Testing of DC Machines / Swinburne's Method / Advantages of Swinburne's Method / Disadvantages of Swinburne's Method

 

Swinburne's Method

One of the easiest ways of measuring no-load losses of a machine is by Swinburne's method.

Since this is a no-load test, it cannot be performed on DC Series Motors. This method is applicable for Shunt or Flat-Compound DC generators and motors. 

In this test, the machine, whether it is a generator or a motor, is run as a motor with no-load at rated speed and excitation.

If machine under test is generator, then

the voltage across the armature = rated terminal voltage + the voltage drop in the armature circuit resistance at rated armature current.

If machine under test is motor, then

the voltage across the armature = rated terminal voltage - the voltage drop in the armature circuit resistance at rated armature current.

Swinburne's method for measuring no-load losses of a DC Shunt Motor

Iao = No-Load Armature Current

If = Field Current

Vt = Terminal Voltage

→ Power input to the machine = Vt ( Iao + If ) 

Power taken by the armature = Vt Iao

Power taken by the armature is equal to the no-load rotational loss Wo plus a small amount of armature circuit copper loss  (Iao^2)Ra.

→ therefore, no-load rotational loss,    Wo = Vt Iao - (Iao^2)Ra

Here, Ra  is the armature circuit resistance including the brush contact resistance.

The Ra is determined by measuring the voltage drop across the armature circuit while rated current is passed through it at standstill. Ratio of this voltage and the current gives the value of the Ra.

→ Shunt field copper loss = Vt If

→ Let, IL be the load current at which machine efficiency is to be determined.

If Machine is run as a generator,

Armature current , Ia = IL + If 

Generator output, Po = Vt IL

Armature circuit loss = (Ia^2) Ra

Shunt field copper loss = Vt If

Therefore, total losses = Wo + (Ia^2) Ra + Vt If

Generator input, Pi =  Vt IL + Wo + (Ia^2) Ra + Vt If

Hence, Generator efficiency  𝜂g = Po/Pi  = (Vt IL) /(Vt IL + Wo + (Ia^2) Ra + Vt If)

If Machine is run as a motor,

Armature current , Ia = IL - If

Motor input, Pi = Vt IL

Armature circuit loss = (Ia^2) Ra

Shunt field copper loss = Vt If

Therefore, total losses = Wo + (Ia^2) Ra + Vt If

Motor output, Po = Vt IL - Wo - (Ia^2) Ra - Vt If

Hence, Motor efficiency  𝜂m = Po/Pi  = (Vt IL - Wo - (Ia^2) Ra - Vt If) /(Vt IL)

Advantages of Swinburne's method

1. Low Power requirement for testing, even for large machines, since only no-load losses are to be supplied from the mains during testing.
2. Efficiency of the machine can be estimated at any desired load.

Disadvantages of Swinburne's Method

(1)  The efficiency estimated by this method is always more than the actual value that can be obtained at the given load:

• Large increase in iron losses when the machine is actually loaded, owing to increase in flux density at the teeth due to armature reaction.
• Flow of Eddy currents within the armature conductors increases the current density value in armature conductors, causing increase in copper losses under actual loading condition. 
• Presence of stray load losses that may take place during actual loading conditions.

(2)  Actual performance of the machine under loaded condition cannot be checked with Swinburne's method since in this test the machine is run under no-load condition only. Certain Performance, such as commutation and temperature rise, can only be correctly checked once the machine is actually loaded.