Radiocommunications Agency  EMC Awareness

Electric vehicles

Description

In the future, electric traction will probably become the main source of motive power for motor cars and other road vehicles. The electricity may be supplied by rechargeable batteries, fuel cells running on hydrocarbons or hydrogen, an internal combustion engine (the so-called ‘hybrid’ powertrain), or some combination of all three. Overall, these are known as ‘alternative powertrain’ vehicles (the internal combustion engine driving roadwheels via a gearbox being the default powertrain.)

A common aspect of electric traction will be the variable speed electric motor drive that uses pulse-width modulation (PWM). These are already widely used in fixed locations in industry and HVAC (heating ventilating and air conditioning) systems, and are in mobile use in some trains, trams and light railway systems.

The Radiocommunications Agency sponsored two studies, in 2000 and 2002, into the interference issues that might be caused by widespread use of such electric drive technology.

These studies found that there are some areas of concern, where improvements need to be made to the current design and construction practices used in such vehicles, to prevent them from causing an interference problem.

Also, the studies found that existing test methods are not ideal for measuring the true effect of alternative powertrain vehicles on the electromagnetic environment. They propose that new testing methods need to be developed to measure their electromagnetic emissions quickly and meaningfully.

Commentary

The edge-rate (switching speed) of the PWM signals are made very fast to minimise heat loss in the switching devices and improve power conversion efficiency. Unfortunately this means that their harmonic spectrum extends to very high frequencies, typically more than 1000 times their basic switching frequency. A typical switching frequency is 20kHz, and this would mean that the PWM motor drive waveform could contain significant levels of energy at frequencies greater than 20MHz.

The studies measured levels of emissions that would be likely to cause a problem for radio broadcast and communication services at up to 144MHz. At such frequencies the energy in the PWM waveform is relatively small, but on the other hand at such frequencies the cables and structure of the vehicles often make very effective ‘accidental antennas’.

References and links

“A study to assess the possible effects on radio based services of electromagnetic emissions from the proposed increase of electrically powered public and private transport”, by L S Blanchard and D Whitehead of the Transport Research Laboratory, October 2000, unpublished project report PR/SE/186/00, prepared for Radiocommunications Agency. Download from http://www.radio.gov.uk/topics/research/topics.htm#emc

“Investigation of electromagnetic emissions from alternative powertrain road vehicles” by A R Ruddle of the Motor Industries Research Association (MIRA) for the Radiocommunications Agency, 28 May 2002. MIRA report number 01-845060, RA reference AY 4117. Download from http://www.radio.gov.uk/topics/research/topics.htm#emc.

Links to Mitigation Techniques

	Installation	Design & Development	Resources
Design of switch-mode power converters to minimise emissions
Filtering with cable-mounted chokes
Filtering
Shielding of enclosures
Shielding of cables