- 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 is crucial for proper fabrication and ensuring smooth operations.
The main tests used for DC machines are:
- Open circuit test
- Short circuit test
- Load test
- Determination of efficiency
Open Circuit Test
The open circuit test is needed to determine the open circuit characteristic or magnetizing characteristic of a dc machine. The open circuit test gives the mmf and hence the excitation current or field current needed to generate the required voltage on no load at a fixed speed. The open circuit characteristic curve shows the variation of induced emf as a function of field current at constant speed and zero load current.
This curve is practically determined by running the machine as a separately excited generator on on-load. This curve is also called no load saturation curve as it gives the saturation characteristic of the generator.
Short Circuit Test
The short circuit test determines the voltage drop across the armature at any load current. During this test, the armature is short-circuited with an ammeter to measure the short circuit current. The resulting short circuit characteristic curve shows how the short circuit current varies with the excitation current.
Load Test
Load testing determines the machine’s rating. Running a machine generates heat due to energy loss, which can raise its temperature. Excessive heat can damage insulation and cause machine breakdowns. Thus, the load should be set within safe temperature limits. The maximum load the machine can handle without harm is called its continuous rating.
Determination of Efficiency
The efficiency of a DC machine is determined by the ratio of its output power to input power.
There are three methods of determining the efficiency of a machine.
- Direct method
- Indirect method
- Regenerative method
The 1st equation is giving an idea about the direct estimation of the efficiency. In this method the machine is fully loaded and the output is directly measured. This method of measurement is only applied for the small machines.
The 2nd and 3rd equations are giving an idea about the indirect estimation of the efficiency.
Indirect method is helpful of determining the efficiency of shunt wound generator and compound wound generators. In this method it is required to determine to determine the losses only. So, power supply is required to supply the losses only without loading the machine. For the regenerative method of determining efficiency, it is required to have two identical machines. One machine is used as motor and drives the other and the other is used as generator and feedback the power into the supply. Two machines are mechanically coupled. Therefore the losses can be determined because the internal power drawn is only to supply losses of the two machines. Except these testes, the insulation test and the test for making the commutation satisfactory is done while building up the machine.
