EXPERIMENT 17

17.1Object: To Perform a Load Test on a Three-Phase Slip-Ring Induction Motor

17.2The Load Test on a Speed-Torque Curve

The object of the experiment is to find how speed varies with the variation of the torque. When we vary the torque, the power factor of the motor also changes. Since the current and torque are related in the motor, the power factor will vary with the current of the motor.

17.3Experimental Setup

The experiment will require two watt meters for the measurement of power input to the motor. One ammeter and one voltmeter will be required. A torque-measuring device and a tachometer will be required for the measurement of shaft parameters, T and N.

Figure 17.1

To decide the range of instruments, we read the nameplate of the motor. Suppose we have a three-phase, 50 c/s motor with 7.5 hp, 230 V, 22.3 A, and a rotor of 200 V and 18.5 amps. We may determine the range of instruments, wattmeters 0-1000 W, ammeter 0-50 A or 0;12A, voltmeter 0-300 V current transformers. A 50:5 ratio will be required.

Make the connections as shown and vary the torque on the motor. Remember that you have noted the no-load speed of the motor.

EXPERIMENTAL RESULTS

The major part of the load test is the speed-torque curve of the motor. The other part may be the torque-efficiency curve.

The torque-power factor and torque current may be the supplementary graphs. The efficiency graphs for all machines will be similar. The efficiency will be:

Figure 17.2

17.4Speed-Torque Curve

The speed-torque curve of the motor is plotted in Figure 17.2. The slip-torque curve and speed-torque curve will be similar. The no-load speed is nearly equal to the synchronous speed and decreases when torque is increased.

Figure 17.3

Figure 17.4

Figure 17.5

17.5The Torque-Power Factor Curve

When the motor runs at no load, its power factor is low, and the power factor increases with load torque.

17.6Discussion

Figure 17.6

The speed-torque curve of the motor is drawn in Figure 17.2, which is the major part of the experiment. The speed torque is like the load curve of a D.C. shunt motor shown in Figure 17.6. The load-power factor curves of the motor are shown in Figures 17.3 and 17.4. The efficiency curve is shown in Figure 17.5.

17.7Questions and Answers on the Experiment

Q1. What is a shunt characteristic? Is it applicable to the induction motor?

Ans. A shunt characteristic has the nature to slow down the speed slightly when the torque is increased. The induction motor has a similar speed-torque curve.

Figure 17.7

Q2. Where will you use the slip-ring induction motor?

Ans. Variable speed loads are connected with the slip-ring induction motors. Large changes in speeds will require this type of motor.

Q3. Where will you use the squirrel cage induction motor?

Ans. For constant speed loads. Fans, telephones, and so on are coupled with this type of motor.