what is a start capacitor?

Starting capacitor refers to ac electrolytic capacitor or polypropylene or polyester capacitor used for starting single-phase asynchronous motor. Capacitor motors or PSC motors use running capacitors to help motors run more efficiently. The starting capacitor powers the motor at startup.

Designed for momentary use, the start capacitor is what lets a motor start up instantly instead of taking a long time to come up to speed before it can be used. It stays energized long enough to rapidly bring the motor to 3/4 of its full speed and is then taken out of the circuit. It will briefly increase motor starting torque and allow a motor to be cycled on and off rapidly, but is not meant to be used for more than a few seconds.

The primary purpose of a motor start capacitor is to briefly increase the motor starting torque as well as to allow a motor to be cycled on and off very quickly. It operates in the circuit by staying active long enough to allow the motor to be brought to 3/4 of it’s full capacity.

Single phase motors will commonly have both a start and run capacitor. The difference between the two is that the start capacitor has a much higher ability to store charge, also known as its capacitance rating, for its size than that of the run capacitor. Essentially, the start cap gives the motor a bolt of energy in order to get it up and running, while the run capacitor keeps the motor going after the start capacitor shuts down.

Single phase motors through the single phase current does not produce a rotating magnetic field, need to take the capacitor is used to split phase, the purpose is to make the two in the winding current to produce nearly 90° phase difference, in order to produce a rotating magnetic field.

The capacitive induction motor has two windings, the starting winding and the running winding.The two windings differ 90 degrees in space.A capacitor with a large capacity is connected to the starting winding. When the running winding and the starting winding pass through single-phase alternating current, the current in the starting winding is 90 degrees ahead of the current in the running winding in time due to the action of the capacitor, and reaches the maximum value first.Two identical pulsed magnetic fields are formed in time and space, and a rotating magnetic field is generated in the air gap between stator and rotor. 

Under the action of the rotating magnetic field, induced current is generated in the rotor of the motor, and the interaction between current and rotating magnetic field generates electromagnetic field torque, which makes the motor rotate.