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Hi everyone.
I've got a question from an employer. Please give me some explaination. Thanks in advance.

A motor 50kw controlled by v/f method. now F=50hz. If f=40hz. P=?
 

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You know that the speed will be 4/5 of the original speed. You cannot predict the power draw without knowing the nature of the load.
You may be able to predict the max power to the load IF you know the motor design.

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The answer he is looking for, however inaccurate, is 40kW. Power CAPACITY of the system (drive + motor) reduces proportionate to speed. So in EFFECT, your 50 kW motor is now a 40 kW motor. The actual LOAD is another issue altogether, but despite the question being poorly worded, this is what the question is designed to test you on, the EFFECT of using a VFD on motor power capability.
 

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The answer he is looking for, however inaccurate, is 40kW. Power CAPACITY of the system (drive + motor) reduces proportionate to speed. So in EFFECT, your 50 kW motor is now a 40 kW motor. The actual LOAD is another issue altogether, but despite the question being poorly worded, this is what the question is designed to test you on, the EFFECT of using a VFD on motor power capability.
Is this true in all modes or only volts hertz mode?
 
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