Equipment: Power Supply, DAI, Universal motor (8254), Electrodynamometer (8960), timing belt.

Transcription

1 Lab 12: The universal motor. Objective: to examine the construction of the universal motor; to determine its no-load and full-load characteristics while operating on AC; to determine its no-load and full-load characteristics while operating on DC; to compare the starting torque on both AC and DC; to observe the effect of removing the compensating winding Equipment: Power Supply, DAI, Universal motor (8254), Electrodynamometer (8960), timing belt. Theory: The AC/DC universal motor is found in portable tools such as electric drills, saws, sanders, etc., and in home appliances such as vacuum cleaners, electric mixers, blenders, etc., where high speed, power and small size are an advantage. However, it is closer in concept to the DC motor than to the AC motor and, therefore, has some inherent disadvantages, which could be avoided in purely AC induction motors; chiefly, the need for commutation and brushes. The universal motor is basically a series DC motor which is specially designed to operate on AC as well as on DC. A standard DC series motor has very poor characteristics when operated on AC, mainly due to two reasons: a) The high reactance of both the armature and field windings limits AC current to a much lower value than DC current (for the same line voltage). b) If solid steel is used for the stator frame, AC flux will produce large eddy currents in the frame with consequent heating. To insure satisfactory operation of the universal motor from an AC power source, some modifications are necessary. The reactance of the series field and armature windings must be reduced as much as practicable. The reactance of the series field winding can be somewhat reduced by using fewer turns of heavier wire. However, it would not be practical to eliminate the reactance voltage drop due to the series field since that would also eliminate the magnetic field. The reactance voltage drop due to the armature winding can be practically eliminated by use of a compensating winding. The compensating winding is connected in series with the armature winding (conductive compensation) and arranged such that the ampere-turns of the compensating winding oppose and neutralize the ampere-turns of the armature. To realize this compensation, the compensating winding is displaced by 90 electrical degrees from the field winding. Since the motor used in this experiment us a 4-pole motor, the mechanical displacement is The compensating winding also improves commutation considerably. This is a great adventure since the field of a universal motor is weakened by lowering the reactance of the series field winding. If the compensating winding is short circuited (inductive compensation), the alternating currents in the armature are induced by transformer action into the shorted compensating winding, thus, effectively cancelling the reactive armature currents. To reduce losses due to hysteresis and eddy currents, the field structure is laminated. Few universal motors operate at the same speed on AC as on DC. Whether it runs faster on AC or DC is a matter of design. The reactance of the armature winding can be lowered by placing a compensating winding on the stator so that the fluxes oppose or "cancel" each other. This same compensating winding can be Page 1

2 connected in series with the armature winding. In this case, the motor is said to be conductively compensated. Under these conditions, the universal motor will have similar operating characteristics whether on AC or DC power. The compensating winding may be simply shorted upon itself, so that it behaves like a shortcircuited secondary of a transformer (the armature winding acting as the primary). The induced AC current in the compensating winding again opposes or "bucks" the armature current and the motor is said to be inductively compensated. The reactance of the field winding can be kept low by limiting the number of turns. The starting torque of a universal motor is determined by the current that flows through the armature and field windings. Due to the inductive reactance of these windings the AC starting current will always be less than the DC starting current (with equal supply voltages). Consequently, the starting torque on AC power will be lower than the starting torque on DC power. The compensating winding has the important role of reducing the overall reactance of the motor. However, it also has an equally important part in opposing armature reaction, thereby improving commutation. An uncompensated universal motor will lose most of its power. Sparking at the brushes will also be markedly worse. The power output (in horsepower) of the motor delivered to the load is defined as follows: P out, hp 1.4 ωrpm TNm = (12-1) where ω rpm is the motor speed in revolutions per minute, T Nm is its torque in Newton-meters. Keep in mind that one horsepower equals approximately to 746 W. The reactive power [var] can be computed as: 2 2 Q= S P (12-2) where S is the apparent power [VA], P is the real power [W] consumed by the motor. The efficiency of the motor is: P out, W efficiency = 100 % (12-3) P where P out,w is the output power delivered to the load in Watts. The motor losses, therefore, are estimated as: Losses = P P out, W (12-4) Page 2

3 Experiment: 1) Examine the construction of the Universal motor (use Figure 12-1 for reference). Identify the armature, the main series, and the compensating windings and their terminals. Identify the commutator and brushes. Note that the neutral position of the brushes is indicated by the red mark on the housing. The brushes can be positioned on the commutator by moving the lever to the right or to the left. Figure ) We start our study of the Universal motor from determining the neutral brush position by using AC excitation. Construct the circuit as shown in Figure Do not apply power at this time! Page 3

4 Figure 12-2 Couple the motor with the dynamometer by the timing belt. Do not close the motor s front panel. Using thin red wires, connect the torque and speed outputs of the dynamometer to the T and N terminals of the DAI; connect ground terminals of dynamometer and DAI. Set the MODE switch of the dynamometer to the DYN position and the dynamometer load control switch to the MAN. Set the dynamometer control knob to its utmost counterclockwise position for a minimum load for the motor. Move the brush positioning lever to its utmost clockwise position. 3) Turn ON the PS and adjust the output voltage to approximately 50 V. The motor should not be rotating. Note that the AC voltage that appears across the compensating winding is induced by the current flowing through the armature. Carefully and slowly adjust the brush position such that the induced voltage as measured by E 2 is at its maximum. This is the neutral point of your universal motor. Do not readjust the brush position during the experiment. Each time using the Universal motor, the brushes should be placed in the neutral position. Return the voltage to zero, turn OFF the PS, and close the front panel of your motor. 4) Connect the armature and compensating windings in series as shown in Figure Figure 12-3 Turn ON the PS and adjust it for approximately 30 V. If the line current measured by I 1 is less than 1 A, the compensating winding is producing a flux in the same direction as the Page 4

5 armature winding, thereby increasing the inductance (and reactance). If this is the case, interchange the leads connected to the armature or to the compensating winding. Measure and record the line current. Note: if the armature revolves, the brushes are not exactly at the neutral position. Return the voltage to zero and turn OFF the PS. 5) Construct the circuit shown in Figure Figure 12-4 Turn ON the PS and adjust for 120 V AC. Measure and record in a Data table the values for line current, line voltage, real power consumed by the motor, its speed, and the torque applied to the motor. Repeat the same measurements for the load torques of 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, and 1.4 Nm while recording the values in a Data table. Do not keep the load values of 1.2 and 1.4 Nm longer than it is necessary to make measurements! Return the voltage to zero and turn OFF the PS. 6) In your Metering window, set your meters to operate in DC mode. Instead of AC output of the PS, connect the motor to the DC output as indicated in Figure Figure 12-5 Turn ON the PS and adjust the voltage to approximately 120 V DC. Note: use caution while adjusting the DC voltage since the PS is capable for supplying DC voltages higher than 120 Page 5

6 V. Measure and record in a Data table the values for line current, line voltage, real power consumed by the motor, its speed, and the torque applied to the motor. Repeat the same measurements for the load torques of 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, and 1.4 Nm while recording the values in a Data table. Do not keep the load values of 1.2 and 1.4 Nm longer than it is necessary to make measurements! Return the voltage to zero and turn OFF the PS. 7) Set the dynamometer control knob to its utmost clockwise position for a maximum starting torque. Turn ON the PS and adjust for approximately 30 V. Measure and record the motor current and the torque developed. Return voltage to zero and turn OFF the PS. 8) Connect the motor to the AC output of the PS again as shown in Figure Set your meters to AC mode. Keep the dynamometer control knob in its utmost clockwise position for a maximum torque. Turn ON the PS and adjust for approximately 30 V. Measure and record the motor current and the torque developed. Return voltage to zero and turn OFF the PS. 9) Next, we assess the dependency of the universal motor speed on the voltage while operating on AC. Having your circuit unchanged, set the dynamometer control knob to its utmost counter-clockwise position for a minimum starting torque. Turn ON the PS and adjust the voltage to 20 V. Measure the motor speed and record its value together with the value of the line voltage in a new Data table. Repeat the same measurements while increasing the voltage to 120 V with increments of 10 V while recording your measurements in the Data table. You will collect total of 11 measurements to plot a smooth curve later. With the input voltage of 120 V, gradually increase the load to 1.0 Nm. Measure the motor speed and record its value together with the value of the line voltage in your Data table. Decrease the voltage by 10 V and repeat the same measurement while recording the results for line voltage and motor speed. Keep decreasing the voltage until the motor stalls. Record the values of line voltage and motor speed at each step. 10) To study the universal motor s uncompensated operation, eliminate the compensating coil winding by reconnecting your circuit as shown in Figure Figure 12-6 Page 6

7 Set the dynamometer control knob to its utmost counter-clockwise position for a minimum starting torque. Torn ON the PS and adjust for 120 V. Carefully increase the motor load to 0.4 Nm. Measure and record the motor current and speed. Return the voltage to zero and turn OFF the PS. 11) Reconnect your circuit for DC operation as shown in Figure Figure 12-7 Set the dynamometer control knob to its utmost counter-clockwise position for a minimum starting torque. Torn ON the PS and adjust for 120 V. Note: use caution while adjusting the DC voltage since the PS is capable for supplying DC voltages higher than 120 V. Carefully increase the motor load to 0.4 Nm. Measure and record the motor current and speed. Return the voltage to zero and turn OFF the PS. 12) To observe effects of the universal motor inductive compensation, reconnect your circuit for AC operation as shown in Figure Short out the compensating coil by connecting a lead directly across its terminals. Turn ON the PS and adjust for 120 V. Set the motor load to 1.0 Nm. Measure and record the motor current and its speed. While the motor is operating, remove the short across the compensating coil. Make notes on your observations (you will be asked to comment on them later on). Caution: hold the lead by the insulated connectors; do not touch the exposed terminals! Return the voltage to zero and turn OFF the PS. 13) Reconnect your circuit for DC operation as shown in Figure Short out the compensating coil by connecting a lead directly across its terminals. Turn ON the PS and adjust for 120 V. Set the motor load to 1.0 Nm. Measure and record the motor current and its speed. While the motor is operating, remove the short across the compensating coil. Make notes on your observations (you will be asked to comment on them later on). Return the voltage to zero, turn OFF the PS, and disassemble your circuit. Page 7

8 In your report: 1) From your observations in Part 3 and material discussed in the class, explain the purpose of determining the brush neutral position. 2) Report the line current you have measured in Part 4. Was the armature revolving? 3) Using Matlab and the data collected in Part 5, plot the speed of the universal motor as a function of its load while operating on AC. 4) Using Matlab and the data collected in Part 6, plot (on the same axes as in Part 3) the speed of the universal motor as a function of its load while operating on DC. 5) For the AC operation of the universal motor (data collected in Part 5), compute its full-load (1 Nm) power factors, power delivered to the load, motor efficiency, and motor losses under the full-load conditions. 6) For the DC operation of the universal motor (data collected in Part 6), compute the power delivered to the load, motor efficiency, and motor losses under the full-load conditions. 7) Compare the universal motor operating characteristics (speed, delivered power, efficiency) on AC and DC and list the major differences. 8) Compare the starting torque of the universal motor while working on DC and AC as assessed in Parts 7 and 8. Explain these results. 9) Using Matlab and the data collected in Part 9, plot (on the same axes) the dependence of the universal motor speed on the line voltage for the no-load and full-load AC operations. Make two curves distinguishable. Comment on their shape. 10) For the uncompensated operation of the universal motor, report the line current drawn by the motor and its speed while operating on AC and DC as measured in Parts 10 and 11. Comment on your observations and compare these results with the corresponding values measured previously for the normal operation of the universal motor. 11) For the inductive compensation of the universal motor studied in Parts 12 and 13, report and compare the values of the motor current and speed while operating on AC and DC. Report your observations on those experiments. Does the inductively compensated DC motor works as well as the AC inductively compensated motor? Explain. 12) Explain the difference between an inductively compensated and a conductively compensated universal motor. Why a compensating winding is necessary in an AC series motor? Page 8