RAC Electrical System
Mechanical component of the system needs external energy in order to operate. Compressor piston can never push and pull vapor refrigerant if without the electric motor connected to the compressor shaft. Fan blades will never rotate and draw air in the absence of electrical power in the fan motor. Solenoid that closes and opens refrigerant line uses electrical energy as well, so as relays and other devices using magnetism to activate.
In some units using compressor driven by external engine, electric motor is optional or sometimes no longer used to drive the mechanical parts. Working with electricity in the system needs extra care and safety consciousness. It is advisable and recommended that safety practices must always be integrated in every task that is about to be done. It requires analysis in order to trace the flow of current based on the function of every component involved and the sequence of its operation based on the units electrical circuit design.
Some units are using timers, time delay, relays and other automatic switches that respond to a certain condition in the system in order to activate. Electrical components in the system can be categorized into three based on their function. Electric motor inside the compressor housing that drives the compressor shaft and the electric motor that drives the fan blade in the condenser and evaporator are called load.
The main purpose of any electrical circuit is to carry the flow of current to the load to perform its function. If this will be altered by any means (open circuit), the magnetic energy that causes the rotor of the motor to rotate will be also disturbed and the mechanical function could not be performed. In order for the load to accomplish its role in the system, electrical circuit must be completely secured.
Single Phase Motor
This electric motor uses line-to-line voltage power supply. It has two terminals line 1 and line 2 to where the current flows. Single-phase motors are commonly used on evaporator and condenser fan motors in which capacitors are used. Domestic refrigeration systems also using hermetic motor compressors are using single-phase electric motors.
3 Phase Motors
Larger refrigeration and air conditioning systems rarely used single phase motors due to the fact that mechanical components inside the compressor is a lot heavier than those of domestic units. Three-phase motor has 3 electrical winding inside that could be interchangeable. It has no polarity, current flows directly to the winding from 3-phase source without any need of some devices like capacitors and relays.
Motor Starting Device
Motor starting devices are essential part of the circuit to supply power and aid single-phase electric motors to start. Single-phase motor has 2 electrical windings in its stator, the running and starting winding. When motor compressor is at rest, static pressure in the system serve as additional load for the motor to break. This requires extra power to start the operation. Motor starting devices such as capacitors and relays has played a very vital role in this condition to aid the electric motor in its starting activity. As the motor rotates and reaches 75 % of its normal operating speed, starting winding will be deactivated and let the running winding do the job.
Capacitor is a device that stores electrical energy used to start single-phase motors. It has two terminals in which starting and running terminal of the compressor is connected. As the current enters the capacitor from the line 2 of the source, the stored energy will be released into the starting terminal of the compressor, activating the starting winding. Capacitor has different capacity measured in Micro Farad depending on the electric motor where it is being used.
Fan capacitor is used to aid electric motors that drives condenser and evaporator blower in most of refrigeration and air conditioning units. Its capacity also depends on the motor horsepower where it is being used. Fan capacitor has smaller capacity than running capacitors.
Running capacitor is used to start motor compressor using Permanent Split Capacitor motor circuit. Some motors are using additional devices in the motor starting circuit to satisfy its need upon start up but the principle of operation of motor start up using capacitors is one.
Starting capacitor is used on larger capacity hermetic motor compressor using Capacitor Start Induction Run starting circuit, in which potential relay is also used to aid the running capacitor in turning the rotor. Starting capacitor has much larger capacity than running capacitor in the circuit. Its operation is controlled by the potential relay.
All capacitors mentioned above are mono capacitors that have 2 groups of terminal for starting and running.
Dual capacitor is a combination of fan and run capacitor that drives both compressor and fan motor. Since fan and run capacitor is common among window type air conditioning units, some designers integrate the two devices into one casing serving both compressor and fan motor. It has 3 groups of terminals; one for the auxiliary terminal of the fan motor indicated with the word “fan” and the other is for the starting terminal of the compressor having “herm” marking. The center is for both running terminal of the compressor and common terminal of the fan motor.
Relays are used to start some split phase motors usually in smaller capacity. This is also a magnetically operated device that throws current in split seconds to ignite starting winding of the compressor. When the motor runs, relay disconnects the supply going to the starting winding. Most domestic refrigerators with smaller capacity uses relay instead of capacitor. Relays are available in many designs; there are current relays that are electronically operated called IC relay. Some relays however are used for some purposes in the circuit; it is also used to switch direction of current flow to deactivate some devices while activating the others like in defrosting mode.
Potential relay is used on some larger hermetic motors having CSIR circuit to aid running capacitor to start the motor. CSIR circuit uses also starting capacitor connected in series to terminal 1 of the potential relay going to the running terminal of the running capacitor.
Time Delay Relay
The purpose of this device is just to delay the compressor from running minutes away from the condenser motor and evaporator. In any refrigeration and air conditioning system, evaporator and condenser motor must run first before the compressor. If both has reach their operating speed, that is the point where compressor must start. Time delay relay has also magnetic coil that runs its small mechanical gear inside. When the contact time is set for 3 minutes after the condenser has started. Time delay will close the circuit going to the magnetic contactor coil of the compressor within 3 minutes initiating contact between load side and power supply line.
Magnetic starter is a manually operated device to stop and run electric motors. It has a push to start and push to stop button to open and close electrical circuit.
This is the most important device in the electrical system. Magnetic contactor separates the load side from the power supply when the compressor or the motor is not running. This is the control point where the flow of current going to the load is altered by opening the contact point. This is normally open device having magnetic coil supplied by the same power from the line (Some contactors are using step down transformers for its magnetic coil). When magnetic coil is energized, it pulls the armature and initiate contact between line contact points allowing the current to flow directly to the load or motor winding. All safety control devices in the circuit are connected in series with the contactor magnetic coil supply. When any of those devices opens, in response to various critical condition, the supply going to the magnetic coil of the contactor also open, deactivating it and releasing the armature, opening the power line going to the load that stops the motor.
Control devices are used to close and open the electrical circuit to stop and run the load in due time. Some control devices also controls the function of other parts that controls the motor and other magnetically operated components. Control devices are classified into two, the cycling control and safety control devices.
Safety Control Devices
These components are used to keep safe the system against critical condition. Critical condition refers to a certain time that the unit undergoes abnormalities of some attribute such as pressure, voltage, current and temperature beyond its capacity to withstand and may cause damage to some major components specially the compressor. When voltage suddenly increases due to some factors, it creates high amount of current flowing to the circuit going to the load, or the motor winding.
The sudden increase in current flow creates heat to the winding of the motor that could cause insulation to melt and burn. Aside from power abnormalities, when mechanical parts driven by the electric motor creates resistance to the force exerted by the motor, it causes the motor to draw more current flow that may also start heat to develop as current increases. This leads to burning of the motor winding and some other devices.
There are also some conditions in refrigeration system that pressure and temperature goes beyond normal that would cause damage to the system if no attention and necessary action will be given right away. Because of the above situation one action that would save the electric motor and the whole system abruptly is to cut the circuit to stop the electric motor from drawing current and the whole operation. It is the role of safety control device
This device closes and opens the circuit in respond to increase of current flowing to the electric motor. When there is a resistance to the rotating action of the motor due to some mechanical defects, poor lubrication of moving parts, unnecessary increase of pressure due to internal or external causes, motor winding will draw more current from the circuit in order to drive the load. Increase in current flow also increases the temperature of the conductor. Before temperature reaches to critical point overload protector opens the circuit in respond to the temperature increase. This device uses bi metal strip that bends when heat is sensed to open the contact and stop the flow of current. When temperature of the conductor and the current flow gets back to normal, contact will be closed again.
The principles of operation among all electrical safety devices are almost similar in which abnormal build up of heat in the conductor is monitored. Circuit breaker is used to cut the flow of current from the power line when there is a short circuit. Short circuit is an occurrence in which negative and positive line of current gets in contact with each other that may cause fire and severe damage to the system and even the building. When short circuit occurs, the positive and negative electrons that must flow in two different paths combined together. This causes extreme and sudden build up of heat beyond the capacity of any conductor. Before fire occurs, circuit breaker reacts with the heat that flows to the circuit and bends its contact to stop the flow of current from the source. This device is manually operated to turn on.
Fuse is also protective device against high current flow that may damage some components, it could be used to protect the whole circuit or for some other devices with in the circuit. All safety devices have their corresponding amperage rating that identifies their capacity and application. 30 amperes fuse and above are used to protect the power line in the absence of circuit breaker based on the total current load that must be lesser. There are also fuses available ranging from 20 amperes and below that is used to protect some devices within the circuit.
These are automatic electrical switches that close and open the circuit in respond to abnormal pressure build up in the system. As the refrigeration system runs, excessive pressure might be developed in the system due to a lot of unexpected condition. This condition may harm the electric motor winding that drives the compressor. Before damage occurs, high-pressure switch open the circuit to stop the whole operation. Low-pressure switch is also used to prevent the compressor from running if the pressure of the low side is not enough due to the absence of refrigerant. This may cause oil pumping on the discharge line going to the whole system if not being prevented.
Oil Failure Safety Switch
This device is used to maintain the operation of the compressor under normal oil pressure. Compressor is a mechanical device driven by electric motor. Every moving part needs lubrication to minimize wear and tear as two surfaces rub each other. As the electric motor rotates and drives the compressor, it also drives the oil pump and pushes oil to the moving metals. If the passage of the oil is blocked normal oil pressure will never be attained, poor oil pressure means poor lubrication and may damage the compressor. In this condition, oil failure safety switch opens the circuit to stop the flow of current to the electric motor driving the compressor and stop the whole operation.
Water Flow Switches
These safety devices is used on package type units using water cooled condenser and flooded type evaporator. This is to ensure that the compressor will be running only if the water flowing inside the condenser and evaporator has reached its desired level to efficiently allow heat transfer. Water is driven by external pump to circulate, if the water pump fails, or when water runs out of supply or by any means, the required water level and flow is not maintained, compressor stops from running to prevent damage. Continues operation of the compressor without having enough water flow may cause the chilled water to freeze inside the evaporator shell. This causes poor heat absorption and eventually breaks the water pipe when extreme cold temperature occurs as it directly exposed with liquid refrigerant. Chilled water flow switch is used to stop the compressor when critical condition happens in the evaporator, while condenser water flow switch is used to protect the compressor when critical condition happens in the condenser.
Cycling Control Device
In any refrigeration and air conditioning system, the ultimate purpose is to lower the temperature of the certain room or compartment into a certain specified temperature set point. Room air conditioner cools the room where human body becomes comfortable which is 15C to 20C. Meat when refrigerated should undergo temperature lower than -18 C, ice cream must reach –25C, banana is 13C, egg is 5C, and most of the fruits and vegetables needs only temperature between 5-10 C or called chilled grade products.
When this temperature is attained after hours of operating, that is the point where the unit must rest, to minimize power consumption and to maximize the lifespan of the unit. Cycling control devices are responsible for this operation. When set point temperature is reached the circuit will opened to stop the whole operation. When temperature rises above set point, the unit starts again automatically to maintain the temperature. Cycling control device is usually dependent on temperature.
There are also times that the unit must run in defrost mode to eliminate the build up of ice in the evaporator since it could serve as insulation from heat transfer to occur efficiently. During defrost mode the compressor stops and the heater activates except those units that are using hot gas defrost system, after defrosting the unit must run again in cooling mode.
This is electrically operated device that closes and opens the refrigerant lines. In automatic pump down system, in which refrigerant is collected to the liquid line during off cycle to avoid liquid refrigerant from entering to the compressor when the system runs again, solenoid valve serves as both control and safety device.
When the system reaches its set point temperature solenoid valve and low-pressure switch must be deactivated in order to completely stop the compressor from running. Closing the liquid line creates low pressure on low side as the compressor continuously collects refrigerant to the liquid line. When low side is empty low-pressure switch cuts off the line and stops the compressor. When temperature of the compartment rises again thermostat sends power to solenoid valve, the solenoid valve opens and allow some liquid refrigerant to flow into metering device. As the refrigerant enters the low side it builds up pressure, low-pressure switch will now react with the pressure build up and closes the circuit to run the compressor.
Thermostat is an electrical control device that is dependent on temperature. Its operation is designed to respond to temperature change. It uses a sensor containing highly volatile fluid that reacts with heat. Sensor is extended to the refrigerated compartment or room via capillary tube. As heat is sensed by the volatile fluid inside the sensor, it evaporates increasing its pressure, increase of pressure causes its bellows to expand, as bellows expand, contact point closes allowing current to flow. When sensor senses no heat bellows will contract and open the circuit. Thermostat and thermostatic expansion valve has similar principles of operation.
Timer activates based on the length of time the compressor operates. During power interruption timer also stops. It is used to run and stop the compressor based on the interval of defrost mode and cooling mode. When defrosting should occur every after 6 hours of compressor operation, timer accomplishes it and starts the system again in cooling mode after defrosting.
Manual switch is the most accessible device to turn off and on the unit, manual switch controls the operation of the thermostat and other safety devices connected in series that supplies power to the magnetic contactors of the motor. Before all other electrical devices must activate, manual switch or remote control switch should be turn on.