What is a Swinburne’s Test: Calculations & Its Applications

The simple and indirect method of testing DC machines with constant flux is Swinburne’s test of DC shunt and compound wound DC machines. It is named as Swinburne’s test after Sir James Swinburne. This test helps to pre-determine the efficiency at any load with constant flux. The most important advantage of Swinburne’s test is, the motor can be used as a generator and no-load losses can be measured separately. This test is very simple and economical because it operates in no-load power input. This article describes the Swinburne’s test of DC machines.

What is Swinburne’s Test?

Definition: The indirect test used in the measurement of no-load losses separately and pre-determination of efficiency at any load in advance with constant flux on the compound and shunt DC machines are called Swinburne’s test. Mostly this test is applied for large shunt DC machines for efficiency, load losses, and temperature rise. It can also be called a no-load loss test or load loss test.

Swinburne’s Test Theory/Circuit diagram

The circuit diagram of Swinburne’s test is shown below. Consider that, the DC machine / DC motor runs at rated voltage with no-load input power. However, the speed of the motor can be regulated using the shunt regulator as shown in the figure. The no-load current and the shunt field current can be measured at the armatures A1 and A2. To find the armature copper losses, the resistance of the armature can be used.

                                            

Swinburne Test of DC Machine

Using the Swinburne’s test, the losses occurred in DC machines can be calculated with no-load power. Since DC machines are nothing but motors or generators. This test is applicable only for the large shunt DC machines which have constant flux. It is very easy to find the efficiency of the machine in advance. This test is economical because it requires a small input power with no-load.

 

Swinburne Test on DC Shunt Motor

The Swinburne’s test on DC shunt motor is applicable to find the losses in the machine with no-load power. The losses in the motors are armature copper losses, iron losses in the core, friction losses, and winding losses. These losses are calculated separately and efficiency can be pre-determined. As the output of the shunt motor is zero with no-load power input and this input no-load is used to supply the losses. Since the change in iron losses cannot be determined from no-load to full-load and the change in temperature rise cannot be measured at full load.

Calculations

Swinburne’s test calculations include calculation of efficiency at constant flux and losses of the DC machines. From the above circuit diagram, we can observe that the DC machine/DC shunt motor runs at rated voltage with no-load. And the speed of the motor can be controlled using the variable shunt regulator.

 

It is a simple method in which losses are measured separately and from their knowledge, efficiency at any desired load can be predetermined in advance. The only running test needed is no-load test. However, this is applicable to those machines in which flux is practically constant i.e. shunt and compound-wound machines.

 

The machine is running as a motor on no-load at its rated voltage i.e. voltage stamped on the name plate. The speed is adjusted to the rated speed with the help of shunt regulator.

 

The no-load current Io is measured by the ammeter A1 whereas shunt field current Ish is given by ammeter A2. The no-load armature current is (Io – Ish) or Iao.

 

Let, supply voltage = V

       Power input to armature = V (Io – Ish)

       No-load input = VIo watt

       Power input to shunt = VIsh

 

No-load power input to armature supplies the following:

(i)   Iron losses in core

(ii)  Friction loss

(iii) Winding loss and

(iv) Armature Cu loss

 

In calculating armature Cu loss, ‘hot’ resistance of armature should be used. A stationary measurement of armature circuit resistance at the room-temperature of, say, 15degree c is made by passing current through the armature from a low voltage d.c. supply.

 

Swinburne’s Test is not suitable for DC series motor?

 

Swinburne’s Test is to be conducted on no-load condition. The purpose of the test is to estimate constant losses comprising iron and mechanical losses which fairly remain constant in DC shunt motors, since shunt motors operate almost at constant speed at all load conditions.

 

DC series motor cannot be run on no-load condition since the motor speed start increasing on light load condition. There is no constant loss. Hence scope of Swinburne’s Test is not suitable for DC series moors.

 

At No-Load

Consider, the no-load current is ‘Io’ at armature A1

Shunt field current measured at Armature A2 is ‘Ish’

The no-load armatures current is the difference between no-load current and shunt field current at A2, given as = (Io – Ish

The input power at no-load in watts = VIo

The equation for armature copper losses at no-load power input is, = ( Io – Ish ) ^2 Ra

Here Ra is the resistance of the armature.

The constant losses at no-load are the subtraction of armature copper losses from the no-load input power.

Constant losses C = V Io – ( Io – Ish )^2 Ra

At Load

The efficiency of the DC machine/ DC shunt motor at any load can be calculated.

Consider the load current I, to determine the efficiency of the machine at any load.

When the DC machine acts as a motor, the armature current Ia = ( Io – Ish )

When the DC machine acts as a generator, the armature current Ia = ( Io + Ish )

Input power = VI

For DC motor at on load:

Armature copper losses are Pcu = I^2 Ra

Pcu = ( I – Ish )^2 Ra

Constant losses C = VIo – ( Io – Ish )^2 Ra

Total losses of the DC motor = armature copper losses + constant losses

Total losses = Pcu + C

Hence the efficiency of the DC motor at any load is, Nm = output/input

Nm = ( input – losses ) / input

Nm = ( VI – ( Pcu + C ) ) / VI

For DC Generator on Load

Input power at no-load = VI

Armature copper losses = Pcu = I^2 Ra

Pcu = ( I + Ish )^2 Ra

Constant losses C = VIo – ( I – Ish )^2 Ra

Total losses = armature copper losses Pcu + Constant losses C

Hence the efficiency of the DC machine when it acts as a generator at any load is

Ng = output / input

Ng = ( input – losses )/ input

Ng = ( VI – ( Pcu + C ) / VI

These are the equations for no-load losses and the efficiency of the DC machines at any load.

Swinburne’s Test Applications

The applications of this test include the following.

1. This test is used to find efficiency and no-load losses of the DC machines at constant flux.

2. In DC machines when runs as motors

3. In DC machines when runs as generators

4. In large shunt DC motors.

 

Swinburne’s Test Advantages & Disadvantages

The advantages of this test include the following.

1. This test is very simple, economical and most commonly used

2. It requires no-load power input or less power input when compared to Hopkinson’s test.

3. Efficiency can be determined in advance because of the known constant losses.

 

The disadvantages of this test include the following.

1. The change in iron losses from no-load to full-load cannot be determined because of the armature reaction

2. It is not applicable for DC series motors

3. Commutation conditions and temperature rise cannot be checked at full-load with the rated voltage.

4. It is applicable for the DC machines which have constant flux.

 

Limitations of Swinburne’s Test

1. Machines having a constant flux are only eligible for Swinburne’s test. For examples – shunt machines and level compound generators.

2. Series machines cannot run on light loads, and the value of speed and flux varies greatly. Thus, the Swinburne’s Test is not applicable for series machines.