Master Electrician Exam (NEC)General Electrical KnowledgeMedium
A technician is installing a new 480V, 3-phase, 60 Hz motor with a nameplate RPM of 1750. What is the approximate synchronous speed of this motor?
- A3600 RPM
- B4800 RPM
- C1200 RPM
- D1800 RPM
Show answer & explanationAnswer & explanation
Correct answer: D. 1800 RPM
Synchronous speed (Ns) is calculated using the formula Ns = (120 * f) / P, where f is the frequency (60 Hz) and P is the number of poles. Since 1750 RPM is slightly less than synchronous speed, the motor is likely designed with 4 poles (120 * 60) / 4 = 1800 RPM. A 2-pole motor would be 3600 RPM, and a 6-pole motor would be 1200 RPM.
Why the other options are wrong
- A. This would be the synchronous speed for a 2-pole motor (120*60/2).
- B. This value does not correspond to a standard number of poles for a 60 Hz motor.
- C. This would be the synchronous speed for a 6-pole motor (120*60/6).
Synchronous Speed of AC Motors
Synchronous speed is the speed at which the magnetic field rotates in an AC motor, determined by the frequency of the AC supply and the number of poles in the motor.
- Formula: Ns = (120 * f) / P, where Ns is synchronous speed (RPM), f is frequency (Hz), and P is number of poles.
- Induction motors always operate slightly below synchronous speed (slip).
- Common synchronous speeds for 60 Hz motors are 3600 RPM (2-pole), 1800 RPM (4-pole), 1200 RPM (6-pole).
Memory trick: Synchronous Speed: Fast Field, Poles Count.