Testing of Dc machines

TESTING
OF D.C.
MACHINES

Swinburne’s* Test (or No-load Test or Losses Method)

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 test 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 nameplate.
The speed is adjusted to the rated speed with
the help of shunt regulato

The no-load current I0 is measured by the
ammeter A1 whereas shunt field current Ish is given by ammeter A2. The no-load armature current is
(I0 ? Ish) or Ia0.
Let, supply voltage = V no-load input = VI0 watt
 Power input to armature = V (I0 ? Ish) ; Power input to shunt = VIsh
No-load power input to armature supplies the following :
(i) Iron losses in core (ii) friction loss (iii) windage loss and
(iv) armature Cu loss, (I0 ? Ish)2 Ra or Ia0
2Ra
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, 15?C is made by passing current through
the armature from a low voltage d.c. supply




Advantages of Swinburne’s Test
1. It is convenient and economical because power required to test a large machine is small i.e. only
no-load input power.
2. The efficiency can be predetermined at any load because constant-losses are known.
31.4. Main Disadvantages
1. No account is taken of the change in iron losses from no-load to full-load. At full-load, due to
armature reaction, flux is distorted which increases the iron losses in some cases by as much as 50%.
2. As the test is on no-load, it is impossible to know whether commutation would be satisfactory at
full-load and whether the temperature rise would be within the specified limits


Regenerative or Hopkinson’s Test (Back-to-Back Test)

By this method, full-load test can be carried out on two shunt machines, preferably identical ones,
without wasting their outputs. The two machines are mechanically coupled and are so adjusted electrically
that one of them runs as a motor and the other as a generator. The mechanical output of the motor drives the
generator and the electrical output of generator is used in supplying the greater part of input to the motor. If
there were no losses in the machines, they would have run without any external power supply. But due to
these losses, generator output is not sufficient to drive the motor and vice-versa. The losses are supplied
either by an extra motor which is belt-connected to the motor-generator set or as suggested by Kapp,
electrically from the supply mains.


The two shunt
machines are connected in parallel.
They are, to begin with, started as unloaded
motors. Then, the field of one
is weakened and that of the other is
strengthened so that the former runs
as a motor and the latter as a generator.
The usual method of procedure is
as follows :
Machine M is started up from the
supply mains with the help of a starter
(not shown) whereas main switch S of
the other machine is kept open. Its
speed is adjusted to normal value by means of its shield regulator. Machine M drives machine G as a
generator and its voltage is read on voltmeter
V1. The voltage of G is adjusted
by its field regulator until voltmeter V1
reads zero, thereby showing that its voltage
is the same, both in polarity and magnitude
as that of the main supply. Thereafter,
S is closed to parallel the machines.
By adjusting the respective field regulators,
any load can now be thrown on to
the machines. Generator current I1 can
be adjusted to any desired value by increasing
the excitation of G or by reducing
the excitation of M and the corresponding values of different ammeters are read