Compressor Motor Controls

Question 1

What are the advantages of 3 phase motors?

Answer 1

Low operating costs, lower amperage circuit,high starting torque,no start components needed

Question 2

What are the disadvantages of three phase motors?

Answer 2

Phase loss and voltage unbalance

Question 3

What is the formula for voltage unbalance?

Answer 3

Voltage unbalance (Vu)=Vd/Va
Vd =voltage deviation (the greatest difference from average)
Va = voltage average

Question 4

What voltage unbalance can damage a 3 phase motor?

Answer 4

Over 2%

Question 5

WhT components have an Inductive rating

Answer 5

Those with high starting current, including fan motors, compressors, etc

Question 6

In a 1 phase motor how many legs must be opened to stop the motor?

Answer 6

One

Question 7

In a 3 phase motor how many legs must be opened to stop the motor?

Answer 7

Three

Question 8

What percentage of power worldwide is consumed by motors?

Answer 8

About 40%

Question 9

What defines a split phase motor?

Answer 9

There are two windings, one start and one run

Question 10

How do you determine the speed of a motor?

Answer 10

G

Question 11

When is a current relay used?

Answer 11

Split systems usually under 3 hp

Question 12

When are potential relays used?

Answer 12

For split systems over 3 hp

Question 13

What is the purpose of a start capacitor?

Answer 13

To provide more phase shift to the start winding (this increases the starting torque)

Question 14

What is the purpose of a bleed resistor ina start capacitors?

Answer 14

For safety: to discharge the

Question 15

What are potential relays used for?

Answer 15

Starting single phase compressors up to five hp

Question 16

When do you use a potential relay for a 3 phase motor?

Answer 16

Never

Question 17

How do you check a run capacitor

Answer 17

Using a capacitor tester, capacitance should be within 10% of the rating, unless the manufacturer specifies otherwise

Question 18

What happens when capacitors are connected in parallel?

Answer 18

The capacitance is additive

Question 19

What happens when capacitors are connected in series?

Answer 19

Their

Question 20

What is the purpose of an overload?

Answer 20

To stop the motor if it becomes too hot

Question 21

What is a contactor?

Answer 21

A large relay that opens and closes the circuit to a motor; an electromagnetic coil in the contactor pulls to contacts together and power is sent to the compressor. In refrigeration, the thermostat, high pressure controls, and low pressure controls are wired in series with the contactor coil.

Question 22

What factors should be considered in choosing a contactor?

Answer 22

1. Poles (3 for 3 phase, 1 or 2 for single phase)
2. An amperage rating (Inductive for compressors) of compressor RLA plus any accessories operated through the contactor.
3. The line voltage rating of the load circuit
4. The coil voltage rating

Question 23

How can you test whether the contacts in a contactor are good?

Answer 23

The voltage between L1 and T1, and between L2 and T2, will be zero. Burning or putting will cause a nonzero voltage rating and indicates replacement

Question 24

How is a NEMA rated general purpose contactor different from a definite purpose contactor?

Answer 24

NEMA rated contractors are for severe industrial usage and designed for a life of 2 million cycles; definite purpose contractors may be smaller and cheaper and have a shorter lifespan appropriate to their purpose

Question 25

What is a motor starter?

Answer 25

Basically a contactor with overloads. Usually requires manual reset accompanied by diagnosis

Question 26

What is a stator?

Answer 26

The stationary winding in a motor

Question 27

What is a rotor?

Answer 27

The rotating permanent magnet in a motor

Question 28

How are the windings of a single phase motor located in relation to one another?

Answer 28

At 180 degrees

Question 29

What is the purpose of a start winding?

Answer 29

To exert magnetic power from the stator at a slight angle to start the rotor turning