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Why do single-phase motors need to add capacitors?
- Two line diagrams get power at the same time
For single-phase asynchronous motors, there is only one way of alternating current.
What happens when it is connected to the stator line diagram A and B at the same time (the coil is energised to produce a magnetic field that attracts the rotor for rotation)?
Yes, that’s right, the rotor may rotate clockwise or counterclockwise. This type of motor cannot be used in actual production.
- The two coils are energised in sequence
If I let coil A get power first and coil B second, then the rotor must be turning counterclockwise. By the same token, the rotor will turn clockwise.
This way the direction of the motor is determined and there is no possibility of left or right rotation.
The happy thing is that our power supply is AC, for AC, the current overrun or hysteresis is achieved by using an inductor or capacitor, the motor itself is inductive and can only be achieved by a capacitor.
Although it is called starting capacitor, this capacitor is mainly used to shift the phase, that is, let the coil get power in a different order.
This explains why the starting capacitor is needed.
A single phase motor cannot be started properly by running the winding alone and must be fitted with a start winding and then phase split by a capacitor to help the motor start.
By design, some motors will start by breaking the circuit of the starting winding and capacitor via a centrifugal switch and work on the running winding alone.
Some motors, on the other hand, do not have a centrifugal switch and the starting winding will continue to help the motor run properly as a secondary winding and capacitor after the motor has started.
Some motors also have a running capacitor installed on top of the starting capacitor, which is generally smaller than the starting capacitor, the purpose of which is to increase the motor torque and to work with the secondary winding to help the main winding complete its operation.
In fact, this can also be interpreted as adding an extra starting capacitor to the inside of a normal capacitor-run motor.
how to choose the capacitor of a single-phase motor?
The formula for calculating the working capacitance of a single-phase motor: GC=1950I/Ucos∮ (microfarads)
I: motor current
U: single-phase supply voltage
cos∮: power factor, take 0.75
1950: constant
If single-phase power is taken as 220Vrms
then GC=1950I/Ucos∮=1950P/(U^2)cos∮=1950P/(220220)0.75≈0.03P(uF)
where P is the motor power
After calculating the working capacitance of the single-phase motor, the starting capacitance is 1-4 times the working capacitance (the larger the starting capacitance, the larger the starting current and the greater the interference with the outside grid), and the starting torque increases, the faster the start.
Conversely, if the starting capacitor is smaller, the lower the starting current (and the lower the interference with the external grid) and the lower the starting torque, the slower the start. The withstand voltage must be greater than the maximum peak AC input voltage (220Vrms*1.414≈311), 400V withstand voltage or higher is desirable.
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