why do some single-phase motors have two capacitors while others have only one?
a motor with two capacitors, one is the starting capacitor and the other is the running capacitor. the starting capacitor is connected to the centrifugal switch.
at present, there are three main categories of single-phase asynchronous motors:
the first type is the single capacitor phase-shifting type without centrifugal switch, such as electric fans, which are usually used on small motors.
the second type is centrifugal switch and single capacitor phase-shifting starting type, such as some fans and other equipment, but at present, due to various reasons, this kind of motor seems to be less and less. but in some special places, he does exist.
the third type is centrifugal switch, double capacitance and double value phase-shifting type. at present, it is the most common in many places, such as air compressor, cutting machine, desktop electric drill and so on.
first, simply put,
first, due to this design, the starting button torque is not large, so it is not suitable for high load equipment, especially for those with large starting button torque, such as air compressors.
in the second category, this kind of starting performance is greater than the former, but it is only suitable for stable operation after starting, because its auxiliary winding is used as starting, which completely depends on the rotating magnetic field of the main winding after starting. there is no so-called commutation, because the capacitor and auxiliary winding do not work through centrifugal switch and separation after the motor speed reaches a speed. the fatal disadvantage of this kind of motor is that, once carrying some high load equipment, such as air compressor, it often slows down after rotating, and then starts again through the auxiliary winding, so it is not suitable for many use scenarios. it is usually only used in fans and other places, but it has been replaced by the third type of motor.
in the third category, its principle is that it has both main winding, auxiliary winding and centrifugal switch. the auxiliary winding and main winding work together, which is similar to that in the first category, but what if the starting performance is reduced? the solution is to connect a large capacity capacitor in series when starting through the centrifugal switch. this kind of motor uses a large capacitor when the motor is at low speed. the current provided by this large capacitor usually exceeds the rated current of the winding. driven by such a high current, the rotating magnetic field is very strong, so as to drive the high torque output of the rotor to rotate.
however, after starting, in order to avoid the disadvantages of the second type of motor, the advantages of the first type are used. the centrifugal switch is centrifugally connected to another contact, and then incorporated into a capacitor with relatively small capacity. in this way, the auxiliary winding is still working, but the current is smaller than that at starting.
in this way, the motor has the advantages of the first type and the second type at the same time. at present, this kind of motor is widely used in single-phase power system.
the current cutting machine and air compressor are generally equipped with the third single-phase motor. there are two metal boxes and cylinders on the motor. each is equipped with a capacitor, and one of them is the starting capacitor (large capacity) and the other is the running capacitor (small capacity).
to sum up, in the second category, the centrifugal switch only connects and disconnects the auxiliary winding (also known as the starting winding) and the connection between the capacitor and the circuit, while in the third category of motor, it controls whether the capacitor used in the auxiliary winding is of large capacity or small capacity.
the wiring of one starting capacitor and one operating capacitor of single-phase motor is as follows:
look at the letter u1 in the wiring group u2 is respectively connected to two outgoing lines of the operating winding, v1 v2 is connected to two outgoing lines of centrifugal switch, w1 w2 is connected to two outgoing lines of the starting winding, and the starting capacitor is connected to w1 v2, the operating capacitor is connected to v1 w1。
put u1 w2 is connected with a connecting piece and then connected with a power cord, v1 u2 is connected with a connecting piece and then connected with another power cord. if the motor wants to reverse, connect the two connecting pieces horizontally or vertically, i.e. u1 v1 connected, w2 u2 together.