Motor Start Capacitor Sizing Formula

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.

Motor start capacitor sizing formula.

Start capacitors increase motor starting torque and allow a motor to be cycled on and off rapidly. Start capacitors ratings of 70 microfared or higher have three voltage classifications. Divide the start up energy requirement in joules of the motor by x to arrive at the capacitor size needed in farads. One often used method is the split phase motors.

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. We know about the activity of a capacitor in a pure a c. How to sizing the starting capacitor. We also can use this basic formula to calculate capacitor sizing.

The first two numbers are the capacitance 35 µf for the compressor and 5 µf for the fan motor. Hence 1 hp motor required 24 66 µf capacitance to start the motor smoothly. Start capacitors are designed for momentary. This is where the rule of 10 of the rating came from for start capacitors only.

Use our capacitance calculation formula. 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. To select the correct capacitance value start with 30 to 50μf kw and adjust the value as required while measuring motor performance. 1 hp 746 watts.

These capacitors usually have capacitance values of over 70 µf. 125v 250v and 330v. Calculate the rated required capacitance value for the single phase 220v 1 hp 50hz 80 of the motor. Another method is the capacitor start induction run motors.

But in the market you can get 25 µf. C µf 746 x 80 x 1000 220 x 220 x 50 24 66 µf. 1 a rule of thumb has been developed over the years to help simplify this process. The start up energy of the motor is found either in its documentation or written on the motor itself.

The voltage rating should be no less than the listed amount for the motor for central heat pumps and air conditioners this is usually a minimum of 370vac.