AC drives can easily vary the speed of a motor.

AC drives can easily vary the speed of a motor.

• A. Single-phase
• B. Either a or b
• C. Three-phase
• D. Neither

Answer: (B)

AC drives control motor speed by changing the supply frequency (and voltage to maintain the V/Hz ratio). For an induction motor, the speed is set by the frequency and the number of poles; lowering the frequency reduces the synchronous speed, and the rotor follows with a small slip, so the motor slows down. This method works for both single-phase and three-phase induction motors. Many drives can take single-phase input for small machines or provide a three-phase output to the motor, so speed control is available in either case. Because a drive can adjust speed in either configuration, this statement is best supported by saying AC drives can easily vary the speed of a motor in both single-phase and three-phase setups.

AC drives control motor speed by changing the supply frequency (and voltage to maintain the V/Hz ratio). For an induction motor, the speed is set by the frequency and the number of poles; lowering the frequency reduces the synchronous speed, and the rotor follows with a small slip, so the motor slows down. This method works for both single-phase and three-phase induction motors. Many drives can take single-phase input for small machines or provide a three-phase output to the motor, so speed control is available in either case. Because a drive can adjust speed in either configuration, this statement is best supported by saying AC drives can easily vary the speed of a motor in both single-phase and three-phase setups.