Why We Need to Install a Starter with a Motor?
Essential or need of starter with 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)
Ia =Armature current
V= Supply voltage
Ra = Armature resistance
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………