STARTING OF DC MOTORS

 Important Points

• When a differentially compounded motor is started, the starting current may be high enough to reverse the flux polarity and thus motor would start in reverse direction. However, since reverse field is weak, the acceleration may be low and thus starting current would remain high for long time and can thus damage the machine. So, series field winding is shorted while starting and when motor acquires normal speed, shorting may be removed.

• Sometimes, due to sudden overload in uncompensated shunt machine, the demagnetizing effect may be dominant and can cause sudden runaway. This is prevented by providing a few series turns to neutralize armature reaction (AR) and hence make it cumulatively compounded.

Starting of DC Motors

At Starting, the speed of DC Motor is zero and hence the induced emf which is proportional to speed of Motor is also zero. Due to which,

Vt = Ia Ra     for Shunt Motor and Separately Excited Motor

Vt = Ia (Ra + Rse)     for Series and Compound Motor

Ia = Vt / Ra  or  Vt / (Ra + Rse)    Which is very high as Ra  and  (Ra + Rse) are very small

Such heavy current may cause,

• Sparking at Commutator
• Damage to armature winding and insulation 
• High starting torque and acceleration 
• Large dips in supply voltage

So this current must be limited by inserting a resistance in the armature circuit, But this resistance must be cut off as rotor accelerates else

• Operating speed of motor will be reduced 
• Losses will be higher and efficiency will be less

For shunt and compound motors, field excitation must be maximum at starting, as large flux results in less operating speed. Also, since T ∝ ⲪIa , for a given load torque, requirement of Ia is reduced at higher Ⲫ.

Shunt and Compound Motors

The simplest type of starter is a rheostat inserted in series with the armature circuit.

The other method which is more commonly used is Four Point Starter.

Four Point Starter

• There are four terminals L, N, F and A and hence it is called as Four Point Starter.
• Initially, all the resistances are connected in series with the armature so that starting torque is low. As the motor speeds up the resistance are cut out with the help of starter handle.
• Finally, the soft iron piece attached to Starter Handle is attracted by 'No Volt Coil' which is excited by supply voltage so that handle is kept static. In case supply voltage is removed this coil gets demagnetized and spring connected to handle brings the handle back to its original position.
• In case of overload the overload piece attracts the lever and triangular iron piece short circuits the holding coil causing release of starter handle and thus turning off the motor.