Capacitors are used on equipment with single-phase electric motors powered by alternating current. The five major types of electric motors are:
Shaded-pole motors. Small induction motors use shaded poles for the purpose of starting. They have very low starting torque and are used to drive a fan directly. These motors are available in fractional horsepower sizes.
Split-phase motors. These are induction motors with separate windings for starting. The start windings are removed from the electrical circuit with the use of a centrifugal switch when the full running speed is approached. The split-phase motor still doesn’t have enough starting torque to be used with heavy loads. They are in the fractional horsepower range and may be used to power direct-drive fans or small belted fans.
Capacitor-start motors. These induction motors have a separate start winding with a capacitor (electrical condenser) connected in the start winding for added torque. When the motor approaches full running speed, the start winding is disconnected and the motor operates as a straight induction motor. These motors have a heavy current demand upon starting. They too are sized mostly in fractional horsepowers.
Capacitor-start and run motors. This motor is similar to the capacitor-start motor except that the capacitor and starting windings are designed to remain in the circuit thus eliminating the switch to disconnect the starting windings.
Repulsion induction motors. They were the work horses of the single-phase alternating current line. These motors are high torque and are used to drive larger equipment.
These capacitors are usually cylindrical in shape and have a microfarad rating from 90 to 400. There are variations of this range from time to time. The plastic outer case encloses alternate layers of paper and a dry electrolyte. These capacitors are designed to stay in the circuit momentarily and are removed electrically by means of a transfer switch. This is accomplished with the use of a relay or a centrifugal switch. The start capacitor should remain in the circuit only long enough to get the motor up to two-thirds to three-quarters of its rated rpm. If it is not removed from the electrical circuit at that time, the capacitor would rupture, burst, or explode due to overheating.
The run capacitor is a constant-duty capacitor. This means that it is designed to stay in the circuit electrically while the motor is in operation. The run capacitor is the direct opposite of the start capacitor. The run capacitor has high impedance and low capacitance, for this reason they are usually rated between five and 40 microfarads. Their construction enables them to dissipate heat. With the oil-filled metal body, this capacitor can stay in the circuit without the danger of bursting. There are times when the capacitor will burst, and it will spew oil all over its general location. Figure 4-1 shows a typical run capacitor.
Fig. 4-1. Typical run capacitor.
When selecting the voltage ratings of motor capacitors, choose ones with a working voltage at least equal to the operating voltage of the unit or more. For instance, when replacing a run capacitor on a unit rated to operate at 240 volts ac, capacitors with voltage ratings of 370 or 440 volts ac can be used. In fact, it is sometimes difficult getting the exact voltage rating on a capacitor. Don’t replace a 220-volt ac capacitor with one rated for 110 volts ac. Remember a higher voltage rating is alright; using an underrated capacitor may be dangerous. In Fig. 4-2 three types of capacitors are shown. It also shows the multimeter hook-up for the test below.
Fig. 4-2. Run capacitor E is a dual capacitor. Terminal C is the common and H is hooked to the compressor, while F is wired to the fan motor. This capacitor is very common in window units. Capacitor S is a dry electrolyte start capacitor. The small capacitor G is a fan capacitor.
When checking capacitors, the first step is to set your multimeter selection switch to the highest resistance scale (ohms).
Check your meter to zero and infinity.
Always discharge the capacitor that is to be checked before handling. If you don’t and the capacitor has a charge in it, you will get quite a shock. The recommended way of discharging a capacitor is to place a 15,000 ohm, two-watt resistor across the capacitor terminals. Some people use a small piece of insulated wire, other just place the blade of a screwdriver across the terminals. The resistor is the best method to use, especially with those capacitors that have an internal fuse. Sometimes shorting with a screwdriver can cause this fuse to fail rendering the capacitor useless. With the capacitor discharged, you must remove one side of the shunt resistor from one of the terminals.
Place one probe of your multimeter on one terminal. Now place the other probe on the opposite terminal. The meter needle should deflect fully, then in a split second start returning to infinity. This is caused by the small dc battery in the meter placing a charge in the capacitor. If you want to repeat the test or if it didn’t work, reverse the probe positions which will reverse the polarity. If the needle reacts in the described way, the capacitor is serviceable. If not, the capacitor will need to be replaced. If there is a slight deflection of the needle and it doesn’t return to the infinity position, the capacitor is shorted and should be replaced. With metal case capacitors, check continuity from the terminals of the capacitor to the case. There should be absolutely no reading. If there is a slight deflection, replace the capacitor.
Run capacitors usually have an index mark on their tops, something that designates where the line wire should be placed to charge the capacitor. It is sometimes a red dot, an index bump, or a plus sign. It is true that the capacitor will function whether the line is indexed or not, but for safety reasons the line wire that supplies the voltage to the capacitor should be indexed. The reason is the way the capacitor is constructed. The terminal indexed is electrically attached in the center of the metal casing. The other terminal is wired very close to the inside of the metal shell. If for any reason the wire should come loose inside the capacitor, it could blow a hole in the case spewing hot oil. It’s simple enough to wire the line to the indexed terminal. Figure 4-3 shows capacitor mounting in a General Electric condensing unit.
Fig. 4-3. Actual mounting arrangement of capacitors in a General Electric condensing unit.
When checking resistance in the capacitor, you already know that you’ve placed a charge in the capacitor from the meter battery. To check if the capacitor retains this charge, turn the selector switch of your multimeter to the dc voltage scale for 10 volts. Touch the probe to the terminals of the capacitor; the needle should deflect to about the 10 volt mark. If the needle deflects towards infinity, reverse the probes to correct the polarity.
Those of you that have been in the field a while are very familiar with what happens to the identification markings of the capacitors once they have shed their packaging and begin sliding around the shelves in the back of a service truck. After a short time, they become unreadable. Knowing the frustration of this experience, you will all take extreme caution in keeping your capacitors pampered. On the other hand you might know all the numbers on your capacitors, but the piece of equipment doesn’t have any identification of the capacitors that need replacement. For this reason, two tables have been placed in this book showing capacitor ratings for fractional horsepower compressors (see Tables 4-1 and 4-2). If you have a compressor that won’t start but has indications of locked rotor, you might want to check the capacitors as described in this chapter.
Table 4-1. Run Capacitors Approximate Sizing for Horsepower of Compressor.
mF stands for microfarads
Table 4-2. Start Capacitors Approximate Sizing for Horsepower of Compressor.
mf STANDS FOR MICROFARADS
Due to the fact that a start capacitor is only in the circuit electrically for just an instant of time, it could be replaced with a capacitor of a slight difference in microfarad rating. The run capacitor is more critical, and the same size should be used for replacement. There should not be a hum in the motor or the compressor when operating. The run current should also be watched for excessive amount over factory specifications. If the compressor starts alright and then develops a hum, it is possible that the run capacitor has a too high a value. Lower it and check it again.
Three phase (polyphase) motors do not start components, they are only used on single-phase equipment. Many units are using a P.S.C. (permanent split capacitor) compressor, this means there is only one capacitor used in the unit. The disadvantage is that the compressor will not start under a load. The single capacitor requires that the unit shut down for several minutes before trying to restart. If it short cycles, it kicks out the circuit breaker, or the overload might open, or even blow a fuse. In certain regions, power supply demand is so high that the power companies are not supplying full power ratings. It is this reason that a 10% electrical design fluctuation is built into the equipment. When power supply borders on this 10%, problems can be witnessed with hard starting. Hard start kits have been installed in many areas. The kit consists of a start capacitor and relay, and at times a run capacitor. In extreme cases of short cycling, a time delay is used to keep the unit off until the pressure in the system equalizes.
Too many people think fuses are all alike. In fact some will even say, “a fuse is a fuse,” but that is not true. There are many types of fuses, and each of them is designed for a specific purpose. The most important reason for using the proper fuse is to have a device that will melt and open a circuit before sufficient heat is generated that could melt the conductors and perhaps ignite the entire structure. For this reason, three factors are very important at this time. Wire size, load amperage, and fuse size. They are closely related. The fuse sizing must be below wire ampacity. For the protection of the appliance, the fuse should be rated close to the f.l.a. of the appliance. In Fig. 4-4 some of the different types of fuses are shown. The job of the fuse is to open the circuit when a load exceeds the rating of the fuse.
Fig. 4-4. Different types of fuses used in air conditioning field.
The glass screw type fuse is still in service in many regions. The biggest problem with this type of fuse was that the base of it was the same for all the different ampacities. If a homeowner plugged an appliance into a wall receptacle, and it caused a circuit overload, the fuse would open. If the fuse that blew (opened) was rated for 15 amps, and all the owner had was a 20 or 30 amp replacement, he would use it. I’m sure this practice caused many structural fires or wiring damage.
The fuse-stat type was then introduced. Basically the difference between it and its predecessor was the threading on the screw-in base. Each fuse-stat had its own color that indicated its ampacity. The safety feature was that each ampacity fuse had a different threading on its base. This meant, if a 15 amp (blue) fuse was to be changed, nothing but a 15-amp fuse could be screwed into the socket (fuse holder). There are many areas still using this type of fusing.
Newer homes use circuit breakers. This type of protection was widely accepted due to the fact there was no longer a need to find an open hardware store late at night when a fuse would fail. With the circuit breaker, the homeowner waits until it cools, turns it to the off position and then to the on position, and he has service again. If the breaker trips again, he knows there is a problem on the circuit that will need correction. Some residential-type circuit breakers can be reset in a single motion.
In both fuses and circuit breakers there are two types of elements placed inside of them. One is called a quick-blow element and the other a slow-blow element. Remember this difference, for it might save you a lot of headaches someday. The quick-blow fuse is designed for the lighting circuits or any circuit that doesn’t carry an extremely high resistive load. This type fuse will open the instant the ampacity is exceeded. The slow-blow fuses are designed for heavy starting loads. It is sometimes labeled dual-element fuse. It is constructed to carry an overload for a couple of seconds. This allows it to carry a locked rotor amperage when a motor starts. You can see what havoc can be initiated, with the use of a wrong type of fuse. The mistake can be made by both the homeowner and the technician.
Tubular fuses are designed for a wide range of ampacities. They are made in very low ampacities and range into very high ampacities. They too are designed for quick and slow-blow service. Some tubular fuses have removable ends that enable the element or link to be replaced. This type of fuse body is usually found in commercial and industrial applications.
To test a fuse, the ohmmeter or continuity light must be used. Turn your ohmmeter selector switch to the highest resistance scale. All fuses have two external contact points where the electricity can enter, then exit the fuse. With the screw type fuse, one contact is in the center of the screw base and the other contact is a small metal tab on the side of the threading where it butts to the receptacle. Tubular fuses have a metal contact on each end of the fuse.
With the ohmmeter on highest resistance scale, or continuity light switch turned on, touch one probe to one contact, at the same time touch the other probe to the other contact. In the case of the ohmmeter, a maximum needle deflection will be shown if the fuse is serviceable. The continuity light will illuminate if the fuse is serviceable. If nothing happens the fuse is open and should be replaced. If there is a slight deflection of the needle, the fuse should be replaced.
