Control of Speed, Torque, and Horsepower
Control of Speed
The speed of a squirrel cage motor depends on the frequency and the
number of poles for which the motor is wound. The higher the frequency,
the faster the motor operates. The more poles the motor has, the slower it
operates. The smallest number of poles ever used in a squirrel cage motor
is two. A two-pole 60-Hz motor will run at approximately 3600 rpm. As
soon will be seen, the motor will always operate at a speed less than 3600
rpm.
To find the approximate speed of any squirrel cage motor, the formula for
synchronous speed can be used, which is actually the speed of the rotating
magnetic field:
| | N | = | synchronous speed (rpm) |
| | F | = | frequency of the power supply (Hertz) |
| | P | = | number of stator poles |
Squirrel cage induction motors are wound for the synchronous speeds
found in Table 3-1.
Most standard induction motors (NEMA 143T through 445T frame sizes)
are wound with a maximum of eight poles.
The actual speed of the motor shaft is somewhat less than synchronous
speed. This difference between the synchronous and actual speeds is
defined as slip. If the squirrel cage rotor rotated as fast as the stator field,
the rotor bars would be standing still with respect to the rotating magnetic
field. No voltage would be induced in the rotor bars, and no magnetic flux
would be cut by the rotor bars. The result would be no current set up to
produce torque. Since no torque is produced, the rotor will slow down
until sufficient current is induced to develop torque. When torque is
developed, the rotor will accelerate to a constant speed. Figure 3-29 is a
graphical representation of slip.
To summarize: There must be a difference between the rotating magnetic
stator field and the actual rotor bars' position. This allows the rotor bars to
cut through the stator magnetic fields and create a magnetic field in the
rotor. The interaction of the stator and rotor magnetic fields produce the
attraction needed to develop torque.
When the load on the motor increases, the rotor speed decreases. Then the
rotating field cuts the rotor bars at a faster rate than before. This has the
effect of increasing the current in the rotor bars and increasing the magnetic
pole strength of the rotor. Basically, as the load increases, so does the
torque output.
Slip is usually expressed as a percentage and can easily be calculated using
the following formula:
Squirrel cage motors are built with the slip ranging from about 3-20%.
Motors with a slip of 5% or higher are used for hard-to-start applications.
A motor with a slip of 5% or less is called a normal slip motor. A normal
slip motor is often referred to as a constant speed motor because the speed
changes very little with variations in load.
In specifying the speed of the motor on the nameplate, most motor manufacturers
use the actual speed of the motor at rated load. The term used is
base speed. Base speed is a speed somewhat lower than the synchronous speed. It is defined as the actual rotor speed at rated voltage, rated hertz,
and rated load.
Direction of Rotation
The direction of rotation of a squirrel cage induction motor depends on the
motor connection to the power lines. Rotation can easily be reversed by
interchanging any two input leads.
Control of Torque and Horsepower
As discussed earlier, horsepower takes into account the speed at which the
shaft rotates. It takes more horsepower to rotate the shaft fast, compared
with rotating it slowly. Note: Horsepower is a rate of doing work.
By definition, 1 HP equals 33,000 ft-lb per minute. In other words, lifting a
33,000-pound weight 1 foot, in 1 minute would take 1 HP.
By using the familiar formula below, we can determine the horsepower
developed by an AC induction motor.
| | T | = | torque in lb-ft |
| | N | = | speed in rpm |
For example, a motor shaft turns at 5 rpm and develops 3 lb-ft of torque.
By inserting the known information into the formula, we calculate that
the motor develops approximately 0.003 HP (3 5 5252 = .0028). As the
formula shows, horsepower is directly related to the speed of motor shaft.
If the shaft turns twice as fast (10 rpm), the motor will develop almost
.006 HP, twice as much.
We can see the general rules of thumb for torque developed versus speed
by reviewing Table 3-2.
Torque developed will vary slightly on lower HP and rpm motors or nonstandard
motors.
As seen in Table 3-2, at higher synchronous speeds, the induction motor
develops less torque compared with lower speeds. We can also see that the
higher the number of poles, the larger the amount of torque developed.
Basically, more poles mean stronger magnetic fields that will be produced.
With more magnetic flux interacting with rotor flux, a stronger twisting
motion will result, thereby developing more torque.
Regarding the issue of motor torque, there are several areas on the standard
speed/torque curve that should be reviewed. An induction motor is
built to supply this extra torque needed to start the load. The speed torque
curve for a typical induction motor is seen in Figure 3-30.
Figure 3-30 shows the starting torque to be about 250% of the rated-load
torque.
Peak (Breakdown) Torque
Occasionally a sudden overload will be placed on a motor. To keep the
motor from stalling every time an overload occurs, motors have what is
called a breakdown torque. The breakdown torque point is much higher
than the rated load torque point. For this reason, it takes quite an overload
to stall the motor. The speed/torque curve shown in Figure 3-30 indicates
the breakdown torque for a typical induction motor to be about 270% of
the rated load torque.
Operating a motor overloaded for an extended period of time will cause an
excessive heat buildup in the motor and may eventually burn up the
motor windings.
The NEMA definitions and ratings for an induction motor's characteristic
torque is given later in this chapter.
Locked Rotor Torque (Starting or Breakaway Torque)
The locked rotor torque of a motor is the minimum torque, which it will
develop at rest for all angular positions of the rotor. This capability is true
with rated voltage and frequency applied.
Pull-Up Torque
The pull-up torque of a motor is the minimum torque developed by the
motor when accelerating from rest to the breakdown torque point. For
motors that do not have a definite breakdown torque, the pull-up torque
is the minimum torque developed up to rated speed.
Peak (Breakdown) Torque
The breakdown torque of a motor is the maximum torque that it will
develop. This capability is true with rated voltage and frequency applied,
without an abrupt drop in speed.
Rated Load Torque
The rated load torque of a motor is the torque necessary to produce the
motor's rated horsepower at rated-load speed. (Note: Rated load speed is
normally considered base speed. Base speed means actual rotor speed
when rated voltage, frequency, and load are applied to the motor.)
The above torque designations are all very important to the motor
designer. Essentially, motors can be designed with emphasis on one or
more of the above torque characteristics to produce motors for various
applications. An improvement in one of these torque characteristics may
adversely affect some other motor characteristic.
Control of Speed, Torque, and Horsepower
Control of Speed
The speed of a squirrel cage motor depends on the frequency and the
number of poles for which the motor is wound. The higher the frequency,
the faster the motor operates. The more poles the motor has, the slower it
operates. The smallest number of poles ever used in a squirrel cage motor
is two. A two-pole 60-Hz motor will run at approximately 3600 rpm. As
soon will be seen, the motor will always operate at a speed less than 3600
rpm.
To find the approximate speed of any squirrel cage motor, the formula for
synchronous speed can be used, which is actually the speed of the rotating
magnetic field:
| | N | = | synchronous speed (rpm) |
| | F | = | frequency of the power supply (Hertz) |
| | P | = | number of stator poles |
Squirrel cage induction motors are wound for the synchronous speeds
found in Table 3-1.
Most standard induction motors (NEMA 143T through 445T frame sizes)
are wound with a maximum of eight poles.
The actual speed of the motor shaft is somewhat less than synchronous
speed. This difference between the synchronous and actual speeds is
defined as slip. If...
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