MotorsAndVariableSpeedDrives

MotorsAndVariableSpeedDrives
From Crosscut – www.wwa.org.za - February 2005
Motors and Variable Speed Drives
©Trevor Pope (tpope AT iafrica.com) – Feb 2005
Inside woodworking machines, you
are likely to encounter two types of
motors – Induction motors and
Universal motors.
I’ll deal with so-called “universal
motors” first. A universal motor has
an armature (the part that rotates)
with copper windings on it and a
commutator that conducts current to
the armature. Typically the
commutator has brushes of carbon
that rub against a segmented copper
ring that switches the current to the
right armature windings for the position of the armature, to generate a magnetic field. The stator field
is the stationary field generated by the field poles, to provide the opposing magnetic field that
generates the torque (turning force) that makes the rotor want to turn. In smaller motors, the field is
created by permanent magnets such as on your battery screwdriver. In bigger motors, the field is an
electro-magnet – it has copper windings as shown in the diagram. (See
http://www.claytonengineering.com/Training/myweb6/101basics.html for more explanations.)
Universal motors have high power-to-weight ratios and tend to turn fast (26000 rpm on your router)
so they are well suited to portable power tools. They are also used in some stationary power tools
such as the club’s Ryobi thicknesser. They are noisy because they turn fast. They also have good low
speed torque characteristics that make them less likely stall. Universal motors can operate on Direct
Current (DC) from batteries and from the Alternating Current (AC) mains supply. Because of their
small size they are inclined to overheat if worked at their maximum ratings for extended periods.
Over time, the brushes wear out and need attention. To keep them cool, they need lots of air blowing
through them, so they are vulnerable to clogging with sawdust.
For the technically minded, I have included a graph that
shows the torque – speed characteristic for a series wound
universal motor (taken from: Electric Machinery 3rd Ed,
Fitzgerald, Kingsley and Kusko which was the text for my
3rd year course in 1978).
The torque and speed are given in percentages, so the rated
torque and speed are 100%. This motor works better with
DC, due the lower impedances of the windings at DC, so a
separate curve is shown for DC operation. You can see that
at lower speeds, the motor is able to develop considerably
more torque, which is helpful to prevent stalling and to start up heavy loads more quickly. Due to
heating effects, the motor will not be able sustain low speeds and high torque demands for very long
without damage. So you can expect high torques for short periods to help through a sticky patch, but
don’t overdo it or you will smell an expensive electrical-insulation-burning smell! Better machines
will have some sort of thermal fuse or resettable cutout to prevent this.
Crosscut – February 2005 – Page 2
In contrast, induction motors don’t have brushes and copper windings on the rotor. The opposing
magnetic fields in the rotor are induced by the changing fields in the stator causing currents to flow,
using the same principle of magnetic coupling as a transformer. Hence the term: Induction Motor.
On smaller motors, the rotor is a composite iron and aluminium assembly (with copper used on
bigger motors) that is one solid piece as shown in the picture
on the right. The stator is a set of copper field windings that
are arranged around the rotor in the body of the motor. The
current that flows in the windings is organised so that it
creates a rotating magnetic field that rotates at the speed of
the frequency of the supply current (50 cycles per second in
South Africa). This rotating magnetic field induces currents
in the armature that flow in the aluminium (or copper) parts
in the rotor. These currents create opposing magnetic fields
that act against the rotating magnetic field from the stator,
thereby creating the torque that tends to turn the rotor. The
induction motor is mechanically simpler than the universal
motor, with only the bearings subject to wear on most motors. Some also have a speed sensitive
switch that is used during starting.
Open frame motors (see left) have holes for air to flow through for
cooling – these are subject to clogging with
sawdust like universal motors. Better motors
for a dusty environment are the so-called
TEFC (Totally Enclosed, Fan Cooled)
motors (see right) that are sealed against dust
and have a fan at the non-driving end that
blows air over the motor to cool it. Some have fins on the outside, to help
conduct heat away. TEFC motors are larger than open-frame motors for the same power output, but
because they don’t get clogged with sawdust, they need less maintenance. In the right conditions, a
TEFC motor should last almost indefinitely.
Because of their need for a rotating magnetic field, induction motors only work on AC. The
maximum speed of the motor is governed by the frequency of the AC field and the number of poles
of the motor. Common motors you will encounter have two poles such as in an electric bench grinder
or four poles such as in a lathe. An induction motor operates with slip, which means that it can only
approach the speed of the rotating magnetic field and cannot exceed it.
The field in a two-pole motor rotates at 3000 rpm (50 cycles per second x 60 seconds per minute =
3000). With slip, the motor rotates at about 2800 rpm when loaded. In a four-pole motor, the field
rotates at 1500 rpm, so the motor rotates at about 1400 rpm when loaded. In bigger industrial
applications, you will encounter 6 and 8 pole motors, but these are not common in smaller machines.
As the load on the motor increases, it slips more and slows down, until the limiting torque is reached
and the motor quickly stalls. Compared to a universal motor, the limiting torque is lower, so the
induction motor is better suited to constant speed applications.
In a three-phase induction motor, the windings are arranged around the stator in three sets, so that the
rotating magnetic field rotates clockwise or counter-clockwise, depending on how the phases are
wired to the motor. Swapping two of the three wires changes the direction of phase rotation and
hence the motor.
Crosscut – February 2005 – Page 3
When a motor starts, the excess torque (if any) over that absorbed by the load accelerates the motor until
the torque demanded by the load exceeds that available from the motor and a steady speed is reached.
This is shown in the graph for an induction motor, where the motor
has reached close to the synchronous speed, where the torque
drops off steeply. The speed settles at the intersection of the two
curves at ωo∞. Should the load demand a lot more torque, the speed
will drop slightly and the motor will be able to deliver it – a fair
approximation to a constant speed is achieved over a wide range of
loads. When the torque demanded exceeds that available from the
motor, the motor will quickly stall.
The rotating field is not so easily arranged in a single-phase motor,
so extra windings are used to simulate the phase rotation, sometimes with a capacitor to induce a phase
shift. To change the direction of single-phase motor, the sense of one of the windings needs to be
changed, but this is not as simple as for three-phase, due to the variety of motors types. Consult with the
supplier to confirm the required wiring changes.
Induction motors generally have low starting torques, and single-phase
motors are worse. Without a start winding, the torque – speed curve
looks like the symmetrical one on the right. At zero speed there is no
torque to start rotation. Without some rotation, the motor will just sit
and hum. This is what happens when the start circuit fails. If the motor
is given a small impulse forward or backwards, some torque is
generated, and it will slowly accelerate up to speed.
Start windings can be arranged without or with capacitors, depending
on the type of load. With a start winding, the torque – speed curve is a
composite of the start (called auxiliary in the diagram) and the main
winding. You can see from the curve on the right, for this particular
motor, there is considerable starting torque. However, the motor
cannot sustain this without being considerably larger, so to prevent
overheating, the start winding is disconnected by a centrifugal switch
when the motor gets up to speed, as shown in the diagram. This high
starting torque is useful to get a machine with a lot of inertia up to
speed quickly, however a lot of current will be drawn for a short time,
dimming the lights, or even tripping a circuit breaker. Also a lot of heat
will be dissipated in the motor, which is why you may need to be
cautious about starting and stopping too frequently. This sort of motor
would be suitable for starting a load like a compressor or a lathe.
If the centrifugal switch fails, you may see starting problems or
overheating of the motor, depending on whether it fails in the open or
closed position.
Single-phase motors generally have two sets of windings, as opposed
to the three in three-phase motors, so they are less smooth. They have lower starting torques and are
generally bigger and more expensive for the same power output. This is why, when three phase supplies
are available in industrial situations, three-phase motors are always used.
Consult the nameplate of the motor to see all the vital information on the motor – voltage, current,
power, speed, insulation and duty cycle ratings.
The type of speed control used depends on the motor type.
Universal motors are mostly insensitive to the frequency of the supply, operating off DC or AC mains,
or a battery, so simply limiting the voltage to them controls the speed of these motors. In the past, a
Crosscut – February 2005 – Page 4
variable resistor was used, which was rather inefficient and resulted in the loss of low speed torque.
Nowadays, an electronic circuit is used that uses AC phase control or DC pulse-width modulation
(PWM). This is much more efficient and preserves the low speed torque of the motor. Even the most
simple battery drill or screwdriver can boast such a circuit that allows you to control the speed from
almost zero to full speed with excellent torque.
Induction motors are more difficult to control. They respond to the frequency of the supply and only
slightly to the voltage changes, so to change the speed, you need give them a variable frequency
supply. In the past, this was difficult, as the mains supply frequency is fixed. However, nowadays, we
have that electronic marvel called an inverter.
So how does an inverter work? Basically it feeds the induction motor with a variable frequency input,
so that it rotates at the corresponding speed. Changing the frequency changes the speed.
An inverter has an input power supply that converts the AC supply to a constant DC high voltage
supply, shown by the diode block on the left and the capacitor. The DC voltage is then chopped into
an AC supply for the motor at a variable frequency as required by the user to control the speed of the
motor. The chopping circuit uses high voltage transistors controlled by the control/power interface
and the microprocessor. It uses Pulse Width Modulation (PWM) to generate an approximation to a
sine wave that keeps the motor running smoothly and reduces losses in the windings. Most chopping
circuits operate in the higher audible range, which is the high-pitched whine you can often hear when
the inverter is operating.
The inverter has a microprocessor that monitors the voltages to the motor and keeps these within
safe-levels to protect the motor against overheating and over-voltage stresses. The microprocessor
can ramp up the speed slowly at start-up to prevent massive start currents stressing the motor and
the mechanical components. It can also implement dynamic braking to slow down the motor more
quickly to a stop if required. This is where the motor is turned into a generator that dumps energy
into a resistor, generating a braking torque to slow it down more quickly. The inverter can be
programmed to control the speed against an input or control signal to keep it constant if needed, by
compensating for slip at different loads.
Crosscut – February 2005 – Page 5
An inverter has the advantage of a widely variable speed range that is continuously variable from
some practical minimum, such as 30 rpm up to the rated speed of the motor (1500 rpm for a 4 pole
motor). The inverter can be set up to compensate for speed variations at different loads. It can ramp
the speed up and down smoothly to prevent mechanical shocks to the drive train. You can use it with
a speed control knob that is quick and easy to adjust. When used with a three-phase motor, if you
want to reverse the direction for sanding, just press reverse. The inverter will safely slow down to a
stop and reverse up to the same speed turning in the other direction. Obviously, on a lathe, you need
to make sure that your work will not unscrew from the spindle when operating in reverse!
Attaching an Induction Motor
What an inverter cannot do is give you more low-speed torque than the motor is capable of. The
torque is limited by the magnetic fields in the motor that are created by the currents that can flow in
the windings, so the inverter is programmed to limit this to safe levels for the
motor, to prevent overheating. Fan cooled motors rely on the fan operating
at near to rated speed, so if the motor is operating at much below rated
speed, cooling is drastically reduced. A fan is a square law device – the
airflow increases with the square of the speed, so at low speeds a fan is
largely ineffective. Special inverter rated motors are supplied with
independent fans to maintain cooling at low speeds (see right where an auxiliary fan can be seen on
the left of the motor). So with a standard induction motor, you cannot get large torque outputs at slow
speeds just using an inverter. You need some other sort of torque multiplying device.
A mechanical torque-multiplying device is still needed if large torque at slow speeds is required.
What is a torque-multiplying device? Examples are a reduction gearbox or a pulley drive with
different pulley sizes – a smaller one on the motor shaft, and a larger one on the spindle shaft. The
gearbox on your car multiplies the engine torque in low gears for better acceleration. You can see
examples of a multiple speed pulley drive on your drill press or lathe. Most lathes fitted with a
variable speed drive still retain two or more pulley options, so that you get high torques at low
speeds if you need them (such as the VB36 and the Stubby lathes). Those that don’t often have a
higher minimum speed at reasonable torque outputs to stay within the limitations of the motor such
as the Nova DVR (100 rpm). The DVR has a special 1.3 kW motor and controller that are carefully
matched for optimum performance. The controller is programmed to keep the range of operating
speeds and torques within the safe operating region of the motor, so you know that you cannot
overheat the motor. The DVR is designed to give you good low speed torque characteristics that you
won’t find just by adding an inverter to a three-phase motor alone.
Adding an inverter to an induction motor.
The speed flexibility offered by an inverter is a great asset on a wood turning lathe. You can rapidly
change the speed and also make small changes to avoid vibrations that can set up a resonance.
Most lathes come with single-phase motors, and these motors are not really suitable for use with an
inverter. It can be done, but the inverter needs to be larger and is more expensive that the equivalent
three-phase inverter, so the combination of an inverter and a new three-phase motor will be cheaper
and better. When I last priced these, a combination one horsepower (0.75kW) inverter and threephase motor cost between R3000- and R4000-, with the motor coming in at less than a R1000-. If
you are converting an existing lathe, you may be able to sell off the single-phase motor to recover
some of the cost.
Smaller inverters usually run off a single-phase supply, and there are some that will run off both
single and three-phase. If you are converting a single-phase machine, it makes sense to buy the
inverter and a new three-phase motor as a matched combination, so that they will be appropriately
rated for your application (power, duty cycle and torque characteristics). There are several suppliers
and the ones I have contacted offer good technical support. The inverter manuals that I have seen
are comprehensive. Inverters are very sophisticated these days, but most of the options can be left
at the default settings, and the supplier should help you with any changes needed for your
application.
Crosscut – February 2005 – Page 6
If you have an existing three-phase motor that you wish to connect to an inverter, you need to be
more cautious. Modern motors come in “Inverter rated” versions, which is what you should get when
you buy a combination as above. The grade of insulation needs to be higher, due to the extra voltage
stresses imposed by the high switching frequencies generated when the inverter generates the
approximate sine waves.
You may wish to add an inverter onto an existing machine that is fitted with a three-phase motor,
such as a saw bench to operate on a single-phase supply. These usually operate at a constant
speed, so there is little to be concerned about. It might be cheaper to change the motor to a single
phase one, unless you have several three-phase machines. These can all be operated from a single
inverter, with the appropriate power ratings. You must check the cable lengths, as long cables can
induce high voltage spikes in motors driven by inverters (leading to insulation failures) – check with
the suppliers.
When you are using an existing three-phase motor, as mentioned above, and you wish to vary the
speed, then you need to worry about the operating speed range of the motor, and particularly cooling
at low speeds. If you want to operate at low speeds only for light finishing operations on a lathe, you
should be OK. That should not stress the motor too much. However, if you plan on roughing out
large blanks with heavy cuts at 200 rpm, then you need to be careful. (Large torque requires large
currents, which have large I2R losses, and without the required cooling air at low speeds, the motor
will overheat.) If you have a data sheet on the motor, you can often program the inverter to protect
the motor. For reliability, you also should consider either a larger motor with a better duty cycle rating
and/or a torque-multiplying device, such as a lower pulley ratio.
In other words, for heavy cuts at low speeds, choose a pulley ratio that runs the motor quickly and
the work-piece slowly. Then the motor runs fast enough to stay cool, and the work-piece runs slowly
enough, but with all the torque you need for heavy cuts.
You may also want to add on separate start, stop and speed controls. These can be housed in a
separate box located in the most convenient place, and are more robust than the membrane keys on
the panels of most inverters. The supplier may be able to sell you some parts and advise you on how
to wire them up if the manual is not clear.
All this may sound rather technically daunting, but if you ask around the club, you are bound to find
one or two electrical engineers and electricians who are comfortable with this technology, who would
be able to help you. Also the suppliers are helpful. When you buy, take along the switchgear and
cables you have and ask them to show you how to wire everything up. They may also help you
configure the inverter for your application. Before you switch on, get an electrician to check your
wiring is correct and safe.
Peter Middleton suggested that I give more information on how
induction motors are wired up, so that you know what to look for.
Three phase induction motors can be wired up into two
configurations, called star and delta.
Star: The star configuration (a) has the windings connected to one
common point at one end, called the neutral point (0), and the other
ends are connected to the phases. The phase-to-phase voltage
here is commonly 400V, which is used in industrial installations (it
was previously 380V which relates to 220V). The voltage across the
individual windings is still 230V (The phase to neutral voltage). Due
to the phase relationships between the phases, the line-to-line
voltage is √3 x phase voltage. If you draw a vector diagram and
calculate the voltages between the three phases that are 120º out
of phase, you can show this.
Delta: If you look at the diagram (b), you will see that the delta configuration has the three windings
arranged in the form of a triangle, hence the delta term. The windings are connected directly across
Crosscut – February 2005 – Page 7
the phases. For the motors that interest us, the phase-to-phase voltage is 230V. (The new IEC
standard is 230V, which harmonizes the mixture of standards previously implemented, which were
220V and 240V.)
Usually, if you have a three-phase induction motor that is wired for
400V as a star configuration, it can be wired for 230V in a delta
configuration. Inside the motor terminal box are six terminals for the
ends of the three windings, and these can be rewired by changing the
jumper strips. There seems to be a standard way of doing this as
shown in the diagrams (to BSS822), but it is just as well to be
cautious and measure the windings to make sure if no information is
given with the motor. The upper diagram shows the links connected
for a star configuration. The lower diagram shows the links connected
for a delta configuration. You can check this by removing the links
and checking continuity to identify the individual windings. For the
individual windings A1-A2, B1-B2, and C1-C2 the locations should
correspond to the diagrams.
Some motors only have three terminals, and these are often wired
internally in a star configuration for 400V, with the neutral buried in
the motor. By dismantling, the neutral points can sometimes be
brought out, but this should be done with caution: you need to
know what you are doing, to get the phasing right.
To summarize, for domestic applications with a 230V single-phase
supply, using a 230V inverter, you need to connect the motor for
230V in a Delta configuration (lower diagram). In an industrial
application, where you have a three- phase supply, 400V phase-to-phase, the same motor will be
wired in a Star configuration.
Bear in mind we are dealing with 230V AC Mains here which is potentially dangerous, so
you must know what you are doing and work carefully, to guard against errors. If you are
unsure, before you switch on, measure all the current paths to look for shorts and imbalances. Ask a
licensed electrician to check the circuit for you. (There is an advert for Stan Lewis in this issue.)
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