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We really struggle with the electrical side of fridges and freezers. You always write to the people who fix big plants but we are in the domestic market. Please help.
Hi Albert, let us complete:
Electrical systems, components and some procedures for the domestic market.
Again – watch that R600a and R290 refrigerants are being used in domestic applications.
Capacitor start motor
The capacitor start motor and resistance start motor are practically the same in that once they have been started they operate the same. Therefore, the capacitor, which is connected in series with the starting winding, is used only to modify the starting characteristics of the motor.
With the correct size capacitor, the starting winding's current can be made to lead the running winding's current by 90 degrees instead of 30 degrees as in the case of the resistance start motor.
Since the starting torque of a split phase motor is directly proportional to the angle between the starting and running windings currents, it follows that the starting torque of the capacitor start motor is much greater than that of the resistance start motor.
The capacitor start motor is normally fitted with an electrolytic type capacitor. This type of non-polarised AC capacitor consists of two sheets of aluminium foil separated by an electrolyte. Because these capacitors are designed for intermittent operations only, excessive start-stop operations of the motor or prolonged connection of the capacitor to the supply will cause the capacitor to fail.
The permanent capacitor motor
The permanent capacitor motor is similar to the capacitor start motor except that it is designed to operate with the starting winding and its series capacitor permanently connected to the supply. This motor is also known as a capacitor start capacitor run motor or simply a capacitor motor.
The capacitor motor has no relay because the start and run windings remain connected to the supply. Because the two windings currents are 90 degrees out of phase with each other, the stators magnetic field will be more constant and as a result, the motor will operate smoother and quieter than either the resistance start or capacitor start split phase motors, and it is more efficient.
The type of capacitor used on these motors is of the oil type rather than the electrolytic type used on capacitor start motors. The oil type capacitor is rated for continuous duty. It consists of aluminium sheets separated by oil impregnated paper dielectric; it is also larger in physical size and more expensive than the electrolytic type.
Testing the compressor with relay and overload
The compressor should always be tested with the relay and overload connected into its circuit. It is good practice to connect an ammeter into this circuit as it will immediately indicate if a fault exists.
Switch the electrical supply on while taking note of the ammeter reading. If the compressor circuit is good, the meter should kick over to a current value of between six to 10 times the normal running current and almost immediately drop back to the running current, which will be determined by the size and make of the compressor – usually between one and two amps.
If the compressor is fitted with a solid state relay, the meter will indicate the same on starting as before but take a little longer to drop back, even this should not take more than about 1,5 to two seconds.
When the supply is switched on and the current indicated on the ammeter remains high and does not drop back at all, either the compressor or the relay is faulty. Faults could include the following:
A seized compressor pump or motor.
An open circuit on the starting winding of the motor.
A starting relay that does not close when energised.
If the compressor has been fitted with a solid state relay and the compressor is switched off, always allow at least five minutes before switching on again. This gives the relay time to cool down. Failing this will give the same symptom as having an open circuit on the starting winding.
The thermostat is a device that senses the temperature in a refrigerator or freezer cabinet, and by switching various electrical components on or off maintains the temperature in the cabinet according to the setting of the thermostat.
All new equipment operating on R600a will have the thermostat situated on the outside of the cabinet to prevent any leakage of R600a from been exposed to electric arcs when the contacts open or close.
Thermostats are manufactured having two, three, four and sometimes more terminals. The amount of terminals is dependent on the design of the refrigeration's electrical circuit. It is always good practice to mark the terminal conductors or make a drawing of where they must be connected to the thermostat.
Most modern systems have electronic boards with relays. Thermostats are also manufactured having a manual defrost push button. This button is mounted in the centre of the control knob. When the button is pushed in, the thermostat switches off the compressor, which restarts only once the evaporator has defrosted completely.
Many ‘frost free’ double door refrigerator/freezer units control the temperature in the refrigerator section, not by switching the compressor on and off, but by regulating the amount of cold air circulated between the freezer and refrigerator section. This is accomplished by a flap between the freezer and refrigerator sections that is thermostatically controlled to open or close relative to the temperature in the refrigerator section.
Practically all ‘Frost free refrigerator/freezer’ units have a defrost timer connected into the electrical circuit.
This defrost timer is essentially an electric clock which operates a two-way switch every six, eight, 12 or 24 hours – depending on its design – thereby switching the compressor off and the defrost heating element on for between 15 and 30 minutes.
To test if a defrost timer is operating correctly, or to distinguish between terminals, consider this:
When replacing a defrost timer. If the exact replacement can not be obtained make sure of the following points:
The rated voltage must be the same;
The defrost cycle must be the same. In other words, the time the defrost heater remains on and the compressor remains off; and
The time from one defrost to the next must be the same.
As with the thermostat, electronic boards are found in newer applications.
Interior light and light switch
All refrigerators have a light in the cabinet that comes on when the door is opened and goes off when the door is closed.
The light bulb is usually of the small, round, 15 watt, Medium Edison Screw (MES) or Edison Screw (ES) pearl glass type, and should never be replaced with a larger one, because the extra heat dissipated by it may damage the plastic lamp shade. The light switch is of the plunger type, and is mounted so that the door when opened or closed activates it.
On most refrigerators that have an evaporator fan, the type of light switch used has two sets of contact points. One set is normally closed and the other set normally open.
The normally closed contact points are used to operate the light in the usual way. The normally open contact points are used on the fan circuit to switch the fan on when the door is closed and to switch the fan off when the door is opened.
Always check if the plunger is the correct length. If it is too long it can be cut shorter. If it is too short, glue a piece of plastic to the door so that it can strike the plunger and push it in far enough.
Freezers don't normally have lights in their cabinets, but some manufacturers have fitted lights into the lids of chest type freezers. The light switch is usually of the mercury in a glass tube type, and built into the light fitting. When the freezer lid is opened, the mercury runs to the contact points at the back of the tube, thus closing the light circuit. When the freezer lid is closed, the mercury runs away from the contact points, thus opening the light circuit.
Almost all refrigerators and freezers manufactured in South Africa use a static condenser that is placed at the back of the cabinet. This condenser is cooled by natural air flow over it.
Many imported refrigerators and freezers have their condenser placed next to the compressor at the bottom of the cabinet. Because the condenser is located in a confined space, natural air flow is restricted, and it becomes necessary to cool the condenser by means of a fan.
The fan motor is usually connected to the same terminals as the compressor; therefore it should run or stop in unison with the compressor. If the fan fails to operate, the condenser and compressor will overheat, thus, the cooling efficiency of the refrigerator/freezer will be low, and it could cause permanent damage to the compressor. The fan motor is usually rated between seven and nine watts and is of the shaded pole type with four poles, that rotates at approximately 1 300 RPM.
If the fan motor fails to turn with the compressor, test it for open circuit or bearing failure. Most fan motors are not worth repairing and should be replaced when found to be faulty. Make sure the replacement fan motor turns in the same direction as the one it replaced.
All ‘frost free’ refrigerator/freezers use a fan to circulate air over the evaporator and through the cabinet. Some of the cold air is also circulated through the refrigerator section by this fan.
To prevent the fan from blowing cold air out of the cabinet, a switch is fitted to stop the fan as soon as the door is opened. This switch may be separated from the interior light switch or it may be part of the light switch. The fan will not operate when the refrigerator is on a defrost cycle, and on some models it will also not run when the thermostat has switched off. However, it should run whenever the compressor is running, except when the refrigerator or freezer door is opened.
If the fan motor fails to run when the compressor is running, check to see if the door switches are operational and that their plungers are not broken off. If the fault is not at the switches, remove the cover over the fan and, disconnect the conductors and test the coil of the fan for an open circuit using an ohmmeter. Also check that the rotor is free and the bearings are not worn. If it is necessary to replace the fan motor make sure the replacement rotates in the same direction and is rated at the same voltage and wattage as the original. Most fan blades are made of plastic and push on to the motor shaft. Care must be taken to ensure the fan blade is a tight fit on the shaft.
Mullion heaters are very low wattage electrical elements of the flexible silicon insulated type. They are usually placed in contact with the metal face of the cabinet where the rubber door-seal touches the cabinet. The purpose of this heater is to raise the temperature of the metal where it is placed to a point where condensation will not form.
Case and liner heaters
Some refrigerator/freezer units have case and liner heaters connected in parallel with the Mullion heater. These heaters are also of the low wattage flexible type and are placed at points to prevent condensation on the case and liner of the cabinet. Some refrigerator/freezer units have a ‘HiLo’ humidity switch, to switch some of the heaters on when the humidity is high and off when the humidity is low. To replace faulty Mullion case and liner heater elements, it is necessary to remove the breaker strips around the cabinet. However, the breaker strips on most modern refrigerator units cannot be removed because they form part of the liner and it is impossible to replace the heaters. Therefore, in most cases the heaters are disconnected and ignored.
Faulty Mullion, case and liner heaters will not affect the cooling of the refrigerator/freezer unit at all, but condensation – sweating – around the freezer section could be quite bad.
Note: Instead of heating elements some manufacturers make use of a ‘hot-gas’ pipe around the face of the cabinet. This ‘hot-gas’ pipe is known as a ‘throat heater’.
Defrost heaters are also low wattage electrical elements of the flexible silicon insulated type, usually known just as ‘flex heaters’. They are glued directly to the evaporators and are operated by defrost timers or thermostats.
Some units use heater elements as illustrated below:
Faulty defrost heaters will not affect the cooling of the refrigerator/freezer unit at all but there will be a build up of frost on the evaporator. A faulty defrost heater can be replaced by peeling it off the evaporator and re-gluing the replacement heater in its place.
Albert, I hope this helps with your domestic electrical work.
Thanks to everybody for the overwhelming response. I receive on average, over 60 questions a month and cannot publish all of them. But keep them coming, as I may answer you directly.
Looking forward to hearing from you.