Keeping brush DC motors low cost

Inexpensive and easy to operate, brush DC motors provide the ideal balance of performance at the right price in industries such as automotive, aerospace, medical and industrial. As a result, billions are manufactured annually worldwide – a figure that is expected to increase over the next 10 years.

However, increasing electromagnetic compatibility (EMC) requirements along with more crowded and "noisy" electronic environments are threatening to upset the balance by driving the cost of these low-end solutions to a level on par with more expensive brushless alternatives.

The issue is the electromagnetic interference (EMI) generated by the brushes as they rub the commutator – an inherent drawback of the design. To counteract the noise generated, a combination of shielding and filtering components is required. This not only drives up the cost, but many EMI/RF filtering solutions for brush DC motors on the market are also not satisfactory to meet today's higher EMC requirements.

"Many EMI filtering solutions do not filter out all forms of noise that are generated and many cannot handle higher DC currents without a corresponding escalation of the cost," explains Christophe Cambrelin of Johanson Dielectrics, a company that manufactures a variety of multi-layer ceramic capacitors and EMI filters.

To address these concerns, companies such as Johanson Dielectrics are now offering more advanced EMI filtering solutions that increase costs of brush DC motors slightly, while meeting the evolving EMC requirements.

When electronic devices receive strong electromagnetic waves, unwanted electric currents can be induced in the circuit and interfere with intended operations. EMI can even cause physical damage in operational equipment.

Exacerbating the issue are increases in operating circuit frequency, noises of higher frequencies that expand the affected frequency range and the miniaturisation of electronic devices that shrinks the distance between source and victim. If that wasn't enough, many electronic devices are more easily affected by noise, even with less energy, due to circuits today that operate at lower voltages.

As a result, industries such as the automotive sector are increasingly turning to brushless DC motors. With these motors, the commutation is done electronically. Therefore there is significantly less noise generation (no noise generated by mechanical commutation), but the complexity and cost of implementation are increased.

So, given a choice, OEMs would prefer solutions that would maintain the relatively low price of the brush DC motors given the quantities involved.

EMI/RFI interference is either radiated or conducted in wide frequency range from several hundred hertz to several gigahertz. Radiated noise occurs when voltage is applied at varying levels to the wiring. To keep the radiations confined in the motor housing, several precautions should be taken by the manufacturers of brush DC motors. The most important is the material used for the motor housing, which should be metal, as well as a metal cap (not plastic) on top of it. When the cap is made of plastic, the user needs to cover it with a metal shield (that may be a metallised PCB).

When EMI/RFI is conducted, the noise generated travels along the electrical power leads and is then radiated. Shielding is ineffectual against conducted noise, so filtering is required with a separate device.

Traditional common mode filtering approaches include low pass filters comprised of capacitors that pass signals with a frequency lower than a selected cutoff frequency and attenuate signals with frequencies higher than the cutoff frequency.

Among the options for OEMs are two-capacitor differential, three-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 two-capacitor differential filters are insufficient because unmatched capacitors generate a different filtering of each line, and therefore mode conversion (i.e. part of common-mode noise is transformed into differential-mode noise, and vice-versa). Traditional three-capacitor filters are adequate, provided the EMC requirements are only at relatively low frequencies (i.e. <150MHz, such as AM/FM radios in automotive).

Although they provide good filtering performance, three-capacitor filters are generally ineffective when filtering noise in telecom frequency bands. Other solutions, such as feed-through filters, offer good rejection over a wide frequency band, but become expensive when the power line must carry a current of several Amps. Additionally, feed-through filters are single-ended devices, and therefore may introduce mode conversions (like two-cap filters). "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 brush DC motor as a low-cost solution," says Cambrelin. For brush DC motors, a possible alternative to low-pass filter is a common mode choke.

When a common-mode signal (same AC current) is going through each winding of the common-mode choke, the magnetic field coming from each winding adds up, and therefore the impedance increases significantly. On the other hand, when a differential signal (opposite AC current) is going through each winding, the magnetic field coming from each winding will subtract to each other and therefore the impedance decreases significantly.

That is why common mode chokes block common mode noise, but let a differential signal go through. Similar to feed-thru filters, a bigger and more expensive common choke is required to be able to carry a significant current (i.e. more than 1A rms). Despite the popularity of common mode chokes, a better alternative may be monolithic EMI filters.

Compared to common mode chokes, monolithic EMI filters provide significantly more RFI suppression in a substantially smaller package. A monolithic EMI filter also rejects a much wider frequency band and is not affected by the amount of DC current required because it is mounted in shunt (between lines and ‘ground’).

EMI filters combine two balanced shunt capacitors in a single package, with mutual inductance cancellation and shielding effect. These filters from Johanson Dielectrics use two separate electrical pathways within a single device attached to four external connections.

Like other EMI filters, monolithic EMI filters attenuate all energy above a specified cut-off frequency, only selecting to pass required signal energy while diverting unwanted noise to ‘ground’. The key, however, is the very low inductance and matched impedance. With monolithic EMI filters, the terminations connect internally to a common reference (shield) electrode within the device, and the plates are separated by the reference electrode.

Monolithic EMI filters can be effective from 50KHz to 6GHz and it filters both common-mode and differential mode noise. The filter also has virtually no limit to the amount of DC current, because it is designed to work in parallel to motor and no DC current flows through it.

Regardless of the type of filter, an often-overlooked factor is the fact that many brush DC motors are controlled by Pulse Width Modulated (PWM) Signal. With PWM signal, the voltage is switched on and off at a very fast rate between a few kilohertz (kHz) and tens of kHz. The total power supplied is based on the time the switch is on compared to the off periods. PWM signal is particularly suited for motors because the time constant of a motor is very long compared to the period of a PWM signal. That is why the brush 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," explains Cambrelin. "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." This can be a problem for users that are not used to developing differential LC filters.

With monolithic EMI filters, there is no need for special skills in filtering: the response of the filter (i.e. rejection of common-mode noise versus frequency) is provided directly by the manufacturer. Cambrelin says Johanson Dielectrics is also working on an integrated solution for brush DC motors that will allow to mount the monolithic EMI filter directly on the housing without the need to manufacture a PCB.

"EMI issues are just going to become more of a problem with the higher frequencies with Bluetooth, Wi-Fi and now 5G devices. That means EMI filters will have to handle wider frequency ranges while allowing the appropriate signals to pass through. This also helps OEMs meet regulatory standards that exist in most countries that limit the amount of noise that can be emitted."