**D. C Motor Starting:**

Why Starter Needed:

Let V

_{t }= Terminal Voltage
I

_{a }= Armature Current
r

_{a}= Armature Resistance
E

_{a}= Back emf = K_{a}ØW_{m}_{}
At the time of starting, speed of motor is zero hence Wm
= 0, therefore there will not be any Back / Counter emf. Hence if we look into
the equation,

V

_{t}= E_{a}+ I_{a}r_{a}
= 0+ I

_{a}r_{a}
Hence Ia = V

_{t}/ r_{a}_{}
Since the value of armature resistance is very less of
the order of 0.1 to 0.5 hence if rated voltage is applied to the Motor then
motor will draw excessive current from the Supply mains. Such heavy inrush
current may result into

1. Sparking
at commutator.

2. Damage
to armature winding and damage of insulation over it due to overheating.

3. Large
dip in supply voltage.

4. High
starting torque and quick acceleration which may result in damage of rotating
part of the motor.

**General Philosophy of Starter:**

The primary function of a Starter should
therefore be to limit the starting current and acceleration time of motor.
Therefore if we insert a resistance in series with the armature circuit, it will
limit the starting current but as the motor accelerates back emf will be
developed which in turn will reduce the armature current, therefore as the
motor accelerates the inserted resistance must be decreased.

While starting the DC Shunt Motor / DC Compound
Motor, field flux should be kept maximum as the result in low operating speed
which means less acceleration time. Also as Torque = KaØIa, hence starting
torque needed to overcome the frictional force will be met by less armature
current. So rheostat in series with field circuit should be kept at zero
position while starting the machine.

DC Shut / Compound DC Motor Starter: Mind
that staring resistor is inserted in armature circuit not in field circuit.

There are two types of starters for Shunt and
Compound DC Motor:

- Three point starter
- Four point starter

For DC Series Motor No load Release type
starter is used. Mind that staring resistor is inserted in armature circuit not
in field circuit.

Wanna know detail of these starters? Just
write in comment box, it will encourage me friends

## 2 comments:

Thanks sir can you please explain the circuit diagram of starter

Thank you Mayur! I will post on the circuit of DC starter soon. So be there and follow me...thank you!

Post a Comment