The circuit diagram for the direct loading test is shown in the figure. Connect the test transformer and meters as per circuit diagram.
As the transformer is loaded, this test is known as direct test. The rated voltage is given to transformer primary side with the help of single-phase auto-transformer and secondary side is loaded.
The load is increased step-by-step till secondary ammeter shows the full load secondary current. Note down the reading of wattmeter, voltmeters and ammeters.
Determine the performance i.e., full load efficiency and regulation of transformer.
Efficiency:
% Efficiency = [ Output Power / Input Power] × 100%= [ W2 / WI] × 100%
Voltage Regulation:
Let us consider that no load secondary voltage is 0V2 and full load secondary voltage is V2
% Voltage Regulation = { 0V2 –V2 / 0V2 }× 100 %
Direct Loading Test on Three Phase Induction Motor
Connect the circuit as per the circuit diagram.
Ensure that the motor is unloaded and the variac is set at zero output voltage.
Switch-on three phase ac mains and start the motor at reduced applied voltage.
Increase the applied voltage, till its rated value.
Observe the direction of rotation of the motor. In case, it is reverse, change the phase sequence of the applied voltage.
Take-down the readings of all the meters and the speed under no load running.
Increase the load on the motor gradually by turning of the hand wheels, thus tightning the belt.
Record the readings of all the meters and the speed at every setting of the load. Observation may be continued up to the full load current rating of the motor.
Reduce the load on the motor and finally unload it completely.
Switch-off the supply to stop the motor.
Note-down the effective diameter of the brake drum.
Perform necessary calculations to determine performance parameter.
Merits of Direct Loading Test
- The efficiency can be determined at any load because constant losses are known.
- Power required for this test is small.
- This method is quite economical and convenient.
- Results are actual / Gives most accurate results.
- Very simple method.
- Very useful for small capacity machines.
- Very simple and easy calculations.
Demerits of Direct Loading Test
- It does not take into account stray load losses that occur when the machine is loaded.
- More power consumption.
- Wastage of energy in testing.
- More time is required for testing.
- Very difficult for high-capacity machines as that much load is difficult to arrange.
- This test is not giving accurate efficiency of the machine. because of iron losses underactual load are greater than the measured efficiency. It is mainly due to armaturereaction distorting the field flux.
- At full-load, due to armature reaction flux is distorted, in some cases which is increases the iron losses as much as 50%.