There is a type of motor powered by a single-phase 220V AC power supply called a single-phase asynchronous motor. Its structure is similar to that of a cage-type three-phase asynchronous motor, and it is also composed of a stator and a cage-type rotor.

　　There are two sets of coils on the stator, namely the main winding (working winding) and the auxiliary winding (starting winding).

　　When the main winding passes through a single-phase AC, an alternating pulsating magnetic field will be generated. Its spatial axis is fixed, and the intensity and direction of the magnetic field change according to the sine law. This pulsating magnetic field cannot generate enough electromagnetic torque when the rotor is stationary. There must be a secondary winding with a spatial phase difference of 90 degrees to participate in the starting. In the stator winding, the main winding and the auxiliary winding are arranged with a phase difference of 90 degrees, thereby forming a rotating magnetic field. The starting torque is increased. The circuit schematic diagram with a starting capacitor is shown in Figure 2.

　　In order to make full use of the phase shifting function of the auxiliary winding, improve the power factor and efficiency of the single-phase asynchronous motor, and reduce electromagnetic noise. A running capacitor C2 is connected between the starting winding and the main winding. When S is disconnected, the starting capacitor is disconnected. The running capacitor C2 allows the auxiliary winding to continue to work and increase the torque of the motor. The schematic diagram is shown in Figure 3. In general, the starting capacitor C1 is 5 to 10 times the capacitance of the running capacitor C2.

　　S is a centrifugal switch. The switch is installed on the front cover of the motor, and the centrifugal block is installed on the rotor shaft. When the motor is stationary, the centrifugal switch is closed. When the rotor speed reaches 80% of the rated value, the swing block on the rotor shaft is expanded under the action of centrifugal force, causing the slider to retract. Switch S automatically pops open due to loss of pressure. If the centrifugal switch is damaged, it can only be disconnected when the rated speed is reached. If it takes too long, the starting winding will be burned due to overcurrent.

　　According to the request letter, this single-phase motor is a two-pole (2850 rpm) running capacitor. Its winding structure is a double-layer concentric type. The main winding wire diameter is relatively thick and is in the lower layer of the stator slot. The secondary winding has a thinner wire diameter and is located on the upper layer of the stator slot. The main winding and the secondary winding are embedded at an electrical angle of 90 degrees (the electrical angle and mechanical angle of the two-pole motor are the same).

　　According to the request for help, the author drew the wiring diagram of the motor (as shown in Figure 4). When inspecting, first check whether there is a centrifugal switch inside the motor. If there is, the two gray wires are connected to the starting capacitor C1, with a capacitance of 100 μF~150 μF and a withstand voltage of 500VAC. If there is no centrifugal switch inside the machine, the two gray wires are connected to the PTC starting resistor. Its characteristics are: small cold resistance, about a few ohms to more than ten ohms. Large hot resistance, about tens of thousands of ohms or more (the two-pin demagnetization resistor of a color TV with fixed pins can be used as a substitute). Its function is: when starting, the cold resistance of the PTC is small, which can be regarded as the switch is turned on. Soon the PTC heating resistance increases, which can be regarded as the switch is turned off. The starting resistor plays the role of the centrifugal switch S.

　　According to the fault phenomenon analysis, the start-up is slow, the speed is slow, and the heat is severe in a short time, but the color of the winding coil is normal. There is no smoke or odor. The author initially determined that the problem lies in the starting circuit. When the motor starts, the starting circuit is not connected. The starting power provided by the running capacitor is limited. The motor cannot reach the rated speed. The speed is low. The current is large and there are electromagnetic noise and other phenomena.

　　According to the cause of the fault, the following points should be checked:

　　1. When the machine is stopped, is the centrifugal switch in a closed state? Is there any poor contact?

　　2. Is the starting capacitor missing or the capacitor internally broken?

　　3. Is there a cold solder joint between the starting circuit wire and the winding connection wire? Is the wire broken or in poor contact?

　　4. Is there a wrong wiring?

　　Check these possible fault locations one by one.

　　The possible fault locations are: the main and auxiliary windings are short-circuited.

　　The inspection method is to cut the casing of the common connection point of the main and auxiliary windings (where the winding is connected to the black line). Weld the connection points and use a megohmmeter to measure the insulation resistance between the main and auxiliary windings. If the inspection is normal, restore the connection points and replace the rotor with a normal motor rotor of the same specification to determine whether the above fault is caused by the broken rotor bar.