If an AC motor runs slowly, which condition is listed as a potential cause?

• A. Low applied voltage
• B. Excess lubrication
• C. High ambient temperature
• D. Strong magnetic field

Answer: (C)

When an AC induction motor runs slowly, the torque it can produce under load is a key factor. The speed of such a motor is set by the supply frequency and the design (synchronous speed), with the actual speed being a little less due to slip. Slip increases when the motor can’t develop enough torque to meet the load demand.

High ambient temperature makes a big difference because the resistance of the windings rises as temperature goes up (R = R0[1 + α(T − T0])). That extra resistance reduces the current in the windings for a given applied voltage, which lowers the magnetic torque the motor can produce. With less torque available, the rotor must slip further to generate the needed torque, so the actual operating speed decreases. In hot environments, motors are often thermally derated, meaning their usable torque at a given ambient is reduced, which also contributes to slower speeds under load.

Low applied voltage also reduces torque and can cause slippage, but the given answer highlights the temperature effect as a common and direct cause of decreased speed. Excess lubrication can add drag but is less typically responsible for a sustained slow run, and a strong magnetic field would not generally cause a slower speed in a standard AC motor.