WATER LEVEL CONTROLS
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PRESSURE SWITCHES & CONTROLS
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COMMMERCIAL HVAC VALVES
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Basic Electrical Controls of Air-Conditioning Units
Basic Electrical Controls of Air-Conditioning Units
Most residential air-conditioning systems come with a minimum of electrical controls to keep the cost down at the OEM level. The typical “builder’s model” condensing unit for a cap tube system will have a one pole contactor and run capacitor, maybe fixed setting automatic reset head pressure control, and what the OEM calls a “loss of charge control”, an automatic reset fixed setting low-pressure control. Many units will not have crankcase heaters, but use “off cycle” heat by utilizing the run capacitor to keep a current flowing in the start winding of the compressor during the off cycle. This keeps the start winding warm and therefore, keeps the oil warm. The capacitor is carefully sized so not enough current is passed to allow the compressor to run. In these instances, you will see a one-pole contactor used, or a two-pole contactor jumped. See Figure 1.
When replacing run capacitors, always use a capacitor with the same MFD rating as the one you are replacing. The MFD rating of a run capacitor is critical. A higher MFD rating will cause a compressor to run faster and a low rating will result in either no start or increased amp draw, resulting in burnout.
The typical “loss of charge” switch can be detrimental to a system. Most systems do not lose their charge all at once. Small leaks are much more prevalent than catastrophic failures that dump the charge of refrigerant. As the charge is slowly lost, the compressor will eventually pull the suction pressure down below the setting of the low-pressure control. When the compressor shuts off, the suction pressure will immediately rise. If the low-pressure control is an automatic reset type, the low-pressure control will close and restart the compressor. Once again, the suction pressure will drop, and the cycle of the compressor going on and off. This will continue until there is no longer a call for cooling (unlikely since there is no cooling going on), or until enough of the charge is lost to keep the low-pressure control below its cut-in setting (depending on the size of the leak, a long time). The compressor will overheat, due to the short cycling, and go off on its internal overload, the most likely scenario. By the time the internal overload reacts, the compressor’s motor windings have been damaged, greatly shortening its life. If the short cycling isn’t noticed and the compressor left to short cycle on its internal overload, it will burn out.
Loss of charge automatic reset low-pressure controls should be changed to manual reset type. This should also be done for high-pressure controls. A high-pressure cutout should be manual reset. Some contractors will change a manual reset to an automatic reset to prevent “the nuisance call just to push the button”. Remember that these controls are safety devices. They operate only if an abnormal condition exists and prevent something else, usually the compressor, from continued operation under the abnormal condition and consequently getting ruined. The manual reset control demands someone’s attention and is a signal that something else is wrong. Correct the reason for excessive head pressure.
Time delays should be installed on all air conditioners to prevent short cycling. They will prevent short cycling and in the “loss of charge control” scenario, help prevent burnout. Two delay schemes can be used. They are “delay on make” or “delay on break.” Either scheme can be set for at least five minutes. One advantage of a “delay on break” is the delay timing starts after the compressor shuts off. Many people listen for the start-up of the unit when they turn the thermostat down. If a “delay on make” is on the unit, they may think something is wrong when the unit doesn’t start-up right away. (Most cap tube units will equalize within the five-minute delay period.
Many contractors have found that installing a solid-state start assist on every unit they put in can prevent a call back. When the weather is hot and everyone is running their air conditioners, the voltage may drop, causing hard start conditions. The solid-state start assists may be the additional boost needed to get the compressor going under low voltage conditions. These start assists wire across the terminals of the run capacitor in parallel. See Figure 2.
The start assist is a Resistive Thermal Device (RTD) with a positive temperature coefficient (PTC). When the contactor closes on a call for cooling, the PTC is below switching temperature. It is at a low resistance level and allows a large current to flow through the start windings. The current flow creates heat in the PTC. A temperature is soon reached (usually in about a second), so that the resistance becomes one thousand times greater than cold start resistance. The current flow drops to a few milliamperes so the PTC is self-regulating, passing just enough current to keep its temperature up thus its resistance high. It will remain in this condition until the voltage is interrupted. It then cools down and its resistance drops. The PTC is sensitive to ambient temperature. Its cool down time can be five minutes or more. If it is not cooled down to about 150°F or less before the next start, it may not be fully effective. This is why they are not used on commercial refrigeration systems that may deliberately short cycle. One additional PTC may be placed in parallel with the first to improve performance.
If more than two PTC’s are required, it is best to put in a “hard start” system. Although often referred to as “hard starts”, PTC’s are not true hard starts. They are start assists. A real hard start system consists of an added start capacitor and relay. See Figure 3.
Hard start components for a particular compressor must be selected from the manufacturer’s information. Each compressor motor manufacturer specifies the correct start capacitor and potential relay. Only he knows the electrical characteristics of his motor, and any other capacitor or relay may result in severe damage to the motor. In an emergency, a start capacitor rated 10% over or 10% under the recommended MFD rating can be substituted, but never substitute a potential relay with different ratings. Potential relays are designed to remove the start capacitor from the circuit at about 85% of rated speed. A varying voltage through the potential relay’s coil does this; then the coil’s magnetic field pulls a set of contacts apart. The calibration for potential relays concerns pickup, dropout, and continuous coil voltage ratings. Any variations from recommended relay specifications will result in a blown capacitor and welded relay at best, and at worst, a burned-out compressor.
A word about capacitors . . . Capacitors have two ratings of interest: the microfarad rating (MFD or UF) and voltage rating (VAC). The voltage rating of a capacitor is the maximum voltage the capacitor can withstand without breaking down and shorting out. Therefore, a voltage rating higher than specified can always be used when selecting a capacitor, run or start. The MFD rating of a run capacitor must not be varied from the one specified. The tolerance of a start capacitor can vary plus or minus 10% from the specified MFD rating without adverse results. Never go greater than 10%. A 20 % tolerance is acceptable for start capacitor values less than 200 MFD. Note that most start capacitors are labeled with a range of microfarad values such as 108 to 130 MFD.
It is perfectly acceptable to achieve the correct MFD rating by parallel wiring capacitors. Parallel wiring capacitors add up each capacitor’s MFD rating, and the voltage rating will be the rating of the lowest voltage rated capacitor. Series wiring of capacitors is almost never used. See Figure 4.
Some manufacturers found that using automatic reset controls and then putting in a lockout circuit was less expensive, just as effective, and more convenient than using manual reset controls. See Figure 5.
The lockout, or reset relay coil (R1) is in parallel with the circuit through the reset relays closed contact (1R1) and any other overloads, such as the high-pressure cutout, low-pressure cutout, etc. When the cooling stat closes, the current flows through the contactor coil and freely through the other overloads. The coil (R1) of the lockout relay is high impedance and not enough current flows in this circuit to activate the relay. Should an overload open, the only path for the current is now through the lockout relay coil, activating it and causing its contacts (1R1) to open. Even if the overload re-closes, the current continues to flow through R1, keeping 1R1 open. The only way the relay can be reset is by killing the power in the circuit. Turning the thermostat up until it opens its contact does this. This de-energizes R1, and 1R1 closes resetting the circuit. (This circuitry is known as the “Scott-Willette Circuit”). The overloads must be automatic reset, since the circuit cannot be remade until their contacts are closed. Since the reset or lockout relay has a special coil, do not substitute any old relay of the same coil voltage for a lockout relay.
Because it can be remotely reset, it is a popular circuit in rooftop units, but a problem with this circuit is it is too easy to reset. Repeated need to reset should clue the owner in that something is wrong and needs correcting. Also, it doesn’t pinpoint the overload that is opening.
Some newer and more expensive residential units will have condenser fan speed control. This is usually an ambient air temperature control that switches the condenser fan from low to high speed as the ambient goes up, usually around 85° to 90°F, when more capacity is needed.
Residential units do not incorporate a low ambient lockout, although it may be a good idea to add one to prevent accidental running of the unit in winter.
Commercial air-conditioning units will usually have more electrical controls than residential units. The number and complexity of these devices will vary according to the tonnage and use of the unit. Fan cycling, solenoid valves, unloader controls, low ambient lockouts, low ambient controls, etc., may be on or can be added to units to meet the needs of each particular installation.
Air-cooled condensing units for air-conditioners should not be run below an ambient air temperature of 60oF unless equipped with some kind of low ambient control system. As mentioned earlier, a low ambient lockout can prevent a unit from accidentally being turned on at low ambient. An A19ABC-74 is an effective, low cost, low ambient lockout.
Commercial air-conditioners with more than one condenser fan are usually equipped with fan cycling controls. These can be temperature or pressure controls. Pressure controls predominate, such as the P70AA-118. Units equipped with fan cycling can be run down to +20° to +30°F ambient, if certain precautions are taken.
Turning off a condenser fan stops airflow through the condenser, but the condenser is still left as a fully effective heat exchanger. If a wind blows through the condenser, it can be as effective a condenser as if the fan was running. Fan cycling also causes sudden large changes in head pressure that has an adverse effect on the TXV. Fan cycling is the most common method of head pressure control because it is the cheapest method. The best low ambient head pressure control method is to flood the condenser with liquid refrigerant. This is the only method that effectively makes the condenser inactive.
(Flooded systems will be discussed in a later Info-Tec, as they are pressure-controlled system, not electrical controls.)
Contactors and starters are found on all condensing units. Starters are contactors with overloads added. Contactors are usually used on units up to 7 1/2 H.P. Units 10 H.P. and above should have starters, although many OEMs will provide only contactors regardless of size. The added protection of a starter’s overloads is well worth the cost to protect an expensive compressor, even if there are other overload devices. Compressor burnout is the catastrophic failure of any refrigeration system, and every precaution should be taken to prevent it!
The power poles of a contactor should be rated at least 50% more than the FLA (Full Load Amps) rating of the compressor, preferably 100% greater.
Again, due to the cost, the OEM will tend to size a contactor almost to the load. For example, a compressor rated 18 FLA may have a 20 AMP inductive load rated contactor as original equipment. When the contactor needs replacing (probably very soon after the unit is put into operation) it should be replaced with at least a 30 AMP rated contactor; a 40 AMP is even better. At the wholesale price level, there is a minimal price difference between a 20 and a 30 AMP contactor. Sometimes physical size will be a consideration and prevent putting in a higher rated contactor, but an effort should be made to put in a reasonably rated one.
Another electric controller found on commercial condensers, usually 7 1/2 H.P. and up, is an oil failure switch. This control senses the net oil pressure of the compressor and stops it, if the oil pressure stays below a minimum set point for a specified length of time. They are a combination time delay and differential pressure control. The time delay feature gives the compressor some time to build up operating oil pressure after start-up. If, for any reason, the oil pressure does not get to the compressor’s manufacturer’s specified oil pressure in his specified time, the switch will shut down the compressor. If during run, oil pressure is lost, the delay period begins and if oil pressure is not restored in time, the compressor will be shut down. Oil failure controls are manual reset.
While crankcase heaters are not an electrical control, they are an electrical component found on most commercial units and some residential units. There is a widely held belief that crankcase heaters are only needed on outdoor compressors in cold ambient temperatures. This is wrong! In fact, the hotter it gets, the more a crankcase heater is needed! Crankcase heaters should be on all compressors to prevent excess refrigerant in the oil. There is always some refrigerant in the oil. That’s desirable as long as the percentage by weight stays below 10%. Above that level, the oil loses its lubrication qualities, and on start-up, excessive wear will occur.
On a cool spring morning of about 50°F, in a compressor without a crankcase heater, on an equalized system using R-22, there would be about 29-30% refrigerant by weight in the oil. On a 75°F day, this figure would be about 35%, and on a 90°F day, we’d have over 60% refrigerant in the oil! By raising the temperature of the oil 25° to 45° F, we drop the amount of refrigerant in the oil to an acceptable level. It’s good practice to use crankcase heaters on all compressors, indoors or outdoors, cold or hot ambient.
Unloaders and the unloader controls are found only on large commercial condensing units that have compressors with unloading capability.
All systems are sized for maximum expected load. Studies show that most systems run at maximum load only about 15% of run time. Running at low load conditions for too long will damage a compressor and ultimately lead to failure. The electrical system employed on large systems to alleviate these problems, associated with low load, is cylinder unloading using pressure controls and solenoid valves. (There are pressure-controlled systems, but we are discussing electrical controls in this issue of Info-Tec.)
When the unloader solenoids are de-energized, the compressor is loaded. To unload the compressor, the solenoids must be energized. Suction pressure controls that “make on pressure drop” control the unloader solenoids. As the suction pressure drops due to low load, the control makes and energizes the unloader solenoid unloading the compressor. (The P70CA-1 closes low, opens high.) Depending on its size, a compressor may be equipped with more than one unloader. Each unloader requires its own unloader control so the unloading can be sequenced as needed by setting the controls at different pressure settings.
Follow manufacturer’s specifications to set pressure switches, if available. If not, Figure 6 on the next page shows a typical example of an R-22 air-conditioning system at standard conditions: 45° F evaporator temperature and 105°F condensing temperature.