Motor capacitors are common components in single-phase electric motors, with two categories: starting and running. Start capacitors provide an initial impulse to start the motor rotating, while run capacitors increase the power factor of the engine. Capacitor starting motors use a capacitor and starter motor winding to create a bias in the magnetic field to spin the rotor. Double run capacitors are used in HVAC systems to start and run both fan and compressor motors.
A motor capacitor is a component commonly found on single-phase electric motors. There are two categories of motor capacitor: starting and running. Start capacitors are used to give a single-phase motor the initial impulse to start the motor rotating and are left in the circuit until the motor reaches approximately 75% of its normal running speed. At this point a centrifugal switch or an electronic relay disconnects the capacitor from the circuit. Run capacitors are left in the circuit while the engine is running and serve to increase the power factor of the engine. The motor capacitor is usually of the electrolytic type and is often easily identifiable as a two-lead tubular component mounted outside the motor casing.
Single-phase induction motors achieve their rotation due to the constantly inverting magnetic field created in the motor stator windings by the alternating current (AC) supply. A permanent magnet rotor located inside the stator is forced to “follow” the north/south orientation of the alternating field, rotating it as it does. While very effective, the rotor needs a push, so to speak, to start spinning when power is first applied. In capacitor starting motors, this impulse is provided by a capacitor and starter motor winding which create a bias in the magnetic field to spin the rotor. Once the motor has reached 75% of its rated rotational speed, the start capacitor and winding are disconnected from the motor circuit via a mechanical centrifugal switch or electronic relay.
The running motor capacitor performs a slightly more complex function involving motor power factor. Put simply, motor power factors involve the relationship between the theoretical and actual power of the device. Leaving the capacitor and winding in a circuit during normal operation allows motors to run at higher power factors. Known as cap-start, cap-run motors, they have a single capacitor that performs both start and run functions or two separate capacitors. In the case of separate arrangements of the start and run motor capacitors, the start capacitor is still disconnected from the circuit after starting.
There is another motor capacitor configuration known as a double run capacitor which is in the category of run capacitors. These are often seen on heating, ventilation and air conditioning (HVAC) systems where a capacitor is used to start and run both fan and compressor motors. These capacitors have three instead of two leads, one common and one each for the two motors. Apart from the fact that two motors are involved, the principle of the run capacitor remains the same.
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