Dcmachines

Electrical Machines I

Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao

9

Testing of d.c. machines
A d.c. machine has to be tested for proper fabrication and trouble free operation.

From the tests one can determine the external characteristics needed for application of these machines. Also, one can find the efficiency, rating and temperature rise of the machine.Some of the tests are discussed in sequence now.

9.1

Measurement of armature resistance
Measurement of winding resistances of field windings and armature winding are

performed by v-i method. Field is not excited during this test. Even though any value of applied voltage can be used, the highest permissible voltage/current is chosen during the test to minimize the errors. The armature circuitconsists of two resistances in series. They are armature winding resistance and resistance due to the brushes and the brush drop. The brush contact drop behaves like a non-linear resistance. To separate this from the armature circuit resistance and brush resistance a number of v-i readings are taken. An equation of V = Vb + IRa form is fitted through these test points shown graphically in Fig. 55.For large values of I the equivalent armature resistance is taken to be V /I ohm. If the value of brush drop Vb can be neglected then the armature resistance Ra = V /I ohm.

9.2

Open Circuit Characteristic (OCC)
The OCC is of great value as it shows the mmf and hence the field current required

to generate a given voltage at any speed, on no load. It is a graph showing the variation 123Indian Institute of Technology Madras

Electrical Machines I

Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao

A

A2
V

DC Supply

A1
(a)Physical connection
I.V. Characteristic
Ra =

dv di

dv di

V

Vb

0

I

(b)Characteristics

Figure 55: Measurement of Armature resistance and Brush drop

124

Indian Institute of Technology Madras

v+

-

Electrical Machines I

Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao

of the induced emf as a function of excitation current, when the speed is held constant, with the load current being zero. It is also called the no-load saturation curve or no load magnetization characteristic. This is experimentally determined by running the machine as a separatelyexcited generator on no-load at a constant speed and noting the terminal voltage as a function of the excitation current. This curve can be used to find the OCC at other speeds and also the self excited voltage when the machine works as a shunt generator.

9.3

Short circuit characteristics:(SCC)
In the case of short circuit test the armature is kept short circuited through an

ammeter. Themachine is demagnetized and an extremely small field current is passed through the field. The variation of the short circuit current as a function of excitation current is plotted as the SCC. The speed is to be held constant during this test also. The short circuit test gives an idea of the armature drop at any load current.

9.4

Load test
To assess the rating of a machine a load test has to beconducted. When the

machine is loaded, certain fraction of the input is lost inside the machine and appears as heat, increasing the temperature of the machine. If the temperature rise is excessive then it affects the insulations, ultimately leading to the breakdown of the insulation and the machine. The load test gives the information about the efficiency of a given machine at any load condition.Also, it gives the temperature rise of the machine. If the temperature rise is below the permissible value for the insulation then the machine can be safely operated at that load, else the load has to be reduced. The maximum continuous load that can be

125

Indian Institute of Technology Madras

Electrical Machines I

Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara...