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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.




There are two types of R4795’s.  The R4795A is recycling and the R4795D is non-recycling.  (Honeywell has informed us that the R4795D will no longer be available by about the end of 1995.  (Note: R4795A are still available as of 5-5-00.)  The R4795’s were superseded by the R7795 series (discussed later) and now the RM7895 series is the latest upgrade. 
You may be asked how long purge timing should be for the plug-in purge card.  Purges are supposed to be long enough to make four air changes before lighting the burner.  If you knew the CFM rating of the blower, and the volume of air in the boiler and chimney, you could calculate the purge timing needed to charge the air four times.  Since most of the R4795’s will be replacements, use the same timing as the existing card. 
Low voltage controllers cannot be used with R4795’s.  The T & T terminals on an RA890 are now 6 & 7, where an airflow switch is connected.
Purge timing does not start counting until the airflow switch closes these contacts.  Once the purge has timed out, the lighting sequence is the same as the RA890’s.  The amplifier circuit is energized during purge so we have safe start check.  If a flame simulating condition is present during purge, the flame relay coil, 2K, will energize preventing ignition, but the burner motor will continue to run.  This will give continuous purge, a “safe” failure condition.  The relay will not “lock-out”.  If the flame simulating condition, or real flame, goes out, the start-up will proceed.  If a purge card fails or is not installed correctly, the burner motor, on a call for heat, will run but pre-purge cannot be completed so ignition cannot occur resulting in a continuous purge.
If the airflow switch doesn’t close, or opens during pre-purge, the purge cannot be completed, and once again, the burner motor will run but no ignition can take place.
If the airflow switch opens during the run period, terminals 3, 4, and 5 will be de-energized, dropping out the main valve, pilot valve, and ignition.  Terminal 8 will remain energized so the burner motor will continue to run.  If the airflow switch closes, the purge timing will start and the start-up sequence will begin again.  Note that no lockouts have occurred which have to be manually reset.  Lockout requiring manual reset happens when no flame is detected after purge.  Flame relay, 2K, will not energize and the safety switch will heat and lockout the control in about 15 seconds. If there is a flame failure during run, terminals 3, 4, and 5 are de-energized; pilot, ignition, and main valve.  If airflow is still proven, an R4795A will begin purge timing and attempt to re-light.  It will make only one try.  An R4795D will not recycle.  An R4795D will lockout on flame failure during run.
An R4795D differs from the A series in safe start check.  If a flame is detected during pre-purge (2K relay energizes), the purge will stop and safety lock-out will occur in about 15 seconds—the time it takes the safety switch to heat up.  These two things are the only differences between R4795A and D.
The next upgrade of the R4795’s was the R7795 series.  The R7795 series used more solid-state technology.  The R7795’s still used plug-in purge timers, ST795A’s, but the amplifier is not plug-in or interchangeable.  Therefore, an R7795 has to be selected with the correct amplifier to match the scanner.  R7795A’s are used with UV detectors and B’s are flame rectification.  A’s and B’s are intermittent pilot models.  R7795C’s and D’s are interrupted pilot models, the C’s for UV detectors, the D’s with flame rectification detectors.  R7795’s require a Q795 sub-base.  Their operation is the same as the R4795’s.
In light of the RM7800 series, do not upgrade a customer from an R4795 to an R7795.  Always upgrade to the 7800 series.  Honeywell is only keeping the R7795 available due to O.E.M. demand.  To an O.E.M., the R7795 is less expensive than the 7800 series and OEMs are very, very price conscious.  With the demise of the R4795 series, the 7800 series will be the service industry’s control of choice.
To select an RM7895 system to replace an R4795 system, some decisions have to be made.  All R4795’s were intermittent pilot.  We can now choose intermittent pilot, the RM7895A or B, or interrupted pilot, the RM7895C or D.  Intermittent pilot means the pilot is on during the run period.  Interrupted pilot means the pilot is shut off during the run period.  All RM7895’s have an initiate sequence that lasts at least 10 seconds on initial powering of the relay.  During this ten seconds, the relay is checking that the line voltage is within 132 VAC and 102 VAC and line frequency is within plus or minus 10%, or 66 HZ and 54 HZ.  If any of these tolerances are not met, the 10 second initiate sequence will go into a hold condition until the tolerances are met, and if not met the RM7895 will lock-out in four minutes.  If, at any time during this hold period the tolerances are met, the 10-second initiate sequence will restart checking voltage and frequency again.
After passing the initiate sequence, the relay goes into stand-by.  Stand-by can be any length of time.  Stand-by simply means the control is waiting for a call for heat.  On a call for heat, terminal 4 is powered; the blower motor and pre-purge begins.  Pre-purge timing is whatever ST7800A plug-in card was selected, from 2 seconds to 30 minutes.  The airflow switch (AFS), installed between terminals 6 and 7, must close within the timing of the short timing purge cards, 2, 7, or 10 seconds, or within 10 seconds for longer timing purge cards.  The purge timing does not start to count until the AFS closes.  Should the AFS not close within the specified time or 10 seconds, whichever is shorter, the control will recycle or lock-out, depending on jumper 3 being intact; recycle or cut; lock-out.
All RM7895’s have three configuration jumpers.  Jumper number 3 is the jumper that governs what happens if there is AFS failure.  If the AFS opens at any time after it has been made, that is in pre-purge, ignition trials, or during run, the RM7895 will recycle if jumper number 3 is left intact or if the jumper is cut the control will lock-out.
All RM7895’s have three jumpers that can be cut or left alone.  They are labeled JR1, JR2, and JR3.  Cutting a jumper enhances the level of safety.  Cutting a jumper never makes the control inoperative!  Jumper number 1 configures the PFEP (Pilot Flame Establishing Period).  Left intact, terminal10 will be powered for 10 seconds, the terminal that the ignition transformer is connected to.  If this jumper is cut, terminal 10 is powered for only 4 seconds.  Jumper number 2 configures the control to be a recycle or lockout control.  If left intact, the control will recycle on flame failure.  If cut, the control will lockout on flame failure.  Just like the RM7890, this jumper must be cut if an amplifier with 3 second flame response timing is used.  Jumper number 3 has been discussed.
The RM7895B and D have a feature the A and C series do not; an air flow switch check.  What this means is that on a call for heat or in stand-by, the control checks for a closed circuit between terminals 6 and 7.  If this circuit is closed, the RM7895B or D will lockout in 2 minutes.  Remember this: In the “old days”, to check R4795 nuisance shutdowns, we often jumped out the AFS for a while to see if bouncing contacts in the AFS were causing the problem.  Obviously, you can’t do this when dealing with the RM7895B or D.
The block diagram, Fig. 6 on page 11 of Honeywell’s form 65-0086 on the RM7895 has an error.  The “Airflow Interlock” is shown as a closed circuit.  It should be shown as an open circuit.  Another error is on the top of page 4.  For the RM7895B under “Flame Establishing Period” “main” it says “yes”.  This should be “no”.  Under “AFSC”, it says “no”.  This should be “yes”.
Attached is the Gordon Piatt diagram that shows the results of converting from the T3 or T4 timer system to the R4795
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