Ben asks this question concerning the starting current of an AC induction motor at no load. It has been answered by ABB (Theme – Electrical Control):
This Q & A is one of thousands posted in our Technical Expertise area, and answered on a daily basis by our Voltimum Experts.
Question: If the Direct On Line (DOL) starting current or Locked Rotor Current (LRC) is six times the full load current of a 3PH squirrel cage induction motor, will the starting current at no-load also be six times no-load current?
Answer: DOL starting applies full voltage directly onto the motor, so starting is carried out at full voltage and frequency, developing a high starting torque and short acceleration times.
The problems with this are that it leads to high inrush currents that can lead to values of 10 -12 times the rated Full Load Current (FLC), which then decrease to 6 - 8 times FLC, which can persist until maximum torque speed is achieved.
The loading on the motor will affect the length of time the 6 - 8 x FLC persists, rather than increasing the current level.
To see ABB’s white paper regarding protection of motor starters, please use the link at the bottom of this web page (PDF doc - 1.4MB file size).
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