CH. 16 Flashcards
The ____ is the value at which a device begins operation.
cut-in
A control system’s ____ is the set of numbers between and including cut-in and cut-out values.
range
In electronic controls, range adjustment is a function of the ____.
processor
A system’s ____ is the number of units between the cut-out and cut-in values.
differential
A device used to change one form of energy into another form is a ____.
transducer
The desired condition in a control system is called the ____.
set point
The difference between the desired condition and the present condition is called the ____.
offset
The present condition as measured by the sensor in a control system is the ____.
control point
Below-atmospheric-pressure elements ____.
contain volatile fluid in a partial vacuum
A bimetal coil ____ as the surrounding air gets warmer.
expands
Which of the following statements about electronic sensors is true?
They can accurately sense extremely high and low temperatures.
The sensing device is usually a thermistor.
Correct!
All of the above.
They operate at a low voltage.
All of the above.
When high head pressure in a hermetic system causes oil and refrigerant breakdown, the presence of moisture or dirt can result in ____ forming.
sludge
carbon
acids
Correct!
All of the above.
All of the above.
In a low-pressure motor control, the cut-out and cut-in pressures are set to maintain proper ____.
evaporator temperature
Which of the following statements about the terminals of a current relay is not true?
A current relay is wired by connecting an ungrounded wire to the M terminal.
Current relays are ____.
All of the above.
typically wired in series with the run windings
usually found on low-torque, single-phase motors
used to close and open the start winding circuit
All of the above.
The pickup voltage required to operate a potential relay is generated when a motor reaches around ____ of its normal operating speed.
3/4
Contactors with ____ of contacts are used to control three-phase motor operation.
three or four sets
After internal motor overload protection opens the motor circuit due to overheating, a technician should ____ to speed up compressor cooling.
use dry ice and forced air
A(n) ____ motor control is mounted on the suction side of the compressor to monitor evaporator suction pressure.
low-pressure
A ____ fuse blows immediately after the maximum rating of the fuse is exceeded.
fast-acting
A ____ fuse will not blow unless an overload condition exists for a certain period of time.
time-delay
Any standard relay may be used to replace a lockout relay.
False
An above-atmospheric-pressure element is used to control heating units.
False
When the pressure inside an above-atmospheric-pressure element increases, it closes an electrical circuit.
True
A flat bimetal strip deforms or bends as temperature rises because the copper expands more than the steel.
True
Bimetal discs are often used for fast snap-action applications.
True
If a thermistor’s resistance increases as temperature increases, the thermistor has a negative temperature coefficient (NTC).
False
An oil pressure motor control connected to a compressor’s lubrication system shuts off the compressor if the oil pressure drops to low-side pressure.
True
A low-pressure safety control is set to maintain a specific evaporator temperature.
False
A low-pressure safety control is set to shut down the compressor in the event of refrigerant loss.
True
Three-phase motors may use the same starting relays that single-phase motors use.
False
A PTC relay has a low resistance when the motor circuit is first energized, so it is easier for the current to pass through the PTC relay than through the run capacitor.
True
After a PTC relay increases its resistance, the motor current seeks the path of least resistance, which is through the run capacitor.
True
In order for a PTC relay’s resistance to drop low enough for a motor to start again, it must have enough time to cool down.
True
The fuses for a motor in an HVACR system are usually factory-installed within the compressor dome.
False
A fast-acting fuse is commonly used on motors with a high starting current.
False
A time-delay fuse may allow a motor to be damaged from high current before the time delay is over.
True
Current-limiting fuses use temperature-sensitive resistors that close the motor circuit as temperature rises.
False
An internal bimetal protection device will open the motor circuit when the temperature reaches 200°F to 250°F (93°C to 121°C).
True
External bimetal protection devices only open a motor circuit if the current is too high.
True
If the contact points of a thermostatic motor control were to open very slowly, electric arcing could burn the contacts.
True
When the thermistor in a PTC relay has high resistance, it acts as a closed switch or contact.
False
If a thermistor’s resistance decreases as the temperature increases, the thermistor has a positive temperature coefficient.
False
If a thermistor’s resistance increases as temperature increases, it has a positive temperature coefficient.
True
A fuse is an automatic switch that opens a circuit if the current draw exceeds a predetermined level and must be manually reset.
False
