Overload class of Induction Motors and RMS overload calculation

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This overload cycle is common for mill duty motors. The motor manufactures usually get enquiry from the users with corresponding duty cycle. it is advisable to send such enquiries to the design team for proper selection not only in terms of frame size and KW, but also he temp rise of the motor.

for such cases RMS overload needs to be calculate. I am providing example for better understanding of the calculation

Condition 1:

160% overload for 60 sec in every 600 sec

so in a 300 sec cycle , the motor will run with 100% load for (600-60) = 540 sec

So the RMS overload is = √((160^2) x 60 + (100^2) x 540) / 600) ( √ for total calculation to be considered ) = 107.5%


Condition 2:

150% overload for 60 sec

175% for 20 sec

so 100% loading for a 600 sec cycle will be = (600-60-20)=520 sec

So the RMS overload is = √((150^2) x 60 +(175^2) x 20 + (100^2) x 520) / 600) ( √ for total calculation to be considered ) = 109%


For any clarification please ask question.




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