Starting relays in commercial units (unlike residential types) are not always fastened directly to the compressor terminals. They are usually installed in a box next to the capacitors and are wired to the compressor terminals. (See fig. 151) The relays shown in figure 47 are connected directly to the compressor terminals.
There are four types of relays:
1. Current (magnetic) relay
2. Potential (voltage) relay
3. Thermal relay
4. Solid-state relay
1. Current (magnetic) relays. The compressor draws up to 600% more current when it is in its starting stage than what it draws during its normal speed. This causes a high amperage flow into the motor (through the starting relay) when the compressor is energized. The high amperage flowing through the relay winding creates a magnetic field, causing the contacts within the relay to close and supply power to the compressor start winding. As the compressor motor picks up speed, it draws less current. By the time it reaches two-thirds of its normal run speed, the reduction in current flow causes the magnetic field in the relay winding to become weak and then gravity separates the contacts and disconnects power to the compressor start winding. At this point, the compressor run winding takes over and brings the motor to its full running speed. Current-type relays are widely used in residential units.
2. Potential relays are often called voltage relays. These relays are used with larger capacitor-start compressor motors which require more torque to start. They are widely used in commercial refrigeration and appear similar to the current (magnetic) relays but work by voltage fluctuation. As the speed of the motor increases, voltage is increased. This causes the winding in the relay to create more magnetism, forcing its contacts to open, disconnecting the power to the compressor start winding. By the time the motor reaches two-thirds of its normal speed, the contacts within the relay are opened. (The contacts are normally closed.) (See figs. 48 and 49.)
3. Thermal relays. A bimetal-metal strip is installed within the relay. A resistance wire is mounted close to the bimetal strip. As current flows to the compressor start winding through the resistance wire, the resistance wire heats the bimetal strip causing it to flex and open the contacts within the relay, interrupting the flow of current to the compressor start winding. By the time the contacts open, the motor reaches its operating speed. (Its contacts are normally closed.)
4. Solid-state electronic relays. These relays are not sensitive to motor size; therefore, they can be used for a variety of motors from 1/12 to 1/3 hp.
Diodes and triacts are used in their construction.
When the motor reaches a predetermined speed, it automatically disconnects the power supply to the compressor start winding. Solid-state relays are replaced and not repaired. (See fig. 50.)
The best way to test the starting relay is to test other components in the circuit first, such as overload protector, the capacitor, thermostat (or pressure control if used in the system). If everything else checks out good, then replace the relay.
However, if a more sophisticated method is preferred, use an electronic relay tester. They are relatively inexpensive and are sold by many supply houses.
NOTE: Relays are interchangeable provided that one with the same rating is used.
A quicker way of checking the relay that experienced technicians use today is outlined below.