The participant will:
· Understand the difference between motor control using control products and AC drives
· Understand the benefits of AC drives
· Recognize applications for AC drives
· Understand the fundamentals of how AC drives work
· Understand the basics of load considerations and drive braking.
All electric motors require a control system. That control may be as simple as an ON/OFF
switch, such as for an exhaust fan. Or, the operation may be so complex that a computer must
be used in the control system, such as in an automobile assembly plant application. Both the
exhaust fan and assembly plant electric motors are provided with start and stop control by their
control systems. But the difference between their control systems is how they provide that
control. In addition to start/stop control, a motor control system may also provide motor overload
protection as well as motor speed and torque regulation.
Square D has a very comprehensive line of control products such as motor starters, contactors,
switches, disconnects etc. These products are used in applications which include on/off controls
with jog and reverse capabilities for pumps, compressors, fans, conveyors, meat cutters, textile
looms, and wood and metalworking machines, just to name a few. Sometimes these types of
control devices (switches, contactors, etc.), are called “across-the-line” starters. This is because
full voltage and current are applied directly to the motor. An AC motor, as you learned in
Chapter 1, when switched on like this tends to run at it’s maximum rated speed and torque. For
many applications this is a perfectly acceptable situation. But there can be problems:
· The locked rotor current (also called “inrush current”) for a motor during starting can be six to
thirteen times the normal operating current. This can cause problems if the electrical
distribution system is already loaded near capacity, because excessive current draw can
cause interruption of the whole system. The excessive starting current (multiplied by the
number of motors in the facility) can cause the demand factor on the electric meter to
become very large which may double or triple the electric bill.
· Another problem created by large locked rotor currents is the wear and tear on switchgear.
When motors are allowed to draw maximum current, they can cause arcing and heat buildup
that stresses contacts and switchgear. This stress causes equipment such as disconnects,
and motor starters to wear out prematurely.
· A problem may also arise when loads are started at full torque. The starting torque of a
squirrel cage motor can be as high as 140 percent of the normal operating torque. Sudden
starting torque can damage the equipment or in the case of a conveyor, spill the materials
being conveyed.
· It may also be important to control the time it takes a motor to stop. In some applications it is
important that the load stop at exactly the time and location when the motor is de-energized.
In normal motor operation, when a motor is de-energized, the load is allowed to coast to a
stop, which means that the larger the load is, the longer the coasting time. This causes the
load to be located at random, which may be unacceptable in certain applications. For
example, if large cutting blades are turning at high speeds and are allowed to continue
rotating after power is removed, then an unsafe condition could exist.
Because of one or more of these concerns, over the years, various methods have been tried to
exert some control over motors. Some of the types of adjustable speed control that have been
used are:
· Mechanical - Mechanical methods involve using devices such as brakes, clutches, and
gearing to control motor speed. As you can imagine, these methods are not particularly
efficient.
· Hydraulic transmission control - Physically the configuration includes a torque converter.
This method is not used widely today:
¾ Advantages: infinite speed control and moderate cost
¾ Disadvantages: complex installation and high maintenance.
· Electro-mechanical speed control - A combination of mechanical and electrical controls are
used. This method of control has fallen from current favor, because it is very inefficient:
¾ Advantages: simple and moderate cost, easy to maintain.
¾ Disadvantages: discrete incremental control only. Uses wound-rotor motors which are
usually non-stock, creating higher expenses and more maintenance problems.
· Eddy current - A magnetic clutch is used to adjust motor output speed in infinite increments.
It is a simple idea, and it works. However, it is very inefficient and a lot of energy is lost
through heat.
¾ Advantages: simple
¾ Disadvantages: poor efficiency and requires cooling, either by water or air.
· Electronic speed control (AC Drives) - Direct electronic controls are used most often to
control speed:
¾ Advantages: most efficient speed control, low maintenance, most flexible of all control
schemes.
¾ Disadvantages: can be more expensive at initial purchase, but saves money over time.
Energy savings and reduced wear and tear on machinery will quickly repay the initial
investment.
What Is The Difference Between a Soft Start and an AC Drive?
Square D makes different types of electronic motor control devices, two that you need to make
sure you can tell the difference between are:
· Soft start devices (i.e., Altistart 23 and 46)
· AC drives (i.e., Altivar 16, 18, 56 and 66).
A soft start device reduces the voltage, thus reducing the current, at startup to relieve the stress
on the motor and machinery. There are many, many applications where it is critical to do just
this. In addition, the soft start can allow a motor to have smooth acceleration up to 100%
operating speed and can control a motor’s smooth deceleration back down to zero as well.
An AC drive can also be a “soft start” device but it can also vary the speed and torque of the
motor according to changing machine requirements. In other words, after start up, the motor
does not have to run at 100% speed and torque but these elements can be varied to suit the
application.
The soft start device reduces the voltage thus reducing the current at startup. While an AC drive
does not reduce the startup torque. This can be a very significant factor depending upon the load
application.
In short, an AC drive can act as a soft start, but a soft start cannot act as an AC drive.
Benefits of AC Drives
The AC drive has been around only since the 1970’s. The growing popularity of AC drives is due
chiefly to their ability to provide adjustable speed control with standard NEMA B design squirrel
cage motors. Other names for AC drives are variable frequency drive (VFD) and variable speed
drive (VSD), but we’ll just call them AC drives.
Some of the reasons for the growing popularity of AC drives are:
· Energy savings, particularly for fans and pumps.
· Extended equipment life through reduced mechanical stress (belts, bearings).
· Elimination of excessive motor inrush current which in turn, extends useful motor life.
· Standard AC motors can be used. This means that off-the-shelf motors, which are easier to
repair, purchase and maintain, not to mention less expensive, can be used.
· With an AC drive, retrofits from a DC or wound-rotor motor to a NEMA B squirrel cage motor
are relatively easy.
· Solid state device which has no moving parts or contacts to wear out.
APPLICATIONS
of many of the types of applications and machines AC drives are used on:
The participant will:
· Be able to identify the components of an AC motor
· Understand how an AC motor operates
· Understand AC motor terms and concepts
· Identify meaning of motor nameplate terms
PREREQUISITES
AC drives control AC motors. It’s that simple. So, in order to understand what AC drives are, how
they work and where they are applied, you need to first understand the parts of an AC motor and
how AC motors function. If you are unsure about AC motor theory here are few suggestions on
where you can go to get that information:
· If you are enrolled in Square D Technical Institute, then motor theory was covered in Module
3. You may want to review that module before continuing on with this course.
· For a more in-depth coverage of AC motors (whether or not you are part of Square D
Technical Institute or not) you’ll find that Square D has available an excellent self paced
course on AC Motor Theory - Course # AUTM 100. It is highly recommended that you
complete this course prior to starting this course on AC drives. AUTM 100 is available on
either 3 1/2” disks or a CD ROM.
While we will present a brief review of AC motor theory and terminology, it is assumed that you
meet one or more of the above prerequisites and have a clear understanding of AC motors.
REVIEW OF AC MOTOR THEORY
Some of the reasons you need to understand AC motor theory in order to understand AC drives
are:
· To provide customer satisfaction
¾ The AC drive and the AC motor work together as a system
· To select the correct drive
¾ Must have knowledge of NEMA A, B, C, and D motor speed and torque characteristics
· To ensure desired motor performance
¾ Because an AC drive effects:
· Speed, torque, current, voltage, heating and horsepower
We all know that there is a relationship between a motor, a machine and motor control. The
machine does the actual work. The motor is the device which causes the machine to operate.
And, the motor controller is the intelligence that directs the motor. That is, it determines when
and in what direction the motor will operate. And, it may provide protection for the motor, branch
circuits and the operator.
Did you know that the average household has more than 25 electric motors. A medium sized
manufacturing plant, such as the Square D plant in Raleigh, NC may have from 4,000 to 5,000
electric motors. And a large automated plant, like an automobile assembly plant could have
25,000 electric motors in it.
And now, let’s get on with learning about the fundamentals of AC motors.
AC MOTOR FUNDAMENTALS
Components Of An Electric Motor
Electric motors are really quite simple. There are only four basic parts to an electric motor:
· There is the housing or external case that surrounds the other components.
· Mounted inside the housing is the stator. The stator is the stationary or non-moving part of
the motor’s interior. It is made up of wire windings. The moving parts of the motor are the
rotor and the shaft.
· The rotor, like the stator, also has windings.
· The rotor is connected to the fourth component, the shaft. The shaft is a metal rod held in
position within the stator by bearings connected to the case. The bearings allow the shaft to
rotate inside the stator. The rotor and shaft are often referred to as the armature of the
motor.
How An Electric Motor Operates
The electric motor operates by converting electrical energy into mechanical energy. Let’s
represent the motor’s stator as an iron block “S” and the rotor as an iron block “R”. Both of these
iron blocks are wrapped with wire coils. When electrical current is passed through the wire coils,
an electromagnetic field is created and the iron blocks become magnetized. All magnets have a
North and a South pole. A North pole is always trying to get next to a South pole and visa versa.
Two North or two South poles will push away or repel each other. In other words, opposite poles
attract and like poles repel. It’s this magnetic pull and push principle that makes an electric motor
operate.
Suppose that “S” is fastened such that it cannot move. On the other hand, “R” is allowed to move
freely. When electricity is passed through the coils and the blocks are magnetized, the opposite
poles try to pull together. Block “R” will move towards block “S.” If the blocks get together the
movement will stop. What if block “S” were mounted in such a way that block “R” couldn’t
contact it? Block “R” would move until it’s positive pole were as close as it could get to block “S”
and then motion would stop.
Let’s add more “S” blocks (S1, S2, S3, and S4). If S1 were demagnetized just as “R” reached it,
and S2 is magnetized, “R” would continue moving toward “S2.” If this same process of
demagnetizing and magnetizing S1, then S2, then S3 and finally S4, were continued then block
“R” would be moving all the time until it reached S4.
In an electric motor the “S” magnets are formed in a circle and the “R” magnet is placed inside
this circle and is attached to a shaft. The stator and rotor are magnetized as current flows
through the coil windings. The rotor moves so that the opposite poles of the windings can try to
move closer to the stator magnets. Just as the magnets are close the magnetic field moves on in
the stator, and the rotor chases after it. Since the rotor and shaft are fastened together, the shaft
moves. The rotation of the shaft is the mechanical energy created by the conversion of the
electrical energy by the motor.
To summarize, the rotor “chases after” the changing magnetic field of the stator which causes
the rotor and shaft to rotate. The magnetic fields of the stator and rotor are changed according to
the frequency of the AC voltage applied to the motor. Changing the frequency of the voltage
applied will alter the speed at which the stator’s magnetic fields change. This will, in turn, change
the speed of the rotor. Changing the current will alter the strength of the magnetic fields of the
rotor and stator. The stronger the magnetic fields the greater the turning force applied by the
rotor to the shaft. This twisting or turning force is called torque.
Types of AC Motors
The four principle types of motors (not including single phase types) found in commercial and
industrial applications are: squirrel cage induction, wound rotor induction, synchronous, and
direct current (dc).
The squirrel cage induction motor is by far the most widely used motor because of its low cost
and proven reliability. The wound-rotor induction motor has been used in applications that
require high starting torque, controlled starting torque, or speed control. The synchronous
separately excited motor has been used in high-horsepower applications where it is
advantageous to overexcite the motor to provide power factor correction in an industrial facility.
The synchronous permanent magnet and reluctance motor is used in applications that need
precise speed for a number of motors operating in combination.
But the squirrel cage motor is by far the simplest, most reliable, least expensive, most readily
available and easiest to maintain. In addition, with improvements in AC drives, squirrel cage
motors are now applied in the majority of the applications your customers are involved with.
MOTOR TERMS AND CONCEPTS
Motor Terms and Concepts
· Electric service is a term used to describe or define electrical power supplied to a motor.
The selection of motor control products depends upon the information that is included as part
of electrical service. This information includes:
¾ Current - the current used by the motor is either AC or DC. Square D currently only
makes drives for AC motors.
¾ Phase - a motor can be powered by either single or polyphase electric power. The term
polyphase means more than one phase and typically refers to 3 phase.
¾ Frequency - is the number of electrical pulses that are transmitted over a given period
of time. Frequency is measured in hertz (Hz) or cycles per second (cps).
In this example, you see that the voltage builds from zero, in the positive direction up to
a peak positive value of + 460 V. Then it starts to decline in value until it reaches zero
volts again. Next the voltage starts in the negative direction until it reaches a peak value
of
- 460 V. Finally the voltage starts to move back in the positive direction until it reaches
zero volts. The change in voltage from zero to a peak positive value, back to zero, to a
peak negative value and back to zero is called 1 cycle. It has taken time for a cycle to
occur. In our example, that time is one second. Frequency is measured in terms of cycles per second and the frequency of this example is one cycle per second. The more
common term for frequency is called Hertz. One Hertz equals one cycle per second.
Alternating current completes these cycles very rapidly and the number of cycles per
second is known as the frequency. Throughout the United States AC current typically
goes through a cycle 60 times per second, so the frequency is 60 Hertz. In many foreign
countries, the AC current cycles 50 times per second, so the frequency is 50 Hertz.
¾ Voltage - electric motors are designed to operate using a specific voltage. Motor control
devices are also rated according to the voltage that can be applied to them.
· Locked rotor current (LRC) is the current flow required by a motor in order for the motor to
start. Locked rotor current may be called Locked rotor amps (LRA).
· Full Load Amps (FLA) - this is the current flow required by a motor during normal operation
to produce its designed HP. Full load amps (FLA) is also called Full Load Current or (FLC).
· Speed (in revolutions per minute), Torque (ft.lbs.) and Horsepower (HP) are all terms that
are used to define motor performance:
Let’s start with Horsepower. Motors and engines are measured in horsepower. Horsepower
is a standard unit of power which is used to measure the rate at which work is done. One
Horsepower is the equivalent of 550 foot-pounds per second --- that is the ability to lift 550
pounds one foot in one second. For example, if an electric motor can lift 550 pounds 10 feet
and it takes 10 seconds, then the motor has a horsepower rating of 1 hp.
In any electric motor the motor torque can be multiplied by the motor speed and the product
divided by 5250 (a constant) to determine the rated horsepower.
HP =Torque (ft.lbs.) X Speed (RPM)\5250
Before continuing the discussion about the horsepower equation, let’s look at torque.
Torque is formally defined as: “the force tending to rotate an object, multiplied by the
perpendicular radius arm through which the force acts.” In the case of a motor, torque is the
force which acts on the shaft and causes rotation. Remember that the amount of torque
created is directly related to the amount of current applied to the motor. The greater
the current the stronger the magnetic fields of the stator and rotor, and therefore the greater
the turning force of the shaft. A motor is a dumb device. As the load is increased on the
shaft, the motor will draw more current (to increase the torque) to try and keep the load
moving. If the load were to continue to be increased, the motor will literally destroy itself
trying to create the necessary torque to move the load.
Consider how a motor generates torque vs how it generates Speed. Motor speed is
measured in rpm (the revolutions per minute the rotor turns) and is the speed at which the
rotor rotates inside the stator. This rotational speed will depend upon the frequency of
the AC voltage applied and the number of stator poles. If the motor has no load, this
speed will approach the synchronous speed of the stator field.
· Synchronous speed is the speed of an AC induction motor’s rotating magnetic field. It is
determined by the frequency applied to the stator and the number of magnetic poles present
in each phase of the stator windings. This can be expressed by the formula:
Synchronous Speed =120 X Frequency\Number of Poles.
For example:
Synchronous Speed =120 X 60 Hz\4 pole motor = 1800 rpm
· Motor Slip
Slip is the difference between the rotating magnetic field speed in the stator and the rotor
speed in AC induction motors. This is usually expressed as a percentage of synchronous
speed. If the rotor were rotating at exactly the same speed as the stator’s rotating magnetic
field (for example, 1800 rpm) then no lines of magnetic force would be cut, no voltage would
be generated in the rotor and no current would be present. However, if the rotor slows down
by 50 rpm it would now be running at 1750 rpm vs 1800 rpm of the stator field. The rotor
bars are now cutting the rotating field at a 50 rpm rate. Now voltage and current would be
generated in the rotor, with a resulting magnetic flux pattern. The interaction of these
magnetic fields would produce torque. The difference between the synchronous and actual
rotor speeds is called slip.
· Torque vs Speed Relationship:
Torque, remember is a force exerted on the motor’s shaft when a load is added to the rotor.
The tendency is for the rotor to slow down, which will create more slip (difference between
the stator magnetic field speed and rotor speed), thus creating more torque within the motor.
As the load is increased, the rotor will continue to slow down, which would result in even
greater slip as the rotor lags behind the synchronous speed of the rotor. The increased
resistance to rotation increases the slip and therefore increases the torque.
Now, lets go back to the horsepower equation again.
HP =Torque (ft.lbs.) X Speed (RPM)\5250
This formula will help you select the proper motor for a job. Notice the relationship
between torque and speed. It is obvious that a 5 hp motor, designed to run at high speed,
will have very little torque. To maintain the equation, torque must decrease as speed
increases:
HP =Torque (ft.lbs.) X Speed (RPM)\5250
Conversely, a 5 hp motor with high torque must run at a slow speed.
HP =Torque (ft.lbs.) X Speed (RPM)\5250
An important relationship for you to remember is that:
SPEED IS RELATED TO FREQUENCY
and
TORQUE IS RELATED TO CURRENT
You’ve already seen that increasing the frequency at which the magnetic fields change will
cause an increase in the speed of rotor and shaft rotation. If the frequency were decreased,
the motor speed would slow down.
If the current drawn by the stator and rotor is increased, this would cause a strengthening of
the magnetic fields. This, in turn, would cause the torque generated by the motor to increase.
Likewise, if the current were decreased, the torque would be decreased as well.
In fact the horsepower formula can also be expressed in electrical terms of voltage and
current, as:
HP (Output) =Volts X Amps X 1.732 X Power Factor X Efficiency\746
· Constant and Variable Torque
If you look at a motor’s usage based on the torque requirements of an application, you will
find that you may need constant torque or variable torque. One application might require
normal starting torque and a normal running torque, for example, a drill machine. This category requires that a motor starts with a normal amount of torque and then continues to
run at the required speed.
Another application category might require a high starting torque but a normal running
torque. For example, a conveyor that is first loaded up and then started. When the loaded
conveyor is started the motor must provide a big push of torque to get the conveyor and its
load moving. Once moving, inertia has been overcome and the resistance of friction falls,
therefore normal running torque provides adequate power to keep the conveyor running.
The third torque category would be an application that requires a very high starting torque,
and a normal running torque.
Starting and running torque can be plotted. As the starting torque increases, motor speed
decreases --- remember the equation: speed times torque equals horsepower. As torque
increases, the motor speed decreases.
Notice that at zero speed the starting torque is very high. This is needed to get the load
moving from a dead stop. As the speed increases the torque curve fluctuates until the full
load torque and full load speed are reached.
· The breakdown torque is the maximum torque that a motor can produce. Higher torque
requirements will slow motor speed to a stop. Breakdown torque is the point where speed
stops as torque requirement increases.
· Full load torque
· NEMA Design Ratings
The NEMA ratings refer to torque ratings. These rating apply to motors which are started
across the line.
The design areas of the nameplate refer to the NEMA rating of the motor which is
comparable to the torque performance of the motor. NEMA has five design ratings of AC
induction motors. Each of these designs has a different characteristic for starting current,
locked rotor current, breakaway torque, and slip. These designs are NEMA A, B, C, D, and
E. Each has a distinct speed vs torque relationship and different values of slip and starting
torque.
The most common is the NEMA Design B motor.
The NEMA B motor’s percentage of slip ranges from 2 to 4%. It has medium values for
starting or locked rotor torque, and a high value of breakdown torque. This type of motor is
very common in fan, pump, light duty compressors, various conveyors, and some light duty
machines. The NEMA B motor is an excellent choice for variable torque applications.
The NEMA A motor is similar in many ways to the NEMA B motor. It typically has a higher
value of locked rotor torque and its slip can be higher
NEMA C motors are well suited to starting high-inertia loads. This is because they have high
locked rotor torque capability. Their slip is around 5%, and their starting current requirement
is average.
The NEMA D motor is found in heavy duty, high-inertia applications. It has high values of
slip (up to 8%), and very high locked rotor torque capability. Typical applications include
punch presses, shearing machinery, cranes, and hoists.
· Motor Load - a motor provides the conversion of electrical energy to mechanical energy
that enables a machine to do work. The energy that a machine requires from a motor is
known as the motor load. For example, the motor in a clothes dryer turns the dryer drum.
The energy required by the dryer motor to turn the drum is called the dryer’s motor load.
· Motor Overload - An electric motor for all its other fine qualities has no intelligence and will
literally work itself to death. If there is a heavy load on a motor, say when the clothes dryer is
full of clothes, the motor will try to produce whatever torque is needed to keep the dryer
drum turning. Because the motor load may be increased above normal, a motor overload
condition exists. More torque is required from the motor to turn the drum, so the motor draws
more current to produce more energy. The higher than normal current flow, which is above
the FLC, increases the temperature in the dryer motor. The electric motor could be damaged
when the temperature rises above its designed limit.
· Motor Cooling - Whenever electrical current is passed through an electrical motor there is a
buildup of heat. The amount of heat produced is a function of the work, or loading, done by
the motor; the type of electrical signal being sent to the motor; and the eventual changes due
to bearing wear and friction. Whenever AC drives are used to control motors it means that
the speed of the motor is going to be changed. And, depending upon motor loading, special
attention needs to be given to how the motor is going to be cooled. Generally speaking, less
speed means less cooling.
Different motor cooling designs are available:
· Many motors are sized for a particular application, or horsepower rating, so that the heat
produced from the current can be accepted and dissipated by the metal content of the
motor. Normal convection and radiation dissipate the heat with the aid on an internal
mixing fan. These motors are classified as “open drip-proof” or “totally enclosed
nonventilated (TENV).”
· Other electric motors incorporate a fan blade that rotates at the same speed as the
motor shaft. This fan blows air across the outside of the motor, cooling it as it runs.
However, if an AC drive is used, the lower in speed the motor is made to run, the slower
the cooling fan will run also. This can result in a buildup of heat in the motor. These
motors are called “totally enclosed fan-cooled (TEFC).”
· Some types of motors use elaborate means for cooling. These are called “totally
enclosed water-to-air cooled,” “totally enclosed air over,” and “totally enclosed
unit cooled.” Obviously, the more complex the cooling method, the more expensive the
actual motor will be.
There are a couple of different strategies used for selecting a motor that will be adequately
cooled during operation:
· One approach is to size the motor with a service factor. A service factor of 1.15 means
that the motor has 15% more capacity when operating conditions are normal for voltage,
frequency, and ambient temperature. This 15% extra capacity means that the motor is
built and sized when the duty cycle is severe, or the loading and speed range is
moderate.
· Another strategy is to simply go up in horsepower, which is how motors are sized. This
might put a motor into a larger frame designation, thereby making it weigh more and
allowing it to handle a greater amount of heat.
The concern of both of these strategies is that you could end up with a motor that is well
oversized for the application. This would cause wasted energy and increase the cost of the
motor. Another answer might be to add auxiliary cooling equipment to the motor.
MOTOR NAMEPLATE DATA
Motor Nameplate Data
Squirrel cage motors, like any other type of electrical equipment, require proper application
for successful operation. Understanding the nameplate information, which identifies the
motor’s important features and characteristics, will aid considerably in proper application.
A nameplate is attached to each AC motor and includes information such as:
· Full load speed
· Torque ratings
· Type of enclosure
· Type of insulation
· Temperature Rise Rating
· Service Factor
· Time Rating
· Locked Rotor KVA
· Frame sizes - In 1972 NEMA Standards included numbers for various frame sizes that
range from 140 to 680. These are commonly called “T” frame motors. There is a relation
between the numbers assigned and their frame dimensions. For example, the first two digits
of the number equal four times the dimension in inches from the center line of the shaft to
the bottom of the feet. A series of letters is used immediately following the frame size
number to help identify certain features.
The motor nameplate identifies a frame size number and letter which are indicative of
dimensions and some features. NEMA has specified certain dimensions for motor frame
sizes -- up to 200 hp. These are identified by the numbers listed below.
Frame Number Series
140 220 400
160 250 440
180 280 500
200 320 580
210 360 680
The physical size and consequently the cost of a squirrel cage motor is determined by its
frame size. The actual horsepower rating for each frame size will vary and will be
determined by several design parameters, which have been standardized by NEMA. Above
approximately 200 hp, electrical standards apply for motors, but the frame sizes are not
standardized.
· Full Load Speed - The motor nameplate identifies the rated full load speed. This speed isone of the key considerations in determining the motor horsepower required for a given load.
The motor synchronous speed is influenced by the number of magnetic poles in the stator.
The synchronous speed is slightly higher than the motor shaft speed. As the number of poles
in a motor design is increased, the rated synchronous speed is decreased per the formula:
Synchronous Speed =120 X Frequency\ Number of Poles
Because of this the physical size of a squirrel cage motor is inversely related to its speed ---
meaning, the frame size may be larger as the rated synchronous speed becomes lower. For
example, a 100 hp, 600 rpm twelve pole motor will be considerably larger than a 100 hp,
3600 rpm two pole motor.
· Torque Ratings - The motor nameplate identifies the type of design motor (A, B, C, D, E),
which is indicative of its locked rotor and peak torque ratings. In addition the nameplate
identifies the rated horsepower at rated speed and from this information, rated full load
torque can be determined. The full load torque rating will determine the full load current at
rated voltage. The physical size of the motor is directly related to its full load torque rating.
For example, in comparing the torque ratings of the 100 hp 600 rpm and 3600 rpm motors in
the full load speed example, you may recall that motor horsepower is proportional to torque
time speed. Since the speed rating of the larger twelve pole motor is 1/6 that of the two pole
motor, the torque rating is approximately six times that of the two pole motor.
· Enclosures and Ventilation - The motor nameplate usually identifies the type of enclosure
and ventilation system, such as open type self-ventilated, totally enclosed fan cooled
(TEFC), totally enclosed non-ventilated (TENV) and others. See “Motor Cooling” in previous
discussion entitled “Motor Terms and Concepts.”
· Insulation and Temperature Rise Ratings - The motor nameplate identifies the Class of
Insulation material used in the motor and its rated ambient temperature. Various types of
materials can be used for insulation which are defined as Class A, Class B, Class F and
Class H. IEEE Standards list temperature ratings as follows:
Class A - 105° C
Class B - 130° C
Class F - 155° C
Class H - 180° C
When the rated temperature of the insulation materials is exceeded, it is estimated that the
insulation life is decreased by 1/2 for every 10 degrees above the rating. By using higher
temperature rated materials, more heat losses in the motor can be tolerated. Consequently,
the horsepower rating of a motor can be increased in a given frame size.
NEMA Standards specify permissible temperature rises above a 40° C ambient for motors.
This is determined by the type of insulation in the motor, and other motor design and
application considerations. Some motors operate at higher temperatures then others, but
none should exceed the temperature rating of the insulation.
· Service Factor - A motor is rated by the manufacturer to produce a certain HP over a long
period of time without damage to the motor. However, occasionally a motor might be
operated intentionally or unintentionally above the rated HP. To protect against motor
damage caused by the occasional excess current an electric motor is usually built with a
margin of safety. The margin of safety is called the motor’s service factor.
The motor nameplate identifies a service factor; 1.0 or 1.15. This indicates overloads which
may be carried by a motor under nameplate conditions without exceeding the maximum
temperature recommended for the insulation. For example, a 100 hp motor with a 1.15
service factor can sustain a 15% overload (100 X 1.15 = 115 hp) continuously and will not
exceed the temperature rating of the insulation in the motor, provided the ambient
temperature is no greater than 40° C. Frequently motors are specified with 1.15 service
factor to provide additional thermal capacity.
· Time Ratings - The motor nameplate identifies its time rating which can be continuous duty
or short times, such as 60 minutes, 30 minutes, 15 minutes and 5 minutes. Obviously, at a
specified horsepower, a motor operating continuously will generate more total losses and will
require a larger frame size, compared to a motor operating intermittently. The short time
ratings indicate the motor can carry the nameplate loads for the time specified without
exceeding the rated temperature rise. After the short time, the motor must be permitted to
cool to room temperature.
· Locked Rotor KVA - The motor nameplate identifies the locked rotor KVA with a code letter
- A thru V. The locked rotor KVA may be a consideration when applying motors where
limitations exist in the power distribution system. NEMA Standards have designated inrush
KVA’s for the various code letters.
