Compressor Motor Controls Flashcards
What are the advantages of 3 phase motors?
Low operating costs, lower amperage circuit,high starting torque,no start components needed
What are the disadvantages of three phase motors?
Phase loss and voltage unbalance
What is the formula for voltage unbalance?
Voltage unbalance (Vu)=Vd/Va Vd =voltage deviation (the greatest difference from average) Va = voltage average
What voltage unbalance can damage a 3 phase motor?
Over 2%
WhT components have an Inductive rating
Those with high starting current, including fan motors, compressors, etc
In a 1 phase motor how many legs must be opened to stop the motor?
One
In a 3 phase motor how many legs must be opened to stop the motor?
Three
What percentage of power worldwide is consumed by motors?
About 40%
What defines a split phase motor?
There are two windings, one start and one run
How do you determine the speed of a motor?
G
When is a current relay used?
Split systems usually under 3 hp
When are potential relays used?
For split systems over 3 hp
What is the purpose of a start capacitor?
To provide more phase shift to the start winding (this increases the starting torque)
What is the purpose of a bleed resistor ina start capacitors?
For safety: to discharge the
What are potential relays used for?
Starting single phase compressors up to five hp
When do you use a potential relay for a 3 phase motor?
Never
How do you check a run capacitor
Using a capacitor tester, capacitance should be within 10% of the rating, unless the manufacturer specifies otherwise
What happens when capacitors are connected in parallel?
The capacitance is additive
What happens when capacitors are connected in series?
Their
What is the purpose of an overload?
To stop the motor if it becomes too hot
What is a contactor?
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.
What factors should be considered in choosing a contactor?
- Poles (3 for 3 phase, 1 or 2 for single phase)
- An amperage rating (Inductive for compressors) of compressor RLA plus any accessories operated through the contactor.
- The line voltage rating of the load circuit
- The coil voltage rating
How can you test whether the contacts in a contactor are good?
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
How is a NEMA rated general purpose contactor different from a definite purpose contactor?
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
What is a motor starter?
Basically a contactor with overloads. Usually requires manual reset accompanied by diagnosis
What is a stator?
The stationary winding in a motor
What is a rotor?
The rotating permanent magnet in a motor
How are the windings of a single phase motor located in relation to one another?
At 180 degrees
What is the purpose of a start winding?
To exert magnetic power from the stator at a slight angle to start the rotor turning
