• What is Swinburne's test and how it required for DC machine ?

Machine are tested for finding out losses, efficiency and temperature rise. Direct-loading tests may be performed on small machines. For large shunt machines indirect methods are used. 

• Swinburne test.
• Hopkinson test. 

Swinburne test:-

It is an indirect method of testing dc machines. In this method the losses are measured separately, and the efficiency at any desired load  is predetermined.

The  machine is run as a motor at rated voltage and speed. 

Let, V = supply voltage

        I0 = no-load current

    Ish = shunt field current

Swinburne test

No-load armature current

    In0 = I0 - Ish

No-load input = VI0.

The no-load power input to the machine supplies the following :

• iron loss in the core,
• friction losses at bearings and commutator.
• Windage loss.
• armature copper loss at no load.

When the machine is loaded, the temperature of the armature winding and field winding increases due to I²R losses. In calculating the I²R losses hot resistances should be used. 

A stationary measurement of resistance at room temperature of, say, t° C is made by passing current through armature and then field from a low voltage dc supply. Then the hot resistance allowing a teperature rise of say 50°C is found as follows :

Rt1 = R0 (1+ α0t1)

Rt1 + 50° = R0 [1+ α0 (t1 + 50°)]

Where α0 = temperature coefficient of resistance at 0° C

Stray loss = iron loss + friction loss windage loss =input at no load - field copper loss - no load armature copper loss

= VI0 - pf - pa0 = ps

Also, constant losses

Pc = no-load input - ( no-load armature copper loss)

pc = ps + pf

By knowing the constant losses of the machine, its efficiency at any other load can be determined as follows :

Let I be the load current at which efficiency is required.

Efficiency when running as motor

Motor input                    = VI

Armature copper loss  = I²aRa = (I - Ish)² Ra

Constant losses              = pc 

Total losses                     = (I - Ish)²Ra + pc 

Motor efficiency  

ηm = input - losses / input

= VI - (I - Ish)²Ra + pc / VI

Efficiency when running as generator

Armature current         Ia = I + Ish

Generator output              = VI

Armature copper loss      = (I + Ish)² Ra

Constant losses                  = pc

total losses                          = (I + Ish)² Ra + pc

Generator  Efficiency

ηg = output / output + losses

= VI / VI + (I + Ish)²Ra + pc

Advantages of Swinburne's Test

The main advantages of Swinburne's test are :

• It is a convenient and economical method of testing dc machines since power required to test a large machine is small.
• The efficiency can be predetermined at any load because constant losses are known.

Disadvantages of Swinburne's Test

• No account is taken of the change in iron loss from no load to full load. At full load‚ due to armature reaction‚ flux is distorted which increases the iron losses.
• As the test is on no load‚ it doesn't indicate whether the commutation on full load is satisfactory and whether the temperature rise would be within specified limits.

Limitations of Swinburne's Test

• Swinburne's test is applicable to those machines in which the flux is practically constant‚ that is shunt machines and level compound generators.
• Series machine can't be tested by this method as they can't be run on light loads and secondly flux and speed vary greatly.

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