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racm > ELEC UNITS 9 to 11 > Flashcards

ELEC UNITS 9 to 11 Flashcards

1
Q

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

A

PSC motors (permanent split capacitor)

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2
Q

(9.1) What type of motors use centrifugal switches?

A

open motors

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3
Q

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

A

enclosed motors (like compressors)

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4
Q

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

A

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

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5
Q

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

A

BEMF (back electromotive force)

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6
Q

(9.1) Define BEMF

A

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

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7
Q

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

A

PTC (positive temperature coefficient)

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8
Q

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

A

N.O.

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9
Q

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

A

0 to 1 ohms

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10
Q

(9.2) Define LRA?

A

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

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11
Q

(9.2) Define FLA?

A

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

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12
Q

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

A

LRA by 5 to 9 times that of FLA.

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13
Q

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

A

potential relay

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14
Q

(9.3) Define pickup voltage

A

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

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15
Q

(9.3) Define dropout voltage

A

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

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16
Q

(9.3) Define continuous coil voltage

A

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

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17
Q

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

A

a potential relay and a start capacitor.

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18
Q

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

A

1000 to 10,000 ohms

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19
Q

(9.4) How does a PTC work?

A

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.

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20
Q

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

A

they use a timer.

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21
Q

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

A

do visual inspection, check resistance, check current

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22
Q

(9.5) How often should bearings be lubricated?

A

1 to 2 times a year.

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23
Q

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

A

because there is no risk of sparks using sleeve bearings.

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24
Q

(9.6) What are motor drives?

A

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

25
Q

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

A

direct-drive and V-belt.

26
Q

(9.6) What loads use direct-drive methods?

A

fan motors, pumps, hermetic compressors

27
Q

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

A

flexible hose coupling and flange coupling.

28
Q

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

A

it uses two pulleys to transfer rotation

29
Q

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

A

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

30
Q

(10.1) What 2 ways can an armature move?

A

swinging and sliding.

31
Q

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

A

125% its max capacity.

32
Q

(10.1) How do you troubleshoot a contactor?

A

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

33
Q

(10.3) Can fuses be used for inductive loads?

A

No.

34
Q

(10.3) What does a line break overload mean?

A

the overload cuts power to the motor.

35
Q

(10.3) What does a pilot duty overload mean?

A

the overload cuts power in control circuit.

36
Q

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

A

current overload and magnetic overload

37
Q

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

A

a magnetic starter has its own overload protection.

38
Q

(10.4) What equipment are magnetic starters used on?

A

three-phase equipment

39
Q

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

A

bimetal relay, thermal relay, molten-alloy relay.

40
Q

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

A

a multitap transformer.

40
Q

(11.1) How are transformers rated?

A

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

41
Q

(11.2) What are low voltage thermostats used for?

A

residential applications

42
Q

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

A

the thermostat controlling element.

43
Q

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

A

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

44
Q

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

A

using a permanent magnet, or mercury in a bulb.

45
Q

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

A

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.

46
Q

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

A

thermistors.

47
Q

(11.2) How do line voltage thermostats work?

A

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

48
Q

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

A

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

49
Q

(11.2) What is a low voltage thermostat?

A

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

50
Q

(11.2) What is a line voltage thermostat?

A

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

51
Q

(11.2) What does “differential” mean with thermostats?

A

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

52
Q

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

A

with a small magnet or a mercury bulb

53
Q

(11.2) What do heating and cooling anticipators do?

A

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

54
Q

(11.2) Define overshoot

A

when a thermostat makes the temperature swing too high.

55
Q

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

A

a mercury bulb and a diaphragm.

56
Q

(11.2) Define system lag

A

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

57
Q

(11.2) How do anticipators work?

A

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

racm (23 decks)

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