DC motor is very common and we use it for different purposes. But problem is, when used with a sensitive circuit, this DC motor creates problems in the working of the circuits. Such as communication disturbed, signal distortion, noise in amplifiers, and so on. In this article, we are going to learn how to reduce noise from DC motors.
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Table of Contents
Why DC motor produce noises?
In DC motors, especially the brushed motors, there is a commutator and two brushes. When the motor rotates and the brash is connected from one commutator’s teeth to another, then it generates a spark. This spark depends on the current flow through the brash. So large the motor, the large the spark, and the noise intensity is according to the spark. This spark generates noise in nearby audio amplifiers and can hamper data communication circuits electronically connected nearby.
What type of noise arises due to the DC motor?
When you run the DC motor, due to the spark, there is a noise created in the power supply terminal like in this image.
This noise is really dangerous for electronic circuits. If it is not eliminated or filtered, this will definitely hamper other circuits nearby. This noise increases with the current flow through the brush.
So, how we can eliminate this noise?
There are a few ways you can follow to eliminate the noise. One with some extra components and another is with some design practices.
Reduce noise from DC motor with components:
1. Using a Line filter to reduce noise from the motor:
Line filters are common for AC power lines. But you can use this line filter with the DC motor too to reduce noise. Usually, the line filter is made with a side-by-side inductor coil wound in a toroidal coil like this:
For AC power lines, it is important to use the Ground/Earth, but for the DC motor, you do not need those. So just use the filter coil and two non-polar capacitors rated double the motor operating voltage. Omit the C3 & C4.
2. Using the L-C filter to reduce noise from the motor:
If you can not manage the line filter coil, then you can simply use the common inductor coils with a capacitor like this image.
I’ll add a video in this article that shows, how to reduce noise from DC motor effectively with this LC filter. Anyway, the filter design is a little bit complex if you are new. So, you can try different values to find a suitable one. But note that a large inductor will also have some power loss. So use suitable inductance as low as you can. And the capacitor? Start from common 0.1uF. Besides, you can use this online tool to find your values within seconds, but it is wise to use trial and error to find your best one.
3. Using capacitor with the Motor terminals to reduce noise from the motor:
In small motors, you may find there is already a ceramic capacitor between the two terminals. This capacitor works as a noise suppressor. But this method is not so effective compared to the LC/Line filters. But you can use this for small motors such as toy motors.
For better results, you can add two more capacitors to the body of the motor to the terminals. But remember again, this method is suitable for small motors. But you can use it anyway for small toy motors.
4. Isolate Motor diving from the control circuit:
There is another method used in the industrial sector, keeping the motor driving circuit isolated from the control circuit. In this case, Opto-Isolators are used to communicate between the driver and control circuit. But that is an expensive way that is suitable only for industrial-grade devices.
You can use this type of isolated motor driver when you have a digital control circuit that controls multiple motors or inductive loads that generate noises. Isolation keeps the noise out of your control circuit.
Reduce noise from DC motor using these design practices:
1. Keeping motor lines short:
Keeping the motor leads and PCB traces short reduces the acting of the trace/line as an antenna. So noise will be less surrounded. Also, take care of corners, you can use vias or decoupling capacitors with traces if required to reduce the antenna effect.
2. Keep the power lines away from signal lines to reduce noise from the motor:
It is a common practice to keep the signal lines away from the power lines while you are designing the PCB. Because, in power lines especially when inductive loads are on the power line, it generates noises. If the signal line is near the power lines, then it will catch some of that noise. Which can hamper signals. That is why a minimum clearance should be maintained between a power line and a signal line. Besides PCB, no signal cables should be placed side by side with the motor power cables. Better arranging them in the opposite directions while cabling the circuits.
3. Twist motor cables to reduce noise from the motor:
From your board to the motor, there are power cables. Twist them with each other. And if possible use co-axile cables and connect the shield to the ground. This practice will help reduce noise but it will not suppress all the noises. It will simply reduce the antenna effect.
4. Use decoupling capacitors across power and ground:
It is another common practice to use decoupling capacitors with the VCC/VDD to GND in PCB. You should use the capacitors (use multiple for better results) near the power entring points. Use a 100nF and 10nF ceramic capacitor and put the lower one near the component. Besides non-polar, use polar capacitors in suitable intervals. Use larger values near high current consuming devices.
Example: You can use a 10uF, 100nF, and a 10nF with a microcontroller, but you should use a large electrolyte capacitor such as 100uF beside the Driver ICs (like ULN2003A).
Also, use non-polar capacitors in different midpoints when traces are long.
So, now you know how to reduce noise from DC motor. I’ve tested with an LC filter that worked excellent, here is a video of the test.
In this video, you can see the practical test result with an LC filter. See how effective this type of filter is.
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