How to eliminate noise from DC motor?

DC motors are connected to the power supply through the commutator brushes. When current flows through the coils, the magnetic field generates force, and the force causes the DC motor to rotate and generate torque. The speed of brushed DC motors is achieved by changing the operating voltage or the strength of the magnetic field. Brushed motors tend to generate a lot of noise (acoustic and electrical noise). If these noises are not isolated or shielded, electrical noise will interfere with the motor circuit, causing the motor to operate erratically. The electrical noise generated by DC motors can be divided into two categories: electromagnetic interference and electrical noise. Electromagnetic radiation is difficult to diagnose, and once a problem is detected, it is difficult to distinguish from other noise sources. Radio frequency interference or electromagnetic radiation interference is due to electromagnetic induction or electromagnetic radiation emitted from an external source. Electrical noise may affect the effectiveness of the circuit. These noises can cause simple degradation of the machine.

When the motor is running, sparks occasionally occur between the brushes and the commutator. Sparks are one of the causes of electrical noise, especially when the motor is started, and relatively high currents flow into the windings. Higher currents generally lead to higher noise. Similar noise can also occur when the brushes remain unstable on the commutator surface and the input to the motor is much higher than expected. Other factors include the insulation formed on the commutator surface, which can also cause unstable current.

Electromagnetic radiation interference can couple to the circuit part of the motor, causing motor circuit failure and performance degradation. The level of electromagnetic interference depends on various factors, such as the type of motor (brush or brushless), drive waveform and load. Generally, brushed motors generate more electromagnetic interference than brushless motors. Regardless of the type, the design of the motor greatly affects electromagnetic leakage. Small brushed motors sometimes generate large RF interference. Most of them use simple LC low-pass filters and metal casings.

Another source of noise is the power supply. Since the internal resistance of the power supply is not zero, the non-constant motor current will be converted into voltage ripple on the power supply terminals during each rotation cycle. DC motors will generate noise during high-speed operation. In order to reduce electromagnetic interference, the motor is placed as far away from sensitive circuits as possible. The metal casing of the motor usually provides sufficient shielding to reduce airborne electromagnetic interference, but additional metal casing should provide better electromagnetic reduction capabilities.

The electromagnetic signals generated by the motor can also be coupled into the circuit to form the so-called common-mode interference. This type of interference cannot be eliminated by shielding, but can be effectively reduced by a simple LC low-pass filter. In order to further reduce the electrical noise, filtering is required at the power supply. This is usually done by adding a larger capacitor (such as 1000uF and above) across the power supply terminals to reduce the effective resistance of the power supply, thereby improving transient response. The filter-smoothing circuit diagram (see the figure below) is used to complete overcurrent, overvoltage, and LC filtering.

Capacitors and inductors usually appear symmetrically in the circuit to ensure the balance of the circuit, forming an LC low-pass filter to suppress the conducted noise generated by the carbon brushes. The capacitor mainly suppresses the spike voltage generated by the random disconnection of the carbon brushes. At the same time, the capacitor has a good filtering function. The capacitor is generally installed in a ground connection. The inductor mainly prevents the sudden change of the gap current between the carbon brush and the commutator copper sheet. Grounding can increase the design performance and filtering effect of the LC filter. Two inductors and two capacitors form a symmetrical LC filtering function. The capacitor is mainly used to eliminate the spike voltage generated by the carbon brushes, and the PTC is used to eliminate the impact of excessive temperature and excessive current surge on the motor circuit.

in conclusion

To reduce the level of EMI, the motor should be placed as far away from sensitive circuits as possible to reduce interference, and an additional metal enclosure should be provided. To suppress EMI in case of common mode interference, a simple LC low pass filter is built in. Other electrical noise can also be eliminated by connecting the motor with a simple speed controller. A higher order LC filter can further improve the noise filtering performance.