A four-pole induction motor operates on a 60-hertz supply. Synchronous speed in revolutions per minute equals 120 times the supply frequency divided by the number of poles. What is the synchronous speed of that motor?
- A3,600 rpm
- B1,200 rpm
- 1,800 rpm
- D900 rpm
Where to look it up
https://www.energy.gov/documents/doe-hdbk-1011-92vol4
Why C is correct
Synchronous speed is the speed of the rotating magnetic field: 120 × supply frequency / number of poles. Thus 120 × 60 / 4 = 1,800 rpm. At 60 Hz, two, six, and eight poles give 3,600, 1,200, and 900 rpm respectively. In normal induction-motor operation, the rotor runs below synchronous speed; the difference is slip. Use 7,200 divided by pole count for a 60 Hz supply. Source: https://www.energy.gov/documents/doe-hdbk-1011-92vol4