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Why a Motor Need a Start Capacitor?

A capacitor is use to give a motor an extra electrical boost for starting. A starting capacitor is used in the motor circuit only for one or two seconds when it starts rotating. Once the motor has reached speed, the starting capacitor is disconnected and not used again until the next motor starts.

If you use an AC pump to remove water from the sewer, you can use a condenser motor to power the boiler. This creates a rotating magnetic field in the capacitors of the single-phase motor, creating the artificial second phase to rotate the rotor. As the rotor begins to rotate, the interaction between the stator and the rotor keeps the magnetic field rotating.

single phase motors

Single-phase motors have primary and secondary windings. When connect to AC power without a capacitor, both coils produce the same phase magnetic field and zero rotation. The magnetic field generate by the capacitor in relation to the secondary winding is smaller than the magnetic field generate by the primary winding. This phase difference generates starting torque and starts the motor.

The capacitors that guarantee the operation of the motor are call the starting capacitors of the motor. Small motors usually have a starting capacitor. That is permanently connect to the secondary winding. Larger motors require larger capacitors to generate initial torque. But smaller capacitors are most efficient. When they are call performance capacitors. Normally, both capacitors are in the same housing with three terminals instead of the usual two. These motors have a centrifugal switch that starts when the motor reaches 70-75% of its maximum speed. Initial capacitors are usually above 100 microfarads, but controlled capacitors are low, ranging from 25 to 47 microfarads.

There are motors with large starting use in many applications that require high torque to move the load. These applications include mechanical conveyor belts, jet belts and commercial garage door keys. These are usually electrolytic capacitors in plastic or metal containers. Inside the box, there are two metal plates cover with insulating paper. Because elastic paper is us to insulate the sheet. Paper impregnate with electrolyte forms the dielectric of the capacitor. The two metal leaves are connect by two terminals. The assembly is seal with epoxy resin. And has two terminals for external electrical connections.


Larger HVAC units may have two capacitors because they have a fan and a compressor motor. To save space, manufacturers combine two physical capacitors into one box. This double capacitor has three terminals, call common, fan, and compressor.


You can find different combinations of 40 + 5uF, 370V or 100 + 25uF, 440V and other dual capacitors. This shape can be cylindrical, circular, or oval. The capacitance of the capacitor was measure with a microfarad. The performance of electrolytic capacitors deteriorates over time. As a result, the engine will not start and will not run at full speed.

Motors do not have many choices when choosing the value of capacitor capacity to use. However, do not use low voltage replacement to replace a fail capacitor. Be sure to use parts with a voltage higher than that of the capacitor for replacement. Of course, for best results in terms of performance and safety, it is always best to replace the capacitor with another suitable power.

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