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Why Do We Need to Install a Starter with a Motor?

Why Do We Need to Connect a Starter with Electrical Motors?

Essential and Need of Starter with Motor

Motors below 1 HP (0.746 Watts) are directly connected to the power supply without starter because their armature resistance is very high and they have the ability to afford and safely pass the higher current because of the high internal resistance. So the armature windings are safe and won’t be damaged from the high starting current while starting an electrical motor.

In the case of large motors, they have a very low armature resistance. If we connect these types of motors directly to the power supply (mostly 3-phase supply), then the higher amount of current will start to flow and it will destroy the armature winding due to low resistance at the initial starting stage while the motor is not running at normal position. The motor will not start to run at this stage because there is no back E.M.F in the motor. The back E.M.F of the motor is reached at full rate when the motor is running at full speed and rated load.

So this is the exact reason that’s why we connect a starter with a motor in series.  A starter in series with the motor (i.e. resistance) reduces the high starting current as the armature needs low current due to rating at the initial stage and then runs at normal speed.

But this is not the end of the story. After starting the  motor at low current, the starter resistance is reduced by turning the manual starter handle (the process can be automatic in case of automatic starter). This way,  the rated current will start to flow through armature windings and the motor armature starts to rotate at full speed.

Why Do We Need to Install a Starter with a Motor?


What happens if we don’t connected a Starter with a Motor?

Let’s see the following example.

We know that the armature current can be found by the following formula.

Ia = V – Eb / Ra   .….   ( I = V / R, Ohm’s Law)


  • Ia = Armature current
  • V = Supply voltage
  • Eb = Back E.M.F  
  • Ra = Armature resistance

Related Post: Main Difference between Contactor and Starter


A 5 HP (3.73 kilowatt) motor with 400 Volts having armature resistance of 0.25 ohms resistance and the normal full load current is 50 amperes. if we connect the motor directly to the power supply without starter, the result (of amount of flow of current) will be as follows:

Putting the values in equation given above.

Ia = 400V – 0V / 0.25Ω

 Ia = 1600 A 

Ah! This high current will destroy the armature winding because it is 32 times higher than the normal full load current of the motor.

1600 A / 50 A = 32 Amps.

So that is the reason why we need to install a starter with a motor.

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  1. Wasim khan says:

    thanx dear <br />bohat acha kam kar rahe ho.

    1. Electrical Technology says:

      Welcome and Thanks for appreciation.

      1. Dear sir how work diesel engin of cummins kta50.if have vedio than share please.

  2. Anonymous says:

    Really. Thanks. Its totally easy to understand.

  3. Anonymous says:

    How can one calculate the suitable contactor for different selected motors that will operate effectively<br />

  4. Anonymous says:

    Why is it the a smaller motor has a higher armature resistance while a larger motor has a lower armature resistance?



  6. What do you consider high currerentnt version low current

  7. What do you consider a large Moyer to a small moteron

  8. why do we connect motor in different ways. eg star-delta, delta-star.etc pls give d diagrams

  9. please tell me how you get 0.25 ohm resistance

  10. kamaludheen says:

    how we select different types of starters for motors

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