In Chapter 4 different types of motors were discussed. The centrifugal switch was discussed to remove starting components from the circuit when the motor reaches a specific speed. This method cannot be used in a hermetic compressor. Specially designed relays were designed to remove the starting components electrically from the circuit when the compressor reaches almost full speed.
All relays, with the exception of electronic ones, have an armature of iron core with a conductor wrapped around it with a specific amount of turns. When electricity flows through the wire, a magnetic field is developed in the iron core. This is the way a switching action is accomplished. The current relay is designed to activate upon high current demand. The switch is in the open position when no current is applied, this is n.o. (normally open). When a high current is placed through the windings, the switch immediately closes allowing the starting components to be placed into the circuit with the starting windings. As the speed of the compressor drive motor increases, the current drops. This allows a small spring to apply pressure on the moveable contacts and open the switch. The one disadvantage of this type of relay is the current surge across the contacts causes excessive arcing that deteriorates the contacts. These relays are rated by current values which differ with the horsepower of the compressor to be controlled. Figure 5-1 shows the typical terminal layout. The internal parts of this relay are shown in Fig. 5-2.
Fig. 5-1. Typical relay with terminal arrangement. This unit has dpdt (double-pole, double-throw switching action).
Fig. 5-2. Simple workings of a single-pole, single-throw relay.
This relay is similar in appearance and construction to the current relay. The main difference is that the contacts are normally closed thus eliminating much of the arcing that is exhibited by the current relay. This is accomplished by using a coil-wound armature that is magnetized when high voltage is applied. The coil is series-wired to the common winding of the compressor motor. As the compressor starts, supply voltage drops. This condition permits the contacts to remain closed leaving the start capacitor electrically connected in the circuit. When the compressor reaches about 60% of its full speed the voltage also rises. This higher voltage causes the armature to open the contacts that remove the starting capacitor electrically from the circuit. These two relays look alike at first glance. When replacement is needed, examine them very carefully to determine which type relay is needed. Figure 5-3 shows how the potential relay is wired into a circuit.
Fig. 5-3. Wiring schematic of a potential relay in a circuit.
This relay works on an entirely different principle. Note Fig. 5-4 and you will immediately see the difference. The hot wire relay has bimetal contacts. Located close to the bimetal is a chrome nickel high resistance wire. Without an applied voltage, the contacts remain in a closed position. When voltage is applied, it flows through the chrome nickel wire causing it to heat. It is designed to each maximum heat when the compressor reaches full operating speed, at which time the first set of bimetal contacts open. When these contacts open, the starting capacitor is removed electrically from the circuit. If the compressor is not operating properly and causing an over-heating condition, a second set of bimetal contacts opens and causes the compressor to stop. With the compressor shut down it has time to cool. This condition also allows the bimetal contacts to cool and reset in approximately four minutes. This type of starting relay also gives the additional protection of the thermo-overload.
Fig. 5-4. Wiring schematic of a hot-wire relay.
Some homeowners might be scratching their heads at this point as to the meaning of bimetal. Let me clear up the mystery of the meaning of this word. Different types of metals expand (stretch) and and contract (shrink) with heat and cooling at different rates. For example, at the same temperature copper wire and aluminum wire expand at different rates. If the two wires were made into flat pieces of metal and bonded together, you would have bimetal strip, made of two dissimilar pieces of metal. With one side expanding at a faster rate than the other side, the strip will bend. This is how the switch action is accomplished with a bimetal switch. This is exactly how a klixon motor overload operates. The factor that determines when the switch actuates is the heater. The resistance factor in the material used for the heater determines the amount of heat generated. The material will allow electricity to flow until the current causes a heating effect exceeding the designed amount. This action is similar to what happens to a fuse when its rating is exceeded.
The relays discussed in this chapter were designed for the specific job of starting a compressor motor. They provide an automatic way to place a start capacitor into a circuit electrically and remove it the same way. There are many different types of relays, but they are all switches that are activated by an electro-magnetic force, or by heat. Most relays are designed to carry very light current loads. They average from a few amps up to about 15 amp loads. Many times in a circuit, a relay actuates a device that is capable of conducting a larger ampacity load. The holding coils of the relays can be wired to conduct 24, 120 or 240 volts. When using a relay, be sure you know the operating voltage in the circuit where it is to be installed.
Contactors are used as electrical switches that can conduct high ampacity across their contacts. The larger the load becomes, the larger the physical size of the contactor becomes. The contactor can also be wired with a holding coil that will operate with 24, 115, 230 and sometimes higher voltages. In Fig. 5-5 a relay is shown; compare with the contactor shown in Fig. 5-6 and you will definitely be able to see the difference in their construction and size.
Fig. 5-5. A common relay.
Fig. 5-6. Contactor. 62
Contactors and relays are electrically rated the same way. First is the ampacity rating, which is the amount of amperage each set of contacts can conduct safely without causing damage to themselves. Some have silver-plated contacts that begin to melt when load is exceeded. For this reason stay below the rated amperage for the load it is to carry. Second, is the voltage rating for the coil; this is the amount of voltage that has to be supplied to the armature in order to set up a magnetic field. Third, the number of circuits that can be wired through the contacts. In the case of a contactor, you might have a single-pole, double-pole, three-pole, or a four-pole switch. In a relay the same rule applies. This is how an abbreviation might be shown on a relay, d.p.s.t.n.o. (double-pole, single-throw, normally-open). The position of the contacts are given in the de-energized position of the relay. It is either n.o. (normally open) or n.c. (normally closed). The number of poles on the relay will depend upon how many functions you want the relay to perform.
In most residential systems you will only be dealing with line voltage which can be 120 volts and/or 240 volts with a control voltage of 24 volts. In commercial units voltage can go higher in both the line and control circuits.
