HVAC Motor Control (Chapters 16 and 19)
What makes up the general-purpose relay?
Holding coil, One "normally open" set of contacts, and One "normally closed" set of contacts.
The unit of measurement used to size a relay is _____.
Horsepower
Where is the high-pressure motor control located?
It's connected to the compressor outlet before the condenser
_____ is the current drawn by the motor just as it begins to start, but before the rotor begins to rotate.
Locked-rotor amperage
If a thermistor's resistance decreases as temperature increases, the thermistor has a _____.
Negative temperature coefficient (NTC)
What are auxiliary contacts generally used for?
1) Open and close control circuits; 2) energize and de-energize other smaller system loads with low current requirements
Two ways in which a control can be adjusted are ____ and____.
1) Range adjustment: set of numbers between and including cut-in and cut-out values. Values range during which system operates after reaching its cut-in value. 2) Differential adjustment: number of units (difference) between the cut-out value and the cut-in value.
_____ are often made of silver cadmium oxide with a higher surface resistance and are physically larger than pilot-duty relays.
Heavier-duty motor switching relay (General purpose relays)
One of the most harmful things that can happen to a hermetic system is to have _____.
High head pressure
____ is a safety control device used to turn off the compressor before dangerously high pressure is reached?
High-pressure motor control
Two types of overload protection are ____ and _____.
Inherent (internal) protection and External protection
What can happen if the head pressure heats the vapor refrigerant too hot?
It can cause the oil and the refrigerant to break down.
What can happen if the head pressure heats the vapor refrigerant too hot and there is moisture present?
It can result in carbon, acid, and sludge forming
____ sets the standard for all electrical installations, including motor overload protection.
NEC (National Electrical Code)
Where are auxiliary contacts located?
Often mounted to the side of the contractor
Where are pressure motor controls often used?
On commercial systems
_____ are designed to disconnect the motor at some current draw value that is slightly more than the FLA value so that the motor can be operated at its FLA design capacity.
Overload protection devices
_____ is a small relay used in control circuits for switching purposes.
Pilot-duty relay
What does PMR stand for?
Potential magnetic relay
____ may be used to regulate compressor motor operation based on pressure measurements in the system.
Pressure motor control
What does the high-pressure motor control do?
Protects compressor from high pressure and high temperatures and from overheating and overloading during excessive current draw.
With a ____ adjustment, cut-in and cut-out temperaturs can be raised or lowered, but the difference between the two temperatures remain the same.
Range adjustment
____ adjustment raises or lowers the cut-in and cut-out values by an equal amount.
Range adjustment
A ____ has a magnetic coil that opens or closes one or more sets of contacts.
Relay
_____ is an automatic component designed to control loads that draw less than 20 amps of current.
Relay
_____, _____, and _____ pass power to the motor by closing a set of contacts.
Relays, Contactors and Starters
____ are devices that react to temperature change and send a signal to the control devices.
Remote temperature-sensing elements
____ is a device that reacts to heat by changing its internal pressure and is connected by a capillary tube to a diaphragm or bellow.
Sensing bulb
What are three common types of temperature sensors used with thermostatic motor controls?
Sensing bulbs, bimetal devices, electronic sensors
____ is a type of contractor that is generally used on many residential air-conditioning units.
Single contact contactor
What method of contactor is used to supply crankcase heat to the compressor during the off-cycle?
Single-pole contactor
A relay with a single set of contacts that close when the coil is energized is called a _____.
Single-pole-single-throw, normally open relay (spst, NO)
What determines the type of switching device used with a motor?
Size of motor and the application
Why are auxiliary contacts generally mounted to the side of a contactor?
So its moving armature can change the position of the auxiliary contacts at the same time the main contacts change position.
Two types of electronic motor starting relays.
Solid-state and Positive temperature coefficient (PTC)
_____ is a three-pole contactor with motor overload protection built into the device.
Starter
What is a general-purpose relay typically used for?
Starting a motor that has a relatively low amperage draw
Motor operating controls measure what two principal values?
Temperature (thermostatic motor control) and Pressure (pressure motor control)
____ is a device that reacts to the heat it senses by closing or opening an electric switch to start or stop the motor.
Temperature motor control
____ is a motor control that uses a sensing-bulb to react to heat instead of pressure.
Temperature motor control
Give three examples of conditions that an HVAC control system would regulate.
Temperature, Pressure, Humidity
What is the most expensive single component in the system?
The electric motor
What is the difference between a motor starter and a contactor?
The motor starter has built in overload protection
____ is an electronic temperature sensing device in which the resistance varies as the temperature varies.
Thermistor
Generally, for large motors there will be how many heavy-duty sets of contactors to start and stop the motor.
Three
What is the purpose of the overload protection device?
To disconnect the motor from the circuit when an overload condition occurs.
A relay with two contacts that close and one that opens is called a _____.
Triple-pole-single-throw with two normally opened contacts and one normally closed contact (tpst)
T/F: Auxiliary contacts are rated at a lower amperage than the main contacts.
True
T/F: Fuses are used to protect the circuit, not the motor.
True
T/F: If a relay fails, it must be replaced; however, if a contactor fails, it's individual components can be replaced.
True
T/F: Relays are designed for light-duty and should not be used to directly start motors.
True
T/F: The higher the current value above FLA, the more quickly the overload should react.
True
T/F: The smaller the motor, the larger the service factor.
True
T/F: You can use a voltmeter to check resistance across a set of contacts under full load.
True
T/F: a high-pressure motor control is used for safety, not for normal operation.
True
What is a pressure motor control used for and where is it located?
Used to monitor the evaporator suction pressure; Connects into the suction side of the compressor
Two metals used to form a bimetal device are ____ and ____.
Copper and steel
What are the most popular types of sensing devices are those that are sensitive to ________?
temperature changes.
What does "service factor" refer to?
the reserve capacity of the motor (expressed as a percentage above the FLA of the motor)
What type of sensing device is used in an electronic temperature sensor?
thermistor
A temperature motor control is often referred to as a _____.
thermostat
Two common types of remote temperature sensing elements are:
1) Above atmospheric pressure element (controls refrigeration temp; closes circuit on temperature rise); 2) Below atmospheric pressure element (controls heating/defrost units; opens circuit on temperature rise)
What are the two motors used in the condenser?
1) Compressor (largest); 2) Condenser fan motor (smallest)
___, ___, and ____ are three types of differential adjustment controls.
1) Cut-in control: allows adjustment to cut-in value only. 2) Cut-out control: allows adjustment to cut-out value only. 3) Combination control: allows adjustment to both cut-in and cut-out values allowing both values to be brought closer together or moved further apart.
Common modes of communications used in a control system include:
1) Direct digital control (DDC), 2) Electrical, 3) Electronic, 4) Hydraulic, 5) Mechanical, 6) Pneumatic, 7) any combination of the above listed.
How is external protection provided?
By thermally activated snap-discs (bimetals) located on the compressor's fifth exterior shell.
Term that refers to specific areas where an HVAC control system is used.
Conditioned space
A ____ is a larger version of a relay that can be rebuilt if it fails, and has both moving and stationary contacts.
Contactor
_____ is an automatic component designed to control loads that draw more than 20 amps of current.
Contactor
How does the bimetal device work using copper and steel metals in combination with one another?
Copper is softer and expands further when heated. This extended expansion causes the bimetal device to bend which breaks the circuit.
_____ monitors current flow through the motors for feedback to a microprocessor providing running status, overload protection, and remote monitoring.
Current sensors (connected between the circuit breaker and the contactor)
Three types of an electromagnetic motor starting relays.
Current, Potential, Contactors (or motor starters)
Which type of adjustments affects only the cut-in value?
Cut-in differential adjustment
A controlled device has a cut-in setting and a cut-out setting. What do each of these settings do?
Cut-in: a condition value at which a device begins to operate (turns on). AND Cut-out: a condition value at which a device stops operating (turns off).
Which type of adjustments affects only the cut-out value?
Cut-out differential adjustment
In the cooling system, if the range adjustment is increased, the run time of the unit will increase or decrease?
Decrease (requires less refrigerant reach cut-out value)
A ____ adjustment increases or decreases the difference between the cut-in and cut-out values.
Differential adjustment
An overload protector opens the circuit if the motor ____.
Draws too much current
_____ is a collection of interacting components that work together to achieve a common purposed such as regulating the operation of an HVAC/R system and protecting its components from operating outside their designed boundaries.
Electrical control systems
What are the two types of motor starting relays?
Electromagnetic and Electronic
What are the two current ratings for electric motors?
FLA (Full-load Amperage) and LRA (Locked-rotor Amperage)
T/R: As the load or torque on a motor increases, the amperage draw required to power the motor will decrease.
False (amperage draw will increase)
A motor normally has a starting current of _____ times the running current, but only for a short period of time.
Five
____ is the corresponding amperage the motor is drawing once it reaches its maximum torque and horsepower.
Full-load amperage (FLA)
What are the typical service factor ratings?
1.15 (aka 15%) to 1.40 (aka 40%)
A contactor's holding coil can be used with which operating voltage amounts?
24 V, 115 V, 208 V - 230V, 460 V
What size motor would a 3 horsepower relay be used for?
3 HP motor
The LRA of a motor is typically ______ times higher than the FLA of the motor, but can be as much as ______ times higher than the FLA.
5 times; 7 times
One horsepower equals _____ watts.
745.7 Watts
What constitutes a "large motor" description?
A motor over 5 hp in air-conditioning, heating and refrigeration systems.
In the cooling system, if the range adjustment is decreased, the average temperature of the condition space (increases or decreases,) and the run time of the unit will (increase or decrease?)
Average room temp decreases; Run time increases (because more refrigerant is needed to reach cut-out value)
What do cut-in and cut-out differential controls affect?
Average temperature, the run time and the cycle interval
Why is the Full-load amperage (FLA) lised on the motor's nameplate?
Because its used to select the current wire size, overload and motor starer devices that serve and protect the motor.
Why should relays not be used to start motors?
Because they can only be used with a current draw of 20 amps or less.
Why can't a fuse or circuit breaker be relied upon for motor protection?
Because they protect the entire circuit and not just the circuits individual components themselves.
____ is a bimetal device used in temperature controls that are often mounted with a mercury switch on one end to open and close the circuit when the set points are reached.
Bimetal coil
___ consists of two different metals bonded together and formed into a particular shape.
Bimetal device
____ is a common bimetal device that consists of a concave disc composed of two metals and are often used as safety switches (e.g., high-limit switch and flame roll-out switch).
Bimetal disc
____ is a popular form of bimetal device formed into a strip that reacts to heat conditions.
Bimetal strip
How is inherent protection provided?
By internal thermal overloads either embedded in or located near the motor windings.
