Swinburne's Test on DC Machines

1.    It is a simple method in which no load dc-machine losses are measured seperately and from this the efficiency at any desired load can be predetermined in advance.

2.    It is a no load test and therefore cannot be performed on a dc series motor.

3.    This test is applicable to those machines in which flux is practically constant i.e shunt and compound-wound machines.  

Lets see how no-load losses and efficiency are determined. The circuit connections are made as shown.

Here,

V_t = rated terminal voltage

I_ao = no-load armature current

I_f  = no-load field current

R_a = armature resistance

W_o = No-load rotational losses

The  machine whether it is a motor or generator, is run at rated speed and with rated terminal voltage V_t. Under this no-load running condition, power absorbed by the armature is  V_t I_ao.

But,
  
armature power absorbed = the core loss (hysteresis and eddy current loss) + mechanical loss (Friction and windage loss) + Stray load loss + armature circuit copper loss ( including brush contact resistance).

The core loss and mechanical loss together called no-load rotational losses W_o. Since these losses depend on motor speed and flux density, therefore this test is applicable to those machines in which flux is practically constant and performed at rated speed. So, that these losses remain constant at all armature currents as long as flux and speed is maintained constant. 

Since the output of the motor is zero and Stray load losses are neglected , the equation reduces to

armature power absorbed = W_o + armature circuit copper loss

                                   V_t I_ao= W_o + I_ao²R_a

Or                                 W_o = V_t I_ao - I_ao²R_a

and shunt field loss = V_t I_f

Efficiency when running as a motor

Let  I_L be current drawn from supply mains by the motor and I_a  be the armature current when efficiency is to be calculated.

I_a= I_L-I_f

η_m = (Input - Losses)/Input = [V_t I_L - W_o - V_t I_f - I_a² R_a] /V_t I_L

Efficiency when running as a generator

Let  I_L be current drawn by the load and I_a  be the armature current when efficiency is to be calculated.

I_a= I_L+I_f

η_g = Output/Input = V_t I_L/[V_t I_L + W_o + V_t I_f + I_a² R_a] 

Drawbacks of Swinburne's test :- The test does not take into account the stray load losses.

Related concepts :-

 (1) What is the need of starter in dc motors ?

 (2) Torque Equation of dc Motor  
 (3) Three Point Starter
 (4)  Ward- Leonard method