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Why We Need to Install a Starter with a Motor?

Why We Need to Install a Starter with a Motor?

Essential or need of starter with motor.Why We Need to Install a Starter with a Motor?

Motors below 1 Hp is directly connect without starter because their armature resistance is very high and they have ability to afford the high current due to high resistance. So the Armature winding safe from the high starting current.

 
But a large size of motors has a very low armature resistance. if connect  this type of motor direct to Supply (3-phase Supply) then the large current will destroy the armature wading due to low resistance because motor is not running in this time. Why motor is not running in this time when we connect motor to supply? Obviously, because their is no Back E.M.F in the motor. the back E.M.F of the motor is reach at full rate when motor is running at full speed.
 
So this is the answer that why we connect a starter with motor in series.  Starter in series with motor ( I.e. Resistance) is reduce the high starting current and armature takes a low current and motor will be start. But this is not end of our story. After starting the  motor at low current, the starter resistance reduce by turning a starter handle ( not in each case, in other system or case, this can be automatically) so the armature will take high current and motor armature will be rotate at full speed ( in other words, the speed of the motor will be increase).

For more Explanation see the example.

We know that the armature current can be finding by this formula,
Ia = V-Eb/Ra       ,             ( I=V/R, Ohm Law)
        Where,      
Ia =Armature current
V= Supply voltage
Eb= Back E.M.F                                           
Ra = Armature resistance
Suppose
A 5 Hp (3.73killowatt) motor with 440 volts having armature resistance 0.25 ohm resistance.
And the normal full load current is 50 amperes.
if we connect to direct to supply without starter the result will be.
So putting the values in equation
Ia= 440-0/0.25 = 1760 A
ahh! This high current will destroy armature winding because its 35.2 times high with respect to normal full load current.
1760/50 = 35.2

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

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11 comments

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

  2. Really. Thanks. Its totally easy to understand.

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

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

  5. MUHAMMAD UMAIR KHAN

    thanks
    .

  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

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