This article presents a simple and effective method for controlling the speed of a single-phase motor without the need for mechanical gears, significantly improving the efficiency and usability of mechanical equipment. The circuit is specifically designed for 220V single-phase motors with a rated current below 6.5A and a power rating of approximately 1kW. It is commonly used in household electric fans, ceiling fans, and similar devices. With some modifications, this circuit can also be applied to dimming systems, electromagnetic vibration control, and automatic temperature regulation in fans.
The core of the circuit is a full-wave bridge rectifier composed of silicon diodes VD1 through VD4. This bridge is connected in series with the motor, providing a full-wave rectified voltage to the thyristor VS. When the thyristor is triggered, it creates a low-resistance path in series with the motor. During the negative half-cycle at point A, current flows through the motor, VD1, VS, R1, and VD3. In the positive half-cycle at point B, the current loop includes VD2, VS, R1, VD4, and the motor. As a result, the motor receives alternating current, and its terminal voltage is primarily determined by the conduction angle of the thyristor. By adjusting the firing angle of the thyristor, the voltage across the motor can be controlled, allowing for smooth speed regulation.
The thyristor's trigger pulse is generated using a simple unijunction transistor (UJT) circuit. Capacitor C2 charges through resistors R4 and R5 to the Zener diode’s stable voltage UZ. Once C2 reaches the peak voltage of the UJT, it triggers the transistor, which in turn activates the thyristor. After the UJT's emitter voltage drops, it turns off, and the capacitor recharges during the next cycle. The charging time of C2 depends on the motor's current, creating a feedback mechanism that helps maintain torque at lower speeds.
The value of resistor R1 is determined experimentally to ensure proper feedback. If the motor current increases during a cycle, the voltage across C2 rises, reducing the time needed to reach the UJT's peak voltage. This results in a smaller firing angle, increasing the average voltage applied to the motor and enhancing torque. Diode VD5 and capacitor C1 prevent unintended triggering during the thyristor's conduction period.
Resistor R2 acts as a current limiter and ensures stable operation of the Zener diode DW1. Increasing R2 reduces the phase shift angle and the motor voltage range. R4 controls the maximum output voltage, while R3 provides temperature compensation for the UJT. This circuit can also be adapted for reversible motor control, offering a continuous voltage adjustment range from 35V to 215V.
If high load voltages are not required, the bridge rectifier can be replaced with a half-wave rectifier, reducing the voltage range to 30V–100V. For fan temperature control, a thermistor-based circuit can be used. As ambient temperature changes, the thermistor’s resistance alters, affecting the conduction angle of the thyristor and adjusting the fan speed accordingly.
When selecting components, ensure that the diodes VD1–VD4 have a breakdown voltage above 400V and a current rating over 0.4A. The thyristor should have a breakdown voltage exceeding 500V and a current rating of at least 1A. The UJT BT35 must have an η ratio greater than 0.5, and the transistor 3CG14 should have a β value above 80.
After assembling the circuit, connect the fan and adjust the potentiometer RP until the fan stops. Then, use a soldering iron to heat the thermistor; the fan speed should increase as the temperature rises. Removing the soldering iron will slow the fan down. Proper adjustment of RP is essential for optimal performance.
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