PRESENTED BY : JESTY JOSE – 130800109027 JOBIN ABRAHAM – 130800109028 SAGAR JAISWAL – 130800109029 DANISH KHATRI - 130800109030

ACTIVE LEARNING ASSIGNMENT SUBJECT – DC MACHINE & TRANSFORMER SUBJECT CODE – 2130904 BRANCH – ELECTRICAL GUIDED BY : Prof. MAYURI TAHILRAMANI

TESTING OF DC MACHINES

CONTENTS : Objectives Of Testing Methods For Testing Brake Test Swinburne’s Test Hopkinson’s Test Field Test For DC Series Motor Retardation Test

OBJECTIVES OF TESTING : A DC 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.

METHODS FOR TESTING : There are three methods for testing of DC Machines. They are : Direct Method Indirect Method Regenerative Method

(1) DIRECT METHOD : In this method, the DC machine is loaded directly by means of a brake applied to a water cooled pulley coupled to the shaft of the machine. It is not practically possible to arrange loads for machines of large capacity. So this method is used only for testing of small dc machines . Brake Test is an example of direct test.

(2) INDIRECT METHOD : In this method of testing, the losses are determined without actual loading of the machine. If the total losses in the machine are known the efficiency can be calculated. Swinburne’s Test is an example of Indirect Test.

(3) REGENERATIVE METHOD : This method requires two identical dc machines. One of the machines is operated as a motor and drives the other machine as a generator. The electrical output of the generator is feedback into the supply. Thus the power drawn from the supply is small only to overcome the losses of two machines. Hence the large machines can be tested at rated load without consuming much power from the supply. Hopkinson’s Test is an example of Regenerative Method .

BRAKE TEST OR LOAD TEST : To assess the rating of a machine a load test has to be conducted. 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. The load test alone can give us the proper information of the rating and also can help in the direct measurement of the efficiency.

LIMITATIONS : Large amount of power is required to test a large machine and the entire output power is wasted at the mechanical brake . Non- availability of large capacity load for testing large motors in the laboratories.

SWINBURNE’S TEST Swinburne's test is the most commonly used and simplest method of testing of shunt and compound wound dc machines which have constant flux. In this test the efficiency of the machine at any load is pre-determined. We can run the machine as a motor or as a generator. In this method of testing no load losses are measured separately and eventually we can determine the efficiency. The following shows the circuit diagram of Swinburne’s Test :

Calculation of Efficiency When the Machine is Motoring on Load Power input = VI Armature copper loss, PCU = I2 Ra = (I - Ish)2Ra Constant losses, WC = VI0 -(I0 - Ish)2 Ra Total losses = PCU + WC ∴ Efficiency of the motor: Calculation of Efficiency When the Machine is Generating on Load Armature copper loss, PCU = I2 Ra = (I + Ish)2 Ra Constant losses, WC = VI0 - (I0 - Ish)2 Ra Total losses = PCU + WC ∴ Efficiency of the generator:

ADVANTAGES : DISADVANTAGES : This test is very convenient and economical as it is required very less power from supply to perform the test. Since constant losses are known, efficiency of Swinburne's test can be pre-determined at any load. DISADVANTAGES : In dc series motor, the Swinburne’s test cannot be done to find its efficiency as it is a no load test. Iron loss is neglected though there is change in iron loss from no load to full load due to armature reaction. We can’t measure the temperature rise when the machine is loaded. Power losses can vary with the temperature. We cannot be sure about the satisfactory commutation on loaded condition because the test is done on no-load.

HOPKINSON’S TEST  It is a full load test and it requires two identical machines which are coupled to each other. One of these two machines is operated as a generator to supply the mechanical power to the motor and the other is operated as a motor to drive the generator. For this process of back to back driving the motor and the generator, Hopkinson's test is also called back-to-back test or regenerative test. If there are no losses in the machine, then no external power supply would have needed. But due to the drop in the generator output voltage we need an extra voltage source to supply the proper input voltage to the motor. Hence, the power drawn from the external supply is therefore used to overcome the internal losses of the motor-generator set.

Efficiency of Generator Total losses in the generator, WG = (I1 + I3)2 Ra + VI3 + W/2 Generator output = VI1 Then, efficiency of the generator : Efficiency of Motor Total losses in the motor, WM = (I1 + I2 - I4)2 Ra + VI4 + W/2 Motor input = V(I1 + I2) Then, efficiency of the motor:

Advantages of Hopkinson's Test : 1. This test requires very small power compared to full-load power of the motor-generator coupled system. That is why it is economical. 2. Temperature rise and commutation can be observed and maintained in the limit because this test is done under full load condition. 3. Change in iron loss due to flux distortion can be taken into account due to the advantage of its full load condition. Disadvantages of Hopkinson's Test : 1. It is difficult to find two identical machines needed for Hopkinson's test. 2. Both machines cannot be loaded equally all the time. 3. It is not possible to get separate iron losses for the two machines though they are different because of their excitations. 4. It is difficult to operate the machines at rated speed because field currents vary widely.

FIELD TEST ON DC SERIES MOTOR The field test is applied to two similar series motors which are coupled mechanically. The connection diagram for the test is shown in the fig.

For Motor : For Generator: One machine is made to run as a motor while the other as a generator which is separately excited. The field of the two machines are connected in series so that both the machines are equally excited. This will make iron losses same for the two machines. The two machines are running at the same speed. The generator output is given to the variable resistance R. The resistance R is changed until the current taken by motor reaches full load value. This will be indicated by ammeter A1. For Motor : For Generator:

RETARDATION TEST : This method is generally employed to shunt generators and shunt motors. From this method we can get stray losses. Thus if armature and shunt copper losses at any given load current are known then efficiency of a machine can be easily estimated. The following shows the circuit diagram of Retardation Test :

The machine whose test is to be taken is run at a speed which is slightly above its normal speed.  The supply to the motor is cut off while the field is kept excited.  The armature consequently slows down and its kinetic energy is used in supplying the rotational or stray losses which includes iron, friction and winding loss.

REFERENCES : Theory & Performance Of Electrical Machines By – J.B. Gupta www.electrical4u.com www.nptel.ac.in