Due to their lightweight, low inertia, and high efficiency, coreless DC motors have become a popular choice across a wide range of industries. Controlling the speed of a coreless DC motor requires a carefully designed control system to optimize performance and ensure smooth operation.

In this article, we will discuss the methods used to control the speed of a coreless DC motor, providing data on common control techniques to help understand which method is suitable for different applications.

Fundamental Principles of DC Motor Speed Control

It’s critical to comprehend how the motor’s speed, voltage, current, and load are related in order to operate a coreless DC motor efficiently. A DC motor’s speed is typically proportionate to the voltage applied:

Fundamental Principles of DC Motor Speed Control

 

 

The following elements affect the management of speed:

  • Supply Voltage (V): Increasing the voltage will result in higher motor speed while reducing the voltage lowers the speed.
  • Load Torque (T): Heavier loads will slow the motor down, while lighter loads allow the motor to run faster.
  • Resistance (R): Resistance in the circuit, whether internal or external, affects current flow, thereby influencing motor speed.
  • Back EMF (E): As the motor rotates, it generates back electromotive force (EMF), which opposes the input voltage, reducing the overall speed.

Understanding Coreless DC Motor Speed Control

The basic idea behind coreless DC motor operation is the transformation of electrical energy into mechanical energy. The motor’s speed is inversely proportional to the load and immediately relates to the applied voltage. Therefore, to control the speed of the motor, we need to adjust either the voltage supplied or manage the current flow within the system.

The following are the primary factors influencing the speed of coreless DC motors:

  • Supply Voltage (V): Increasing the voltage will result in higher motor speed while reducing the voltage lowers the speed.
  • Load Torque (T): Heavier loads will slow the motor down, while lighter loads allow the motor to run faster.
  • Resistance (R): Resistance in the circuit, whether internal or external, affects current flow, thereby influencing motor speed.
  • Back EMF (E): As the motor rotates, it generates back electromotive force (EMF), which opposes the input voltage, reducing the overall speed.

The equation governing the speed of a coreless DC motor is:

equation governing the speed of a coreless DC motor

where:

  • n = Motor speed (RPM)
  • V= Applied voltage (V)
  • E= Back EMF (V)
  • k = Motor constant
  • T = Load torque (Nm)
  • C = Torque constant (Nm/A)

By controlling these parameters, we can effectively control the motor speed.

Speed Control Methods for Coreless DC Motors

There are several methods to control the speed of a coreless DC motor, each with its own advantages and limitations. Let’s review the most common techniques.

Voltage Control (Linear Control)

In voltage control, the motor speed is adjusted by changing the supply voltage. This is a simple method for speed control, but it lacks precision and is less efficient for high-power applications.


Pros


Cons

  • Simple implementation and low cost.

  • Limited efficiency, significant power losses, and poor speed regulation under varying loads.

Speed Control Methods for Coreless DC Motors

where:

  • V is the supply voltage
  • Vback EMFV is the voltage generated by the motor as it rotates,
  • R is the motor resistance.

Data for Voltage Control Method:

Voltage (V) Speed (RPM) Efficiency (%)
6 1,500 85
9 2,300 88
12 3,000 90
15 3,750 92
18 4,500 93

As seen in the table above, increasing the voltage results in a proportional increase in motor speed, but there is a limit to this method’s efficiency as the motor reaches its maximum rated speed.

b. Pulse Width Modulation (PWM)

Pulse Width Modulation (PWM) is a more efficient and precise method of speed control for coreless DC motors. Rather than delivering a constant voltage, PWM quickly turns the motor on and off while adjusting the length of the on-time (duty cycle). This modulated signal efficiently regulates the motor’s average voltage and, consequently, its speed.

The duty cycle  determines the average voltage delivered, calculated as:

Pulse Width Modulation (PWM)

Where:

  • Vavg = Average Voltage
  • Vin = Input voltage
  • D = Duty cycle (%)

For instance, if the input voltage is 12V and the duty cycle is 50%, the average voltage supplied to the motor is 6V.


Pros


Cons

  • Highly efficient

  • Offers smooth speed control

  • Reduced heat generation

  • Requires more complex electronics

  • May introduce noise into the system

Data for PWM Control Method:

Duty Cycle (%) Average Voltage (V) Speed (RPM)
20 2.4 1,000
40 4.8 2,000
60 7.2 3,000
80 9.6 4,000
100 12 5,000

As the table demonstrates, increasing the duty cycle results in a higher average voltage, which increases the motor speed proportionally.

c. Current Control

Controlling the current flow is another way to adjust a coreless DC motor’s speed. In this method, the current supplied to the motor is controlled, which directly affects the torque and speed. This technique is widely used in industries that need torque management and speed control, such as robots and medical equipment.


Pros


Cons

  • Provides control over both speed and torque

  • Suitable for applications with variable loads

  • Complex circuitry required

  • Less efficient compared to PWM

Data for Current Control Method:

Current (A) Speed (RPM) Torque (Nm)
0.5 1,000 0.1
1.0 2,000 0.2
1.5 3,000 0.3
2.0 4,000 0.4
2.5 5,000 0.5

3. Speed Feedback Systems

In more advanced applications, coreless DC motors are often paired with feedback systems to achieve closed-loop speed control. A common method is to use an encoder that monitors the motor’s speed in real time and adjusts the input parameters to maintain the desired speed.

a. Closed-Loop Control with Encoders

  • Encoder: This is a sensor that detects the position or speed of the motor. It provides feedback to a controller, which adjusts the input voltage or current to keep the motor running at the desired speed.

The feedback loop is crucial in maintaining speed consistency, especially under varying loads or input voltage conditions.

b. Proportional-Integral-Derivative (PID) Control

In a closed-loop system, a PID controller is generally employed to improve motor speed performance. By making real-time adjustments, the PID controller alters the motor speed in response to deviations between the target setpoint and the actual speed (error).

  • Proportional (P): Corrects the present error
  • Integral (I): Addresses past errors by summing them over time
  • Derivative (D): Predicts future errors based on the rate of change

PID Control Data for Coreless DC Motor:

Time (s) Set Speed (RPM) Actual Speed (RPM) Error (RPM)
0 3,000 2,800 200
1 3,000 2,900 100
2 3,000 3,000 0
3 3,000 3,000 0

As shown in the table, the PID controller gradually reduces the error, achieving the desired speed within 2 seconds.

Comparative Overview of Speed Control Methods

Control Method Complexity Efficiency Cost Applications
Voltage Control Low Moderate Low Simple applications, low-precision devices
PWM Control Medium High Medium Robotics, drones, automotive, precision devices
Current Control High Low High Torque-sensitive applications, robotics
PID with Encoder Very High Very High High High-precision devices, medical equipment

Conclusion

Controlling the speed of a coreless DC motor requires choosing the right method based on the application, power requirements, and control accuracy. For applications where precise speed control and responsiveness are critical, Gian Transimission offers the most reliable solution.