25.4.2 Run Capacitor The run capacitor is constructed like the start capacitor, but with some important differences. The run capacitor is better-built and is almost trouble-free because its capacitance rating is low, and it must remain in the circuit. The capacitor case is heavy gauge metal, which helps dissipate heat. The aluminum foil strips are shorter and thinner, and the coil is wound less tightly, limiting the amount of electron movement in the capacitor. Most distinctive is that the run capacitor is filled with a dielectric (insulating) oil that helps transfer heat from inside the capacitor to the metal case. Manufacturers of outdoor condensing units for residential air conditioning systems sometimes install a run capacitor with three terminals. Having three terminals indicates that it is actually two run capacitors in one case. The cost-saving procedure is possible because the compressor and condenser fan are both PSC motors. The rating for each capacitor is listed on the case. One terminal of the capacitor is identified as common to each capac- itor and is connected to power supply. Another came from. Electrons are constantly rushing into and out of each side of the capacitor. When one side is full, the other side is empty. When the start capacitor is connected in series with the start winding, one side of the capacitor is discharging its electrons into the winding. Discharging increases current flow in the start winding, strengthening the magnetic field, and producing better starting torque. The other side of the capacitor discharges back into the supply wire. Capacitor Malfunctions Both sides of the capacitor must be able to function, else the movement of electrons stops (no current flow). The movement of electrons into and out of the capacitor causes the capacitor to become hot. If a start capacitor is not removed from the circuit quickly, the capacitor will suffer a burnout or become shorted. A shorted capacitor is one in which the aluminum coils touch, such as a failure or burn-through of insulation. In this case, the capacitor is acting like any conductor, no longer as a capacitor. A burnout, or open capacitor, occurs when one of the terminals separates from its coil. An open capacitor acts like an open circuit, stopping current through it. The start capacitor is a frequent source of motor problems. Anything that causes the motor to short cycle (turn on and off frequently) will cause the start capacitor to burn out. If the motor tries to start and cannot, the capacitor will remain in the circuit and burn out. Bleed Resistor Sometimes a 20,000 Ω, 2 W, solid-state resistor is soldered in place to connect the capacitor termi- nals, Figure 25‑35. Such a resistor does not inter- fere with normal capacitor operation. During the off cycle, the resistor permits electrons trapped on one side of the capacitor to slowly migrate (bleed over) to the empty side until the two sides are equal. Called a bleed resistor, it serves a two-fold purpose: • • It protects the technician by preventing an electrical shock that could occur from touching the terminals of a charged capacitor. • • It prevents the excessive arcing that would otherwise occur when relay contacts close after a very short run cycle. Excessive arcing can cause relay contacts to weld together or destroy the contacts. High resistance bleed resistor Goodheart-Willcox Publisher Figure 25‑35. A bleed resistor is a safety feature that prevents a possible electrical shock from touching the terminals of a charged capacitor. It also helps reduce arcing of relay contacts. 504 Heating and Cooling Essentials Copyright Goodheart-Willcox Co., Inc.