Instrumentation • Electronics
different wheels when cornering for example. In electric vehicles, the power drive usually runs on a high
voltage battery of cells connected in series to produce a PWM output of 400-700V, whereas everything else typically runs off a conventional 12V battery. Higher voltage produces the same power at lower current in order to reduce cable size to save on cost and weight. From a product liability point of view, there is a legal
requirement for auto makers to apply stringent tests according to ISO 11451 Part 2 and CISPR 12 and 25 specifications, but according to Carl Wayland, EMC technical and quality manager at the Motor Industries Research Association (MIRA), they will often exceed these requirements with additional tests devised in house. “The standards call for testing of surges, voltage dips
and interruptions, particularly for to grid where you need to make sure you are not putting harmonics into the network,” Wayland clarifies. “When an electric vehicle is charging, there must also be no radio interference, so it depends on how well the PWM inverter is suppressed, the shielding techniques used and how much engineering has been put into it all. “We are talking of high current at high voltage, so what
gives off EMC emissions is given by the change in voltage, or dv/dt, in a signal to drive a motor. You need expensive test equipment like receivers, signal generators, amplifiers and transient generators to replicate high voltage transients which occur in the environment and surges to simulate electrical strikes, as well as voltage dips and variations on power mains.
“These tests are generally performed in an anechoic chamber kitted out with network analysers, scalar analysers and other diagnostic equipment for monitoring the vehicles under test. It can well over £3 million to fully kit out an EMC test chamber complete with ‘rolling road’ dynamometer and test equipment. We currently have 11 test facilities at MIRA and do 15 to 20 shifts a day to keep up with technology and changing test requirements.”
Over-engineering
Wayland cautions against over-engineering in vehicle design without paying attention to the subsequent test regime this will incur. EMC can be modelled on a desktop computer but until the vehicle is actually in a chamber with RF being directed at it and RF being emitted from it, the position of units and modules means that millimetres can make a significant difference in terms of results. Auto makers therefore ensure that component suppliers test their subsystems at a high level. EMC design therefore needs to be integrated into
silicon, the first part of the vehicle supply chain. Mart Coenen, chief executive officer at EMCMCC in Eindhoven, The Netherlands, is involved with integrated circuit (IC) development, pre-qualification and characterisation for linking to various vehicle sensors like temperature, pressure and fuel flow. Between the ICs and their sensors, the interface may range from simple analogue to sophisticated digital fieldbus protocols. “Most sensors nowadays are provided with four
Fig. 2. The Vauxhall Ampera has 30kg of cable shielding to counter electromagnetic interference.
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