1. Required starting torque is small, but quiet operation is most important.The least expensive motor that meets these requirements is a
a) permanent split capacitor (PSC) motor.
b) split phase motor.
c) capacitor start-capacitor run motor.
d) capacitor start motor.
2. The secondary voltage in a non-saturated transformer is determined by
a) the size of the primary and secondary wire
b) the permeability of the core.
c) the cross sectional area of the core.
d) the turns ratio and primary voltage.
3. Among single phase AC motors, a motor has the highest starting torque.
a) split phase
b) shaded pole
c) capacitor start
d) permanent split capacitor (PSC)
4. An induction motor turning at 1750 RPM consumes 4750 watts of power and produces 14.37 ft-lbs of torque. The efficiency of the motor is
a) 33.7%. b) is 66.6%.
c) 75.2%. d) 133%
5. A three phase transformer is constructed from three lkVA transformers.The secondaries are wired in A (delta) configuration and supply power to a balanced load. The maximum combined power that they can deliver to the load is
a) 1kVA b) 3kVA 3kVA ⁄ √3 d) 3kVA x √3
6. The secondaries of the same three 1 kVA transformers are wired in Y (wye) configuration. Themaximum combined power that they can deliver to the load is
a) 1kVA b) 3kVA c) 3kVA ⁄ √3 d) 3kVA x √3
7 The rated secondary current in a transformer is determined by
a) the size of the primary and secondary wire
b) the permeability of the core.
c) the cross sectional area of the core.
d) the turns ratio and primary voltage.
8. Among DC motors, a motor has the highest starting torque.
a) shunt
b) series
c) cumulative compound
d) differentially compound
9. In a compound DC motor, a diverter
a) controls the shunt field current
b) controls the armature current
c) controls the series field current
d) none of the above
10. One advantage in using a shaded pole motor is
a) they are inexpensive.
b) they have high starting torque.
c) they have near unity power factor operation under full load.
d) they operate synchronously.
11. In a DC generator installation where there is significant resistance in the wiring to the load, the load voltage can be maintained nearly constant by connecting the generator in a configuration.
a) series
b) shunt
c) cumulative compounded
d) differentially compounded
12. The no-load voltage produced by a separately excited DC generator operating at 1000 RPM is 120VDC. The no-load voltage at 1500\ RPM would be
a) 80 VDC b) 100 VDC
c) 150 VDC d) 180 VDC
13. A 5kW 120V DC generator has a no-load voltage of 140 VDC. The voltage regulation is
a) 14.3% b) 16.6%
c) 20% d) 85.7%
14 Caution must be used in designing systems with large series connected DC motors to be sure
a) it will never operate unloaded.
b) it will never start under load.
c) the line voltage VT never drops below rated voltage.
d) all of the above.
15. In a standard automobile electrical system, three phase electrical power is produced by an alternator to operate the 12 volt DC system. The output voltage of the alternator is converted to DC by
a) rectifiers.
b) the battery.
c) the slip rings & brushes.
d) the field current.
16. For any motor, plugging is a method to
a) start the motor.
b) stop the motor.
c) improve motor efficiency.
d) improve power factor.
17. In a DC generator, the commutator and brush assembly
a) converts the output to DC
b) serves as a mechanical bearing
c) powers the stator
d) corrects for lagging power factor.
18. In the schematic symbol for a transformer with one secondary, there are dots shown on one end of each winding. At the instant when the AC voltage is positive at the dot end of the primary, the voltage at the dot end of the secondary will be
a) positive.
b) negative.
c) zero.
d) the dots indicate current direction, not voltage polarity.
19. Increasing the shunt field current in a DC motor causes the motor speed to
a) increase
b) decrease
c) remain the same
20. A separately excited DC generator rated at 125V has an armature resistance of 0.75 ohms and a no load voltage of 140 VDC. At rated load, the armature current will be
a) (140-125) x 0.75
b) (140-125) / 0.75
C) 125 / 0.75
d) 140 / 0.75
21. A three phase induction motor can be reversed by
a) reconnecting phase A to B, B to C, and C to A.
b) disconnecting one phase.
c) it cannot be reversed.
d) reversing any two phases.
22. One advantage in using a three phase synchronous motor is
a) it has a large operating torque.
b) it is easy to start.
c) it is electrically the simplest of all three phase motors.
d) it can operate at a leading power factor.
23. A horizontally oriented conductor ispassed vertically downward between two magnetic poles. The poles are magnetized with north on the left and south on the right. Assuming a complete electrical circuit, current in the conductor will tend to flow
a) toward us.
b) away from us.
c) cannot be determined.
24. In any electric motor, the torque produced is directly proportional to
a) the magnetic field strength.
b) the current through the windings.
c) the length of the conductor in the field.
d) a and b
e) all of the above.
25. A % hp, 120 VAC, 60 Hz single phase induction motor is rated at n. = 1175 RPM. Its synchronous speed n:is most likely
a) 600 RPM. b) 900 RPM.
c) 1200 RPM. d) 1800 RPM.
26. A self excited shunt DC generator outputs the wrong voltage polarity and has been installed such that it is impossible to change the connections of the main terminals or the field. The solution is.
a) to reverse the direction of rotation.
b) to flash the field and reverse the direction of rotation.
c) the polarity cannot be reversed.
d) to flash the field.
27. A 60 Hz single phase induction motorhas an 8 pole stator. Its synchronous speed is
a) 600 RPM. b) 900 RPM.
c) 1200 RPM. d) 1800 RPM.
28. In a balanced three phase 4-wire Ysystem, the measured voltage from any phase to neutral is 480VAC. The line voltage would be ....
a) 240VAC b) 480VAC
c) 831.4VAC d)1440VAC
29. A single phase capacitor start induction motor is rated at 1750 RPM, 120VAC, 1/2hp, and 4.14A line current. With rated voltage applied and the rotor stalled (blocked), the input current is 12A. Under these conditions, the motor efficiency is
a) 0% b) 66.6% c) 75%
d) cannot be determined
30. ...and the power dissipated by the motor windings will be
a) OW b) 248.6W
c) 373W d) 1440W
Show all work on these pages (not separate paper) and circle your answer for each problem. For the next two problems: A single phase capacitor start induction motor is rated at 1750 RPM, 120 VAC, 1/2 hp, and 6.08 A line current at 0.7 lagging power factor.
31. Under rated operating conditions, what is the motor efficiency?
32. A three phase transformer (made out of three identical single phase transformers) converts a 1200 VAC delta system (primary) to a 13,200 VAC Y system (secondary). Find the turns ratio of the transformers, N1/N2.
33. A transformer core with cross sectional dimensions of 0.5 inch by 1 inch has a magnetic fluxdensity of B = 1.5 Wb/m2. What is the flux (in Webers) in the core (1 inch = 25.4 mm)?
34. A DC shunt motor has the following ratings: VT = 150V, IL = 10A, Racir = 0.5 ohm, Rf = 300 ohms, Pstray = 100W. What is the % motor efficiency?
35. A 240VAC-240VAC isolation transformer that is rated at 1.2kVA is to be wired as shown in Figure 1 in order to step-up the 240 input to 480VAC output. If the transformer is operating at rated voltage and current, what is the power delivered to the load.
(Hint: the answer is not 1.2kVA) Figure 1
36. A single phase transformer has a primary voltage of 240 VAC, a secondary voltage of 12 VAC, a primary current of 10 A, and 600 turns of wire on the primary.
a) Find the number of turns on the secondary (assume a perfect transformer).
b) Find the secondary current.
37. A permanent magnet DC motor with a 0.5 ohms armature resistance is to be stopped by dynamic braking (the motor terminals will be shorted by a switch with no external resistance). At operating speed (i.e. at the instant the switch is closed), the counter EMF in the motor armature is 11.5 volts. What will be the peak current through the switch at the instant it shorts out the motor (assume brush drop is zero)?
38. When operating at rated load, a 120 VAC 60 Hz single phase induction motor can be modeled as a 10.8 ohm resistor in series with a 25.3 millihenry inductor. Under these conditions, what will be the power factor?
39. A self excited shunt DC generator has the following characteristics.
Maximum load current: 15 Amperes
Rated output voltage: 150 VDC
Field Resistance: 100 ohms
Armature circuit resistance: 0.75 ohms
Brush drop: 2.0 VDC
Since it is self excited, some of the armature current provides field current. However, if we connect it as separately excited, all the armature current could be supplied to the load. What would be the maximum available load current if the generator were connected as separately excited?
40. In the PLC ladder program rung shown below, certain on/off combinations of IN1, IN2, and IN3 will cause CR2 to be either off or on. For input combinations a) thru h) below, indicate the condition of CR2 by writing ON or OFF in the blanks in the CR2 column. Note: Remember that an ON in an input column does not necessarily mean that the contact is conducting. It simply means the input to the PLC is ON.
1N1 1N2 1N3 CR2
a) OFF OFF OFF ___
b) OFF OFF ON ___
c) OFF ON OFF ___
d OFF ON ON ___
e) ON OFF OFF ___
f) ON OFF ON ___
g) ON ON OFF ___
h) ON ON ON ___
