D. C Motor Starting:
Why Starter Needed:
Let Vt = Terminal Voltage
Ia = Armature Current
ra = Armature Resistance
Ea = Back emf = KaØWm
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,
Vt= Ea+ Iara
= 0+ Iara
Hence Ia = Vt / ra
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.
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