ECE 3072 Lab 2 Page 1 of 3

Lab 2 – AC Circuits and Electric Machines

Procedure:

Single Phase Voltage & Current Measurements:

1. Open LVDAM-EMS on the PC next to the Labvolt setup, and open the scope, phasor meter and harmonic analyzer utilities.

2. Connect circuit. Record voltage magnitude. The source voltage is not a pure sinusoid, and there are some harmonics of 60Hz present. Use the harmonic analyzer to record the magnitude of the 1st through 13th harmonics.

3. Turn on 300 Ohm resistor, record current magnitude and angle with phasor analyzer.

4. Connect 300 ohm resistor and 8.8uF capacitor in series, measure current magnitude and phase

a. Print the voltage and current waveforms from the oscilloscopeb. Calculate apparent power, real power, reactive power and power factor

5. Connect circuit, measure current magnitude and angle. a. Print the voltage and current waveforms from the oscilloscopeb. Does waveform look purely sinusoidal or is it distorted? If it has

distortion, measure the magnitude of 1st through 13th harmonics. Now compare the harmonic currents of a purely capacitive load to those of a purely resistive load by re-connecting the circuit, and measuring the current harmonics. Note the difference in harmonic currents between cap and resistor and explain.

c. Calculate apparent power, real power, reactive power and power factor

6. Connect circuit, measure current magnitude and phase.a. Print the voltage and current waveforms from the oscilloscope

ECE 3072 Lab 2 Page 2 of 3

b. Question: This is an inductor, theoretically, what should be the phase angle of the current? If the measured angle does not match the theoretical value, explain the difference.

c. Calculate apparent power, real power, reactive power and power factor. Why is the real power non-zero?

7. Connect circuit, excluding the shunt capacitor, and measure current magnitude and phase. Be sure to reduce the input voltage to 90Vrms.

a. Print the voltage and current waveforms from the oscilloscopeb. Calculate apparent power, real power, reactive power and power factor.

c. Find the value of shunt capacitance to bring the power factor to unity. Connect the closest available capacitance value and measure the inductor current magnitude and phase. Print the new voltage and current waveforms from the oscilloscope. Measure the magnitude and angle of the voltage and current, and calculate the new apparent power, real power and power factor.

Three Phase Circuits & Induction Motor

8. Disconnect the previous circuit, and now measure VAN and VAB. Record magnitude and phase of each voltage. What is the relationship between the two voltages? What is the theoretical relationship between them?

9. Connect the 120/208V – 15A three phase source to the three phase stator windings of the induction motor. Connections 1 through 3 are the 3 phase stator windings of the induction motor, and 4 – 6 are the neutral points of 1 – 3. Short connections 4, 5, and 6, but leave them floating with respect to the supply neutral. Connect the voltage and current meters to measure the line-neutral voltage of each phase, and the line current in each phase. A schematic has not been provided, so you should figure out how to make the connections.

10. Set the dynamometer to speed control, and adjust the load control knob to minimum. This is the no-load condition. Record each phase voltage and current magnitude and angle. Record the speed from the dynamometer.

ECE 3072 Lab 2 Page 3 of 3

11. Now adjust dynamometer load control manual knob to change the speed to 25 rpm below no-load speed. Record the speed, then switch the dynamometer display to torque, and record the torque output. Record the voltage and current magnitude and angle of phase A only.

12. Now repeat step 11 by reducing in 25 rpm increments until the rated speed (1670rpm).

a. For each of the 5 speeds calculate apparent power, real power, reactive power, power factor, output power and efficiency.

b. Plot torque, power factor and efficiency vs. speed

Table 1. Current, voltage and speed measurements of a DC motor no-load test.Speed Torque S P Q pf Pout Efficiency

Questions:

1. Customers usually only pay for real power consumed, but a power factor less than unity means that reactive power is also being supplied or consumed by the customer, and a portion of the current supplied to the customer is supplying no real power. In the RL circuit from step 7, what percentage of the current magnitude is supplying real power? If a unity power factor were achieved, how much more real power could be transmitted across the line without exceeding the current limit (assuming current in circuit of step 7 corresponds to maximum allowable current)? Find the rating of the shunt capacitor in μF necessary to achieve unity power factor.

Discussions (optional):