Reduce the starting current of an induction motor

During the transient condition associated with acceleration of an AC machine started as an inductive load, current is drawn from the source that can be several times the full-load rating of the machine winding. To handle this current, the protective device (e.g. circuit breaker) located at the juncture of the distribution bus and the feeder to the rotating load should have an instantaneous setting and short time setting that will adequately protect the downstream equipment but will also avoid nuisance tripping during the acceleration transient.

The conductor(s) used to carry the current from the protective device to the rotating load are typically well within their limits if sized according to the local electrical code. In North America and Europe (and probably elsewhere), the cable must be able to carry something more than the rated load current on a continuous basis - typically this translates into a 1.25x ampacity rating.

Both the motor stator winding and rotor winding are designed to accommodate the starting current without excessive thermal stress, provided rotation actually takes place (i.e. the motor does not sit at "stall").

All connecting joints - bolted terminations, crimped connections, brazed or welded connections, etc - should also be sized for full rated current consistent with the actual cable ampacity rating. The additional heating resulting from the high current transient condition will not adversely affect the joint over the long term, as long as the motor actually rotates.

Some of the techniques used to reduce the starting current of a squirrel induction motor:

  1. Soft starting (voltage control)
  2. Variable Frequency Drives (Voltage and frequency control)
  3. Star/Delta Starting
  4. Stator impedance and/or resistance starting
  5. Autotransformer Starting

Additional technique for wound rotor induction motors (slip rings):

  1. Rotor resistance starting

Each has its advantages and disadvantages over direct online starting and some of the factors that need to be considered when choosing a starting method are:

  1. The mechanical loads attached to the motor
  2. Speed, torque and braking requirements
  3. The mechanical ratings of the motor including its mechanical duty rating
  4. The electrical ratings of the motor including its form factor rating if you are using a variable frequency drive or a soft starter.
  5. The speed/torque characteristics of the load and the motor
  6. The cabling and the electrical systems feeding the motor
  7. The costs of the starting methods as they vary considerably between methods.

You will need to do a lot of reading about induction motors and starting methods as there are many factors to consider. These are just a few of them.


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