Plain Rheostatic Starting of Traction Motors (Notching):

As already mentioned in this plain rheostatic starting method the voltage across the motor is gradually increased from zero to full (normal) voltage and the current is kept constant at normal rated value by reducing the external resistance gradually.

Let the plain rheostatic starting period be T seconds, constant current I amperes and line voltage V volts.

At the instant of switching on the supply, the back emf is zero and it starts building up immediately the motor starts rotating. At any instant the supply voltage is equal to emf developed by motor plus voltage drop in the series field and armature of motor plus voltage drop in starting rheostat.

At the end of starting period, the back emf becomes approximately equal to line voltage and is given as.

Therefore, external resistance is reduced to zero, and therefore, voltage drop in the external resistance is zero.

Energy drawn from the supply during starting

Energy lost in external resistance

if voltage drop in series field and armature of motor is neglected.

Energy utilized in driving the motor.

Starting efficiency,

The energy lost and the voltage drop can be illustrated with the help of illustrations shown in Fig. 13.6.