For a d.c shunt motor change of speed from no load to full load is quite small. Therefore, mechanical loss can be assumed to remain same from no load to full load. Also if field current is held constant during loading, the core loss too can be assumed to remain same.
In this test, the motor is run at rated speed under no load condition at rated voltage. The current drawn from the supply IL0 and the field current If are recorded (figure 40.4).
Since the motor is operating under no load condition, net mechanical output power is zero.
Hence the gross power developed by the armature must supply the core loss and friction &
windage losses of the motor. Therefore,
Pcore + Pfriction = (V − I a 0 ra ) I a 0 = Eb 0 I a 0
Since, both Pcore and Pfriction for a shunt motor remains practically constant from no load to
full load, the sum of these losses is called constant rotational loss i.e.,
constant rotational loss, Prot = Pcore + Pfriction
In the Swinburne's test, the constant rotational loss comprising of core and friction loss is
estimated from the above equation.
After knowing the value of Prot from the Swinburne's test, we can fairly estimate the
efficiency of the motor at any loading condition. Let the motor be loaded such that new current
drawn from the supply is IL and the new armature current is Ia as shown in figure 40.4. To
estimate the efficiency of the loaded motor we proceed as follows:
Input power to the motor, Pin=VIL
Cu loss in the field circuit Pfl=VIf
Power input to the armature=vIa
Cu loss in the armature circuit=Ia2 Ra
Net mechanical output power, Pnet mech=Eb Ia- Prot
The estimated value of Prot obtained from Swinburne’s test can also be used to estimate the
efficiency of the shunt machine operating as a generator. In figure 40.5 is shown to deliver a
load current IL to a load resistor RL. In this case output power being known, it is easier to add
the losses to estimate the input mechanical power.
Efficiency of the generator, η= Pin,mech/VIl
The biggest advantage of Swinburne's test is that the shunt machine is to be run as motor
under no load condition requiring little power to be drawn from the supply; based on the no load
reading, efficiency can be predicted for any load current. However, this test is not sufficient if we
want to know more about its performance (effect of armature reaction, temperature rise,
commutation etc.) when it is actually loaded. Obviously the solution is to load the machine by
connecting mechanical load directly on the shaft for motor or by connecting loading rheostat
across the terminals for generator operation. This although sounds simple but difficult to
implement in the laboratory for high rating machines (say above 20 kW), Thus the laboratory
must have proper supply to deliver such a large power corresponding to the rating of the
machine. Secondly, one should have loads to absorb this power.