Research Center, Navsari PRESENTATION ON TESTING OF DC MACHINES Mahatma Gandhi Institute of Technical Education & Research Center, Navsari PRESENTATION ON TESTING OF DC MACHINES DEPARTMENT OF ELECTRICAL ENGINEERING Year 2014-15 Prepared By Name Enroll No. Vachhani Ankur P. 130330109120 Ukani Jignesh M. 130330109119 Vala Urvik B. 130330109121 Vegad Tushar N. 130330109122 Vyas Divyesh V. 130330109123 Yeshi Gopal N. 130330109124 Guided By Mr. Animesh Patel

METHODS FOR TESTING OF DC MACHINES The methods for testing of dc machines can be broadly classified in to three methods: Direct method Indirect method Regenerative method (A) Direct method In this method of testing, the dc machine is loaded directly by means of the mechanical brake coupled to the shaft of the machine. The efficiency of the machine is calculated directly by measuring the power input and output. (B) Indirect method In this method of the 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 .

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. Both the machines are mechanically coupled and electrically connected

SWINBURNE’S TEST This test is the simplest Indirect method for finding out the efficiency of dc machine. In this method of testing , constant losses are determined experimentally by operating the dc machine as motor running at no load . This test is applicable to dc machines in which flux is practically constant i.e. shunt and compound machines . fig (A) circuit diagram of Swinburne's test on dc shunt motor

Fig(B) Circuit diagram for the measurement of armature resistance In this test the machine is run as a motor on No-load at its rated voltage. The speed of the motor is adjusted to its rated value with the help of shunt field regulator. The following observations are taken Observation Table Input voltage V volts No-load current I0 A Shunt field current Ish A

The power input to the armature at No-load supplies the following: Iron losses Mechanical losses No- load Armature Copper loss Power input to the dc motor at No-load , W0 =VI0 Armature Cu loss at No-load = I²a0 ra Constant losses of the machine Wc = VI0 - Calculation of efficiency when running as a motor Power Input to the motor = VIL Armature copper losses at full-load = I²ara Total losses = Wc + Efficiency of the motor I²a0 ra I²ara

Calculation of efficiency when running as a generator Output of the generator = VIL+ Total losses = Wc + I²ara Advantages of Swinburne’s test:- This test is very simple and convenient. The efficiency of the machine can be predetermined at any load condition , since constant losses are known. Disadvantages of Swinburne’s test:- This test is applicable to dc machines in which flux is practically constant. Series machines can not be tested by this method. The temperature rise and the commutation conditions can not be checked under rated load conditions as the test is performed on No-load.

HOPKINSON’S TEST This is a regenerative test which requires two identical dc shunt machines coupled mechanically and connected electrical as shown in fig (C) . One of the machines is operated as a motor and other as a generator. The electrical output of the generator is fed to the motor. Hence the power input from the supply is very small only to overcome the losses of both the machines. In this test , the mechanical output of the generator is fed as input to the motor . Hence this test is also called regenerative test or back-to-back test. Fig(C) circuit diagram for performing Hopkinson's test on two dc shunt machines

Calculation of efficiency hen running as a motor Observations Table Calculation of efficiency hen running as a motor Shunt Field copper loss of the motor = VI3 Total losses of the motor , Ptm = Wc + (I1 + I2)²rm + VI3 Total power input to the motor , Pi = V(I1 + I2 + I3) Efficiency of the motor Input voltage v volts Current drawn from the supply I1A Gen. Arm. Current I2 A Motor (I1+I2) Motor field Current I3 A Field Current I4A

Calculation of efficiency when running as a generator Shunt field copper losses of generator = VI4 Total losses of the generator, Ptg = Wc + I²2 + VI4 output of the generator , P0 = VI2 Efficiency of the generator Advantages of Hopkinson’s test The temperature rise can be estimated during the test. The efficiency of the machine can be accurately determined at various loads. The commutation conditions can be checked under rated load conditions. Disadvantages of Hopkinson’s test Availability of two identical dc machines. It is impossible to separate out iron losses of the two machines which are different because of different excitations.

FIELD’s TEST FOR DC SERIES MOTORS Fig.(D) Circuit diagram for performing Field’s test on dc series machines By above fig. small series machines can be tested by direct load test , but the large series machines cannot be tested by Swinburne’s test because series motor can not run at No- load due to dangerously high speed . Field’s test is applicable to two similar series machines. These two machines are mechanically coupled and electrical connected. One of machines are run as a motor and drives the other machine as a generator. A variable load is connected across the generator terminal. The output power of the generator is wasted in a variable load resistance . This load resistance is varied till the motor carries its full load rated current.

Observation Table Power input to the whole set = VI1 Power output of the set = V2I2 Total losses in the set , Pt = VI1 - V2I2 Armature and field Cu losses of motor = I²1 (ra + rf ) Armature and field Cu losses of generator = I²2ra + I²1rf Total copper losses of the set Pc = I²1ra + I²1rf + I²2ra + I²1rf = I²1 (ra + 2rf) + I²2ra Stray losses (Iron + Friction) per machine , Ps = 0.5 (Pt - Pc) The stray losses are equally divided between the machines because of their equal excitation and speed . Supply Voltage V Motor Armature Current I1 Generator I2 Output V2 V1

Motor Efficiency Generator Efficiency In this test , the two machines are mechanically coupled and electrically connected but it is not a regenerative method of testing because the output power of the generator is wasted in variable load instead of fed back into the motor or supply as in the case of Hopkinson’s test

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