Controlling the Speed of a Brushed DC Motor: Techniques and Implementations

When it comes to controlling the speed of a brushed DC motor, several methods can be employed to achieve the desired performance. Among these, varying the supply voltage, frequency modulation, and using thyristors are particularly effective and widely used techniques.

Varying the Supply Voltage for Speed Control

One of the most straightforward methods of controlling the speed of a brushed DC motor involves varying the voltage supplied to the motor. The basic principle is that the speed of a brushed DC motor is directly proportional to the voltage applied to it. To control the speed, you simply need to ensure that the voltage is higher than the start-up voltage and lower than the maximum voltage that the motor can handle. This method is reliable and often sufficient for many applications.

Historical Methods: Using a Thyristor for Speed Control

I built these control systems over thirty years ago, primarily to manage my train set. Back then, we used non-filtered battery charger supplies, which provided a stable voltage source. For the controller, a thyristor was used, with the motor connected in the emitter path of the thyristor. A variable resistor was used to send varying voltage levels to the thyristor's trigger.

One of the unique features of this setup was its ability to maintain a stable speed even when the motor encountered heavy loads, especially at low speeds. Here’s how the system worked:

When power is not supplied, the motor functions as a generator, producing voltage. The thyristor remains off because the voltage produced by the motor is not sufficient to trigger it. As the motor speed increases, the generated voltage increases, causing the thyristor to turn on and supply power to the motor. When the speed decreases, the generated voltage drops, and the thyristor turns off, thus stopping the power supply. This cycle helps maintain a stable speed, even under varying load conditions.

Modern Control Techniques: Frequency Modulation

In more modern applications, a common method of speed control is frequency modulation, also known as Pulse Width Modulation (PWM). This technique involves rapidly switching the voltage supplied to the motor on and off at a certain frequency, with the duty cycle determining the average voltage applied to the motor. Higher duty cycles result in higher average voltages and thus faster speeds, while lower duty cycles lead to slower speeds.

Variation with Variable Resistor

Another classic method involves using a variable resistor (potentiometer) to control the voltage supplied to the motor. By adjusting the resistance, you can vary the voltage, thereby controlling the motor's speed. This method is often used in simpler, less complex applications where fine-tuned control is not necessary.

Conclusion: Selecting the Best Method

Selecting the best method for controlling the speed of a brushed DC motor depends on the specific requirements of the application. Varying the supply voltage is a reliable and easy-to-implement method, especially for simpler applications. Thyristor-based systems offer more advanced control and stability, particularly in demanding environments where load changes are frequent. Frequency modulation and PWM offer precise control over speed, which is crucial for applications requiring high precision.

Whichever method you choose, understanding the basic principles outlined here will facilitate effective control of your brushed DC motor, ensuring optimal performance and efficiency.