Voltage regulation of alternator by emf method

1. The following test results were obtained on a 12 MVA, 13.8kV synchronous generator:

Open-circuit voltage, V: 6100 10050 13.200 14700 16200

Exciting current, A: 16 25 37.5 50 70

The full-load current was circulated on short-circuit with a field excitation of 22 A. The armature resistance is

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3

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and the mechanical plus iron losses measured at 16 kW.
(a) Calculate the full-load voltage regulation when the power factor is 0.9 lagging using the synchronous impedance method (EMF) Methods.

(b) If the machine is running at full load and unity power factor and if 1.2% of the output is consumed by the excitation, calculate the machine efficiency.

2. A 3-phase, 60-Hz, 6.6 kV alternator is running at rated-load at 6 MW and 0.85 PF lagging. If the machine has a per unit synchronous reactance of 1.1 (resistance ignored), determine;

(a) The MVA rating of the alternator

(b) The number of poles if the alternator is running at 1200 rpm

(c) The synchronous reactance in Ohms.

(d) Explain the EMF method for voltage regulation. Use the EMF method to determine the voltage regulation at full load and 0.8 PF lagging.

3. Explain what’s meant by power factor correction.

An induction motor load is receiving 40-kW and 30-kVAR from the utility line. Determine the power factor at the load Size the capacitor (in kVAR) needed to improve the power factor to unity. Size the Y-connected capacitor in “uFarads/ph) if the 3-phase motor is rated at 60-Hz, 4.16-kV.

4. In the two watt-meter method, prove that one watt-meter will measure zero power if the power factor is 0.5.

5. A 275-kV, 3-phase power transmission line of length 300 miles is rated 850-A. The values of the resistance, inductance, and susceptance per phase per mile are 0.125-

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, 1.7-mH, and 5.92
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m

, respectively. The receiving-end voltage is 275-kV when full-load is transmitted at 0.85 PF lagging. Using the long line model, calculate
a. The ABCD constants

b. The sending end voltage

c. The load angle

d. The voltage regulation

e. The transmission efficiency

6. A 24 hp, 6-pole, 460-V, 60-Hz, 1150 rpm, Y-connected, 3-phase induction motor has mechanical losses of 260 W and stator losses of 1300 W. Determine (a) the slip, (b) the rotor losses, (c) the input power to the motor, (d) the motor line current, and (e) the motor efficiency.

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