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AB-005 : Electromagnetic Compatibility (EMC / EMI) for Vibe Motors

Overview : EMC and EMI Background

Electromagnetic interference (EMI) is the radiation or induction of electromagnetic noise on a system. DC motors are a common source of EMI, as are most electromagnetic circuit components. They are potential sources of noise and can generate common-mode currents. EMI can result in degraded performance, data corruption, or if strong enough can cause the system to fail completely. EMI can be radiated or conducted comes from magnetic and electrical sources, respectively. In the case of DC motors, both radiated and conducted emissions are present.

Electromagnetic compatibility (EMC) is the practice of monitoring and reducing unwanted EMI. There are often varying regulations on EMC, depending on the purpose of the system and the country it is used or sold in. For example, you can read about the EU’s EMC regulations here. EMC performance concerns the complete system or final product, and is therefore usually the responsibility of the OEM (original equipment manufacturer) and not of those who supply the components. Here we will show you some popular EMI reduction techniques for motors to improve your systems EMC performance.

Ceramic capacitor between motor terminals
Ceramic capacitor between motor terminals

The Generation of EMI

Arcing (sometimes referred to as arc discharge or voltaic arc) is an electrical characteristic where current can flow through the air, or other normally non-conductive materials. You may have seen instances of arcing between two wires, or on the power rails of trains or trams. This is not to be confused with a electrical spark, as an electrical arc is continuous, however they do look alike.

Whilst arcing can be useful, used in both welding and strip lighting, in some cases it can be a source of EMI. In DC motors, arcing can be common because of the periodic interruption of the current in the rotor windings. This very high-frequency spectra content, which can appear as wideband noise superimposed onto other signals. DC motors also provide paths for common-mode currents through their frames.

An additional source of radiated and conducted emissions can come from the driver circuit. A typical H-bridge circuit should ideally provide a constant current to the motor, but this current has fast rise time spikes due to the fast and frequent switching of the current in the driver circuit. Another significant problem is that usually the motor is quite far from the driver, and this creates a fairly large loop area between motor leads and device frame. The radiation potential is a direct function of the loop area; the larger the loop, the larger the emissions.

How to Reduce EMI and Improve EMC 

Common-Mode Current Reduction 

To block the common-mode current, a common-mode choke could be placed in the motor leads, as shown in the figure below.

Choke to block common-mode currents
Choke to block common-mode currents

Chokes are used extensively in the cell / mobile phone market; you can read more about this in our analysis of mobile phone vibration motor implementations.

Radiated and Conducted Noise Reduction

Standard Components

The easiest solution is to place a ceramic capacitor between the motor terminals, as close as possible to the motor. This isknown as a decoupling capacitor, and reduces EMI by removing some of the high frequency noise signals. The common value used for these decoupling capacitors is between 100pF and 100nF, depending on the size of the motor. There is an example picture at the top of the bulletin, and some example test results shown below.

310-103 - Capacitor effect on motor electrical noise
310-103 - Capacitor effect on motor electrical noise

If a PWM signal is used for motor control, the capacitor should be chosen to have a resonant frequency with the motor inductance far from the PWM frequency. A value between 10 and 100pF is usually correct. The resonant frequency \( f_{r} \) is calculated as:

$$ f_{r} = \frac{1}{2\pi\sqrt{L_{m}C_{e}}} $$
Where \( L_{m} \) is the motor inductance and \( C_{e} \) is the external decoupling capacitor. For motors with low terminal resistance an additional resistor in series with the capacitor will help with the dumping.

A more comprehensive solution involves two additional capacitors to the one above, one per motor terminal, connected between terminal and motor housing. This will reduce the arcing, especially for carbon brush motors.

X2Y attenuator

A number of more complex filters can be implemented with inductors, capacitors, and varistors as elements. A simpler, cheaper, and more effective solution can be implemented by a single X2Y® attenuator. The following PDF is an application note that describes the X2Y® attenuator operation and implementation, together with tip and trick to minimize the emissions from a DC motor.

More publications and a list of manufacturers can be found on the X2Y® Attenuators website.

X2Y Application Note

The graph below shows the emissions from a 27mm 12V DC motor in different conditions. The best results (lowest emissions) are obtained when a X2Y® attenuator is fitted (1 Component track on the graph). The PDF under the graph describe the test performed and the types of filter used.

Example of radiated emissions graph
Example of radiated emissions graph

Grounding Securing pads for PCB mounted vibration motors

A small improvement in EMC can be seen if the securing pads of the PCB mounted vibration motors are tied to ground. This grounds the case and the internal components which can help act as a shield to the commutation mechanism. It's not perfect though because EMI will still leak through the plastic end-cap; so you can't use it as a substitution for component based EMC suppression.

Conclusion

EMC is an important field in electronics with strict regulations, and DC motors and their circuits are a significant sources of EMI. It is therefore essential that engineers take the appropriate actions to reduce EMI improve EMC as much as possible. We have suggested several common techniques for to achieve this with simple additional circuitry.