Characteristics of AC Motor control
The synchronous speed of AC induction motor is directly proportional to the applied frequency. Below is the formula:
Speed = 120 x Frequency /No. of Motor Poles
The synchronous speed is the speed of the rotating electrical field, not the actual motor rotor speed. The difference between two speeds is called slip, which is normally expressed in percent. The percentage of slip is determined by the design of the motor, primarily the rotor resistance.
NEMA has assigned code letters (A, B, C, D, etc.) to standardize motor characteristics including slip. The type most commonly used is NEMA Design B with 3% slip at rated operating conditions. The figure below shows typical speed/torque curves for NEMA Design Band D motors.
As the applied frequency is changed, the motor will run faster or slower as shown by the figure below. The actual full-load motor slip (as a percent of the motor synchronous speed) varies in inverse proportion to the frequency, where a 3% slip motor 60 Hz would have a 6% slip at 30 Hz or 1 1/2 % slip at 120 Hz. Motor speed is limited only by the maximum inverter output frequency, load torque requirements, and the mechanical integrity of the motor.
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