ELEC UNITS 9 to 11

Question 1

(9.1) What are the only motors that don’t need a starting relay or a centrifugal switch?

Answer 1

PSC motors (permanent split capacitor)

Question 2

(9.1) What type of motors use centrifugal switches?

Answer 2

open motors

Question 3

(9.1) What type of motors use a starting relay?

Answer 3

enclosed motors (like compressors)

Question 4

(9.1) What are the 3 types of starting relays used in hermetic compressors?

Answer 4

current/magnetic relays, solid-state relays, and potential relays.

Question 5

(9.1) What principle does a potential relay operate on?

Answer 5

BEMF (back electromotive force)

Question 6

(9.1) Define BEMF

Answer 6

The voltage produced in the start winding of a motor by induction.

Question 7

(9.1) What material does a solid state relay use?

Answer 7

PTC (positive temperature coefficient)

Question 8

(9.2) Are current relays N.O. or N.C.?

Answer 8

N.O.

Question 9

(9.2) What should the resistance of a current relay’s coil read?

Answer 9

0 to 1 ohms

Question 10

(9.2) Define LRA?

Answer 10

(Locked rotor amperage) the amperage draw of the motor while the rotor is stationary.

Question 11

(9.2) Define FLA?

Answer 11

(Full load amperage) the amperage draw of the motor while the rotor is at full speed.

Question 12

(9.2) Which is higher and by how much: FLA or LRA?

Answer 12

LRA by 5 to 9 times that of FLA.

Question 13

(9.3) What starting relay can be adapted to a wide variety of compressors?

Answer 13

potential relay

Question 14

(9.3) Define pickup voltage

Answer 14

the voltage where a potential relay is taken out of the circuit.

Question 15

(9.3) Define dropout voltage

Answer 15

the minimum voltage required to keep potential relay out of the circuit.

Question 16

(9.3) Define continuous coil voltage

Answer 16

the maximum voltage that the coil of a contactor can operate on without burnout.

Question 17

(9.3) What does a hard starting kit consist of?

Answer 17

a potential relay and a start capacitor.

Question 18

(9.3) What should the resistance reading of the coil of a potential relay read?

Answer 18

1000 to 10,000 ohms

Question 19

(9.4) How does a PTC work?

Answer 19

the ceramic material’s resistance increases as it heats up with current flowing through it, causing it to drop the start winding of a motor.

Question 20

(9.4) How do electronic motor starting relays open the starting winding of a motor?

Answer 20

they use a timer.

Question 21

(9.4) How do you troubleshoot a solid state relay?

Answer 21

do visual inspection, check resistance, check current

Question 22

(9.5) How often should bearings be lubricated?

Answer 22

1 to 2 times a year.

Question 23

(9.5) Why do hermetic compressors use sleeve bearings instead of ball bearings?

Answer 23

because there is no risk of sparks using sleeve bearings.

Question 24

(9.6) What are motor drives?

Answer 24

motor drives are the connections between a motor and the load it is rotating.

Question 25

(9.6) What are the 2 types of motor drives used in the industry?

Answer 25

direct-drive and V-belt.

Question 26

(9.6) What loads use direct-drive methods?

Answer 26

fan motors, pumps, hermetic compressors

Question 27

(9.6) What are the 2 types of direct-drive couplings used to get a tight fit?

Answer 27

flexible hose coupling and flange coupling.

Question 28

(9.6) Describe how a V-belt system works.

Answer 28

it uses two pulleys to transfer rotation

Question 29

(10.1) What causes the armature of a contactor to move?

Answer 29

the current through the coil of the contactor creates a magnetic field, which attracts the armature to open and close.

Question 30

(10.1) What 2 ways can an armature move?

Answer 30

swinging and sliding.

Question 31

(10.1) What percentage of current can a contactor, in theory, run on?

Answer 31

125% its max capacity.

Question 32

(10.1) How do you troubleshoot a contactor?

Answer 32

check for continuity in terminals, check that coil has some resistance, check the mechanical linkage.

Question 33

(10.3) Can fuses be used for inductive loads?

Answer 33

No.

Question 34

(10.3) What does a line break overload mean?

Answer 34

the overload cuts power to the motor.

Question 35

(10.3) What does a pilot duty overload mean?

Answer 35

the overload cuts power in control circuit.

Question 36

(10.3) What are the two most used pilot duty overloads used in the industry?

Answer 36

current overload and magnetic overload

Question 37

(10.4) What is the difference between a magnetic starter and a contactor?

Answer 37

a magnetic starter has its own overload protection.

Question 38

(10.4) What equipment are magnetic starters used on?

Answer 38

three-phase equipment

Question 39

(10.4) What are the 3 types of overloads used in a magnetic starter?

Answer 39

bimetal relay, thermal relay, molten-alloy relay.

Question 40

(11.1) What type of transformer can be applied to multiple different receiving voltage?

Answer 40

a multitap transformer.

Question 40

(11.1) How are transformers rated?

Answer 40

by primary voltage, secondary voltage, and voltamperes (VA).

Question 41

(11.2) What are low voltage thermostats used for?

Answer 41

residential applications

Question 42

(11.2) What part of the thermostat reacts to heat?

Answer 42

the thermostat controlling element.

Question 43

(11.2) How does a mechanical thermostat react to temperature?

Answer 43

by using a bimetal or a pressure system (as temperature correlates with pressure).

Question 44

(11.2) How is snap action accomplished in a thermostat using a bimetal?

Answer 44

using a permanent magnet, or mercury in a bulb.

Question 45

(11.2) How do pressure elements work in a mechanical thermostat?

Answer 45

the thermostat contains a bulb containing a pressure element. A diaphragm reacts to the pressure in the bulb and will open and close a set of contacts.

Question 46

(11.2) What do electronic thermostats use to sense temperature?

Answer 46

thermistors.

Question 47

(11.2) How do line voltage thermostats work?

Answer 47

they open and close contacts to cut control line power to a load in the system

Question 48

(11.1) What are the 3 ways that transformers are rated?

Answer 48

primary voltage, secondary voltage, and voltamperes (VA).

Question 49

(11.2) What is a low voltage thermostat?

Answer 49

A thermostat that operates on the 24V supplied to the unit.

Question 50

(11.2) What is a line voltage thermostat?

Answer 50

A thermostat that gets power from the lines supplied to the structure (like 120V residential).

Question 51

(11.2) What does "differential" mean with thermostats?

Answer 51

the difference between the cut in and cut out temperatures of a unit.

Question 52

(11.2) How is snap action accomplished in a thermostat?

Answer 52

with a small magnet or a mercury bulb

Question 53

(11.2) What do heating and cooling anticipators do?

Answer 53

make heating and cooling more accurate, so the temperature doesn't swing.

Question 54

(11.2) Define overshoot

Answer 54

when a thermostat makes the temperature swing too high.

Question 55

(11.2) What are the pressure elements in a mechanical thermostat?

Answer 55

a mercury bulb and a diaphragm.

Question 56

(11.2) Define system lag

Answer 56

the drop in temperature between the thermostat calling for heat and furnace actually heating the room.

Question 57

(11.2) How do anticipators work?

Answer 57

Heating anticipators slightly heat bimetal to account for overshoot. Cooling anticipators slightly heat bimetal when cooling is off to account for system lag.