break down and become grounded, the fuse in the power supply line will blow before the motor is damaged. Safety Note Run Capacitors and PCB Older run capacitors used polychlorinated biphenyl (PCB), a dangerous fluid, as the dielectric oil. If the metal shell is accidentally pierced or opened, do not touch the fluid or breathe the fumes. Disposal of such run capacitors should be done by properly trained technicians. 25.4.5 Testing Capacitors One method of testing a capacitor is to replace the possibly faulty device with a known good capacitor. If the motor operates properly, then the old capacitor is faulty. Another method of testing is to use an ohmmeter. Before testing, the capacitor must be disconnected from the circuit and discharged by shorting across the terminals. This should be done with a 20kΩ, 2 W, solid-state resistor. Touch the two leads from the ohmmeter to the capacitor terminals. If the capacitor is good, the terminal is marked F (fan) and is connected to the start winding of the fan motor. The other terminal, marked H (hermetic), is connected to the start winding of the compressor. 25.4.3 Capacitor Ratings Both start and run capacitors have two important ratings: VAC (volts, alternating cycle) and microfarads (mfd or µf). The voltage rating of a capacitor indicates the nominal voltage at which it is designed to operate. Using a capacitor at voltages below its rating does no harm. However, excessive voltage causes severe damage. Run capacitors must not be subjected to voltages over 10% of their nominal VAC rating. Start capacitors must not be subjected to voltages over 30% of their nominal VAC rating. Start capacitors are normally rated at 110 VAC or 250 VAC, and run capacitors are normally rated at 370 VAC. The 370 VAC rating allows for the back-emf generated by the start winding. The amount of energy that a capacitor can store is expressed in microfarads. The rating tells how much “kick” the capacitor delivers to the start winding. Start capacitors have microfarad ratings much higher than those of run capacitors. Run capacitors are available from 1.5 mfd to 50 mfd, while start capacitors range from 75 mfd to 600 mfd. Capacitors are selected according to the size of the motor and the load placed on the motor. A replacement capacitor should have exactly the same microfarad and voltage ratings as the capacitor being replaced. If an exact replacement is not possible, the new capacitor’s microfarad rating should be 5% to 10% greater than the one being replaced. 25.4.4 Capacitor Safety The start winding of a motor can be damaged by a shorted and grounded run capacitor. Damage can be avoided by properly connecting the run capacitor. The electrical terminal attached to the aluminum foil closest to the metal case is identified with a special marking, Figure 25‑36. The power supply should always be connected to the identified terminal, which is the terminal most likely to short to the metal case and be grounded in the event of a capacitor breakdown. If the capacitor should Red dot Identified terminal Goodheart-Willcox Publisher Figure 25‑36. The terminal connected to the outer aluminum strip is marked on a run capacitor. The power supply connects to this lead to help protect the motor. Some manufacturers use a dot, while others prefer an arrow or a dash. Chapter 25 Motors 505 Copyright Goodheart-Willcox Co., Inc.