V4.16 - Electric motors Part 2

We left off our discussion by saying that the source voltage is not usually the voltage that makes it to the motor. That's important because induction motors are designed to operate at exactly their nameplate voltage (120 or 240V, in this case), and all of the published parameters are based on exactly that voltage at the motor leads. They also are designed to operate within a range of voltage, above and below that rated voltage, but the current, power factor, efficiency, internal heating, and torque curve will vary (not usually for the better) at voltages other than the nameplate voltage. As the voltage at the motor leads droops, the torque curve also droops, but as the square of the actual voltage to nameplate voltage ratio, meaning voltage at the motor that's 10% lower than the rated voltage will depress the torque curve to 81% of design (.9 squared). A 20% voltage sag results in a torque curve depressed to 64%.

Here's where it gets complicated. Motor current, at least within the normal operating range, depends on rotor speed. The rotor is always trying to spin at the synchronous speed, which is 3600 rpm, or 1800 rpm (and slower, like 1200, 900, etc.), depending on the motor. It can't ever reach that, because the rotor spinning less than synchronous speed is what induces current within the rotor, forming magnetic fields within the rotor which interact with the stator magnetic fields, providing the torque (hence the term 'induction' motor). The slower the rotor goes, the greater the speed difference between the stator fields and the rotor fields, and the greater the current in the stator windings, the greater the strength of the resulting magnetic fields, and the greater the torque. When you put a load on the motor (by cutting wood), you can hear the motor slow slightly. Slightly, because the difference between full-load and no-load is only about 4.2% [(1-(1725/1800)) * 100%].

The slower the motor spins, the greater the current through the windings. BUT, if the torque curve is depressed due to saggy voltage, for a given torque load, the motor will spin slower. Slower rotor means higher current to maintain the output torque. So lower voltage = higher current = greater winding heating.

This isn't a problem for a motor operating within 'normal' conditions, as in torque at or below rated, ambient temperature at or below rated (it's on the nameplate), and voltage at or within tolerance (+/-10%). In fact, the reason your motor is rated 115/230V for operation on a 120/240V system is in recognition of voltage sag over the supply wiring (includes 200V for 208V systems, 460V for 480V, and 575V for 600V, which is common in Canada).

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