The Hopkins on test is another useful way to test the efficiency of a DC machine. This is a full load test and requires two identical machines connected to each other. One of these two machines is operated as a generator to supply mechanical power to the motor and the other is operated as a motor to drive the generator. For this process of running the motor and generator back to back, Hopkins’s test is also called back-to-back test or regenerative test.
If there is no damage to the machine, no external power supply is required. But due to reduction in output voltage of the generator we need an additional voltage source to provide adequate input voltage to the motor. Therefore, the power from the external supply is used to overcome the internal losses of the motor-generator set. Hopkinson test is also called reproducibility test or back-to-back test or heat run test.
Connection Diagram of Hopkinson’s Test:
Displayed beneath is a circuit association for the Hopkinson test. An engine and generator, both indistinguishable, are coupled together. At the point when the machine is begun it is turned over as an engine. The shunt field obstruction of the machine is changed so the engine runs at its appraised speed.
The generator voltage is currently made equivalent to the stockpile voltage by changing the shunt field opposition associated across the generator. This equity of these two voltages of the generator and the stock is shown by the voltmeter as it gives a no perusing at the point associated across the switch. The machine can run at evaluated speed and required load by differing the field current of engine and generator.
As of now, in case of motor, armature copper adversity in motor = .
Ra is the armature obstacle of both the motor and the generator.
The I4 is the shunt field current of the motor.
The shunt field copper hardship in the motor will be = VI4.
Then, at that point, in case of mischief to the generator armature copper generator =
The I3 is the shunt field current of the generator.
Shunt field copper mishap in generator = VI3
As of now, power drawn from outside supply = VI2
Along these lines, the two machines will have stray mishaps.
Efficiency of Generator:
Advantages of Hopkinson’s Test:
- This test requires very little power compared to the full load power of the motor-generator coupled system. Hence it is economical. Larger machines can be tested at rated load without excessive power consumption.
- The temperature rise and change can be observed and maintained within the range as the test is carried out under full load condition.
- The change in iron loss due to flux distortion can be taken into account due to its full load condition gain.
- Performance can be determined at different loads.
Disadvantages of Hopkinson’s Test:
- It is difficult to find two identical machines required for Hopkinson’s test.
- Both machines cannot be equally loaded at all times.
- It is not possible to obtain separate iron losses for the two machines even though they differ in their excitation.
- It is difficult to operate the machines at rated speed because the field currents vary widely.