Automatic control and applications

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

What are three types of switching mechanisms used in low voltage thermostats

Mercury in a bulb, a heat anticipator and a cold anticipator

What does a heat anticipator do

Is a small resistor which is usually adjustable ( Amps ) locate close to the bimetal sensing element, and used to cut of the heat equipment prematurely

What does a cold anticipator do

The cold anticipator starts the system a few minutes early to alow the system to reach full capacity to anticipate the capacity lag

What is system overshoot

System overshoot is room temperature that is way over the set room temperature due to the fan still blowing residual heat from the heat exchanger into the room for a few minutes before it shuts off

What is system lag

The temp. drop of heat below the set point of the thermostat is called system lag

what is meant by a steady state condition when referring to setting a heating anticipator

ignition process has been shut off and the furnace has been running for about one minute

How can you measure current traveling thru a heat anticipator

10 turns of a wire wraped around an ammeter will give a more accurate reading. Don't forget to divide it by ten!

How do microelectronic or electronic thermostat handel system overshoot and system lag

These thermostats have a factory built in microprocessors with software that will determin the cycle rate, another method is a factory installed thermistor wired into an el. Circuit that will determine the cycle rate.

how does a bimetal function

The levers or knobs used to adjust these controls are arranged to apply more or less pressure to the sensing element to vary the temp. range.

what component steps down the voltage to a low voltage value

the step down transformer

why is low voltage desirable for residential control

economy , safety , more precise control of relative still air temp. and in may states a technian does not need a license to install and service low voltage wireing

what is the max amperage usually fond for a low voltage thermostat


a heating thermostat ...... on a rise in pressure

A heating thermostat opens on a rise in temp

a cooling thermostat...... on a rise in pressure

A cooling thermostat closes on a rise in temp.

what two types of switches are normally found on a low voltage thermostat

fan on and heat off cool switch

when is a line voltage thermostat used

rated up to 20 amps used in window ac to start and stop compressor

what are three elements used with a line voltage thermostat

bimetal bellows or liquid filled bulb

what is the max amperage usually found in a line voltage thermostat

20 amps

why do electric motors build up heat?

electrical energy that passes thru the motor is intended to be converted to magnetism and work, but some of it is converted to heat

what are two types of motor temp. sensing devices

bimetal and thermistor

what is the princible of operation for an externally mounted motor temp. protector

to protect the motor from drawing to much current

name a method for speeding up the cooling of an open motor when it overloads

fan or moving air streem can be used to speed up the cooling process

name a method for speeding up the cooling of an hermetric motor when it overloads

Can be cooled with a fan , or water, or turning the unit off and waiting till the next day

how can an electric 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 / the well is packed to get the probe against the wall of the well

A temp sensing device used to measure airflow is the

A fan or limmet switch on a gas, oil or electric furnace

how does a thermocouple change with a temp change

The bimetal warps, the thermistor changes resistance, and the thermocouple changes voltage.

name two methods used to change pressure changes into action

Pressure switches are use to stop and start electrical loads, such as motors, and pressure controls contain a bellows , a diaphragm or a bourdon tube to creat movement when the pressure inside it is changed. Pressure controls may be attached to switches o

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 contac

can pressure below atmospheric pressure be detected ?

Pressure below atmospheric pressure can be detected

Name a function of the low pressure control

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

Describe a high pressure control

The high pressure control closes on a rise in pressure, opens on a fall in pressure and it protects the compressor from high pressure

name two types of water pressure controls

A safety control (boiler relief valve) and operating control ( water regulating valve)

why should a safty control not be adjusted if it has a seal to prevent tampering

If the differential of a motor control is set to small, the compressor may short cycle

will changing the differential change the cut in point ?

The changing of a differential will not change the cut in point of the control

will changing the differential result in a change of the range?

A change in differential will always change the range of the control

can you change the cut in adjustment without moving the differential

Changing the cut out adjustment without changing the differential will not change the cut in adjustment

what is the range of a control

The range is of a control is the difference between the cut in and the cut out point

what does the pressure transducer

The pressure transducer measure 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 devi

list two advantages an electronic oil safety control has over a mechanical bellows oil safety control

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