Commercial Refrigeration Chapter
8 Review Questions and Answers
100% Correct
What are the two main weaknesses of 3Ø motors? - ANSWER - Phase loss and
voltage imbalance.
What formula is used to measure the percentage of voltage unbalance? - ANSWER -
Vu = Vd ÷ Va
What indicates the contacts are bad on a contactor? - ANSWER - Any voltage
reading across the closed contacts
The contactor must be replaced whenever a 3Ø compressor is replaced. - ANSWER -
True
How is a motor starter different from a contactor? - ANSWER - A motor starter has
overloads.
Why does a 1Ø motor need a phase shift to start the motor turning? - ANSWER - The
windings are 180° out of phase; therefore, on start up the stator and rotor have
equal, but opposite, attraction.
How does a 1Ø motor accomplish its phase shift? - ANSWER - The start winding has
more resistance, which causes a phase shift, creating a slightly different angle to
the magnetic pull of the stator.
What is the difference between the windings in a 3Ø motor and a 1Ø motor? -
ANSWER - A 1Ø motor has both start and run windings
What are considered the "poles" of a motor? - ANSWER - The bundles of wires for
the run windings, or for the start windings.
, What is the nominal rpm of a two pole motor? A four pole motor? A six pole motor? -
ANSWER - 3,600, 1,800, and 1,200 rpm.
During start up, as the compressor motor speed increases the amperage does
what? - ANSWER - Decreases
A motor draws locked rotor amps (LRA) only when the motor is mechanically locked
up. - ANSWER - False
What are the two basic functions of a start relay? - ANSWER - The start relay
provides full power to the start winding on start up, and then de-energizes the start
windings when the motor is up to speed.
What size compressors use current relays? - ANSWER - Fractional horsepower
compressors
On a current relay, what do the terminal designations L, M, and S stand for? -
ANSWER - Line, main winding, and start winding
A start capacitor is added to a start relay for compressors on what type of system,
cap tube or TEV systems? Why? - ANSWER - A TEV system, because the system
does not equalize during the off cycle.
How is a start capacitor wired in the electrical circuit? - ANSWER - In series with the
start relay
Why do some manufacturers add a resistor to a start capacitor? - ANSWER - It
bleeds off excess electrical charge when the capacitor is not in the circuit.
Why is the resistor removed before using a meter to check the capacitor? - ANSWER
- It will cause the meter to give an incorrect reading.
8 Review Questions and Answers
100% Correct
What are the two main weaknesses of 3Ø motors? - ANSWER - Phase loss and
voltage imbalance.
What formula is used to measure the percentage of voltage unbalance? - ANSWER -
Vu = Vd ÷ Va
What indicates the contacts are bad on a contactor? - ANSWER - Any voltage
reading across the closed contacts
The contactor must be replaced whenever a 3Ø compressor is replaced. - ANSWER -
True
How is a motor starter different from a contactor? - ANSWER - A motor starter has
overloads.
Why does a 1Ø motor need a phase shift to start the motor turning? - ANSWER - The
windings are 180° out of phase; therefore, on start up the stator and rotor have
equal, but opposite, attraction.
How does a 1Ø motor accomplish its phase shift? - ANSWER - The start winding has
more resistance, which causes a phase shift, creating a slightly different angle to
the magnetic pull of the stator.
What is the difference between the windings in a 3Ø motor and a 1Ø motor? -
ANSWER - A 1Ø motor has both start and run windings
What are considered the "poles" of a motor? - ANSWER - The bundles of wires for
the run windings, or for the start windings.
, What is the nominal rpm of a two pole motor? A four pole motor? A six pole motor? -
ANSWER - 3,600, 1,800, and 1,200 rpm.
During start up, as the compressor motor speed increases the amperage does
what? - ANSWER - Decreases
A motor draws locked rotor amps (LRA) only when the motor is mechanically locked
up. - ANSWER - False
What are the two basic functions of a start relay? - ANSWER - The start relay
provides full power to the start winding on start up, and then de-energizes the start
windings when the motor is up to speed.
What size compressors use current relays? - ANSWER - Fractional horsepower
compressors
On a current relay, what do the terminal designations L, M, and S stand for? -
ANSWER - Line, main winding, and start winding
A start capacitor is added to a start relay for compressors on what type of system,
cap tube or TEV systems? Why? - ANSWER - A TEV system, because the system
does not equalize during the off cycle.
How is a start capacitor wired in the electrical circuit? - ANSWER - In series with the
start relay
Why do some manufacturers add a resistor to a start capacitor? - ANSWER - It
bleeds off excess electrical charge when the capacitor is not in the circuit.
Why is the resistor removed before using a meter to check the capacitor? - ANSWER
- It will cause the meter to give an incorrect reading.
