- Swinburne Test Definition: The Swinburne test of DC machine is defined as an indirect method to test the efficiency of shunt and compound wound DC machines by measuring no load losses.
- Operation Principle: This test runs the machine as a motor or generator to measure its no load losses and calculate efficiency.
- Efficiency Calculation: Efficiency is determined by subtracting the armature copper loss from the no load power input and calculating for different loads.
- Advantages: This test is convenient, economical, and allows efficiency prediction at any load.
- Disadvantages: It neglects changes in iron loss due to armature reaction, can’t assure satisfactory commutation on load, and is not suitable for DC series motors.
The Swinburne test is an indirect efficiency test for constant-flux DC shunt and compound machines. The machine runs as an unloaded motor at rated terminal voltage and rated speed. Its measured no-load input establishes the constant losses, while the calculated armature copper loss allows motor or generator efficiency to be predicted at a selected load.
The circuit for Swinburne’s test includes a shunt-field regulator. Adjusting it brings the machine to rated speed before the no-load readings are taken.
Calculation of Efficiency
Let I0 be the total no-load current, measured by ammeter A1.
Let Ish be the shunt-field current, measured by ammeter A2.
The no-load armature current is 
Let V be the terminal voltage. The no-load electrical input is therefore VI0 watts.
Because there is no useful shaft output during Swinburne’s test, this input supplies the machine losses:
- Iron loss in the magnetic core
- Mechanical friction and windage losses
- Armature copper loss.
The test assumes that iron loss, mechanical loss and shunt-field copper loss remain constant at the specified voltage and speed.
The no-load armature copper loss is 
Here, Ra is the armature resistance at the test condition.
Subtracting this armature copper loss from the no-load input gives the constant loss.
Thus, 
After finding the constant loss, calculate efficiency at a selected load within the machine’s rating.
For motor operation, let I be the input line current.
The armature current Ia is then (I – Ish).
For generator operation, let I be the output load current. Then 
.
Calculation of Efficiency When the Machine is Motoring on Load
Electrical input = VI
Armature copper loss, 
Constant loss, 
Therefore, the motor efficiency is:
Calculation of Efficiency When the Machine is Generating on Load
Electrical output = VI
Armature copper loss, 
Constant loss,
Therefore, the generator efficiency is:
Advantages of Swinburne’s Test
The main advantages of this test are:
- The test is economical because it draws only the no-load input instead of the machine’s full-load power.
- Once the constant loss is known, Swinburne’s test can predict motor and generator efficiency at selected loads without physically loading the machine.
Disadvantages of Swinburne’s Test
The main disadvantages of this test are:
- The method assumes that the no-load iron and rotational losses remain constant. Armature reaction at load can change flux and iron loss.
- A no-load test does not show whether commutation will be satisfactory at full load.
- It does not establish the full-load temperature rise. Higher winding temperature increases resistance and copper loss.
- The Swinburne test cannot be used for DC series motors because a series motor must not run unloaded.
