A+ Answers 93031 Solenoids, Relays, And Motors
Select the best answer to each question.
1. An electromagnet becomes stronger when the
A. current through the magnetizing coil is increased.
B. core of the magnet is removed.
C. size of wire in the coil is increased.
D. number of turns in the coil is reduced.
2. The purpose of the shading coil in some appliance motors is to
A. increase the current flow through the rotor.
B. cause imbalance in the magnetic field.
C. place more weight on one pole.
D. strengthen the iron form of the motor.
3. The rotor in a shaded-pole motor consists of
A. coils energized by alternating current. C. a permanent magnet.
B. a short-circuited loop of copper wire. D. an iron cylinder.
4. Why does a shaded-pole motor run at a constant speed?
A. The lines of force do not change direction.
B. The current through the rotor circuit is constant.
C. The inertia of the load is at a minimum.
D. The frequency of the applied voltage is constant.
5. What will happen if you place too many washers on the end of a shaded-pole motor shaft?
A. The motor will become noisy. C. The bearings will become too heavy.
B. The rotor will jam. D. The shaft will loosen.
6. A shaded-pole motor coil will have a resistance of approximately
A. 2 to 4 ohms. C. 15 to 40 ohms.
B. 5 to 10 ohms. D. 100 to 200 ohms.
7. The device which disconnects the start winding in some split-phase motor circuits after a preset speed is obtained is the
A. thermostat. C. brush spring.
B. commutator. D. relay.
8. It is essential for a relay energized by alternating current to have
A. many turns of small wire.
B. a laminated core and a shading coil.
C. high resistance and light weight.
D. double-pole double-throw contacts.
9. Which one of the following statements is true of a split-phase motor when it is starting?
A. The speed is high. C. The current is low.
B. The torque is high. D. The voltage is low.
10. An important difference between a universal and a split-phase motor is that the split-phase motor has
A. two brushes attached to the stator.
B. a single coil formed on the rotor.
C. two windings on the stator.
D. an armature with a commutator.
11. The function of a start winding in a split-phase motor is to
A. provide the starting torque.
B. reduce the starting current.
C. decrease the strength of the magnetic field.
D. improve the steady running at full speed.
12. Which one of the following parts of a universal motor does not move?
A. The armature C. The shaft
B. The field coil D. The commutator
13. A universal motor uses brushes made of
A. steel. C. copper.
B. brass. D. carbon.
14. Why should you not use emery cloth for polishing the commutator of an appliance motor?
A. It wears the surface too much.
B. Its dust conducts electricity.
C. It fails to remove oily, discolored spots.
D. Its material reacts chemically with copper.
15. The commutator of a universal motor connects each armature coil to
A. the rotor. C. the gearbox.
B. the shaft. D. a brush.
16. The part you should check first when you are servicing a sparking universal motor is the
A. commutator. C. brushes.
B. armature. D. field windings.
17. The range of motion for most pull-type solenoids is approximately
A. 50 mm. (2 in.) C. 3.0 cm. (1.25 in.)
B. 2.5 cm. (1 in.) D. 3.5 cm. (1.5 in.)
18. In a solenoid valve system, if the voltage on the coil is 118 VAC and a continuity test of the solenoid’s coil measures 80 ohms, yet the solenoid will not allow water flow, the most likely cause of failure is a
A. broken spring. C. faulty coil.
B. stuck plunger. D. cracked disk.
19. If, when a relay is energized, you measure 0.05 VAC across a set of normally open contacts, the
A. relay is operating properly. C. contacts must be pitted.
B. contacts must be burnt. D. coil is open.
20. If a shaded-pole motor is chattering or buzzing while it is turning, the problem is most likely caused by
A. an open shading pole. C. a shorted rotor winding.
B. a faulty coil. D. a faulty or worn bearing.
21. The centrifugal switch contacts on a split-phase motor will open when the motor reaches what percentage of full speed?
A. 20% C. 70%
B. 50% D. 90%
22. What will be the normal rotating speed in RPM for a split-phase motor with six poles covered by each run winding?
A. 900 C. 2400
B. 1200 D. 3600
23. The calculated speed of a four-pole split-phase motor is 1800 RPM. However, if you measure the shaft speed, you will find it to be 1725 RPM. What causes this loss of rotational speed?
A. Counter electromagnet force C. Centrifugal mechanism loading
B. Magnetic slip D. Power line frequency changes
24. Which one of the following is NOT a method of universal motor speed control?
A. Tapped field windings with a switch selector.
B. Varying the voltage using semiconductor devices.
C. Dynamic braking of the motor’s shaft.
D. Using an adjustable centrifugal switch in parallel with a resistor.
25. If you need to disassemble a permanent-magnet motor, you should always
A. avoid match marking the case for fear of loosening the magnets.
B. replace the bearings since the case is open.
C. pull the armature out of the case.
D. pull the case from the armature which is held in a vise.