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A motor capacitor, such as a start capacitor or run capacitor, including a dual run capacitor, is an electrical capacitor that alters the current to one or more windings of a single phase A.C. Induction motor to create a rotating magnetic field. It is used in air conditioners, hot tub/jacuzzi spa pumps, or forced air heat furnaces for example. A round dual run capacitor (described below) is used in some air conditioner compressor units, to boost both the fan and compressor motors.
Motor capacitors include two common types, run capacitors and start capacitors. The units of capacitance are labeled in microfarads (µF or uF or mfd).
Run capacitors are mostly polypropylene film capacitors and are designed for continuous duty, and they are energized the entire time the motor is running. Run capacitors are rated in a range of 1.5–100 microfarads (µF or mfd), with voltage classifications of 370 V or 440 V. Single phase electric motors need a capacitor to energize a second-phase winding. If the wrong run capacitor is installed, the motor will not have an even magnetic field, and this will cause the rotor to hesitate at those spots that are uneven. This hesitation can cause the motor to become noisy, increase energy consumption, cause performance to drop, and cause the motor to overheat.
Start capacitors above 20 microfarads (µF) are always non-polarized Aluminum Electrolyic Capacitors  with non solid electrolyte and therefore they are only applicable for the short motor starting time.
Start capacitors briefly increase motor starting torque and allow a motor to be cycled on and off rapidly. Start capacitors have ratings above 70 microfarads (µF), with four major voltage classifications: 125 V, 165V, 250 V, and 330 V. A start capacitor stays in the circuit long enough to rapidly bring the motor to 3/4 of full speed and is then taken out of the circuit, such as by a centrifugal switch that releases at that speed. The motor after that runs more efficiently with a run capacitor.
Examples of motor capacitors are: a 35 µF/mfd, at 370 V, run capacitor, or an 88–108 µF/mfd at 250 V start capacitor.
A dual run capacitor supports 2 electric motors, such as in large air conditioner or heat pump units, with both a fan motor and a compressor motor. The dual capacitor has 3 terminals labeled "C", "FAN", and "HERM" for the common, fan, and hermetic compressor connections.
Round dual run capacitors (shaped as round cylinders) are commonly used for air conditioning, to help in the starting of the compressor and the condenser fan motor. Dual capacitors come in a variety of sizes, depending on microfarads (µF), such as 40 plus 5 µF, and also the voltage. A 440 volt capacitor can be used in place of a 370 volt, but not a 370 in place of a 440 volt. The microfarads must stay the same within 5% of its original value. An oval dual run capacitor could be used instead of a round capacitor, but the mounting strap should be changed to better fit the oval shape.
A faulty run capacitor often becomes swollen, with the sides or ends bowed or bulged out further than usual: it can be clear to see that the capacitor has failed because it is swollen or even blown apart with capacitor oil leaking. Some capacitors are built with a "Pressure Sensitive Interrupter" design that causes them to fail before internal pressures can cause serious injury. One design causes the top of the capacitor to expand and break internal wiring.
"Weak Capacitor" - Over many years of use the capacitance of the capacitor (measured in microfarads) reduces. As a result the motor may fail to start or run at full power.
The motor capacitor, which is a component of a jacuzzi circulating pump, can overheat if defective. This poses a fire hazard, and the U.S. Consumer Product Safety Commission (CPSC) has received more than 100 reports of incidents of overheating of the motor capacitor, with some fires started.