The Benefits of Closed-Loop DC Motor Control

An open-loop brush or brushless DC motor's speed depends on the applied voltage, with higher voltages resulting in higher speeds and lower voltages resulting in lower speeds. These open-loop systems, however, are susceptible to external factors. An applied load, for example, can cause the motor's rotational speed to decrease according to the unit's speed-torque characteristics.

In applications like an electric drill or screwdriver, this decrease in speed may not be an issue. However, for more demanding applications like a peristaltic pump or surgical power tool, it's critical to control and maintain the motor's speed with the utmost stability and accuracy.

Using a controller with closed-loop speed capabilities can overcome these challenges, enabling you to continuously regulate the DC motor’s speed. Here’s a quick overview of the differences between closed- and open-loop systems:

Open-loop Speed Control in DC Motors

To achieve the required movement, many applications require simple DC motor control, which is typically accomplished by generating a defined cycle of voltages. This approach does not take into account the motor's speed, and the motor is expected to run properly according to its speed-torque characteristic. This method is referred to as an open loop.

In this open-loop setup, if the load increases, the speed of the motor will drop. An excellent example is the use of a personal drill: as the motor's load increases and it becomes more difficult to drill, you can feel the speed of the drill decreasing. Sometimes, using a more powerful motor or adding a gearbox can help you mitigate the decrease in speed. However, the effect will still exist and may be a problem for more critical applications.

Closed-loop Speed Control in DC Motors

For some applications, it's important to maintain a stable speed or to provide maximum power while the load torque is increasing. In these cases, you may want to use a controller with a closed-loop speed control function. This kind of controller will continuously measure the motor's speed via feedback from hall sensors and encoders that detect changes in the motor’s electromagnetic field. The controller will then compare this speed to the reference and readjust the voltage or current to keep a constant speed according to the desired speed command.

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Although DC motors are typically driven in an open loop, closed-loop speed control offers an effective option for controlling motor speed — especially in critical applications like surgical power tools that require precise speed control.

Knowing which speed control method to use in your application or product may be tricky, which is why engaging a trusted motor supplier early in the process is key. Questions? Drop us a line, and we’ll be happy to help. You can also read the full whitepaper here.