Which motor type maintains a speed tied to the supply frequency?

Which motor type maintains a speed tied to the supply frequency?

• A. Induction motor
• B. Synchronous motor
• C. DC motor
• D. Servo motor

Answer: (B)

Speed is locked to the supply frequency with a synchronous motor. The stator creates a rotating magnetic field that turns at a speed determined by the frequency (and the number of poles). The rotor, either a DC magnet or a permanently magnetized rotor, locks to that field and runs in step with it. Because they stay in sync with the rotating field, their speed is essentially fixed by the frequency according to the formula n_s = 120 f / P. If frequency changes, the speed changes proportionally; load changes don’t cause the rotor to slip away from the field as they do in other motor types, so the speed remains tied to the frequency. Induction motors, in contrast, run slightly behind the rotating field (slip) and their speed depends on load and frequency, not strictly locked to frequency. DC motors control speed mainly through voltage and back-EMF; servo motors are designed for precise control via feedback rather than inherent frequency-locked speed.

Speed is locked to the supply frequency with a synchronous motor. The stator creates a rotating magnetic field that turns at a speed determined by the frequency (and the number of poles). The rotor, either a DC magnet or a permanently magnetized rotor, locks to that field and runs in step with it. Because they stay in sync with the rotating field, their speed is essentially fixed by the frequency according to the formula n_s = 120 f / P. If frequency changes, the speed changes proportionally; load changes don’t cause the rotor to slip away from the field as they do in other motor types, so the speed remains tied to the frequency.

Induction motors, in contrast, run slightly behind the rotating field (slip) and their speed depends on load and frequency, not strictly locked to frequency. DC motors control speed mainly through voltage and back-EMF; servo motors are designed for precise control via feedback rather than inherent frequency-locked speed.