Lessons · Engineering · Slip, torque and starting current
Slip, torque, and the current at the moment of starting
An induction motor's rotor always turns slightly slower than the rotating field; that difference is slip, and it is what produces torque. At the instant of starting, slip is total and the motor draws several times its running current.
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In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
Starting current is why a large motor dims the lights, why supplies are sized above the running figure, and why soft starters exist. A motor selected on running current alone will trip its protection every time it starts.
How to think about it
Read the nameplate for running current, then ask what the starting multiple is before sizing supply and protection. If the starting current is a problem, the answer is a starting method, not a bigger motor.
Worked example
The rotor lags the field; that lag is slip.No slip, no torque.
More load means more slip and more torque.Up to a limit.
At standstill slip is total: current is several times running.Which the supply must tolerate.
Soft starters and star-delta reduce that surge.Rather than oversizing everything.
Your turn
A motor runs at 12 A and starts at six times running current. Write the starting current in amps.
12 x 6 = A
Solve one, graded on the server
The trap
Sizing the supply on running current. The start is brief and it is still real, and protection that cannot tolerate it will trip on every start.