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RMS and Peak values of Motor


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Dear Members,

Can anyone give

guidelines regarding the Motor Peak Current and motor RMS Currents? During measurment of starting current of a motor in the field it is noticed that the starting current is high. But is the current to be measured Peak or RMS? Urgent reply required. Thanks.

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Hello sharadmn


Welcome to the forum.

When you start an in duction motor from a full voltage supply, you have an initial inrush current which lasts for a few cycles only. This is the current required to etablish the magnetic field in the stator. The current then drops to the starting current of the motor.

At rest, the start current of the motor is equal to the Locked Rotor current of the motor. This can vary between 550% and 900% of the rating of the motor, depending on the design of the rotor.

As the motor accelerates, the strt current begins to fall, but only falls significantly as the motor approaches full speed.

The shape of the speed current curve of the motor is a function of motor design.

The start current is a function of motor design, start voltage and rotor speed. The start current is independant of shaft load.


In terms of whether to measure the peak or the RMS values, it depends on what you are looking for. The inrush current is cycles only and must be a peak measurement. The start current is dependent on the start time, and this is dependent on the attached load, (time, not magnitude) and can be considered a steady state current for the start duration.


Best regards,

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