- Definition of Testing of DC Machines: Definition of Testing of DC Machines
- Open Circuit Test: Determines the magnetizing characteristic and the required field current to generate the needed voltage at no load.
- Short Circuit Test: Measures the voltage drop across the armature by short-circuiting it and provides the short circuit characteristic curve.
- Load Test: Determines the machine’s rating by assessing the maximum load it can handle without overheating.
- Efficiency Determination: Testing of DC machines includes determining efficiency using direct, indirect, and regenerative methods.
Testing of DC machines confirms winding condition, magnetisation, load performance, losses, efficiency and temperature rise. Tests must use rated limits, suitable protection and the manufacturer’s procedure.
Common DC-machine tests include:
- Open circuit test
- Armature resistance and voltage-drop test
- Load test
- Determination of efficiency
Open Circuit Test
The open-circuit test determines the magnetisation characteristic of a DC generator. At constant speed with the armature terminals open, it varies the field current and records the generated voltage. The resulting curve shows residual voltage, the unsaturated region and magnetic saturation. For a self-excited shunt generator, it can also be used to find critical field resistance and critical speed.
In practice, the machine is driven at the specified constant speed and operated as a separately excited generator with no electrical load. The curve is therefore also called the no-load saturation curve.
Armature Resistance and Voltage-Drop Test
Armature-circuit resistance is measured with the machine stationary by applying a controlled low DC voltage and limiting the current. The measured resistance is used to calculate the resistive voltage drop at a stated armature current. A running DC generator must not be short-circuited through an ammeter because its low armature resistance can produce destructive current. Brush-contact drop and armature reaction require separate treatment when predicting loaded terminal voltage.
Load Test
A direct load test measures terminal quantities, speed, torque and temperature while the machine supplies or absorbs a controlled load. The test confirms operating characteristics and whether temperature rise remains within the assigned rating. The load must stay within the machine, drive and test-equipment limits. A manufacturer assigns the continuous rating for defined duty, cooling and ambient conditions; one load test does not by itself establish a universal safe maximum.
Determination of Efficiency
DC-machine efficiency is output power divided by input power for the stated operating point.
Three common approaches are:
- Direct method
- Indirect method
- Regenerative method
The 1st equation represents direct efficiency measurement. The machine operates under load, and both input and output power are measured. This method gives actual loaded performance but wastes the output energy unless the test system recovers it, so it is most practical for smaller machines.
The 2nd and 3rd equations represent loss-based methods.
An indirect test such as Swinburne’s test estimates the losses of a shunt wound generator or compound wound generators without applying full load. It needs only enough supply power to cover the measured no-load losses, but it does not reproduce all full-load temperature and commutation effects. A regenerative test such as Hopkinson’s test uses two mechanically coupled, similar machines. One operates as a motor and drives the other as a generator, which returns most of the power to the supply. The external source supplies mainly the combined losses, allowing near-full-load testing with much lower net input. Other checks include insulation resistance, winding resistance, speed regulation and commutation under load.
