Electric Motor Run Capacitor Sizing

Updated april 24 2017.

Electric motor run capacitor sizing.

A bad motor capacitor may cause starting problems or could shut off the motor while running. Single phase electric motors need a capacitor to energize a second phase winding. The higher the capacitance of the capacitor the more energy it can store. There are two common types of motor capacitors.

Divide the start up energy requirement in joules of the motor by x to arrive at the capacitor size needed in farads. Motor start capacitors are used during the motor startup phase and are disconnected from the circuit once the rotor reaches a predetermined speed which is usually about 75 of the maximum speed for that motor type. Run capacitors and start capacitors. The run capacitor should have the exact microfarad uf that the motor is rated for.

This is why sizing is so critical. Assuming a start up energy of 0 00033 joules you obtain 0 33 divided by 66 1 which equals 5 0 times 10 6 farads. Motor capacitors are used with air conditioners. One often used method is the split phase motors.

It saves space by combining two physical capacitors into one case. It gives the motor an initial push on start up briefly increasing its starting torque and allowing the motor to be cycled on and off quickly. Motor capacitors store electrical energy for the motor to use. The difference is more than 100 times is there a calculation error.

Almost every motor is furnished with either a start capacitor a run capacitor or both. A dual run capacitor supports two electric motors such as in large air conditioner or heat pump units with both a fan motor and a compressor motor. Capacitors with ratings above 70 microfarad mfd are starting capacitors. 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 alternating current induction motor to create a rotating magnetic field.

The voltage classifications are 370v and 440v. Run capacitors are designed for continuous duty and are energized the entire time the motor is running. The first two numbers are the capacitance 35 µf for the compressor and 5 µf for the fan motor. The start capacitor is connected into the motor s electrical circuit at rest.

The start up energy of the motor is found either in its documentation or written on the motor itself. The single phase induction motor can be made to be self starting in numerous ways. Capacitors rated above 70uf are considered start capacitors and are generally removed from the circuit electrically during operation. Motor start and motor run capacitors start capacitors.

You aproximate sizing of the capacitor is in the range of 4 40 mf and at the motors i have 1 1 2 2 kw the run capacitor is sized between 35 70 uf. How to size a capacitor to an electric motor. What capacitors do. Another method is the capacitor start induction run motors.