Motor Controls for HVAC - Residential Air Conditioning and Heating
The Air Conditioning and Heating Thermostat
Motor controls are necessary to start and stop air conditioning and heating motors automatically or manually. The most common manual motor control for air conditioning and heating, which the average person is familiar with, starts and stops the blower motor on their home air conditioning and heating system. It is the Fan - Off - Auto switch on the air conditioning and heating thermostat. This switch when placed in the on position turns the blower motor on and circulates air continuously until the user turns the thermostat fan switch to either the off position or the automatic position. When the thermostat fan switch is placed in the automatic position the blower fan will operate automatically based on the temperature setting of the air conditioning and heating thermostat. In most residential systems the air conditioning and heating thermostat is the primary controller for the whole system.
Heating Motor Controls - Residential Air Conditioning and Heating
When the air conditioning and heating thermostat Fan - Off - Auto switch is set to the automatic position then the control for the blower fan changes from the air conditioning and heating thermostat to a control board or relays located in the air conditioning and heating equipment. Gas furnaces on a call for heat the air conditioning and heating thermostat closes some contacts. This completes a circuit and depending on the age of the equipment and the complexity of the controls an induced or forced drafted motor turns on by either relay or a switch in a control board. This causes a series of sequences, much of them safety related, to be set into motion and if everything is safe the furnace produces a flame in the heat exchanger. There are two different controls used to start the blower motor and, again, depending on the age and type of system you have will depend on the type of control used to start the heating blower motor.
The most common in modern equipment is a time delay on the control board. After the control board detects a flame and senses all is well with the system it allows to the flame to burn in the heat exchanger and a solid state clock in the board begins a count down to close a switch and brings the heating blower motor on. This time delay is necessary to allow the heat exchanger to heat up before turning the heating blower motor on to circulate air throughout the system. The controller on older systems that turns the heating blower motor on controls the motor by a temperature switch. This motor control has a sensor in the heat exchanger and when the temperature in heat exchanger reaches a certain set point it closes a switch and turns on the heating blower motor. This heating blower motor controller also provides a delay after the flame has been lit and heats the heat exchanger so no cold air gets circulated through the duct system. When the thermostat satisfies the blower fan is delayed in the on position while the heat in the system is dissipated from the heat exchanger and the duct work. This adds efficiency to the system by making use of usable heat that would otherwise be left in the system to dissipate into the environment and not the house. This delay for the blower fan is either in the control board or in the temperature sensor of the exchanger depending on the type of system you have.
Air Conditioning Motor Controls - Residential Air Conditioning and Heating
In the average residential air conditioning system there are three motors which need to be controlled. There is the compressor, the condenser fan motor, and the blower fan motor. Again, when the air conditioning and heating thermostat is set to the air conditioning mode and is in automatic the thermostat is the main control switch of the air conditioner. The process for controlling the start and stop is usually all from the thermostat. The compressor and the condenser fan motor are energized when the compressor contactor is energized. The compressor contactor is usually controlled entirely by the thermostat unless the manufacturer or contractor installed a compressor time delay to prevent short cycling of the compressor. Most digital thermostats have a compressor delay in them so this control is unnecessary if you have a digital thermostat. If there is a sudden loss of main power that is suddenly restored the time delay will prevent the compressor contactor from energizing which will prevent damage to the compressor. So when the air conditioning and heating thermostat energizes in the air conditioning mode the compressor contactor is energized and the compressor and the condenser fan motor turn on. The blower fan which circulates air throughout the duct system is on a separate circuit inside the thermostat and at the equipment. At the same time the compressor and the condenser fan motor energize the blower energizes (provided there is no time delay in the thermostat or equipment for compressor protection) and begins to circulate air throughout the duct system. When the air conditioning and heating thermostat satisfies the compressor contactor is de-energized stopping the compressor and the condenser fan motors but the blower should continue to blow. By putting a time delay on the blower in the air conditioning mode it allows the system to dissipate the cool air inside the duct work which adds some efficiency to the system. This delay is either in the thermostat or in the control board of the air conditioning unit.
Air Conditioning Motor Controls – Commercial Air Conditioning and Heating
Commercial air conditioning and heating motor controls are far more complex than residential HVAC motor controls. These commercial HVAC motor controls are generally integrated with an automated system or energy management system. The most popular motor control in commercial HVAC motor control is the Hand-Off-Auto (HOA) switch. These HOA switches are generally located near the motor so that if the motor needs maintenance or repair the HVAC technician can turn power to the motor off. Typically the HOA switch has built in overloads to prevent over amperage on the motor circuit. The overloads are generally removable so that if the motor was replaced with a motor rated for a different amp draw then the overloads can easily be replaced. There are some HOA switches which have solid state overloads. These solid state overloads can monitor the motor for over amperage draw, voltage spikes, phase loss, and inductance. This monitoring is designed to protect the HVAC motor circuit and motor from possible catastrophic failure.
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