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HVAC UNIT 14
| Question | Answer |
|---|---|
| system overshoot | the temp rises of the controlled space above the set point of the thermostat |
| heat anticipator | a device that anticipates the need for cutting off the heating system prematurely so the system does not overshoot the set point temperature |
| snap ot detent action | the quick opening of a control that is used to minimize the arc when making and breaking an electrical circuit |
| net oil pressure | difference in the crankcase pressure and the compressor oil pump outlet pressure |
| range | the pressure or temperature settings of a control defininf certain boundaries of temperature or pressure |
| differential | the difference between the cut in and cut out points of a control , pressure, time, temperature, or level |
| cut-in | the two points at which a control opens or closes its contacts based on the condition it is supposed to maintain |
| cut-out | the two points at which a control opens or closes its contacts based on the condition it is supposed to maintain |
| pressure switch | a switch operated by a change in pressure |
| pressure control | a switch or control that responds to changes in pressure |
| subbase | the part of a space temperature thermostat that is mounted on the wall and to which the interconnecting wiring is attatched |
| steady-state condition | a stabilized condition of a piece of heating or cooling equipment where not much change is taking place |
| temperature swing | the temperature difference between the low and high temperatures of the controlled space |
| system lag | the temperature drop of the controlled space below the set point of the thermostat |
| what is space temperature control | regulates indoor air temperature for comfort by cycling heating or cooling equipment on or off. It functions on a rise in temperature and senses random air currents in the space |
| what is a mercury control bulb | a thermostat switching mechanism containing mercury and fine wires that complete or break an electrical circuit as temperature changes |
| what is the difference between low voltage and high voltage controls | low voltage controls are more sensitive and operate at reduced voltage (typically 24V) high voltage controls are heavy duty, self-contained devices that operate at line voltage and are less sensitive |
| name commonly used low voltage controls | mercury bulb thermostats, electronic thermostats, heat anticipators, and cold anticipators |
| name commonly used high voltage controls | Line-voltage thermostats with mercury bulbs or silver-coated contacts and sensing elements such as bimetal strips, bellows, or liquid-filled bulbs |
| Name two ways motors are protected from high temperature. | Thermistors embedded in windings and bimetal strips mounted externally or embedded in windings |
| What is the difference between a diaphragm and a bellows control? | A diaphragm control uses a flexible membrane that moves under pressure changes, while a bellows control uses a collapsible, accordion-like element that expands or contracts with pressure changes. |
| What are pressure-sensitive controls used for? | They measure and/or control refrigerant, air, gas, and water pressure and can function as operational or safety devices |
| What is a high-pressure control? | A safety device that stops the compressor if high-side pressure becomes excessive. It typically has a built-in differential |
| What is a low-pressure control? | A low-charge protection device that shuts off the compressor when low-side pressure drops below a manufacturer-set minimum |
| What is range in a pressure control? | Range (cut-in adjustment) determines the pressure at which the control starts operation |
| What is differential in a pressure control? | Differential is the difference between cut-in and cut-out pressure settings |
| What is a pressure transducer? | A device that converts sensed pressure into an electronic signal that is processed by a microprocessor to adjust system operation |
| What is a pressure relief valve? | A pressure-sensitive safety device that releases excess pressure in fluid systems (water or refrigerant), usually located at the highest point of a boiler |
| What are mechanical controls? | Controls that operate independently without electrical components, such as water pressure regulating valves |
| What are electromechanical controls? | Controls that convert mechanical movement into electrical action, such as a high-pressure switch |
| What result can be obtained by passing a coil of wire through a magnetic field? | An AC voltage is generated (electromagnetic induction). |
| Three-phase voltage waveforms are separated by __________ degrees phase angle. | 120° |
| As current flows through a conductor, a __________ __________ is created around the conductor. | Magnetic field |
| Current flow in a motor is converted to mechanical motion through the process of magnetic __________ and __________. | Attraction and repulsion |
| The device used to raise or lower AC voltage levels is called a __________ | Transformer |
| Fewer turns in the secondary winding will result in a (higher/lower) secondary voltage. | Lower |
| How many watts of power are required to do one horsepower of work? | 746 watts |
| How many watts of power are being used by a resistance heater operating at 460 volts and drawing 5 amps? | P = E × I P = 460 × 5 P = 2,300 watts |
| A motor is operating on 230 volts and drawing 6.5 amps. At what horsepower is the motor operating? | P = 230 × 6.5 = 1,495 watts HP = 1,495 ÷ 746 ≈ 2 horsepower |
| One watt will generate __________ Btu’s of heat per hour. | 3.414 Btu/hr |
| In a circuit with an inductive load, the voltage will: | Lead the current through the load. |
| When a capacitor is connected in series with a load, the load current (leads/lags) the applied voltage. | Leads |
| The electrical term used to indicate effective resistance in an AC circuit is __________. | Impedance |
| The most common source of residential service has __________ wires and provides __________-phase power. | Three wires; single-phase |
| The closed delta service supplies only three-phase voltage. True/False | True |
| If a 208-volt supply is required, special arrangements must be made with the power company. True/False | True |
| What is the maximum allowable current imbalance in a three-phase supply? | 5% |
| What is likely to occur if the imbalance exceeds the allowable maximum? Motors will overheat (motor damage may occur). | Motors will overheat (motor damage may occur). |
| A current imbalance is more of a problem in a __________ system. | Open delta |
| What voltage is used to power fluorescent lighting systems? | 277 volts |
| Line voltage must remain constant because most appliances are not designed to tolerate any voltage fluctuations. True/False | True |
| Why is a low-voltage thermostat normally more accurate than a high-voltage thermostat? | low-voltage thermostats has less mass in the internal mechanism |
| The three kinds of switching mechanism used in low-voltage thermostats are _______, _________, and ________. | mercury bulb open contacts open contacts in a bulb |
| True or False: A heat anticipator causes the thermostat to shut the heat off early to dissipate the heat in the furnace's heat exchanger and not overheat the conditioned space. | True |
| True or False: A cold anticipator causes the thermostat to start the air-conditioner compressor early so that it will be up to capacity at the correct time. | True |
| Define system overshoot when dealing with thermostats. | the termperature rise in the room above the set point of the thermostat |
| True or False: The temperature drop below the thermostat's set point is referred to as system lag. | True |
| What is meant by the steady-state condition with regard to setting a heat anticipator. | are met when the ignition process has shut off and the furnace has been running for about 1 minute |
| True or False: A bimetal warps, changing position or dimension with a change in temeperature, and makes or breaks electrical contacts. | true |
| What component steps down the voltage to the low-voltage value? | step down transformer |
| Low voltage is desirable for residential control voltages because of ______, ______, ______, and _____. | safety economics more precise control smaller parts |
| What is the maximum amperage usually encountered by a low-voltage thermostat? | 2 amperes |
| A heating thermostat _________ on a rise in temperature. | opens |
| A cooling thermostat ____________ on a rise in temperature. | closes |
| What two types of switches are normally found in the subbase of a low-voltage thermostat? | fan, selector |
| When is a line-voltage thermostat used? | when a remote thermostat is not needed |
| The three sensing elements that can be used with a line-voltage thermostat are _____, _____, and ______. | bimetal, bellows, remote bulb |
| 17 What is the maximum amperage generally encountered in a line-voltage thermostat? | 15 amperes |
| True or False: A line-voltage thermostat is mounted on an electrical box because it keeps the line voltage contained which makes it safer for the technician. | True |
| Why do motors build up heat? | some of the electrical energy is converted to heat |
| The two types of motor temperature-sensing devices are _____ and _____. | internal external |
| What is the main precaution for an externally mounted motor temperature protector? | it must be in good contact with the housing of the motor |
| What three methods are there for cooling an overheated compressor? | forcing air over the motor with a fan spraying water over the housing with the power off and electrical circuits and components protected let the compressor sit until it is cooled down from the ambient air |
| How does a thermocouple change with a temperature change? | the thermocouple output voltage changes. an increase in temperature at the hot and cold junction causes an increase in voltage |
| What methods are used to convert pressure changes into action? | bellows bourdon tube diaphragm |
| True or False: Pressures below atmospheric pressure cannot be detected. | False |
| Which is True of a high-pressure control? | it protects the compressor from high pressures |
| Name two types of water pressure control. | relief valve water pressure regulating valve |
| If the differential of a motor control is set too small, the motor may short cycle. True or False | True |
| A change in the differential setting will automatically change the cut-in point of the control. True or False | False |
| A change in the differential of a control will always change the range of the control. True or False | False |
| Changing the cut-out adjustment without changing the differential adjustment will automatically change the cut-in setting. True or False | True |
| If a high-pressure control has a cut-out of 500 psig and a cut-in of 450 psig, what would be the differential of the control? | Cut-out = 500 Cut-in = 450 Differential = 500 − 450 = 50 |
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