From Motors and Drives

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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Products & Services
AC Motors
AC motors include single, multiphase, universal, induction, synchronous, and gear motors. They also include servomotors.
Brushless Motors
Brushless motors are synchronous electric motors that have a magnetically (AC induction) or electronically (DC) controlled commutation system instead of a brush-based mechanical commutation system.
Stepper Motors (rotary)
Stepper motors use a magnetic field to move a rotor in small angular steps or fractions of steps. They provide precise positioning and ease of use, especially in low acceleration or static load applications.
DC Motors
DC motors are most commonly used in variable speed and torque applications. They include brushless and gear motors, as well as servomotors.
AC Motor Drives
AC motor drives interface controllers to AC motors. They match the control signals (voltage and power levels) as well as the signal type (analog or digital). They also provide power conversion, amplification, and the sequencing of waveform signals.

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