REFRIGERATION AND AIR CONDITIONING Relay & Capacitor Assemblies Pump Down Systems

Relay & Capacitor Assemblies
     Pump Down Systems

Capacitor Assemblies

 -   Relay & Capacitor Assemblies:
Relay and Cap Assemblies are required to start single phase Compressors.  They allow compressors to start up under a load.  They consist of a relay and starting capacitor for compressors that have Induction Run Windings.  However on Capacitor Run Models, a running capacitor is requires.  The relay has a coil and a set of normally closed contacts where the starting capacitor is removed from the circuit after the Compressor has started and has produced enough voltage between the common terminal and the start terminal to energize coil in the relay. This voltage that is produced in all electric motors while running and is called Electro-Motive Force ( EMF).

The starting capacitor is usually the plastic capacitor and is in the circuit for only a second or less This really advances the field inside the motor so it has the torque needed to start.  It is always best to have a Bleed Down Resistor of between 15,000 and 18,000 Ohms of resistance attached. This will expand its life and will also help the relay contacts by removing some of the arch that takes place when the contacts open.  It will always have a higher MFD ( microfarad) reading than the Run Capacitor.

Running Capacitors are usually metal and are wired to the Start and Run terminals all the time. This advances the field inside the motor slightly which in turn reduces the amperage draw lowering the heat produced in the motor.

The failure of any one of these can mimic a bad compressor that fails to start.  It has been my experience that usually the Relay and Starting Capacitor should be replaced first because the Running Capacitor very rarely fails.  Replacing the Relay and Starting Capacitor should always be done before changing the compressor.  For the relatively small expense it is money well spent.

  -    Pump Down Systems:
      Pump Down Systems

Pump Down Systems require a Liquid Line solenoid, a thermostat and, in most cases, a time clock.  In addition, the condensing unit must have a Low Pressure Switch and Receiver Tank. The liquid suction line is wired through the thermostat and time clock so when the box is satisfied or it is time to defrost it drops power to the solenoid causing the system to start a pump down where all the refrigerant is pumped into the condenser and receiver tank as the suction pressure drops.  The low pressure switch shuts down the system.

This design has two advantages:
1.    In the startup mode, the solenoid opens and the low pressure switch starts the compressor up, there is less chance of a Slug of liquid damaging the compressor.
2.    Defrosting the coil is made easier and faster because there is no liquid refrigerant in the evaporator to boil off first before the ice can melt.

Remember, in this type of set up the only things that cuts the compressor off is the low pressure switch and any other safety device such as the Oil Safety Switch.

                                          abo bahaa eddine


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