be made in many different shapes and sizes to fit
easily into machines. Manufacturers also offer control units to make it
easier to operate them.
Document
SKF
The photo (C Fig.42) gives a view of some products offered by one
manufacturer (SKF).
v Selection guide
Choosing the right electric screwjack often requires detailed knowledge of
the application and some calculation.
However, manufacturers’ catalogues can help in making the initial choice
of screwjacks meeting the basic criteria such as load and speed.
A Fig. 42
Electrical screwjacks from SKF
v Screwjack drives and parts
Drives offered by manufacturers.
Electric screwjacks can be driven by:
- direct current motors,
- asynchronous alternating motors,
- brushless synchronous motors,
- stepper motors.
Direct current motors are usually low voltage (12 or 24 volts) for average
forces (approx. 4000N) and medium performance (approx. 50mm/s).
These screwjacks are used on mobile standalone battery-operated devices.
An asynchronous motor drive considerably increases performance up to
50,000N and 80mm/s. These screwjacks are mostly fitted to immobile
machines.
Brushless drives are used for high dynamic performance (approx. 750mm/s)
for forces up to about 30,000N.
Stepper motor drives are used for precision positioning of the load without
recoil.
57
3
3. Motors and loads
3.8
Valves and electric jacks
v Parts and variants
• Built-in controller
Some electric screwjacks have a built-in control device. This is especially
the case in some types of screwjack with a brushless motor drive. These
include a speed controller which can be connected to the automation
system by a field bus.
• Potentiometer
The potentiometer is a movement sensor. This device is used to ascertain
the position of a moving part and align it with precision.
• Thermal protection device
This protects drives and control units from overheating.
• Encoder
This is a sensor which, when it is connected to a control unit, is used to
give the position of the screwjack.
• Stress limiters
Some types of screwjack are fitted with a mechanical safety device similar
to a friction clutch to protect the motor and the reduction unit from
damage.
• Limit switches
These are switches which limit movement in a given direction in mechanical
devices by opening and closing an electrical contact. Limit switches comes
in all shapes and sizes and can be fitted on the inside or outside of the
screwjack.
These safety devices are part of the control system and it is important to
be aware of them when using screwjacks in an automation system or any
other system.
• Mechanical jamming control
This safety device makes the screwjack to stop in case of an excessive
resisting force. It is provided to protect persons from injuries.
• Electrical jamming control
This is a safety option on some electric screwjacks.
It cuts the power to the motor when external stress is applied in the opposite
direction to screwjack movement.
b Valves
Valve operating systems do not enter into the scope of this guide. That
said, as valves can be part of industrial control systems such as regulation
loops or speed controllers, it is useful to have some idea of their structure
and what happens when they work.
v Valve structure
A valve (C Fig.43) consists of a body and a throttle which presses against a
seat. Fluid movement is controlled by an operating rod. This rod is actuated
by electric or pneumatic devices.
Many valves are pneumatically controlled, others are electrically controlled
(solenoid valves).
There are many different valve designs (butterfly, spherical, diaphragm, etc.)
for different types of use, fluid and progression rates (output in relation to
the position of the throttle or the control signal in regulation valves).
The throttle usually has a specific shape to prevent or mitigate any
unwanted effects such as water hammer or cavitation.
A Fig. 43
58
Cross viexw of a valve
• Water hammer
This can occur in hydraulic pipes when the valve is closed. The flow through
the pipe is suddenly stopped and causes this phenomenon known as water
hammer.
3. Motors and loads
Dh
3.8
Valves and electric jacks
As an example (C fig. 44a et 44b)), here is a description of a pumping
station feeding a reservoir above the feed pump.
+C
=
Q
0
When the emptying valve is closed, the water drained from the reservoir
via the pump below the fluid column tends to pursue its movement while
there is no more output from the pump.
Q
This movement causes elastic deformation of the pipe which contracts at
a point near the valve.
0<t<T
Q
A Fig. 44a
=
0
Water hammer (start)
This phenomenon makes the mass of fluid temporarily available and
maintains it in movement.
Depression occurs and spreads throughout the pipe at the speed of elastic
waves C until the entire pipe is affected by it, i.e. after a time T=L/c,
where L is the length of the pipe between the valve and the outlet.
The result is that the pressure where the pipe goes into the reservoir is
lower than the pressure in the reservoir and causes backflow. The wave
spreads from the reservoir to the pumping station and reaches the valve
throttle after a time 2T from the start of the phenomenon.
Dh
The fluid column continues its descent and hits the closed valve again,
causing the pipe to swell and reversing the movement of the fluid.
=
-Q
0
-C
Q
Water hammer would occur indefinitely if the effects of load loss, depression
and overpressure are not gradually dampened.
T < t < 2T
Q
A Fig. 44b
=
0
Water hammer
To overcome this potentially destructive phenomenon, valve closing can be
controlled by a system based on a slow closing law to keep overpressure
and depression within reasonable limits.
Another procedure involves gradually slackening the speed of the feed
pump to enable the valve to close the pipe.
In the case of pumps running at constant speed, the most suitable device
is a soft start device such as Altistart by Telemecanique or Altivar for
speed-controlled pumps.
• Cavitation
Closing a valve results in restricting the section available for fluid flow
(C Fig.45). Applying the Bernoulli theorem, restricting the flow section left
by the valve accelerates the flow and lowers static pressure at that point.
The amount of static pressure drop depends on:
- the internal geometry of the valve,
- the amount of static pressure downstream of the valve.
The pressure when the valve is open is shown on (C curve 1).
Flow is restricted at the point of the closing valve throttle, causing a drop
in pressure and accelerated flow (Venturi effect);
When the throttle closes, the Venturi effect increases and curve 1 is gradually
deformed (C curve 2).
A Fig. 45
Cavitation phenomenul
When the static pressure in the fluid vein reaches the value of the vapour
tension at the flow temperature, vapour bubbles form in the immediate
vicinity of the restricted flow.
When the static pressure rises again downstream of the valve (pressure
P2), the vapour bubbles condense and implose.
Cavitation has the following undesirable effects:
- unacceptably loud noise, rather like pebbles rattling in the pipes,
- vibrations at high frequencies which loosen the valve nuts and other
parts,
- rapid destruction of the throttle, seat and body by removal of metal
particles. Surfaces subject to cavitation are grainy,
- the flow through the valve is related to valve opening.
Regulation valves are often required to operate for a long time in
conditions where cavitation can occur and their lifetime will be seriously
affected by it.
Ways of limiting or preventing cavitation do not enter into the scope of
this guide.
59
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