Speed Control for Fractional HP Motors - FE-1000

Information and Recommendations for the Engineer
Speed Control For
Fractional Horsepower Motors
As fan and blower use has evolved over the years, the
way the speed is controlled has also evolved. For all
practical purposes this article will explain speed control
for the AC, induction, squirrel cage, asynchronous motor.
Before attempting to explain the differences in speed
controls you must have a basic understanding of AC
motors. Below is a brief overview but it is recommended that you read other engineering topics on AC motors
to fully understand how they operate.
Overview of AC
Induction Motors
There are two major AC induction motor groups that are
usually used in fans and blowers: the single-phase motor
and the polyphase motor. In the polyphase group the
one that is the most commonly used is the three-phase
induction motor. In the single-phase motor group there
are five different sub groups that are often used. They
are split-phase, capacitor-start (CS is a variation of the
split-phase motor), shaded pole, permanent split capacitor (PSC) and capacitor start-capacitor run (CSCR is a
variation of the PSC motor).
Usually the split-phase, CS and CSCR motors are
eliminated in variable speed control situations because
they all have a start winding or a contact that needs to
reach 75% of the motor’s full load speed before it disconnects the start winding. Often this 75% is not reached
when controlling the speed. If the start switch is not
disconnected, the result is that the contacts or start
winding will quickly burn out and the motor will overheat
and shut down. We cannot eliminate these motors totally when deciding what induction motor to use, however,
because there are still mechanical means of controlling
the speed of a fan that will allow these motors to reach
their full RPM. Or these motors could theoretically be
used if they would never go slow enough to re-engage
the starting contacts while running in normal operation.
The remaining motors, shaded pole, PSC, and polyphase, are the best choices when selecting a motor that
will be speed controlled. The shaded pole motor is the
most economical to use out of these three motors
because of its inherent design, but it is only available in
subfractional to 1/4 horsepower. This is not the only
limitation it has; it also has a low starting and run torque
and is inefficient. So for single-phase operation where the
horsepower is greater than 1/4 and/or there is a high run
torque, the PSC motor is usually used. Although this type
of motor is one of the more expensive single-phase
motors, it is very reliable, has good running and starting
capabilities, and allows the option of controlling the speed
through an electronic apparatus. The PSC motor functions
the closest to the way the three-phase motor does which
leads to the discussion of the three-phase motor.
The three-phase motor is available in fractional and
integral horsepowers but in general its electrical savings
does not outweigh its cost until you get into the integral
horsepower range. It essentially operates in the same
manner as a single-phase motor although instead of
there being one hot electrical lead there are three.
Controlling the speed can require a different control than
the single-phase controller just because single-phase
and three-phase function slightly differently.
Common Methods
of Speed Control
Now with an idea of the types of fractional horsepower
motors that are most commonly used in fans and blowers we can introduce the common methods of controlling their speed. There are four basic groups of speed
control that are often used with fans and blowers. The
simplest is the variable speed motor. The remaining
three groups are passive device speed controls, solidstate controls, and mechanical devices. Each way is
unique and is used throughout the industry but you must
be careful when trying to match any speed control with
a motor. Whatever the application may be, be sure to
check with the motor manufacturer to be assured their
speed control criteria is being met because some motor
manufacturers will void their warranty if the motor is not
approved for speed control. Also, be aware of all safety issues such as thermal overload protection as this
needs to be part of the speed control system. There is
often a substantial amount of heat generated at the
motor when speed controllers are used. To prevent
accidents or damage it is always recommended that a
thermal overload protector either be part of the motor
or of the controller itself.
A brief list of how an induction motor’s speed can
be changed other than mechanically is: affect the electrical frequency sine wave, change the number of motor
stator poles, adjust the power input, or control the rotor
slip. The formulas for motor speed that all speed control
is based on are:
horsepower (HP) =
speed (RPM) =
torque (lb.ft) x speed (rpm)
5250 (constant)
120 x motor supply frequency
number of motor poles
Basic AC Electricity
To have a good understanding of the technical talk about
speed controllers you must have a basic understanding
of electricity. It is best to refer to other articles to explain
this but for general purposes Figure 1 shows a singlephase AC frequency sine wave which is what most modern controllers affect in some way to control motor speed.
In the U.S. if we were to look on an oscilloscope at the
sine wave of single-phase line electricity we would see
what is represented in Figure 1. The sine wave of threephase electricity is represented in Figure 2.
©2000 Twin City Fan Companies, Ltd.
Figure 1. Single-phase sine wave
trolling the motor’s electromagnetic characteristics by
altering the incoming voltage. This reduces the entire
sine wave in amplitude which really is changing the
frequency sine wave’s height. Figure 3 shows the sine
wave and the current present as a percentage when
using a passive device speed control.
Figure 3. Reduction in sine wave amplitude
Figure 2. Three-phase sine wave
The humps that go up and down represent the current
and the wave is described as going 360 degrees. In the
U.S. this occurs 60 times per second and is referred to
as 60 Hertz. In other parts of the world this speed is 50
times per second (50 Hertz). If you look at the horizontal
line in the center, which is called the “zero line,” you can
see that a single sine wave crosses it 120 times a second. Since motors rotate in a direct relation to the electricity provided we can understand how speed controllers
function by affecting the electrical input to the motor. The
following are the several ways that motor speed is controlled throughout the fan and blower industry.
Multi-Speed Motors
Most motor manufacturers offer motors that have more
than one available speed without actually having a
separate control to alter these speeds. Multi-speed
motors use either a tapped winding or several windings
that are alternated. Speed is changed manually with a
switch or controlled through a circuit board. The motors
normally used for multi speeds are the PSC and the
shaded pole single-phase motor. The PSC motor is the
most commonly used and can have either tapped or
alternated windings. The shaded pole motor can be
manufactured with tapped windings.
One limitation that multi-speed motors present to any
fan situation is that the speed required for the application may not always be available since there are limitations to the combinations of speeds the manufacturers
offer. The other downfall is that the switches that are
used are often the inexpensive kind where the speed
needs to be changed manually. These motors work well
with fans that are used for exchanging a set amount of
air without variations at a given time. This type of fan
motor is relatively economical but an exact airflow is
hard to match because of its speed inflexibility.
Passive Device Speed Controls
Passive device speed controls are often the cheapest to
buy but are often the most expensive to use because
of the way they waste electricity. They operate by con2
This is old technology and because of its efficiency
rating it is being overtaken by more modern methods
using switches that are more efficient and becoming
The most common motors used with passive device
speed controls are the PSC, shaded pole, and the threephase motor. Whichever motor is used, the load should
never be greater than what the motor is rated for at
startup when using the specific type of control or it will
malfunction. This is usually 150-200% of the running
torque at full speed. This control is often applicable with
fractional horsepower motors where we usually don’t
have to worry about the load or the efficiency. Although
passive device controls are cheap and reliable they usually require someone to manually operate them and
exact speeds may be hard to achieve because of inconsistent voltage outputs. Below are the most common
passive device controls used.
Series Resistors
Rheostat: When this device is activated it varies the
voltage across the winding of a motor creating motor
slip. Typically a rheostat consists of a resistance element
equipped with two contacts, one that slides and one
that is fixed. When the sliding contact moves along the
resistance element towards the fixed contact, the distance is changed and so is the speed that the current
leaves the element. This change in current speed can
be translated into variable speed when a motor is
attached in line. The disadvantage of this type of control
is that the motor needs to start at a high speed and
then be slowed down, otherwise the contacts will get
too hot and burn out. Another disadvantage is that
rheostats must be manually operated.
Potentiometer: The potentiometer is very similar to the
rheostat except it has a resistance element that is
attached to the circuit by 3 contacts instead of 2. The
arrangement of the slide and how the current is changed
are also slightly different. The ends of the resistance
element are attached to two input voltage conductors of
the circuit. The third contact, which is attached to the
output of the circuit, is usually a movable contact that
slides across the resistance element. The resulting resistance determines the magnitude of the voltage applied
to the circuit that also can be translated to variable
speed when a motor is attached in line. The disadvantages are the same as the rheostat.
Transformer: This may be used in place of a resistor
Fan Engineering FE-1000
Solid-State Controls
Over the past several years there have been huge
advances in solid-state control technology and how it
can be used to control motors. Most of the present
product development being done on speed controllers
involves solid-state controls. Some controls are even
being incorporated into the motors themselves. Solidstate controls are usually very reliable, can give exact
speeds and are becoming more economically feasible to
use with fractional horsepower motors. In the past these
types of controls were usually only used with integral
horsepower motors where the electrical savings justified
their expense. But the advances in electronic components along with decreases in cost have changed the
way they are used today.
Most of the different styles of solid-state controls
incorporate the varying of the electrical frequency and/
or current to control the motor speed. They can even
vary the frequency to be higher than the normal line
frequency meaning we can increase the speed beyond
what the motor is rated for.
The motors commonly used with solid-state controls
are the shaded pole, PSC and three-phase.
Even though these types of controls are more accurate than less complex controllers used in the past, in
regards to speed and torque control there are still several disadvantages or problems that occur while using
them. For example most manufacturers of these speed
controls do not recognize the damages that can occur
to a motor by changing the harmonic frequency. Some
other disadvantages to be aware of are that a current
over what is specified can be very harmful to the motor
winding laminates, voltage spikes from the inverter can
do a lot of damage to the windings, harmonics higher
than what the motor parts have been tested for can
damage bearings or laminates, sound and vibration can
be altered creating their own damage, and of course
there is a greater risk of fire. So be careful when matching a solid-state control with any motor and always ask
the motor and control manufacturers about the consequences of using a controller.
Variable Frequency Drives (VFD)
or AC Inverters
Triac: This type of control varies the speed of a motor
by chopping part of the frequency sine wave in order
to alternate the current frequency. A triac is really a
switch that turns on but will not turn off until there is
not current flowing through the device. To better understand this we should refer to Figure 1 and look at the
frequency sine wave. We can see that the current does
shut off on a standard 60 hertz cycle 120 times per
second on a 60 Hertz frequency line when the wave is
equal to the zero crossing line. This means that if we
turn the switch on milliseconds from this zero point line
the sine wave will be altered. See Figure 4.
By altering this time that we switch this device on
Figure 4. Chopped sine wave (Triac)
and in general works in the same way. It consists of
two coils of wire, electrically insulated from each other,
and arranged so that a change in the current in one will
change the voltage in the other. This decrease or
increase in voltage is what varies the speed of the
motor. The transformer has the advantage over the
series resistors of not losing power wasted into heat on
a low speed startup which also means that the load can
be started at a slow speed. The disadvantages of this
type of control are that they are usually large in size,
need to be manually operated, and are not as economical as a series resistor.
the sine wave will ultimately be affected and therefore
change the speed of an induction motor. These devices
are relatively inexpensive and can repetitively change the
speed day after day in the same manner. Simple measurement techniques used in conjunction with this type
of control often make it effective enough to justify its
expense. The disadvantage of this control is that by
chopping the sine wave in half it is essentially turning
the motor on and off which damages the motor by
generating a lot of heat at the windings. Some of the
less expensive models of this control you may notice will
buzz because they are not made well and the noise of
this sine wave being chopped can actually be heard.
Pulse Width Modulation (PWM): This is a general purpose AC inverter that incorporates a complex circuit
design into an electronic programming module to vary
the frequency sine wave. It presently is the most popular VFD and has been since the prices of electronic
components and circuitry were lowered allowing it to be
sold at an economical price. One stipulation that there
is to using this control is the motor must be threephase, but the input power to the PWM can be either
one- or three-phase. By converting the AC power to DC
and then converting it back into AC the output closely
approximates a sinusoidal (sine wave) current wave form
allowing the variable speed control of any AC induction
motor. Figure 5 represents the frequency sine wave and
how it is affected with a PWM. A logic circuit and software control the switching to provide this variation in
voltage and frequency.
Figure 5. Pulse width modulated sine wave
(line voltage from inverter)
The disadvantage of the PWM inverter is that there
is a potential danger of inverter-induced voltage spikes
(see Figure 6) which can create serious damage to the
motor. What happens is that when the inverter is rapidly switching it induces a voltage overshoot which when
attached to a motor can cause voltage spikes that are
three times higher than what the motor winding is rated
for. Voltage spikes are accentuated by longer lines
between the controller and the motor so it is important
to have short lines. Voltage spikes can also cause high
voltages to be induced in the rotor which can cause
Figure 6. Inverter duty spikes (actual voltage at motor)
Fan Engineering FE-1000
arcing in the bearings and result in short bearing life.
These high voltages are not the only damaging thing
because along with these spikes comes a phenomenon
known as corona. Corona is a breakdown in air molecules in the air that is between the adjacent conductors
in the motor winding. The resulting damage from corona
and voltage spikes has been known to literally poke a
hole in the motor’s insulation. There are now inverter
rated motors that offer extra insulation and corona treatment to protect from this but in any circumstance you
should be aware of the wave form that the inverter is
giving off and its compatibility with the motor before you
use it. Also there are some PWMs that create more
spikes than others and there are some PWMs that protect against spiking better than others do.
Current Source Six-Step Inverter: The six-step inverter
was one of the first advanced motor controls used to
control motor speed by varying the shape of the frequency sine wave. It was mainly used for high horsepower motors and is now replaced by the Pulse Width
Modulator. It is named from the shape of the waveform
that it generates. See Figure 7. The normal line voltage
is rectified and fed into an inverter and produces an
alternating square wave voltage that looks like six steps
and takes the basic shape of the normal sine wave. This
can be adjusted in height and is read by the motor as
a change in frequency (speed). The disadvantages are
similar to the PWM plus it is very expensive especially
with fractional horsepower motors. The six-step inverter
can also cause torque pulses in the motor resulting in
increased vibration.
Figure 7. Six-step inverter sine wave
Vector Control: This allows an AC motor to act like a
DC motor in relation to the speed and torque being
linear. This is accomplished by allowing independent
control of the field flux and rotor current. To do this the
motor control must regulate the instantaneous magnitude
and phase of the stator’s current. This can be interpreted as torque or slip frequency. The biggest downfall
of this type of control is that along with it being very
expensive the exact speed of the motor must be known
and programmed into the controller in order for it to
function correctly. Since most of the motors we use in
the fan and blower industry are asynchronous the exact
speed is unknown.
Switched Reluctance: This method of motor controlling
is very complex and also allows the motor to possess
the qualities of both the DC and AC motor. The rotor
position of the motor is the most important factor with
this type of control. Because the position of the rotor is
always a hard thing to measure cheaply in an asynchronous motor this controller hasn’t been feasible to use
until recently. It regulates the speed by measuring the
current and voltage in each winding. From this the
inductance (plus the speed and torque) can be esti-
mated through complex software. This type of controller
seems to be the most promising for the near future
since a lot of work is being done to perfect it. This will
undoubtedly make switch reluctance controlled motors a
very popular method of speed control since the AC
motor acts more like a DC motor. Most of us know this
as the ultimate since torque and speed are directly
related in a DC motor. The disadvantages known at this
time are related to expense since this technology is just
becoming popular. The latest breakthrough in this technology is that the position of the rotor is being determined without using a sensor which has decreased the
cost significantly and has made it more reliable.
Mechanical Devices
Mechanical devices are some of the oldest forms of
controlling the speed of fans and blowers. They can be
used alone or with other types of controls. In most
situations they have a lot of advantages over electrical
forms of speed control. For example, you don’t have to
be concerned about electronics failing and having an
expert figure which part has failed. With mechanical
devices you can usually see which part has failed and
most mechanical people could fix the problem if needed.
Unfortunately the speeds are not usually adjustable during normal operation and the mechanisms require a lot
of maintenance such as lubrication.
Sheaves: This is the most common form of controlling
speed in a belt drive situation. There are even sheaves
that are fully adjustable to where you can obtain exact
speeds. But when sheave diameters are changed usually the belt length needs to be changed also. Sheaves
have a tremendous advantage in a large fan or blower
where the speed needs to be slower or faster than what
standard motors offer or where direct drive cannot be
used. They can also carry a large load easily as long
as there is the correct belt tension on the sheaves. Most
of us are familiar with this type of product and know it
is not adjustable while the fan or blower is running which
is a huge downfall. This arrangement is also usually one
of the highest maintenance because of all of the parts.
Transmission Products (Gear Reducers, Gears, Clutches,
etc.): This form of speed control is usually not used in
fans and blowers but it should not be eliminated from
consideration. Transmission products are familiar to most
of us as being used in applications where, unlike fans,
the load does not change when the speed is slowed
down. These are usually reliable but often very expensive and cumbersome. They often require high maintenance and often need to be manually adjusted in order
for the speed to change.
To properly select a speed control you must thoroughly
understand how it will be managed, what kind of motor
it can adapt to, and expense/efficiency limitations
involved. As noted, some controllers may be less costly
and not as user friendly as others but may still get the
job done. Hopefully this paper will help you in determining which speed control and motor to use for your
Twin city fan & blower | www.tcf.com
5959 Trenton Lane N | Minneapolis, MN 55442 | Phone: 763-551-7600 | Fax: 763-551-7601
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