Selection of Starters for Motors

How to select the starters for electrical motors?

Starters Introduction:

Starters are used to limit the starting current of electric motors. In addition to limiting the starting current, they have the following functions

  1. Starting and stopping the motor quickly and safely
  2. Automatic control of the motor
  3. Protection from over load

So the starter is not just simple switch or a circuit breaker. It incorporates motor protective devices like over voltage protection, over current protection, short circuit protection.

Selection of Starters for Motors Tutorial
The important factors influencing the selection of starters are given below

  1. Voltage rating
  2. Thermal current rating
  3. Operational current rating
  4. Frequency of operation
  5. Type of switching device used
  6. Protective relays provided

Voltage rating
As per the Indian standard institution code, the contactors should close positively between 85% and 110% of the rated voltage. The standard starters with AC coils are designed for operation on 50Hz.

Thermal Current rating
A 5HP motor is expected to carry the full load current of 7.5A continuously without getting overheated. This is known as thermal current rating. This current limits the temperature rise of the starter within permissible value.

Operational current rating

  • The operational current is different from rated thermal current.
  • For example the starting current for the above 5HP motor will be about 6 times fo the full load current (7.5 x 6 = 45A). This current(45A) is called as operational current.
  • All the contacts in the starter should be able to make and break this much current.

Frequency of Operation:

  • Starters are designed for intermittent or continuous duty. For some applications, the starters will be switched on and off frequently. Ex. Machine tools, Milling machines
  • During the turn on process, high current will pass through the contacts.
  • The starters should have high breaking capacity to withstand this high current.
  • Starters working for such operations, load factor, duty cycle has to be considered.

Switching Devices:
The type of switching device used in a starter may be

  1. Motor switch (star/delta switch)
  2. Motor protection circuit breakers (over load relay)
  3. Contactor ( making and breaking duty with magnetic coil)

Protective Device provided
The protective device employed will widen the choice of starters. Bimetal relay, despot relay, magnetic relay are some of the protective devices.

Thanks for reading….

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