Swinburne’s Test Of DC Machine (Generator And Motor)

Swinburne’s test is the simplest indirect method of testing dc machines. In this method, the dc machine (generator or motor) is run as a motor at no-load and losses of the machine are determined. Once the losses of the machine are known, its efficiency at any desired load can be determined in advance.

It may be noted that this method is applicable to those machines in which flux is practically constant at all loads e.g., shunt and compound machines.

Read the testing of dc machines before reading this topic.

Steps to find the Efficiency

Let us see how the efficiency of a dc shunt machine (generator or motor) is determined by this method. The test insists on two steps:

Determination of hot resistances of windings
Determination of constant losses

Determination of hot resistances of windings

The armature resistance and shunt field resistance are measured at room temperature (say, at 15°C) using a voltmeter ammeter method (battery, voltmeter, and ammeter).

Since these resistances are measured when the machine is cold, they must be converted to values corresponding to the temperature at which the machine would work on full-load.

Generally, these values are measured for a temperature rise of 40°C above the room temperature. From the data so obtained, different losses are computed and efficiency is determined.

Determination of constant losses

Please go through losses in dc machines for more details.

The machine is run as a motor on no-load with supply voltage adjusted to the rated voltage i.e. voltage stamped on the nameplate.

The speed of the motor is adjusted to the rated speed with the help of field regulator R as shown in figure (for more details please visit speed control of dc shunt motor).

Swinburne’s Test for Determining Efficiency of DC Machine

Let V = Supply voltage

I_o = No-load current read by ammeter A₁

I_sh = Shunt-field current read by ammeter A₂.

∴ No-load armature current, Ia0 = I_o – I_sh

No-load input power to motor = V I_o

No-load power input to armature = V I_ao = V(I_o – I_sh)

Since the output of the motor is zero, the no-load input power to the armature supplies

iron losses in the core
friction loss
windage loss
armature Cu loss [ I_ao²R_aor (I_o – I_sh)²R_{a .}

Constant losses, W_c = Input to motor – Armature Cu loss

W_c = V I_o – (I_o – I_sh)²R_a

Since constant losses are known, the efficiency of the machine at any other load can be determined. Suppose it is desired to determine the efficiency of the machine at load current I. Then,

Armature current, Ia = I – I_sh … if the machine is motoring

= I + I_sh … if the machine is generating

Efficiency when running as a motor

Input power to motor = VI

Armature Cu loss = I_a²R_a = (I – I_sh)²R_a

Constant losses = W_cfound above

Total losses = (I – I_sh)²R_a+W_c

∴ Motor efficiency, η_m= (Input – Losses)/Input = [VI – (I – I_sh)²R_a+W_c]/VI

Efficiency when running as a generator

The output of generator= VI

Armature Cu loss = I_a²R_a = (I + I_sh)²R_a

Constant losses = W_cfound above

Total losses = (I + I_sh)²R_a+W_c

∴ Motor efficiency, η_m= (Input – Losses)/Input = VI/[VI + (I + I_sh)²R_a+W_c]

Advantages of Swinburne’s test

The following are the advantages of Swinburne’s test

The power required to carry out the test is small because it is a no-load test. Therefore, this method is quite economical.
The efficiency can be determined at any load because constant losses are known.
This test is very convenient.

Disadvantages of Swinburne’s test

The disadvantages of Swinburne’s test are given below

It does not take into account the stray load losses that occur when the machine is loaded.
This test does not enable us to check the performance of the machine on full-load. For example, it does not indicate whether commutation on full load is satisfactory and whether the temperature rise is within the specified limits.
This test does not give quite accurate efficiency of the machine. It is because iron losses under actual load are greater than those measured. This is mainly due to the armature reaction distorting the field.