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?

Why C is correct

One hundred twenty times 60 hertz is 7,200, and dividing by the four poles gives 1,800 revolutions per minute for the rotating field. The rotor lags that field by the amount of slip needed to produce torque, which is why the nameplate speed sits a little below 1,800.

What this question is testing

Tests whether you know the relationship between supply frequency, pole count and synchronous speed, whether you apply it correctly, and whether you understand slip well enough to explain the gap between the calculated speed and a real nameplate value.

On the job

Motor speed is set by poles and frequency, not by voltage, which is the point most customers get wrong. That is also why variable frequency drives change speed by changing frequency rather than voltage alone. When you replace a motor, matching the nameplate speed matters as much as matching horsepower, because a two-pole replacement on a four-pole pump or fan will roughly double the flow demand and can overload the driven equipment and the circuit.

Memory technique

Twelve thousand divided by poles, then drop a zero: four poles gives eighteen hundred.

Exam tip

Two poles gives 3,600, four gives 1,800, six gives 1,200, eight gives 900. Double the poles, halve the speed.

Where to look it up

Motor speed is theory, but Article 430 uses horsepower and full-load current tables where the same motor characteristics show up in the sizing rules.