A motor capacitor, such as a start capacitor or run capacitor is an electrical capacitor that alters the current to one or more windings of a single-phase alternating-current induction motor to create a rotating magnetic field. There are two common types of motor capacitors: run capacitors and start capacitors. Motor capacitors are used with air conditioners, hot tub/jacuzzi spa pumps, powered gates, large fans or forced-air heat furnaces for example. A "dual run capacitor" is used in some air conditioner compressor units, to boost both the fan and compressor motors.
Start capacitors
Start capacitors briefly increase motor starting torque and allow a motor to be cycled on and off rapidly. A start capacitor stays in the circuit long enough to rapidly bring the motor up to a predetermined speed, which is usually about 75% of the full speed, and is then taken out of the circuit, often by a centrifugal switch that releases at that speed. Afterward the motor works more efficiently with a run capacitor. Start capacitors have ratings above 70µF, with four major voltage classifications: 125V, 165V, 250V, and 330V. Start capacitors above 20µF are always non-polarized aluminium electrolytic capacitors with non solid electrolyte and therefore they are only applicable for the short motor starting time. The motor will not work properly if the centrifugal switch is broken. If the switch is always open, the start capacitor is not part of the circuit, so the motor does not start. If the switch is always closed, the start capacitor is always in the circuit, so the motor windings will likely burn out. If a motor does not start, the capacitor is far more likely the problem than the switch.
Run capacitors
Some single-phase AC electric motors require a "run capacitor" to energize the second-phase winding to create a rotating magnetic field while the motor is running. Run capacitors are designed for continuous duty while the motor is powered, which is why electrolytic capacitors are avoided, and low-loss polymer capacitors are used. Run capacitors are mostly polypropylene film capacitors and are energized the entire time the motor is running. Run capacitors are rated in a range of 1.5 to 100 µF, with voltage classifications of 370 V or 440 V. If a wrong capacitance value is installed, it will cause an uneven magnetic field around the rotor. This causes the rotor to hesitate at the uneven spots, resulting in irregular rotation, especially under load. This hesitation can cause the motor to become noisy, increase energy consumption, cause performance to drop and the motor to overheat.
Dual run capacitors
A dual run capacitor supports two electric motors, with both a fan motor and a compressor motor. It saves space by combining two physical capacitors into one case. The dual capacitor has three terminals, labeled C for common, FAN, and HERM for hermetically-sealed compressor. Dual capacitors come in a variety of sizes, depending on the capacitance, such as 40 plus 5µF, and also on the voltage. A 440-volt capacitor can be used in place of a 370-volt, but not a 370-volt in place of a 440-volt. The capacitance must remain within 5% of its original value. Round cylinder-shaped dual run capacitors are commonly used for air conditioning, to help in the starting of the compressor and the condenser fan motor. An oval dual run capacitor could be used instead of a round capacitor; in either case the mounting strap needs to fit the shape used.
Failure modes
A faulty run capacitor often becomes swollen, with the sides or ends bowed or bulged out further than usual; it can then be clear to see that the capacitor has failed, because it is swollen or even blown apart causing the capacitor's electrolyte to leak out. Some capacitors have a "pressure sensitive interrupter" design that causes them to fail before internal pressures can cause serious injury. One such design causes the top of the capacitor to expand and break internal wiring. Over many years of use the capacitance of the capacitor drops; this is known as a "weak capacitor". As a result, the motor may fail to start or to run at full power. When a motor is running during a lightning strike on the power grid, the run capacitor may be damaged or weakened by a voltage spike, thus requiring replacement. IEC/EN 60252-1 2001 specifies the following levels of protection for motor run capacitors:
The motor capacitor, which is a component of a hot tubcirculating pump, can overheat if defective. This poses a fire hazard, and the U.S. Consumer Product Safety Commission has received more than 100 reports of incidents of overheating of the motor capacitor, with some fires started.
Labeling
The units of capacitance are labeled in microfarads. Older capacitors may be labeled with the obsolete terms "mfd" or "MFD" which can be ambiguous but are, especially in this context, used for microfarad as well.
