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HVAC&R CONTROLSbyJayson Goff, CM12/12/2012
OBJECTIVESUnderstand the operation of relays and contactorsExplain how to test relays and contactorsLearn the selection and application of transformersFind out how to troubleshoot pressure controlsUnderstand the operation of solenoids
Basic Relay Parts
Common Relay FaultsStuck armatureShorted coilOpen coilBurned or welded contacts
Testing the Contacts
Transformer TypesBoost-and-buck transformersControl transformersAuto-transformersHigh-current secondary transformersBoosting transformerBucking transformer
Setting the Low-Pressure ControlStep 1. Set HIGH EVENT by adjusting range screw.All-Range Controls: Turn screw clockwise to raise high event.Micro-Set Controls: Turn screw clockwise to lower CUT IN setpoint.Step 2. Adjust the differential screw.All-Range Controls: turning the differential screw changes theLOW EVENTTurn screw clockwise to raise low eventMicro-Set Controls: Turning the differential screw changes the differential setting.Turn screw clockwise to increase differentialDifferentialScrewRange Screw
Typical MalfunctionsCoil BurnoutFailure to openFailure to close
HVAC&R CONTROLSEND OF LESSON
This lessons content is found primarily in Technical Institute Manual #2. Some additional information was used from Tech Manual #1 as well as Bulletins published by Sporlan Valve, a division of Parker Valve Company.This figure shows a typical pilot-duty relay. A pilot-duty relay usually controls less than 2 A of current at its contacts. Some relays, however, have more than one set of contacts. As you can see in the figure, there are four basic parts to the relay. The coil of wire becomes magnetized when a voltage or current is applied to it. The armature is connected to the contacts. (Generally, the armature and the contacts are electrically isolated from one another.) The spring keeps the relay in its normally open or normally closed position when no power is applied.Stuck armature. This occurs when the armature (the moveable part of the device) becomes mechanically caught within the housing or frame of the relay.Shorted coil. This occurs when the windings in the coil have either shorted together, or to the frame of the relay.Open coil. This occurs when the wire has broken or has burned off due to excessive voltage being applied to the relay.Burned or welded contacts. This occurs when the relay has cycled too many times, or when too large a current has passed through the contacts, causing the metal points to fuse together.As this illustration shows, in the normal position of the relay-that is, when no power is applied-terminals 1 and 2 are normally closed (N.C.) and terminals 1 and 3 are normally open (N.O.). When power is applied to the coil, these configurations will be reversed. To test for a welded set of points or contacts, first make a diagram of the wiring, then disconnect all the wires. For this relay, use an ohmmeter to measure the resistance between terminals 1 and 2 (with no power applied). Your reading should be 0 . If you measure anything other than 0 , replace the relay. When you measure the resistance between terminals 1 and 3, you should get a reading of infinity ( ). If you get any other reading, replace the relay.A contactor is a heavy-duty power relay with contacts that are usually rated for 15 A or higher. The function of a contactor is to use a relatively small amount of electric power to control the switching of a larger amount of power. Constructed of the same four basic parts as a relay, a contactor can have as few as one set of contacts, or as many as five sets. Contactors are normally open-that is, they do not close until power is applied to the coil. Because a contactor can have more than one set of extra heavy-duty contacts, more power is required to pull in the armature.Contactors can be visually inspected for burned contacts, burned coils, and cracked cases. If you see any of these symptoms, replace the contactor. It is also a good practice to replace the contactor whenever the compressor or motor that the contactor controls has been replaced. A contactor's mating contacts are sometimes referred to as poles (that is, a "three-pole" contactor has three sets of contacts). This figure is a schematic of a typical three-pole contactor. Note that the terminals are marked Tl, T2, T3, and Ll, L2, L3. The "L" represents the line side, and the "T" represents the load side. (The "T" actually stands for the "transformer" side of the system in an older terminology.)Like the coil of a control relay, the coil of a contactor is subject to two faults it may be either open or shorted. With an ohmmeter, test the continuity of the coil. If your ohmmeter displays an infinity reading, the coil is open and you must replace the contactor and/or coil. When you test for a shorted coil, it is doubtful whether you will know the actual resistance reading of the contactor coil. However, if the control circuit fuse has blown, you can assume that the coil is shorted.A magnetic starter is the same thing as a contactor, but with some form of protection built into it. An overload protector is usually a thermally operated switch. As the current passes through the overload protectors, they get hot and open a switch in the starter's control circuit. The full current of the device being controlled is normally passed through them.This figure is a schematic diagram of a three-pole starter that shows how the overloads and the coil are wired. As with all control circuits, they are wired in series. Troubleshooting a magnetic starter is similar to testing a contactor. However, the overloads can create problems if they have been cycled too many times. They begin to lose their current-carrying capabilities, and tend to trip prematurely. The overloads are normally closed switches, and open when the overload element overheats. To test for an open overload, turn off the power, disconnect the switch from the circuit, and take a reading with an ohmmeter.A transformer is a device for transferring electrical energy from one circuit to another at a different voltage. The voltage is changed by means of electromagnetic induction. Transformers are frequently used in control circuits. Their function in such circuits is to step voltage down from line voltage to a lower control circuit voltage. There are no moving parts in a transformer. Its action is determined by its coil windings.
An inadequate transformer may supply abnormally low voltage to the control circuit. This results in improper operation of contactors and/or motor starters. Problems may include chattering or sticking contacts, burned holding coils.or contacts that fail to close properly. Any of these conditions can lead to eventual system failure and possible damage to the compressor. Obviously, it is important to size control transformers correctly.A boost-and-buck transformer has two windings that are tied together in such a way that they act like a transformer with a single tapped winding. This figure shows the schematic for this type of transformer. If you apply 208 V to the input terminals, the voltage at the output terminals will be 240 V. This type of transformer ispopular for applications in which small compressors are run at low voltages, and the voltage needs to be "boosted." Of course, if a lower voltage were needed, the transformer could be connected as shown in this illustration, with an input voltage of 240 V and a resultant output voltage of 208 V. This transformer's "bucking" operation is caused by the counter electromotive force (CEMF) developed in the windings.
A control transformer has several different applications in the HVAC/R industry. The upper figure shows what the schematic connections look like for a standard control transformer. The most common application for this type of transformer is to reduce the line voltage ( 440 V or 220 V) to either 220 V or 120 V. This is especially important where cost is a factor. Since it must handle greater currents, this type of transformer is rated in kilovolt-amperes (kVA). The lower figure shows how a control transformer can be connected for a 440-V primary and a 220-V secondary. The terminals marked "H" are usually the primary windings, located on the high-voltage side of the transformer. The terminals marked "X" are usually the secondary windings, located on the low-voltage side of the transformer.
An autotransformer is similar to the boost-and-buck transformer in that the primary and secondary do not appear to be separate and distinct windings. The primary difference is that an autotransformer has a variable output. Note in this illustration that a single coil is "tapped" to produce the electrical equivalent of a primary and secondary winding. Neither autotransformers nor boost-and-buck transformers reverse the phase, as transformers with separate (primary and secondary) windings do. The movable tap can be used with the fixed winding to select an output voltage that can be adjusted from 0 V to a voltage above the input voltage.
Extremely high currents at low voltages can be achieved with a high-current secondary transformer, a type of transformer in which the secondary winding is wound with heavy wire and just a few turns. There are many applications for high-current secondary transformers, including welding equipment and soldering guns. This figure is a representation of a typical high-current secondary transformer.The most important elements of a system are the components that control the flow of refrigerant in accordance with the requirements of the refrigerat