Question for EE/MEs about AC motors.

Factual Questions
1

Had a super-condensed summer class on power, and the teacher made part of our exam take home to ease the burden of cramming.

One of the questions is “At start-up conditions (locked-rotor), does the start up current rise to very high levels in all machines? Explain.”

Now I fully understand the workings of the high startup current in DC machines, but I’m a bit off on the 3-phase, squirrel cage induction motors. The current is going to be high within the cage itself with the rotational speed differential between the cage and the field, but that doesn’t affect the current drawn from the source voltage though, right? Isn’t what is drawn from the source voltage going to be based on the resistance of the inductors in the stator? Do you think the high current in the rotor really be considered “high startup current?”

2

Put an ohmmeter across the terminals of a three-phase motor and you’ll measure in the tens of ohms. Apply ohms law and see what kind of current you get for a 600V motor. The start-up current is very high, about 4 to 9 times the running current. Once the rotor starts turning it develops a counter-emf the same way a DC motor does. That and the inductive reactance act to bring the current back to running levels.

3

Excellent user name combination !;):smiley: