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What Is a Motor Capacitor?

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  • Written By: Paul Scott
  • Edited By: R. Halprin
  • Last Modified Date: 21 September 2014
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A motor capacitor is a component commonly found on single phase electric motors. There are two categories of motor capacitor: starting and running. Starting capacitors are used to give a single phase motor the initial impetus to start the motor turning and are left in circuit until the motor achieves approximately 75% of its normal running speed. At this point, a centrifugal switch or electronic relay disconnects the capacitor from the circuit. Running capacitors are left in circuit while the motor operates and serve to boost the power factor rating of the motor. The motor capacitor is generally an electrolytic type and is often readily identifiable as a tubular, two lead component mounted on the outside of the motor casing.

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Single phase induction motors achieve their rotation courtesy of the constantly reversing magnetic field created in the motor's stator windings by the alternating current (AC) power supply. A permanent magnet rotor located inside the stator is forced to “follow” the north/south orientation of the alternating field, thereby causing it to rotate as it does so. Although very effective, the rotor needs to be given a nudge, so to speak, to start turning when power is first applied. In capacitor start motors, this impetus is supplied by a starting motor capacitor and winding which create a bias in the magnetic field to turn the rotor. Once the motor has reached 75% of its rated rotational speed, the starting capacitor and winding are disconnected from the motor circuit by a mechanical centrifugal switch or an electronic relay.

The running motor capacitor fulfills a slightly more complex function which involves the motor's power factor rating. Put simply, motor power factors involve the relationship between the theoretical and actual power output of the device. Leaving the capacitor and winding in circuit during normal running allows motors to operate with higher power factors. Known as cap-start, cap-run motors, they feature either a single capacitor which fulfills both start and run functions or two separate capacitors. In the case of separate start and run motor capacitor arrangements, the starting capacitor is still disconnected from the circuit after start up.

There is one other motor capacitor configuration know as a dual run capacitor which is in the running capacitor category. These are often seen on heating, ventilation, and air conditioning (HVAC) systems where one capacitor is used to start and run both fan and compressor motors. These capacitors feature three rather than two leads, a common, and one each for the two motors. Other than the fact that two motors are involved, the running capacitor principle remains the same.

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