Electrical Components of Shop Unit

Electrical for HVACs

Southern Technical College

Abstract
A working overview of our Goodman Package Units located in the lab. The overview will show comprehensive knowledge of the electrical and electronic components associated with our lab equipment.
Electrical for HVACs

The electrical systems of an air conditioning unit are easier to understand if a technician breaks the complex schematic into individual circuits. This will provide an efficient method for diagnosing and repairing systems. The narrative covers the major loads in the system which is the compressor, compressor fan, and evaporator fan.
On heat pump units, when the room thermostat set to the cooling mode, 24Vac is supplied to terminal “O” of the defrost board and to the reversing valve. As long as the thermostat is set for cooling, the reversing valve will be in the energized position for cooling. The constant 24Vac at the “O” terminal of the defrost board signals the board that the unit is in cooling. On a demand for cooling, the room thermostat energizes “Y” and “G”. This supplies 24Vac to terminal “Y” of the defrost board, the compressor contactor and the “G” terminal in the unit. The compressor starts in the cooling mode and the indoor blower motor starts. The contactor closes and turns on the compressor and condenser fan. When the cooling demand is satisfied, the room thermostat removes the 24Vac from “Y” and “G”. The contactor opens and turns off the compressor and condenser fan. The indoor blower will stop after the fan off delay. If the room thermostat fan selector switch should be set to the "on" position then the indoor blower would run continuous rather than cycling with the compressor.
On heat pump units, when the room thermostat is set to the heating mode, the reversing valve is not energized. This is an indication to the defrost board that the unit is in the heating mode. As long as the thermostat is set for heating, the reversing valve will be in the de-energized position for heating except during a defrost cycle. On a demand for first stage heat with heat pump units, the room thermostat energizes “Y” and “G”. This supplies 24Vac to terminal “Y” of the defrost board, the compressor contactor and the “G” terminal in the unit. The compressor starts in the heating mode and the indoor blower motor starts. When 24Vac is present at terminal “Y” of the defrost board during the heating mode, the board accumulates compressor run time. When the first stage heat demand “Y” is satisfied, the room thermostat will remove the 24Vac from “Y” and “G”. The compressor turns off and the the indoor blower will stop after the fan off delay. The defrost board will store the compressor’s accumulated run time in memory.
The contactor and other relay holding coils are wired into the low (24v) or line voltage circuits. When the control circuit is energized the coil pulls in the normally open contacts or opens the normally closed contacts. When the coil is de-energized, springs return the contacts to their normal position.
The closed contactor supplies 120v to the “Run” 120v to the common on the dual capacitor. The “hermetic” side of the capacitor is attached to the start winding of the compressor. A potential or voltage type relay is used to take the start capacitor out of the circuit once the motor comes up to speed. This type of relay is position sensitive. The normally closed contacts are wired in series with the start capacitor and the relay holding coil is wired parallel with the start winding. As the motor starts and comes up to speed, the increase in voltage across the start winding will energize the start relay holding coil and open the contacts to the start capacitor.
Concurrently with compressor start the compressor fan (CM) motor energizes. The CM receives one leg of power from the contactor and has two leads attached to the dual capacitor, one on the common and one attached to the fan side for start assistance. The control boards in our system for this discussion have been bypassed by wiring the components up directly. They essentially would control motors speeds, defrost cycle timing, fan delay, and include control for low and high pressure switching.
The evaporator fan is wired to the contactor, and its run capacitor. The fans purple lead is attached to the motor and that contact is tied to the contactor. The brown lead is also attached to the capacitor.
A run capacitor is wired across the auxiliary and main windings of a single phase permanent split capacitor motor. The capacitors primary function is to reduce the line current while greatly improving the torque characteristics of a motor. This is accomplished by using the 90° phase relationship between the capacitor current and voltage in conjunction with the motor windings so that the motor will give two phase operation when connected to a single phase circuit. The capacitor also reduces the line current to the motor by improving the power factor. The technique we have learned is to bypass the majority of the control circuits which simply are a group of switches that control all the system parameters. The major loads operation can then be accurately be observed. This allows technicians to start the process of isolated defective components and wiring.