To recognize the importance of proper capacitor sizing.

The student will be able to:

1) Understand what capacitors are and how they work
2) Demonstrate the effect improper capacitor sizing has on energy consumption
3) Demonstrate the ability to test capacitors

The simplest way to explain the mechanics of a capacitor would be to compare it to a battery. Both store and release electricity. Capacitors are charged with electricity, then releases its stored energy at a rate of sixty times per second in a 60 cycle alternating current system. The sizing is critical to motor efficiency just as sizing of batteries is critical to a radio. A radio that requires a 9V battery will not work with a 1.5V size battery. Thus, as the battery becomes weaker the radio will not play properly. A motor that requires a 7.5 mfd capacitor will not work with a 4.0 mfd capacitor. Much the same way, a motor will not run properly with a weak capacitor. This is not to imply bigger is better, because a capacitor that is too large can cause energy consumption to rise. In both instances, be it too large or too small, the life of the motor will be shortened due to overheated motor windings. Motor manufacturers spend many hours testing motor and capacitor combinations to arrive at the most efficient combination. There is a maximum of +10% tolerance in microfarad rating on replacement start capacitors, but exact run capacitors must be replaced. Voltage rating must always be the same or greater than original capacitor whether it is a start or run capacitor. Always consult manufacturers to verify correct capacitor size for the particular application.

Capacitors contain two metal plates insulated from each other (See Figure 1). When opened, the inside looks like two sheets of foil with wax paper in between them and rolled tight, similar to a roll of paper towel. Years ago the oil filled type used PCB's as a coolant. Today most capacitors are the dry type.

Figure 1

Two basic types are used in electric motor:
1) Run capacitors are rated in a range of 3-70 microfarad (mfd). Run capacitors are also rated by voltage classification. The voltage classifications are 370V and 440V. Capacitors with ratings above 70 microfarad (mfd) are starting capacitors. Run capacitors are designed for continuous duty, and are energized the entire time the motor is running. Single phase electric motors need a capacitor to energize a second phase winding. This is why sizing is so critical. If the wrong run capacitor is installed, the motor will not have an even magnetic field. This will cause the rotor to hesitate at those spots that are uneven. This hesitation will cause the motor to become noisy, increase energy consumption, cause performance to drop, and cause the motor to overheat.

2) Starting capacitors are housed in a black plastic case and have a mfd range as opposed to a specific mfd rating on run capacitors. Start capacitors (ratings of 70 microfared or higher) have three voltage classifications: 125V, 250V, and 330V. Examples would be a 35 mfd at 370V run capacitor and an 88-108 mfd at 250V start capacitor. Start capacitors increase motor starting torque and allow a motor to be cycled on and off rapidly. Start capacitors are designed for momentary use. Start capacitors stay energized long enough to rapidly bring the motor to 3/4 of full speed and are then taken out of the circuit.

Potential relays are also as important. Potential relays are used to electronically connect and disconnect to starting capacitors from the motor circuit (See Figure 2). Each relay has a specific voltage rating to place the start capacitor in series with the start winding and a specific voltage to take it out of the circuit. Each rating is based on the electromagnetic field generated by the rotation of the motor. The motor manufacturer studies the effect of placing in and taking out the capacitor to increase starting torque with as little winding flex as possible. Potential relays have four ratings; (1) continuous coil voltage, (2) minimum pick-up voltage, (3) maximum pick-up voltage, and (4) drop out voltage. A potential relay is difficult to check and should always be replaced when a start capacitor is replaced. The exact size designed for that particular motor must be reinstalled. The potential relay must also be replaced if contacts are found to be open.

Figure 2

Demonstrate the use of standard 1/2 horse power blower from a residential heater in the following exercises. During each exercise, the student should record the noise level, the speed, the temperature, and the amperage of the motor.

(1) Remove the capacitor and try to run the motor. Be sure to insulate the wire ends. This will simulate an open capacitor.

(2) Run motor with correct capacitor. Block front of blower to obtain correct motor speed and amp draw.

(3) Short the two wires that normally go to the capacitor and insulate the connection. This will simulate a shorted capacitor.

(4) Replace the standard capacitor with one that has half the mfd rating.

(5) Replace the standard capacitor with one that is double the mfd rating.

Before starting the exercise be sure to create proper static pressure to obtain motor plate amperage rating with correct run capacitor.

Exercise #1 Noise Level Speed Temperature Amperage

Exercise #2 Noise Level Speed Temperature Amperage

Exercise #3 Noise Level Speed Temperature Amperage

Exercise #4 Noise Level Speed Temperature Amperage

Exercise #5 Noise Level Speed Temperature Amperage

The activity on the preceding page deals with high voltage. Eye protection must be worn and extreme caution exercised to prevent electrocution. If not properly connected, capacitors may explode and cause severe personal injury. It is recommended that the instructor demonstrate the activity before considering allowing the student to perform it. The instructor should also check a student's work prior to the student's test run.

Modern Refrigeration and Air Conditioning. Goodheart-Willcox Co., Inc. S. Holland, IL. 1988.

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