Motor speed control
This motor speed control circuit uses only seven components to regulate the speed of a motor. Simple and very cheap to contruct. This circuit controls the speed of a motor from 0 to 50 % of its nominal power. Resistor R1 and potentiometer P1 divide the positive halfwave swing of the line voltage. During negative swings, the current is blocked by diodes D1 and D2 and the thyristor receives no triggering voltage. The thyristor, in turn, cannot conduct current.
But why can the circuit control only up to 50 % of the maximal motor power? Well, speed regulation is only practical at low motor speeds. At more than 50 % of the motor’s capacity, speed regulation becomes almost unnecessary. Anyway, below 50%, this circuit has the capability to precisely set the motor’s speed.
Motor speed control circuit schematic
If you want to be able to control the current up to 100 % like in appluing this circuit as lamp dimmer, you only need to connect a bridge rectifier between the line voltage and the circuit. In this case the bridge rectifier delivers both voltage swings intro the circuit. You must choose a bridge rectifier with the correct values.
Four 1N4004 are enough to handle lamps up to 200 watts. If you want to handle power up to 600 watts, you’d better use 1N5404.
Controlling the speed of the motor is quite simple. The voltage driving the motor is varied by a thyristor by changing the phase of its triggering pulse. A difference of few volts between the gate voltage of the thyristor and the cathode voltage (the motor voltage) determines the moment when the thyristor fires.
The rotation of the motor induces a counter-EMF with a force that is proportional to the motor’s speed but opposite to the polarity of the power supply. When motor rotates freely without load, the counter-EMF is equal to the power supply voltage. In this case, the thyristor blocks the whole half wave swing long enough that the energy delivered to the moros is just sufficient to overcome the mechanical and electrical resistance. Once the motor is loades, the speed will go down, thereby reducing the counter-EMF. At this moment, the thyristor conducts longer and deliverrs more energy so that the desired speed is regained and maintained.
Perfect speed compensation is only possible if the selected speed is far below the nominal value. As the selected motor speed comes closer to the maximum speer of the motor, the compensation becomes more difficult.
Motor speed controller PCB
In construction the circuit keep in mid that this motor speed control circuit works with hazardous voltages. A plastic box must be used as housing. The potentiometer must have a plastic handle and the knob must also be plastic. The thyristor must be heatsinked. Capacitor C1 must be able to handle 400 volts.
Random schematics
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