Shunt DC motor

In this arrangement, the armature and field coils are connected in parallel (the motor is called 'shunt connected').

Analysis

With reference to the diagram above,

Normally I_f is a approxmetely constant and Ra is very small (so V>>I_a.R_a).Therefore the motor speed is approximately constant for a given supply voltage, V. The speed may be controlled by reducing the field I_f  by increasing the field resistor R_f   is reduced and the so Φ speed  increases.

• I_f  is approximately constant in this motor configuration, determined by the supply voltage and the resistance of the field coils. 

• Since Ra is very small, V>>I_a.R_a.Hence, although I_a  changes significantly with change in motor speed, (and hence also the torque), the motor speed need only change very slightly to cause a large change of armature current and torque produced. This means that the torque can overcome any imposed mechanical load  with very little change of motor speed (so shunt motor has approximately  onstant speed vs load characteristics)
• Since the armature winding resistance is small, measures must be taken to limit the surge of armature current when it is first
  switched on (i.e. increase the supply slowly to allow the motor to gain speed before full supply voltage is applied). 
• Methods for controlling the speed are:
  
  i)    vary the resistance of the field circuit, R_f . Increasing R_f  a increases the speed.
  
  ii)   change the applied voltage. Increasing V increases the speed.
  
  iii)  change R_a by means of an extra (small) resistance in the armature circuit. Increasing R_a  reduces the speed.
  
  iv)  increasing the mechanical load reduces the speed only slightly. (Approx. constant speed vs load characteristics)
  
  The preferred method of speed control atconstant supply voltage is method (i)because the field current is low compared to the armature current, and a resistance added in series with the field windings results in less power being wasted.