Section 3: Solenoids, Relays, and Motors
Define an armature.
An armature is the rotating part, or the rotor, of the universal motor.
What type of material is used for the magnets of a permanent-magnet motor?
Magnetic powder that is pressed into the U-shaped form
What happens to the magnetic fields surrounding a wire when the current-carrying wire is formed into a loop?
The magnetic fields around the wire add together
What would happen if you placed a compass directly above a wire carrying DC current?
The pointer on the compass would align itself with the magnetic field encircling the wire
Why is the number of washers at the end of the motor shaft important?
If too many washers are used, the motor will not move freely, and if too few are used, there will be too much end play.
How is the starting torque produced in a split-phase motor?
In a split-phase motor, the starting torque is produced by the combination of the starting and the running field windings.
What is the usual cause of worn bearings in appliance motors?
Insufficient oiling of the bearings causes them to wear and become dry
How do normally-open contacts perform in a circuit?
Normally-open contacts will close and complete a circuit when the relay coil is energized.
What is the purpose of the coil in a relay?
The coil produces the magnetic field which pulls the armature and closes or opens the relay contacts
What is the purpose of the commutator?
The commutator makes it possible to connect different armature coils to the brush
What should you use to remove dirt and scratches from a motor shaft?
Use fine emery cloth, jewelers' rouge powder, or crocus cloth.
What method of motor speed control is used with a permanent-magnet motor in a variable speed hand drill?
Varying resistance is a common method of speed control for small drills
Why does the universal motor need brushes?
A brush provides electrical contact between the rotating and stationary parts of the motor.
What influence does a capacitor have on AC current and voltage?
A capacitor causes the current to lead the voltage
What type of capacitor is used with an AC motor?
A non-polarized capacitor
What is the typical resistance of a reed-relay and a DC control relay?
A reed relay will have a resistance of about 100 - 200 ohms. A control relay for DC circuits will have a coil resistance of about 500 to 1000 ohms.
What is the most likely cause of a capacitor-start motor having burnt start winding's?
A shorted capacitor is the most likely cause, followed by a struck centrifugal switch contact or mechanism
What is a universal motor?
A universal motor is a motor which can operate on both AC and DC circuits
Why are AC relay coils laminated and what action does the shading coil perform?
AC coils are laminated and use shading coils to prevent chatter and vibration of the relay armature. The shading coil causes part of the magnetic field to be phase-shifted.
What two things affect the rotational speed of a split-phase motor?
AC frequency and number of poles in the motor.
What is the typical coil resistance for an AC motor-control relay?
Above 2000ohms
How are the electric parts connected in universal motors?
All electric parts of a universal motor (field coils, commutator bars, brushes, and armature) are connected in series with each other.
The magnetic field around an energized solenoid coil can be detected by using what type of tool?
An insulated screwdriver
A cracked or deformed seat on a solenoid valve will cause what kind of problem?
As with a broken spring, the solenoid valve will leak
What is the purpose of having two field winding's in the split-phase motor?
Both windings are energized at the start to produce a starting torque, but the run winding alone is sufficient when the motor has come up to the desired speed.
What does excessive sparking at the brushes indicate?
Excessive sparking at the brushes indicates a worn brush, a loose brush, or a brush not properly seated
Why are shaded-pole motors used to drive light loads only?
Shaded-pole motors have a low starting torque
Why is a shading coil used on the magnetic poles of some small motors?
Shading coils cause an unbalance in magnetic attraction and the rotor starts to rotate
How can the direction of rotation of a split-phase motor be changed?
The direction of rotation will change if the connections to either the start or run windings are reversed.
How is the start winding disconnected when the motor comes up to speed?
The internal cylinder opens the contacts, de-energizing the start winding
A broken spring in a solenoid valve will cause what kind of problem?
The solenoid valve will leak
Why is the speed of shaded-pole motors constant?
The speed of shaded-pole motors depends on the frequency of the applied alternating current; this frequency is kept constant.
Which winding is placed in series with the capacitor in a capacitor-start motor?
The start winding
If a capacitor-start motor does not rotate or hums when power is applied, what could be the problem?
The thermal protector capacitor or centrifugal switch has most likely failed
What will happen if the south pole of one magnet is brought close to the south pole of another magnet?
The two poles will repel each other
How are the permanent magnets attached to the case of the motor?
They are cemented into place
How do you reverse a permanent-magnet motors' direction?
You switch the leads on the armature or from the battery supply
