Industrial Electronics

Speed control techniques for DC motors

27 March 2024
The speed of a motor can be controlled by a potentiometer board. Source: Adobe Stock

Speed control options and the idea of speed regulation vary for different motor types. In a DC motor, the operator or an automated control device must manually adjust the speed of the motor; in a speed-regulated motor, the speed varies naturally. There are several methods to control the speed of a DC motor, each with its own advantages and limitations. This article will discuss some of the common techniques used to control DC motors.

1. Varying the armature voltage:

This is the most fundamental method based on the principle that the speed of a DC motor is directly proportional to the voltage applied to its armature. This technique for regulating speed is also called the Ward Leonard Method.

  • By increasing the voltage, the motor's speed increases, and vice versa.
  • This can be achieved using:
    • Variable resistors (rheostats): Simple but inefficient, as energy is wasted as heat.
    • DC-DC converters: More efficient, allowing for voltage regulation and potentially bi-directional control.
  • Disadvantages:
    • More floor area and a more expensive foundation are needed.
    • Regular upkeep is necessary.
    • Due to decreased efficiency, the losses are greater.

2. Field control method:

A rheostat, which is a kind of variable resistor, is linked in series with the field windings in the flux control method. The motor's speed can be increased by increasing this component's series resistance in the windings, which in turn reduces the flux. It allows for precise and variable speed control of the motor.

  • This method adjusts the strength of the magnetic field generated by the stator, affecting the motor's speed.
  • Applicable to motors with separately excited field windings:
    • Weakening the field: Increases speed but reduces torque.
    • Strengthening the field: Decreases speed but increases torque.
  • This method is less common due to its limited speed range and potential stability issues.
  • Advantages:
    • Rapid changes in speed or load can be accommodated more effectively, contributing to better overall performance.
    • Helps minimize heating and wear on components leading to a longer lifespan of the motor and reduced maintenance requirements.

3. Pulse width modulation (PWM):

This widely used technique rapidly switches the DC power supply on and off, controlling the average voltage delivered to the motor. It allows for efficient control of motor speed by regulating the average power delivered to the motor. Instead of using resistive methods, which dissipate excess power as heat, PWM adjusts the duty cycle to achieve the desired speed, minimizing energy losses.

  • By varying the duty cycle (on-time versus off-time ratio), the effective voltage and, consequently, the motor's speed can be controlled.
  • Advantages:
    • Highly efficient, minimizing energy waste.
    • Enables precise and dynamic speed control.
    • Commonly used with microcontrollers and integrated circuits for sophisticated control strategies.

4. Armature resistance control:

The armature resistance control operates on the basis that back electromotive force is directly proportional to motor speed. It follows that, with a constant supply voltage and armature resistance, the motor speed is proportional to the armature current.

  • This method involves adding a variable resistor in series with the motor's armature.
  • Increasing the resistance reduces the voltage reaching the armature, thereby lowering the motor's speed.
  • Disadvantages:
    • Inefficient, wasting energy as heat in the resistor.
    • Limited speed control range, often used for braking or slow-speed operation.

5. Combination methods:

Combining different techniques can offer more flexibility and control over the motor's speed and torque characteristics. For example, PWM can be used for overall speed control, while field control can be used for fine-tuning or adjusting torque under varying loads.

Further considerations:

  • Motor type: Different motor types (brushed, brushless, PMDC, etc.) may have limitations or specific considerations for speed control methods.
  • Feedback control: Implementing feedback mechanisms (e.g., encoders) allows for closed-loop control, ensuring the motor maintains the desired speed even under load variations.
  • Control circuitry: The complexity of the control circuit varies depending on the chosen method and desired level of precision.

Conclusion

Regulating the machine's speed in such a way that it impacts the motor's rotational speed — and the machine's functionality — is critical for maximum performance. By understanding these different techniques and their trade-offs, engineers can select the most appropriate method for a specific DC motor application and achieve the desired speed control performance.

To contact the author of this article, email GlobalSpecEditors@globalspec.com


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