Capacitor starting principle of single-phase motor
Single-phase electricity does not produce a rotating magnetic field. In order to make the single-phase motor rotate automatically, a starting winding can be added to the stator. The starting winding is spatially separated from the main winding by 90 degrees. The starting winding is connected in series with a suitable capacitor so that the current with the main winding is in phase. The difference is 90 degrees, which is the so-called phase separation principle. Such two currents that differ by 90 degrees in time into two windings that are 90 degrees apart in space will produce a (two-phase) rotating magnetic field in space, and the rotor can initiate rotation by itself under the action of this rotating magnetic field. stand up.
Capacitor Startup As shown, the stator windings of this type of motor have a start winding and a running winding that are spatially separated by 90°.
The starting winding is connected in series with the capacitor in the circuit such that the current in the starting winding branch is approximately 90° out of phase with the current in the working winding branch, ie, the single-phase alternating current is converted to two-phase alternating current, thereby generating a rotating magnetic field Let the motor start itself.
When the motor is rotated to 75% of the rated speed, the starting winding branch is cut off from the circuit by the start relay. When the motor is working normally, it is only connected by the working winding.
This type of motor has a large starting torque and a small starting current. The output power is 40-300W, which is commonly used in air conditioners, small refrigerators, and cold drinks machines.
Capacitor phase splitting start circuit
(a) heavy hammer start; (b) PTC start
1-motor; 2-starting capacitor; 3-heavy hammer starter; 4-PTC starter; 5-lamp switch; 6-lighting; 7-thermometer; 8-thermal protector
It has two windings. Generally, the main winding has a larger diameter. There is also a starting winding (secondary winding). The starting winding is connected in series with a capacitor. Its voltage is delayed by 90 degrees, so that the two sets of windings get different magnetic fields. A rotating magnetic field is formed and the motor is turned.
The starting mode of 220V AC single-phase motor is roughly divided into the following types:The first type, the split-phase starting type, as shown in Fig. 1, is assisted by the auxiliary starting winding, and its starting torque is not large. The operating rate remains approximately constant. Mainly used in electric fans, air conditioning fan motors, washing machines and other motors.
Secondly, when the motor is stationary, the centrifugal switch is turned on. After the power is supplied, the starting capacitor participates in the starting work. When the rotor speed reaches 70% to 80% of the rated value, the centrifugal switch will automatically jump off, and the starting capacitor completes the task. Was disconnected. The starting winding does not participate in the running operation, and the motor continues to operate in the running winding coil, as shown in Figure 2.
Third, when the motor is stationary, the centrifugal switch is turned on. After the power is supplied, the starting capacitor participates in the starting work. When the rotor speed reaches 70% to 80% of the rated value, the centrifugal switch will automatically jump off, and the starting capacitor completes the task. Was disconnected. The running capacitor is connected to the start winding to participate in the operation. This connection is generally used in places where air compressors, cutting machines, woodworking machines, etc. are heavily loaded and unstable. As shown in Figure 3.
A motor with a centrifugal switch, if the motor cannot be started successfully in a short time, the winding coil will burn out quickly.
Capacitance value: Double-valued capacitor motor, large starting capacitor capacity, small operating capacitor capacity, and withstand voltage is generally greater than 400V.
Positive and negative control:
Figure 4 is a wiring diagram with a forward and reverse switch. Generally, the starting winding of this motor has the same resistance value as the running winding, that is, the starting winding and the running winding of the motor are exactly the same in wire diameter and number of coils. This type of motor is generally used in washing machines. This method of forward and reverse control is simple and does not require complicated switching switches.
Figure 1, Figure 2, Figure 3, forward and reverse control, you only need to reverse the 1-2 line or 3-4 line to complete the reversal.
For the determination of the starting and running windings of Figure 1, Figure 2, Figure 3, the starting winding is usually much larger than the DC resistance of the running winding, which can be measured with a multimeter. Generally, the DC resistance of the winding is several ohms, and the DC resistance of the starting winding is ten ohms to several tens of ohms.
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