Page 233 - Electrician - TT (Volume 1)

Eb
α N
Φ
ELECTRICIAN - CITS
Eb V Ia Ra
V Ia Ra
α N
• Ise is bypassed / reduced
Φ
• Field current and flux decrease
• Speed increases α N 1
Φ

output Input Losses
Efficiency
Input Input

VI [(I Ish) 2 Ra Wc]

VI

DC Series motor - voltage control method
In voltage control method, speeds lesser than normal is obtained. For this a resistance is connected in series to
motor. Hence the supply voltage to motor is reduced, field current and flux decrease. So speed decreases. This
method requires high power resistance. Solosses increase

Tapped field control
In this method field turns are tapped. Field flux decrease when tapped. When flux decreased speed increases.
This method is used in traction

DC Shunt motor- flux control method
DC Shunt motor- flux control method

Ward leonard system
In this system a variable voltage is applied across armature. Separately excited Generator supplies variable
voltage.This generator is driven by a motor.Field regulator of generator controls the voltage and polarity of the
supply to motor under speed control. M-G set runs at constant speed in one direction

220

CITS : Power - Electrician & Wireman - Lesson 30-37