Draw a circuit diagram for the following experiment:
1. Set up the dc machine to be operated as a separately excited motor. Use a synchronous generator as the load to the dc motor. (Fig. 2.20 and Fig. 2.21 show the required connection diagram.)
2. Supply the power to the field of the dc motor from a 120 V fixed dc supply through a rheostat. Adjust the field current, by varying the rheostat, to the rated value.
3. Provide the armature current of the dc motor from a 0-120 V dc variable supply. Make sure that the variac reads zero before switching on/off the power supply.
4. Observe the motor characteristics by taking five readings between no load and 100% of rated torque. The load to the motor can be var- ied by changing the resistors connected across stator terminals of the synchronous generator and also varying its field rheostat. Make sure the armature voltage and the field current of the dc motor remain at the rated values as the load is varied. Before taking any measurement, change the resistors and the field rheostat, and observe how the torque changes. Depending upon the deflection of the pointer in the torque meter, the polarity of the field connection of the dc motor may have to be reversed. Record speed, armature current, armature voltage, and torque. Do not increase the armature current higher than the rated value. Take one measurement at rated Ia and calculate the efficiency of the motor.