Refrigerator Troubleshooting Diagram

Refrigerator Positive Temperature Coefficient

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Many commercial motor compressors have this type of starting device. In most cases the motor is designed with an internal overload protector; this gives better protection against overload conditions since it is more sensitive to the temperature of the motor windings. It is less current sensitive than the external overload protector.

When the compressor starts, resistance in the semiconductor is low and current can pass freely to the start winding. As the current flows it heats the semiconductor (in approximately 2 to 3 seconds). This causes an increase in resistance through the semiconductor, thereby reducing the flow of current; by this time the motor is up to speed. The reduced current flow is sufficient to maintain the heat in the semiconductor and prevent high current flow to the start winding, allowing the compressor to operate on the run winding.

In the event of a starting failure the increased current drawn will also cause a rise in temperature of the motor windings and actuate the overload protector. Obviously current will not flow to the start winding after a starting failure. The semiconductor has to cool before another starting attempt can be made, and there is therefore greater motor protection.

Basic PTC circuit and wiring diagram

Basic PTC circuit and wiring diagram

A further advantage of this type of starting device is that it does not have any moving parts, unlike current and voltage relays. This lessens the risk of failure due to wear and corrosion of contacts. It also makes it more acceptable where stringent demands are made for low noise levels; arcing across switch contacts can affect tape recorders, videos and other domestic and industrial electronic equipment.

The normal cooling period before a restart is 3 to 5 minutes. The protector cut-out temperature is about 140o C or 285o F and the cut-in temperature is about 105o C or 220o F At 140o C the cooling period before the compressor can restart may be as long as 45 minutes, depending upon the ambient temperature.

A basic PTC circuit and wiring diagram is shown in Figure 38.

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