Unit 14 Automatic Control Components and Applications

8. Describe the method a service technician should follow when measuring the current that normally travels through a heat anticipator.

10 turns of a wire wrapped around an ammeter wil give a more accurate reading, and remember to divide your reading by 10.

19. What is the maximum amperage generally encountered in a line-voltage thermostat?

25A.

13. What is the maximum amperage usually encountered by a low-voltage thermostat?

2A.

42. 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?

50 psig.

25. Describe a method for speeding up the cooling of an open motor.

A fan or moving air stream can be used to speed up the cooling process.

T2. A cold anticipator is

A fixed resistor in a space-cooling thermostat used to start an air-conditioning system prematurely.

T11. The term fluid pertains to

A liquid, a vapor, and steam.

T15. The oil pressure safety control is

A pressure differential control.

T13. The pressure control can be an operating control or

A safety -type control.

35. Name two types of water pressure control.

A safety control (Boiler relief valve, or a pressure-regulating valve for system pressure), and an operating control (water regulating valve).

T4. Detent is

A snap action.

T5. The low-voltage thermostat passes power from ____ to the loads in the control circuit.

A transformer.

T1. The heat anticipator is

An adjustable resistor in a space-heating thermostat used to stop a furnace prematurely.

1. Why is a low-voltage thermostat normally more accurate than a high-voltage thermostat?

Because unanticipated thermostat cycles shows system lag and system overshoot causing larger temperature swing. (Low voltage acts faster)

30. Name two methods to convert pressure changes into action.

Bellows and Diaphragms.

22. The two types of motor temperature sensing devices are

Bimetal and thermistor.

T8. What are two types of motor temperature protection devices?

Bimetal and thermistor.

28. A temperature-sensing device used in measuring airflow is the

Bimetal disc, or helix coil.

18. The three sensing elements that can be used with a line-voltage thermostat are

Bimetal, bellows, and liquid filled bulbs.

T10. A thermistor motor temperature protection device operates by

Causing a circuit to open when the motor overheats.

15. A cooling thermostat ______ on a rise in temperature.

Closes.

26. Describe two methods for cooling an overheated compressor.

Cooling by air, cooling by water, or time.

T18. Gas pressure switches

Detect the presence of gas pressure before gas burners are allowed to ignite.

T21. The difference between the cut-out and the cut-in of a control is the

Differential.

12. Low voltage is desirable for residential control voltages because of

Economy, safety, more precise control of relative still air temperature, and because in many states a technician doesn't need a license to install and service low voltage wiring.

21. Why do motors build up heat?

Electrical energy that travels within the motor is intended to convert to magnetism and perform, but instead, some of it processes to heat. (Inductive Reactance)

32. Pressures below atmospheric pressure cannot be detected.

False.

38. A change in the differential setting will automatically change the cut-in point of the control.

False.

16. What two types of switches are normally found in the sub-base of a low-voltage thermostat?

Fan-On-Auto, and Heat-Off-Cool switches.

T12. What is used in some applications to sense temperature in a moving airstream?

Helix coil.

T20. An example of an electro-mechanical control is a

High pressure switch.

3. What does a heat anticipator do?

It cuts off heating equipment prematurely.

44. List two advantages an electronic oil safety controller has over a mechanical, bellows-operated oil safety controller.

It eliminates capillary tubes, thus giving it a less chance of a refrigeration leak, and the electronic clock and circuitry are much more reliable.

34. What is true of a high-pressure control?

It protects the compressor from high pressures.

4. What does a cold anticipator do?

It starts the system a few minutes early to allow the system to reach full capacity to anticipate the capacity lag.

2. The three kinds of switching mechanism used in low-voltage thermostats are

Mercury in a bulb, open contacts, and contacts sealed in an inert gas.

36. Why should a safety control not be adjusted if it has a seal to prevent tampering?

Most safety controls are pre-set at the factory and sealed, and should automatically reset w/o tampering.

17. When is a line-voltage thermostat used?

Normally in window AC, rated up to 20 amps, they start and stop the compressor.

14. A heating thermostat ______on a rise in temperature.

Opens.

T3. The heating thermostat _____ on a rise in temperature.

Opens.

T23. A device that senses the pressure in a system and then converts the sensed pressure signal to an electronic signal usually to be processed by a microprocessor is a

Pressure transducer.

T22. A control adjustment that provides for the correct minimum and maximum pressures or temperatures is the

Range.

T14. A switch that detects air movement in a duct with electric heat is a

Sail switch.

T17. The air pressure control on a heat pump outdoor coil

Senses the air pressure drop across the coil to determine when it should go into defrost.

T9. Most hermetic compressors are cooled by

Suction gas.

T7. What is meant by the phrase "temperature of the ambient air"?

Temperature of the surrounding air.

29. How does a thermocouple change with temperature change?

The bimetal warps, and the rise in temperature is converted into voltage.

10. Describe how a bimetal functions.

The controls are arranged to apply more or less pressure to the sensing element that varies the temperature range.

7. What is meant by the steady-state condition with regard to setting a heating anticipator?

The ignition process has been shut off and the furnace has been running for about a minute or the furnace is fired and operating.

33. Name one function of a low pressure control.

The low pressure control can be a safety control or an open type control.

T16. The difference between the suction pressure and the compressor oil pump outlet pressure is

The net oil pressure.

31. Name two actions that can be obtained with a pressure change.

The pressure control can either open or close on a rise in pressure, This opening and closing action can control water or other fluids and the pressure control can sense a pressure differential and be designed to open or close a set of differential contacts.

43. Explain the function of a pressure transducer, and give an example of where one would be used in the HVAC/R industry.

The pressure transducer measures pressure by converting a sensed pressure signal to an electronic signal to be processed by a microprocessor. This signal is then sent to the input/output board to operate relays to either stop or start power consuming devices. EXAMPLE: A pressure transducer would be hooked up to a compressor, in a grocery store, that runs the refrigeration/freezer system.

41. Explain what is meant by the range of a temperature control or pressure control.

The range of a temperature/pressure control is the difference between the cut-in and the cut-out point.

11. What component steps down the voltage to the low voltage value?

The step down transformer.

5. Define system overshoot when dealing with thermostats.

The temperature rise in a room (room temperature) above the set point of a thermostat.

27. How can an electronic thermometer be used in a well to obtain an accurate response?

The well is fastened to the metal plate so that heat will conduct both into and out of the bulb, and the well is packed so that the probe is against the wall of the well.

9. Explain how microelectronic or electronic thermostats handle system overshoot and system lag.

These thermostats have built in microprocessors with software that will determine the cycle rate, or a factory installed thermistor wired into an electrical circuit that will determine the cycle rate.

23. What is the main precaution for an externally mounted motor temperature protector?

To protect the motor form drawing too much current and overheating.

24. What is the principle of operation for most externally mounted overload protection devices?

To protect the motor from drawing too much current.

20. Why is a line-voltage thermostat mounted on an electrical box that has conduit connected to it?

To reduce a fire hazard.

37. If the differential of a motor control is set too small, the motor may short cycle.

True.

39. A change in the differential of a control will always change the range of the control.

True.

40. Changing the cut-out adjustment without changing the differential adjustment will automatically change the cut-in setting.

True.

6. The temperature drop below the thermostat's set point is referred to as system lag.

True.

T19. An example of a mechanical control is a

Water pressure regulating valve.

T6. The cold anticipator is

Wired in parallel with the cooling contacts.