Effect of operating an Induction Motor designed at 415V, 50 Hz in 380V, 60Hz

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Let us consider a situation where a customer is asking your support to deliver a 45KW/4P, 380V, 60Hz motor on immediate basis. The load data is not available. You have one motor readily available which is also 45KW/4P but designed in 415V, 50Hz. Now if we are going to use this motor for 380V, 60Hz operation, let us see what will happen...


1. We know in an Induction motor, the starting torque and the pull out torque is related with voltage as            below

                       Tst ( starting torque ) ∝ V^2

                       Tpot ( Pull out torque ) ∝ V^2


So if we run the existing 415V motor in 380 V, then the Tst and Tpot at 380V will be calculated as per below

Tst ( starting torque ) ∝ (380/415)^2

Tpot ( Pull out torque ) ∝ (380/415)^2

It is very much clear that both the values at 380V will be lower than the designed value at 415V. Hence running this motor without checking the suitability against the load will not be a good idea.

Now, if you are operating the same motor in 60Hz, then the situation can be critical.

Since the driven machine is Fan, it will follow the Law of affinity i.e

T∝N^2 and P∝N3 , where P= Power, N= Speed, T= Torque

The 50Hz motor will run at 60 Hz which means the motor will be at constant power zone hence the torque availability will go down.

T∝(50/60)^2

But the driven machine will demand higher torque at 60Hz. Hence the existing motor will not be suitable at all at higher frequency with same KW.

Such motor must be designed for power output that is excess of BKW at the maximum speed and for a frequency corresponding to the maximum speed of operation.


  


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