I. Dual-speed motor control principle and speed regulation principle
According to the speed formula of a three-phase asynchronous motor: n1 = 60f/p
There are several ways to adjust the speed of a three-phase asynchronous motor, such as using frequency conversion speed regulation (YVP frequency conversion speed regulation motor used with frequency converter), changing the excitation current speed regulation (using YCT electromagnetic speed regulation motor used with controller to achieve stepless speed regulation), or changing the motor's pole changing speed regulation, which is to change the connection method of the stator winding to change the number of pole pairs of the stator rotating magnetic field, thereby changing the motor speed.
According to the formula n1 = 60f/p, the synchronous speed of an asynchronous motor is inversely proportional to the number of pole pairs. Doubling the number of pole pairs reduces the synchronous speed n1 to half its original value, and the motor's rated speed n will also decrease by approximately half. Therefore, changing the number of pole pairs can change the motor speed (this explains why common synchronous speeds are 3000 rpm for 2-pole motors, 1500 rpm for 4-pole motors, and 1000 rpm for 6-pole motors). This speed control method is stepped and cannot provide smooth speed adjustment; it is only applicable to squirrel-cage motors. This is the speed control principle of a dual-speed motor.
The diagram below illustrates the most common single-winding dual-speed motor, where the speed ratio equals the pole ratio, such as 2 poles/4 poles or 4 poles/8 poles. When the stator winding is changed from a delta connection to a YY connection, the number of pole pairs changes from p=2 to p=1.
Therefore, the speed ratio = 2/1 = 2
II. Control Circuit Analysis (Wiring diagram of dual-speed motor shown below)
1. Close the air switch QF to introduce three-phase power.
2. Press the start button SB2. The coil circuit of AC contactor KM1 is energized and self-locked. The main contacts of KM1 close, introducing three-phase power to the motor. L1 is connected to U1, L2 to V1, and L3 to W1; U2, V2, and W2 are left floating. The motor runs in delta connection, at which time the motor p=2 and n1=1500 rpm.
3. FR1 and FR2 are overload protection components for motor Δ and YY operation, respectively.
4. To switch to high-speed operation, press button SB3. The normally closed contact of SB3 opens, de-energizing the KM1 contactor coil. The main contacts of KM1 open, disconnecting U1, V1, and W1 from the three-phase power supply L1, L2, and L3. Its auxiliary normally closed contact closes again, preparing for energizing the KM2 coil circuit. Simultaneously, the KM2 contactor coil circuit is energized and self-locked, its normally open contact closes, connecting the three ends U1, V1, and W1 of the stator winding together, and introducing the three-phase power supply L1, L2, and L3 into U2, V2, and W2. At this time, the motor runs in YY connection, with p=1 and n1=3000 rpm. The auxiliary normally open contact of KM2 opens to prevent KM1 from malfunctioning.
5. In this control circuit, the normally open contact of SB2 is connected in series with the KM1 coil, and the normally closed contact of SB2 is connected in series with the KM2 coil. Similarly, the normally closed contact of the SB3 button is connected in series with the KM1 coil, and the normally open contact of SB3 is connected in series with the KM2 coil. This control is the interlocking control of the buttons, ensuring that the Δ and YY connection methods cannot occur simultaneously. At the same time, the normally closed auxiliary contact of KM2 is connected to the KM1 coil circuit, and the normally closed auxiliary contact of KM1 is connected to the KM2 coil circuit, which also forms an interlocking control.
III. The stator wiring diagram of the dual-speed motor is as follows:
