Locked rotor current rating vs magnetization current


Is the locked current rating of a motor the highest current I should expect when starting a motor? Is the locked rotor current rating the same as magnetization current? If not then how should the magnetization current be considered when selecting motor protection?

Best Answer

The magnetization current of an AC motor would be that current that flows into the stator should the rotor be removed. It's difficult to measure for that reason because any amount of movement/loading of/on the rotor is going to affect it.

Like a transformer, an AC induction motor has an equivalent circuit and it is sensible (for both motor and transformer) that the magnetization current is significantly lower than the load current due to the rotor transferring power to something (or the secondary winding transferring power to a load).

This diagram shows the equivalent circuit and note Im: -

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Current Im flows thru inductor Xo and this current is largely unrelated to what happens on the rotor. It follows that the true magnetization current of the machine is with the rotor removed.