Why We Need to Install a Starter with a Motor?

Why We Need to Connect a Starter with Motors?

Essential and Need of Starter with Motor

Motors below 1 HP (0.7457) is directly connected to the power supply without starter because their armature resistance is very high and they have the ability to afford and pass higher current due to high resistance. So the armature windings are safe from the high starting current while staring a motor.

In case of large size of motors, they have a very low armature resistance. If we connect these type of motors directly to the power supply (mostly 3-phase supply), then the highir amount of current will 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 motor is running at full speed and rated load.

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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 armature needs low current due to rating at the initial stage and then run at normal speed.

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

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.

I_a = V – E_b / R_{a  ………} ( I = V / R, Ohm’s Law)

Where,

• I_a = Armature current
• V = Supply voltage
• E_b = Back E.M.F  
• R_a = Armature resistance

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Suppose

A 5 HP (3.73 kilowatt) motor with 440 volts having armature resistance of 0.25 ohm resistance and the normal full load current is 50 amperes. if we connect the motor directly to the power supply without starter, the result will be as follows:

Putting the values in equation given above.

I_a = 440V – 0 / 0.25Ω

  I_a = 1760 A  

Ah! This high current will destroy the armature winding because its 35.2 times higher than the normal full load current of the motor.

1760 A / 50 A = 35.2

So that’s why we need to install a starter with a motor.

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