Korea University

• ELECTRICAL 0104

• Experiment 12 The Direct Current Motor Part II OBJECTIVE • To locate the neutral brush position. • To learn the basic motor wiring connections. • To observe the operating characteristics of series and shunt connected motors. DISCUSSION In order of a DC motor to run, current must flow in the armature winding. The stator must develop a magnetic field (flux), either by means of a shunt winding or a series winding (or both). The torque developed by a DC motor is directly proportional to the armature current and the stator flux. On the other hand, motor speed is mainly determined by the armature voltage and the stator flux. Motor speed increases when the voltage applied to the armature increases. Motor speed will also increase when the stator flux is reduced. As a matter of fact, the speed can attain dangerous proportions if, accidentally, there is a complete loss of the stator field. DC motors have been known to fly apart under these overspeed conditions. However, your DC motor has been carefully designed to withstand possible overspeed condition. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. PROCEDURE CAUTION! High voltages are present in this Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement! The Direct Current Motor Part II 12-2 Finding the Neutral * 1. You will now determine the neutral brush position for your DC motor by using alternating current. Using your Power Supply, AC Voltmeter and DC Motor/Generator, connect the circuit shown in Figure 12-1. Terminals 4 and N on the power supply will furnish variable 0-120 V ac as the voltage output control is advanced. Figure 12-1. DO NOT APPLY POWER AT THIS TIME! * 2. Unlock the DC Motor/Generator and move it forward approximately 10 cm [4 in]. Reach behind the front face of the module and move the brush positioning lever to its maximum clockwise position. Do not slide the module back in place (you will later move the brushes again). * 3. Turn on the power supply. Place the power supply voltmeter switch to its 4-N position. Slowly advance the voltage output control until the AC voltmeter connected across the shunt field winding indicates approximately 80 V ac. (The AC voltage across the shunt field is induced by the AC current through the armature. This will be covered in a later Experiment). * 4. a. Carefully reach behind the front face of the module (preferably keeping one hand in your pocket) and move the brushes from one extreme position to another. You will notice that the induced AC voltage across the field drops to zero and then increases again as you approach the other extreme counter-clockwise position. b. Leave the brushes at the position where the induced voltage is zero. This is the neutral point of your DC Motor/Generator. Each time you use the DC Motor/Generator the brushes should be set at the neutral position. c. Return the voltage to zero and turn off the power supply. Slide your DC Motor/Generator back in place and disconnect your circuit. The Direct Current Motor Part II 12-3 Series Motor Connections * 5. Using your Power Supply, DC Voltmeter/Ammeter and DC Motor/Generator, connect the circuit shown in Figure 12-2. Notice that the armature is connected in series with the series field winding, across the input voltage. Figure 12-2. * 6. Turn on the power supply. Place the power supply voltmeter switch to its 7-N position. Adjust the output voltage to 120 V dc. * 7. a. Does the motor turn fast? * Yes * No b. Using your hand tachometer, measure the motor speed in revolutions per minute. Series speed = r/min Note: The operating instructions are enclosed within the tachometer container. * 8. a. Reduce the power supply voltage and note the effect on motor speed. Comments: b. Reduce th