Capacitor single phase AC motor starts after the starting capacitor has no effect

The capacitors can be disconnected after a normal start of a low-power device, such as a fan capacitor! High-power ones are not good, such as air conditioning compressor capacitors!

The principle of rotation of a single-phase AC motor.

Single-phase AC motors have only one winding, and the rotor is squirrel cage type. When the single-phase sinusoidal current passes through the stator winding, the motor will produce an alternating magnetic field, the strength and direction of this magnetic field change with time as a sinusoidal law, but it is fixed in the spatial orientation, so it is also called the alternating fluctuating magnetic field. This alternating pulsating magnetic field can be decomposed into two rotating magnetic fields that are opposite to each other at the same speed and direction of rotation, and when the rotor is stationary, these two rotating magnetic fields produce two torques of equal magnitude and opposite directions in the rotor, so that the resultant torque is zero, so the motor cannot rotate.

When we use external force to rotate the motor in a certain direction (such as clockwise rotation), then the movement of the cutting magnetic field lines between the rotor and the rotating magnetic field in the clockwise rotation direction becomes smaller; The movement of the cutting magnetic field lines between the rotor and the rotating magnetic field in the counterclockwise direction of rotation becomes larger. In this way, the balance is broken, the total electromagnetic torque generated by the rotor is no longer zero, and the rotor rotates in the direction of propulsion.

To make the single-phase motor can rotate automatically, we can add a starting winding in the stator, the starting winding and the main winding are 90 degrees apart in space, and the starting winding should be connected in series with a suitable capacitor, so that the current with the main winding is approximately 90 degrees different in phase, that is, the so-called phase splitting principle. In this way, two currents that differ by 90 degrees in time are fed into two windings that differ by 90 degrees in space, and a (two-phase) rotating magnetic field will be generated in space, and under the action of this rotating magnetic field, the rotor can start automatically, and after starting, when the speed rises to a certain level, the starting winding is disconnected by a centrifugal switch or other automatic control device installed on the rotor, and only the main winding works normally. Therefore, the starting winding can be made to work for a short time.

But there are many times, the starting winding is not opened, we call this kind of motor capacitive single-phase motor, to change the steering of this motor, you can change the position of the capacitor in series.

According to the electrical schematic diagram, the starting winding and its capacitor still have a role after the completion of the starting task, although no test has been carried out in this regard, it is generally believed that if the starting circuit is disconnected during normal operation, the load capacity of the motor will be affected.

After the wire diameter of the starting winding increases, the current becomes larger, and the capacity of the capacitor should also increase, otherwise the starting time is long, which will cause damage to the capacitor, and can be replaced with 250uf*2. Whether the power supply voltage of the motor is too low, or the starting time of the motor is too long due to excessive load, and the centrifugal switch cannot be disconnected in time, resulting in overheating and damage to the starting capacitor; It is also necessary to consider the selection of a starting capacitor with a higher withstand voltage. The capacitance of the starting capacitor can be increased appropriately.

Capacitor start motors can be divided into capacitance split-phase starter motors and permanent split-phase capacitor motors. It is characterized by simple structure, fast start-up and stable speed, and is widely used in household appliances such as electric fans, exhaust fans, and range hoods.

The stator winding of the capacitance split-phase motor is equipped with a main winding and a secondary winding (starting winding), and a large-capacity starting capacitor is stringed in the starting winding, so that the voltage phase difference between the main and auxiliary windings is close to 90 ° after being energized, so as to produce a large starting torque and make the rotor start to run.

The start-up time is too long [more than 5 seconds], or the capacitor withstand voltage is too low. The load is reduced to minimize the start-up time, and high-quality capacitors with a withstand voltage of 400V are selected.

The quality of the starter capacitor is not good, or the withstand voltage value is too low, buy a capacitor with a more expensive and higher withstand voltage value.

Select a higher withstand voltage value for the capacitor. The capacitor of the 220V motor is 250V and 400V.

If the motor is fine, change the brand of capacitor, and choose a higher withstand voltage. I choose 400v.

There are several reasons for this:

1) The capacitor has low withstand voltage or poor quality;

2) The motor has a large load, resulting in too long starting time.

Start and then stop, listen carefully to the sound of the centrifugal switch disconnecting? If the centrifugal switch cannot be disconnected normally, the motor will not reach high speed, and there will be a vibration sound, and the coil will be burned. At this time, it is necessary to remove the end cover and replace the centrifugal switch, or use a sand step to remove the platinum contacts on it.

If the centrifugal switch can be disconnected normally, change the starting capacitor. Change the withstand voltage to 500V, and see if there is any problem with the capacity of the selected starting capacitor.

Is there a centrifugal switch? Is it that it didn't shake off.

The capacitance causes the current in the two windings to produce a phase difference of nearly 90 to create a rotating magnetic field. The single-phase current flowing through a single-phase motor cannot generate a rotating magnetic field, and capacitors are required to be used for phase separation.

Capacitive induction motors have two windings, namely the starting winding and the running winding.

The two windings differ by 90 degrees spatially. A capacitor with a large capacity is connected in series on the starting winding, and when the running winding and the starting winding pass through a single-phase alternating current, the current in the starting winding is 90 degrees ahead of the current of 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, so that a rotating magnetic field is generated in the air gap between the stator and the rotor, and under the action of the rotating magnetic field, an induced current is generated in the rotor of the motor, and the current interacts with the rotating magnetic field to produce electromagnetic field torque, which makes the motor rotate.

1. The running winding of the single-phase motor can only produce a pulsating magnetic field - the electromagnetic field corresponding to any point of the motor stator can only be made at that point, including the change in polarity direction and amplitude. In other words, a rotating magnetic field cannot be formed.

Three-phase power supplies, on the other hand, can generate a rotating magnetic field. ]

2. The capacitor winding, in space, is orthogonal to the running winding, that is, it is 90 degrees [electric angle, for motors with 4 poles and more than 4 poles]. If the capacitor winding is connected with the same power supply as the running winding, then the magnetic field generated by the synthesis of the two windings is still a pulsating magnetic field.

3. In fact, a capacitor with a certain capacity is connected in series in the capacitor winding, so that the equivalent circuit of the winding becomes a circuit in series between the resistance inductance and the capacitor, as long as the capacitance value meets the phase angle of the current at a certain angle ahead of the voltage, such as 30 degrees;

4. At the same time: if the current lag voltage of the inductive running winding is 60 degrees, then the current phase difference between the two windings reaches 90 degrees [close to it]. In this case, the maximum point of the electromagnetic field in the circumference of the stator occurs periodically at intervals of 90 degrees of electric angle – thus creating a rotating magnetic field.

At this point, it is only natural that the motor will spin.

5, and without the capacitor, 3 cannot be realized. So there will be no result of 4. Therefore: capacitors are required.

Because the capacitor has the function of phase shifting, the voltage at the initial two ends of the capacitor is zero, and the current will be 90 degrees ahead, resulting in a difference of 90 degrees between the current of the starting coil and the current of the running coil. Without this capacitance, it is impossible to form a rotating magnetic field formed by the front and back currents, and the direction of rotation cannot be determined.

There is an impact, if the capacitance is too large, the motor current is large, and the capacitance is too small, the starting force of the motor will become smaller, and it will not start, so the capacitor must be suitable.

The capacitance of the capacitor running motor should meet the original requirement of 10%, and the starting capacitance can be slightly larger than the original value but not too small, so as not to cause starting difficulties or weakness.

When repairing a motor, if the capacitor is not selected properly, it will have serious consequences for the motor. When replacing the start-up and running capacitors, it is best to use the same capacitors as the original configuration parameters. If the capacitor is damaged, and you don't know or can't see the annotated parameters clearly.