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How to calculate motor overload

Updated April 17, 2018

Electric motors have a rated voltage and frequency and a rated full load current which can all be found on the motor nameplate. Motors must be operated at rated voltage and frequency, and the motor overload can then be calculated by comparing the actual motor current to the rated full load current. Motors heat up when overloaded, reducing the life expectancy of their insulation. However, if the nameplate of the motor gives a service factor, the motor can be overloaded by that amount without any damage.

Get the rated voltage and frequency from the motor nameplate. Check that the voltage of the electric supply feeding the motor matches the rated voltage. For the US and Canada, the rated frequency should be 60Hz. If these figures don't match, then measuring the motor current may not give a true indication of motor heating and overload.

Measure the motor current. Divide by the rated full load current from the motor nameplate. This will be the load factor for the motor. If the motor current is 22A and the rated full load current is 20A, then the load factor is 22/20 = 1.1. This means the motor is overloaded by 10%. If the load factor is 1.0 or less, the motor is not overloaded.

Check the motor nameplate for a service factor. Many motors are rated with a service factor of 1.15 which means the motor can be overloaded by 15% without damage. If the motor in the example from step 2 had a service factor of 1.15 then its overload would be acceptable and the motor could be operated without damage.

Check the motor nameplate for the ambient temperature rating and the insulation class ratings. Electric motors are commonly rated for operation in an ambient temperature of 40 deg. C (104 deg. F) or 50 deg. C (122 deg. F). Their insulation rating is usually class B or F. Class B insulation motors are designed for an operating temperature in the insulation of up to 130 deg. C (266 deg. F) and class F insulation motors for 155 deg. C (311 deg. F).

Tip

If the motor is always operating at an ambient temperature well below the rated ambient temperature, it may be possible to overload the motor without damage. Check with the manufacturer for new nameplate information based on a lower rated ambient temperature. If the motor is operating with an insulation temperature well below the maximum temperature, it may be possible to overload the motor without damage. This is especially true if the motor is insulated to class F but utilised to class B. Check with the manufacturer for new nameplate information based on utilising the motor to class F.

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About the Author

Bert Markgraf is a freelance writer with a strong science and engineering background. He started writing technical papers while working as an engineer in the 1980s. More recently, after starting his own business in IT, he helped organize an online community for which he wrote and edited articles as managing editor, business and economics. He holds a Bachelor of Science degree from McGill University.