Commercial Refrigeration Ch 8
Questions and Answers (Graded
A)
What are the two main weaknesses of 3Ø motors? - ANSWER - Phase loss and
voltage unbalance.
What formula is used to measure the percentage of voltage unbalance? - ANSWER -
Vu = Vd / Va
The amperage load rating of a compressor contactor is listed under which rating,
the inductive rating or resistive load rating? - ANSWER - Inductive load rating
What indicates the contacts are bad on a contactor? - ANSWER - There is a voltage
rating across the contacts of the contactor when pulled in.
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 degrees out of phase; therefore, 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.
Are the contacts (L-S) of a current relay normally open (NO) or normally closed
(NC)? - ANSWER - Normally Open
, Are the contacts (1-2) of a potential relay NO or NC? - ANSWER - Normally Closed
What is a good rhyme to remember the wiring terminals of a potential relay? -
ANSWER - 5, 2, 1, can she run?
What is the difference between the windings in a 3Ø motor and a 1Ø motor? -
ANSWER - A single phase motor has both start and run windings.
What are considered the "poles" of a motor? - ANSWER - The bundles of wires for
the start windings, or the run windings.
What is the nominal rpm of a two pole motor? A four pole motor? A six pole motor? -
ANSWER - 3,600, 1,800 & 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 windings on start-up, and then de-energizes the
start windings when the motor is up to speed.
What are three types of start relays? - ANSWER - Current relay, Potential relay, and
Positive temperature co-efficient relay.
What size compressors use current relays? - ANSWER - Fractional HP compressors
On a current relay, what do the terminal designations L, M, and S stand for? -
ANSWER - Line, main winding, and start winding.
Questions and Answers (Graded
A)
What are the two main weaknesses of 3Ø motors? - ANSWER - Phase loss and
voltage unbalance.
What formula is used to measure the percentage of voltage unbalance? - ANSWER -
Vu = Vd / Va
The amperage load rating of a compressor contactor is listed under which rating,
the inductive rating or resistive load rating? - ANSWER - Inductive load rating
What indicates the contacts are bad on a contactor? - ANSWER - There is a voltage
rating across the contacts of the contactor when pulled in.
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 degrees out of phase; therefore, 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.
Are the contacts (L-S) of a current relay normally open (NO) or normally closed
(NC)? - ANSWER - Normally Open
, Are the contacts (1-2) of a potential relay NO or NC? - ANSWER - Normally Closed
What is a good rhyme to remember the wiring terminals of a potential relay? -
ANSWER - 5, 2, 1, can she run?
What is the difference between the windings in a 3Ø motor and a 1Ø motor? -
ANSWER - A single phase motor has both start and run windings.
What are considered the "poles" of a motor? - ANSWER - The bundles of wires for
the start windings, or the run windings.
What is the nominal rpm of a two pole motor? A four pole motor? A six pole motor? -
ANSWER - 3,600, 1,800 & 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 windings on start-up, and then de-energizes the
start windings when the motor is up to speed.
What are three types of start relays? - ANSWER - Current relay, Potential relay, and
Positive temperature co-efficient relay.
What size compressors use current relays? - ANSWER - Fractional HP compressors
On a current relay, what do the terminal designations L, M, and S stand for? -
ANSWER - Line, main winding, and start winding.
