What is the function of a starting capacitor?
1 Answers
The starting capacitor in a starter is used to create a rotating magnetic field in a single-phase motor, enabling the machinery to rotate. This is necessary because the single-phase current flowing through a single-phase motor cannot generate a rotating magnetic field on its own. A capacitor is employed to split the phase, aiming to produce a phase difference of approximately 90 degrees between the currents in the two windings, thereby generating the rotating magnetic field. More details are as follows: 1. A capacitor-start induction motor has two windings: the starting winding and the running winding. These windings are spatially separated by 90 degrees. A large capacitor is connected in series with the starting winding. When the running winding and the starting winding are energized with single-phase alternating current, the capacitor causes the current in the starting winding to lead the current in the running winding by 90 degrees in time, reaching its peak value first. 2. This creates two identical pulse magnetic fields in both time and space, generating a rotating magnetic field in the air gap between the stator and the rotor. Under the influence of this rotating magnetic field, an induced current is produced in the motor rotor. The interaction between this current and the rotating magnetic field generates an electromagnetic torque, causing the motor to rotate. 3. To enable a single-phase motor to start rotating automatically, a starting winding can be added to the stator. This starting winding is spatially separated from the main winding by 90 degrees and is connected in series with an appropriate capacitor. This ensures that the current in the starting winding is approximately 90 degrees out of phase with the current in the main winding, following the principle of phase splitting. 4. When these two currents, which are 90 degrees apart in time, flow through two windings that are 90 degrees apart in space, a (two-phase) rotating magnetic field is generated in space. Under the influence of this rotating magnetic field, the rotor can start automatically. Once the speed reaches a certain level, a centrifugal switch or other automatic control device installed on the rotor disconnects the starting winding, leaving only the main winding operational during normal operation. 5. Therefore, the starting winding can be designed for short-term operation. However, in many cases, the starting winding is not disconnected. Such motors are called capacitor-run single-phase motors. To reverse the direction of rotation of such a motor, the position of the capacitor in series can be changed.
