Noise in Electric and Hybrid Electric Vehicles

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December 11, 2012

Noise in vehicles (of all types) is fairly well understood. Heavy inductive loads (and corresponding inductive noise) can be found, such as in a variety of motors used for everything from accessories, such as window motors, to high-speed motors or pumps used in antilock braking or electric power steering systems. Ignition systems in internal combustion vehicles are notorious for broadcasting the brief but powerful discharge of the ignition coil. Alternators by nature “alternate” and require filtering to make sure their AC is properly converted to DC and not delivering an AC waveform downstream. A lot has been done over the years to mitigate these issues in vehicles. Critical signals are shielded, inductive loads are properly suppressed with blocking diodes, and alternators are almost universally filtered now to make “alternator whine” filter kits a distant memory (ask a modern teenage car-audio aficionado if he knows what that is).

Enter the electric car.

A few things have happened with the introduction of the electric car. First… the electric motor. That in itself is not very revolutionary or even an issue in itself. This, though, combined with the lack of historical knowledge, is. Developers of EVs tend to be young by nature- new, innovative technology on new programs. New startup companies. New ideas. Unfortunately, all this “new” may push out some of the “old.” In addition to this, you have the introduction of vehicles engineered in China, originally powered by internal combustion, transplanted to the US for a conversion to electric (the Wheego and Coda come to mind).

EVs are subject to many of the same noise sources that other (internal combustion) vehicles are. Where items specific to internal combustion (ignition noise, alternator whine) are omitted, they are replaced by noise from the electric motor and motor controls. This technology has transitioned overwhelmingly to synchronous AC motors. DC motors have the same noise issues but to a much lesser extent, and they tend to be focused on a single PWM frequency in the tens of kilohertz. AC controls can vary much more widely and with load.

EV drive motor noise is relatively easy to combat; like many other noise sources, cable routing is key. We all know that every wire is an antenna… so keep your antenna routed away from the transmitter you don’t want to receive from! Routing unrelated circuits away from drive motor cables and power supply cables near the motor controller electronics is key. Sensitive or low-voltage/high-impedance circuits will need to be shielded. Software averaging of sensor signals, where possible, is also of great benefit.

My other, more abstract points about the loss of knowledge and importation of vehicles/chassis is based on the trend towards startups and building teams from scratch. Larger, well-funded companies like Tesla had the ability to hire seasoned veterans who brought a lot of knowledge with them. They also had a long development or incubation period to develop a lot of knowledge. On the other hand, many other smaller startups or some freshly minted teams at larger companies are starting with a deficit of the basic knowledge of vehicle systems and handling noise in vehicles. Similarly, vehicles are being brought over that were designed for internal combustion but fitted with an EV powertrain stateside. Included accessory wiring may not provide for necessary noise immunity when an EV powertrain is laid in where available space allows. This may also not show up until multiple vehicles are built and minor variances in those vehicles place cables just within reach for inductively coupled noise. These are all items that have to be considered for anyone working on one of these teams.

While the modern car audio nut may not have to be bothered with filters for the alternator whine like their fathers, they will still have to listen to stories about them (just as we had to listen to stories about multivibrators).

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