Testing of DC Machines

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Direct method ( Brake test ) and Indirect method ( Swinburne's method ) are explained briefly.

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Testing of DC Machines Part 1 By - Siddhang Tendulkar

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There many methods for testing d.c. Machines In this presentation, 2 most commonly used methods are described briefly. These two methods are : Direct Method & Indirect Method. Testing of d.c. machines part-1

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1) DIRECT METHOD This method is suitable for small d.c. machines. In this method, the d.c. machine is subjected to rated load and the entire power output is wasted. For a d.c. generator, the output power is wasted in resistors. For a d.c. motor, a test called ‘ Brake test ’ is carried out . Testing of d.c. machines part-1

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As shown in the figure, a belt around the air- or water-cooled pulley has its ends attached to spring balances S 1 & S 2. The belt tightening hand wheels H 1 & H 2 , help in adjusting the load and, therefore on the motor. If spring balances are calibrated in kilograms, the motor output is given by, Motor Output= ω( S 1 – S 2 )r x 9.81 Watts Where S 1 & S 2 are the tensions on the tight and slack sides of the belt, r is the effective radius of the brake-pulley in meters and ω is motor speed in rad/sec. BRAKE TEST Testing of d.c. machines part-1

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If V t is the motor terminal voltage and I L is the line current, the power input to motor is given by, Input power = V t I L Now the efficiency of motor is, Ƞ m = ω( S 1 – S 2 )r x 9.81 x 100 V t I L For a series motor, the brake should be sufficiently tight before the motor is switched on to the supply. The size of the motor that can be tested by this method is limited from the consideration of the heat that can be dissipated at the brake drum or pulley. Testing of d.c. machines part-1

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DISADVANTAGES OF BRAKE TEST. The disadvantages of this method are: The spring balance readings are not steady and, The friction torque, at a particular setting of the hand wheels H 1 & H 2 , does not remain constant. Testing of d.c. machines part-1

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2) INDIRECT METHOD In this method, the no-load machine losses are first measured by a suitable test and then the additional losses on load are determined from the machine data, in order to calculate the machine efficiency. The simplest method of measuring the no-load machine losses is by ‘ Swinburne’s method ’ Testing of d.c. machines part-1

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Swinburne’s Method As this is a no-load test, it cannot be performed on a d.c. series motor. In this method, the machine, whether it is a motor or generator, is run as a no-load shunt motor at rated speed and with rated terminal voltage V t , Testing of d.c. machines part-1

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If I ao & I f are the no-load armature current and field current respectively, then the power absorbed by the armature( = V t I ao ) is equal to the no-load rational loss W o plus a small amount of armature circuit loss I 2 ao r a. Therefore, No-loads rational loss W o = V t I ao - I 2 ao r a Here armature circuit resistance r a includes the brush contact resistance also. Shunt field loss = V t I f Let I L be the load current at which the machine efficiency is required. Testing of d.c. machines part-1

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Generator efficiency Generator output = V t I L Armature current I a = I L + I f Armature circuit loss = I 2 a r a Shunt field losses = V t I f Total losses = W o + I 2 a r a + V t I f Therefore, efficiency Ƞ g = 1 - W o + I 2 a r a + V t I f V t I L + W o + I 2 a r a + V t I f Motor efficiency When the machine is working as motor then I a = I L - I f Motor input = V t I L Therefore, efficiency Ƞ m = 1 - W o + I 2 a r a + V t I f V t I L Testing of d.c. machines part-1

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Advantages of Swinburne’s method The advantages are: Low power is required even for testing large machines, since only no-load losses are to be supplied from the mains and, The efficiency of the machine can be calculated at any desired load. Disadvantages of Swinburne’s method Since d.c. machines in this method is running at no-load, Stray load losses can’t be accounted for. The actual performance of the machine on rated load can’t be checked. Testing of d.c. machines part-1

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THE END Thank you Testing of d.c. machines part-1