The relay in the above control diagram controls a blower motor as shown. Some relays are controlled by other relays depending on the components in the system and the sequence of operation. It works through the 24-volt control circuit. The relay or contactor is nothing more than a switch that turns on a motor or compressor. Control Circuits for Air Conditioning and Heating - How It All Works If a relay or contactor needs to be replaced it needs to be replaced with an exact match relay or contactor. If a relay with a 5 amp rating for the contacts is used to control a motor with a 10 amp rating then the contacts will burn out and the relay or contactor will be useless. One is the voltage that controls the coil on the relay or contactor and the other is the amp rating of the contacts.Ī relay or contactor with a 24-volt coil will blow out if 120 volts is used on the coil. Relays and contactors have a few very important ratings on them. They are both normally controlled by 24 volts and they typically pass line voltage to a motor or compressor.
Furthermore, contactors and relays work in much the same way in a typical HVAC control circuit. Relays and contactors are used in HVAC control circuits to turn things on and off such as a blower motor, condenser fan motor, or compressor. Resource Link: Ohms Law and HVAC | Heating and Cooling Technical Furthermore, in case there is a dead short in the control circuit the fuse will prevent the transformer from blowing. Additionally, it is important that an inline fuse is placed on the hot leg or 24V hot side of the transformer. If the VA rating on the transformer is exceeded by the number of loads in the system then the control circuit will not work properly and the transformer will fail. The sum of these VA ratings for all the loads in the circuit cannot exceed the VA rating on the transformer. Control transformers always come with a VA rating and that gives the maximum rating for which the maximum number of loads can be used for that transformer.Įach relay or load in the control system has a VA rating. Some control transformers are multi-tap transformers that can be used on either 120V or 240V to step down the voltage to 24 volts.
Transformers (step-down) can be 120V to 24V, 240V to 24V, and many other combinations. The above points can be fulfilled by understanding the electrical wiring diagram of individual HVAC equipment and of the whole system also.Īlso the HVAC designer will need to know the size of the electrical loads to assess the impact of the heat generated by the electrical system on the HVAC load.Step-down transformers are used for air conditioning and heating systems to step down the voltage from a line voltage to a safer and more efficient voltage for use in the control of the system. Understanding how different HVAC equipment operates in a certain HVAC system.Knowing the size of the HVAC system (equipment types, locations, …),.So, to determine the electrical equipment and power supply required for the HVAC system proper operation, the electrical designer needs: This information designed by the electrical designer will be and must appear on the electrical drawings for proper installation by the electrical contractor. In the detailed design phase, the electrical designer must size and select the wires/cables, conduits, starters, disconnects and switchgear necessary for supplying power and control to HVAC equipment.
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