Inexpensive and easy to operate, brushed DC motors balance performance and price for the automotive industry. Manufacturers make billions of these simple devices each year, a figure expected to increase throughout the next 10 years.
However, increasing electromagnetic compatibility (EMC) requirements, along with more crowded and noisy electronic environments, could drive brushed motor costs closer to more expensive brushless alternatives.
Multiple factors are increasing the amount of electromagnetic interference (EMI) that can disturb functionality or damage electronic devices, starting with the amount used in close proximity at any time. Today’s automobiles include Wi-Fi, Bluetooth, satellite radio, GPS systems, LED lights, air conditioning, power steering, anti-lock brakes, rearview cameras, and other instruments. Numerous items using brushed DC motors include power seats, adjustable mirrors, windshield wipers, power windows, and sunroofs.
Eliminating EMI/radio frequency interference (RFI) with traditional methods no longer suffices, given increases in operating circuit frequency – higher frequency noises that expand the affected frequency range – and electronic device miniaturization that shrinks the distance between source and victim.
Traditional brushed DC motors generate EMI as they work, an inherent drawback of the design. Brushed motors are less expensive than brushless designs because they don’t require controllers. A stator creates a static magnetic field (with permanent magnets or electromagnetic windings), and a rotor rotates inside the stator, reacting to magnetic polar shifts. In brushless models, controllers vary the magnetic field to create rotation. Brushed models use carbon brushes attached to the rotor to activate alternating magnetic fields.
Unfortunately, brushes rubbing against the DC motor’s commutator (the component that controls the magnetic field variation) generate EMI noise, requiring shielding and filtering. Mitigation strategies increase cost, and many EMI/RFI filtering solutions for brushed DC motors can’t meet EMC requirements.
“Many EMI filtering solutions don’t filter out all forms of noise that are generated, and many can’t handle higher DC currents without a corresponding escalation of the cost,” says Christophe Cambrelin of Johanson Dielectrics, a company that manufactures multi-layer ceramic capacitors and EMI filters.
Johanson Dielectrics and other electronics suppliers now offer advanced EMI filtering solutions. The challenge is keeping DC motors affordable while meeting evolving EMC requirements.
EMI/RFI is radiated or conducted in frequencies from several hundred Hertz (Hz) to several GigaHertz (GHz).
Radiated noise occurs when voltage is applied at varying levels to the wiring. To keep the radiations confined in the motor housing, brushed DC motor makers should take several precautions. Motor housing materials should be metal, and the housing should have a metal cap. When using a plastic cap, manufacturers should cover the cap with a metal shield or metalized printed circuit board (PCB).
Conducted EMI/RFI occurs as generated noise travels along the electrical power leads. Shielding is ineffectual against conducted noise, requiring filtering with a separate device.
Common mode filtering approaches use low-pass filters comprised of capacitors, which pass signals at a lower frequency than a selected cutoff and attenuate higher-frequency signals.
Options include 2-capacitor differential, 3-capacitor (one X-cap and 2 Y-caps), feed-through filters, common mode chokes, LC filters, or combinations of these.
To meet increasing EMC requirements, however, low-cost solutions such as 2-capacitor differential filters are insufficient because unmatched capacitors generate a different filtering of each line, and therefore mode conversion (part of common-mode noise transforms into differential-mode noise, and vice-versa).
Traditional 3-capacitor filters are adequate, provided the EMC requirements are only at relatively low frequencies, such as AM/FM radios that operate at less than150MHz. At telecom frequency bands, 3-capacitor filters generally do not suffice.
Feed-through filters offer good rejection throughout a wide frequency band but become expensive when the power line must carry several amps. Additionally, feed-through filters are single-ended devices and may introduce mode conversions like 2-cap filters do.
“Regardless of the noise generated, if a high DC current is required, you will need a very large, expensive feed-through filter, which eliminates the brushed DC motor as a low-cost solution,” Cambrelin says.
A possible alternative to low-pass filters is a common mode choke. When a common mode signal goes through each winding of the common mode choke, the magnetic field coming from each winding adds up, increasing impedance significantly. When a differential signal goes through each winding, the magnetic field subtracts, reducing impedance. So, common mode chokes block common mode noise but let a differential signal go through. Brushed DC systems that need more than 1A current require common mode choke filters.
Monolithic EMI filters suppress more RFI in a substantially smaller package than common mode choke systems. They also reject a much wider frequency band and are not affected by the amount of DC current required because they mount in shunt (between lines and ground).
Such filters combine two balanced shunt capacitors in a single package, with mutual inductance cancellation and shielding effect. Johanson Dielectrics filters use two separate electrical pathways within a single device attached to four external connections.
As with other filters, monolithic EMI filters attenuate all energy above a specified cut-off frequency, passing required signal energy while diverting unwanted noise to ground. The key, however, is the very low inductance and matched impedance. Terminations connect internally to a common reference (shield) electrode within the device, and the plates are separated by the reference electrode.
Monolithic EMI filters operate from 50kHz to 6GHz, filtering common mode and differential mode noise. Because monolithic filters work in parallel to the motor, no DC current flows through it, so the filters have virtually no DC current limits.
Pulse-width modulated signals (PMW) control many brushed DC motors. The voltage switches on and off rapidly between a few kHz and tens of kHz. Total power supplied is based on the time the switch is on compared to off. PWM signaling is particularly suited for motors because the time constant of a motor is very long compared to a PWM signal. The brushed DC motor acts as if the average of the PWM signal was applied on the power leads.
“When you first test the motor in the lab, the EMI filter may perform well, but everything changes when you apply a PWM signal on the power leads,” Cambrelin explains. “You want to filter out the noise, but not unintentionally filter out the PWM signal. If you don’t choose the right filter, the motor may not even start.”
Monolithic EMI filters handle such challenges without requiring in-depth filtering knowledge, Cambrelin says. The response of the filter (rejection of common mode noise versus frequency) comes directly from the manufacturer. Johanson Dielectrics provides an online tool that simplifies such choices.
Cambrelin adds that future solutions could include monolithic EMI filters mounted directly on the motor housings, eliminating the need for PCBs.
“EMI issues are going to become more of a problem with the higher frequencies with Bluetooth, Wi-Fi, and now 5G devices,” Cambrelin concludes. “EMI filters will have to handle wider frequency ranges while allowing the appropriate signals to pass through. This also helps manufacturers meet regulatory standards that exist in most countries that limit the amount of noise that can be emitted.”
Johanson Dielectrics https://johansondielectrics.com