**Hopkinson’s test or regenerative test**is a full load test and it requires two identical machines which are coupled to each other.In this test two identical d.c. machines mechanically coupled to each other and simultaneously tested.One is operated as generator another one as the motor, hence we can find the efficiency of two dc machines simultaneously.So output power of dc machines is going to be wasted.The mechanical output of motor given to generator through the shaft to shaft mechanical coupling.And generator’s electrical power supplied to run the motor, where losses will be supplied by the external power source.

**Hopkinson’s test**. The two shunt dc machines are connected in parallel. In that two machines, one is started as a motor another one operated as the generator.Here the only rotor connections are mentioned, stator connections are not shown for simplicity.

## Connection Diagram of Hopkinson’s Test

## Calculation of Efficiency by Hopkinson’s Test

**Case 1:**

*Assuming that the efficiency of both the machines is same.*

**Case 2:**

*Assuming both the machines have the same iron loss, friction loss, and windage loss.*

__Case 1:__

__Case 1:__

**Assuming that the efficiency of both the machines is same.**

**The above expression is used to determine the efficiency satisfactorily perfect for a rough test. If the case needs to find more accuracy then the efficiency of the two machines can be determined separately using the below expressions.**

__Note:____Case 2:__

__Case 2:__

**Assuming both the machines has the same iron loss, friction loss, and windage loss.**

**To find the efficiency of Motor in **Hopkinson’s test

### To find the efficiency of Generator in Hopkinson’s test

## Advantages of Hopkinson’s Test or Regenerative Test On DC Motor

1. This test is very economical because it requires only very small power just to compensate the losses which are the very small value when compared to the full-load power of the motor-generator coupled system.

2. This is performed at full load condition so we can take flux distortion into account.

3. There is no need for arranging any actual load. Similarly, by changing the field currents of two machines, the load can be easily changed and a load test over the complete range of load can be taken.

4. This test is better suited in case of large machines.

## Disadvantages of Hopkinson’s Test or Regenerative Test On DC Motor

1. It is a big task to check for two identical machines needed for Hopkinson’s test.

2. We can not load two machines 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 excitation.

4. It is difficult to operate the machines at rated speed because field currents vary widely.

5.The machines are not loaded equally in case of small machines which may lead to difficulty in analysis.