Questions about Magnetization Current in Induction Motor

bodoo23 said:

I can not understant two points about magnetization current in induction motors.

1-It is said magnetization current is almost constant in induction motors. I can understand that if we run motor in v/f scalar mode or directly by contactor. But if we use vector control , as you know v/f ratio changes according to need. So According to my understanding, magnetization current also should change when we are running in vector control mode. What do you think about it ? (Do not consider field weakining point, My question is for any point under field weakining)

2- I have a 1PH8 motor that I used in a project. In manual of motor it says magnetization current 215A but when I calculate magnetization current from motor nameplate (Motor rated current = 460A and P.F = 0.85), I found about 240 A. What is the reason of this difference ? They should not be the same ?

Thanks in advance.

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Lynx777 said:

There are others here who know more about this than I do but your post has gone unanswered, so I will tell you what I believe to be true.



1. As you said the magnetizing or flux current for standard motors remains relatively constant. My understanding is that any control method (v/f, FV, SV etc.) gets "layered" on top of the flux current to control the motor.

As an example: Assume you have an uncoupled motor. Once the VFD has established the flux current it then can hold the motor at zero speed with only the flux current. Any extra current is only needed to move or hold a load.

2. The flux current for any motor can be calculated based on the nameplate data, but those calculations make a lot of general assumptions and can only get you in the ballpark of the true flux current. If the flux current is listed on the nameplate that means that the manufacture has taken the time to test that particular motor design and give you the true flux current requirement.

I hope this helps answer your questions. And if anybody sees something I've written here is wrong, please feel free to correct me.

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mylespetro said:

I assume that this is one of the things a VFD is checking when you do an autotune procedure on it?

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