Swinburne's Test of DC Machine - Advantages & Disadvantages

For small ratings of dc machines, we can perform the tests directly by loading. In the case of large dc machines, it is difficult to perform tests by loading directly. For this purpose, there are indirect methods of testing a dc machine without actually loading the machine so that losses and efficiency can be calculated at any desired load. The most commonly used indirect methods of testing dc machines are,

     In this article let us discuss briefly Swinburne's test and how it is performed.

Swinburne's Test of DC Machine :

It is an indirect method (without actual loading) of testing the dc machine where the losses are calculated and efficiency can be determined at any load either the machine running as a generator or motor. Since the test is performed without loading the machine it is also known as No-load Test. However, without load, we cannot start a dc series motor. Hence this test is limited for dc shunt and compound machines where flux remains practically constant.

Swinburne's Test of DC Machine

The connection diagram for Swinburne's test is shown above. At no-load with rated voltage, the machine runs as a dc motor. By adjusting the rheostat connected in series with the shunt field the motor speed is adjusted to its rated value.

Let,
  • V = Input supply voltage
  • Ia = No-load armature current
  • Ra = Armature resistance
  • Ish = Shunt field current
  • Rsh = Shunt field resistance
  • Power input = V I (Ia + Ish) watts

Since it is a no-load test there is no output at the motor, but the input as to supply,
  • Iron loss or constant loss,
  • Friction and windage loss,
  • Copper loss in the shunt field (Ish2 Rsh), and
  • Armature copper loss (Ia2 Ra).

Since, the iron, friction, and windage losses are independent of load. The losses remain constant throughout the operation i.e., at any load. Constant losses Wc are obtained by subtracting the shunt field copper loss and no-load armature copper loss from the no-load input.

Swinburne's Test of DC Machine

Here, the copper loss will not remain constant through the operation. As the load on the motor varies the temperature of the field and armature winding increases, this affects their winding resistances. If Ra' and Rsh' are the new resistances due to a change in temperature. Consider, that the temperature change is about 40⁰.

  • The new armature resistance due to change in temperature is,
  • Swinburne's Test of DC Machine
  • Similarly, the new field winding resistance is,
Swinburne's Test of DC Machine
Where α1 = resistance temperature coefficient of copper at room temperature. Therefore, by calculating the constant losses, the efficiency can be determined at any load current I.

Efficiency When Machine Running as a Motor :

Let,
  • Input supply = VI
  • Constant losses = Wc
  • Armature copper loss = Ia2 Ra
  • Field copper loss = Ish2 Rsh

The efficiency of the motor is,

Swinburne's Test of DC Machine

Efficiency When Machine Running as a Generator :

Let,
  • Output power = VI
  • Constant losses = Wc
  • Armature copper loss = Ia2 Ra
  • Field copper loss = Ish2 Rsh

The efficiency of the generator is,

Swinburne's Test of DC Machine

Advantages of Swinburne's Test :

  1. The method of testing is a dc machine is simple and convenient.
  2. It is very economical because the power required is very low and large machines can be tested with a small amount of energy.
  3. By knowing the losses efficiency at any load can be determined.

Disadvantages of Swinburne's Test :

  1. In Swinburne's test, the iron losses are kept constant at any load (from no-load to full-load). But in practice due to armature reaction, there will be a distortion of flux which increases the iron losses.
  2. since it is a no-load test, it is not possible to determine the temperature rise and whether the commutation is satisfactory at full-load.

Limitation of Swinburne's Test :

  1. This test is limited for the machines where flux remains constant i.e., for dc shunt and compound-wound machines.
  2. A dc series machine cannot be tested because it is not advisable to run a series motor without load.

Also check out, Hopkinson's Test of dc machines.


Previous Post Next Post