Case Studies

The smarter way to test DC motors

Battery powered vehicles have been with us for many years and acknowledged as the workhorses for the milk delivery industry, as golfing buggies and in many other niche or specialist applications.

Latterly the impact of two major drivers has focused greater attention on the acceptability and benefits of electric vehicles, thrusting them once more into the headlines; these being the rocketing price of hydrocarbons and city pollution.

One leading edge UK manufacturing company, Zytek, saw the need for electric vehicles in 1994 when Co-founder Bill Gibson, a long-time exponent of light-weight, very high performance electric vehicles, began the development of what was to become the highest power/density ratio, electric vehicle drivetrain to date.

Gibson had realised that two of the major challenges in this field, precision engineering and control systems, were areas where Zytek had exceptional expertise, gained from work with Chrysler on hybrid street vehicles and from Zytek's own F1 racing engine management experience.

Initial development and test work centred on a Lotus Elise followed by the ultra compact smart car chassis, the EV10-15. The successful completion of this project led to Mercedes-Benz, the brand owner, commissioning Zytek to covert 100 'smart fortwo' cars to electric propulsion at their production facilities in Fradley, Staffordshire. The vehicles are being delivered in a market trial to selected British customers as lease vehicles.

The drivetrain installed in the smart electric vehicle utilizes state-of-the art brushless DC motor technology, designed and manufactured in house by Zytek. This innovative drivetrain integrates the electric motor, power and control electronics, into one, compact assembly with only three connections: water, high voltage (300V) and low voltage (12V).

With each motor being assembled by Zytek prior to fitting, there was a requirement for a test instrument to run a number of tests on the wound stator, prior to shrinking it into its aluminium case and fitting a brushless rotor, and then again after the motor was potted and assembled prior to fitting into the vehicle. A subsequent hi-volt test on the whole electronics loom completed the sequence.

Using the accumulated data from the tests, feedback was initiated between Zytek and the UK motor windings' manufacturer resulting in a virtually nil rejection rate as the build continued.

The use of Whitelegg Machines Baker AWA 4 tester for this purpose came about through a visit by Zytek engineering staff to the Berlin Coil Winding Show. The AWA was chosen as it had the versatility to test short production runs, was easily portable and had the flexibility and capacity to run resistance, Hi Pot, Meg Ohm, peak to peak tests etc. The software was seen as straightforward with reports written in MS Access easily downloadable on a windows platform.

Commenting on the utility of the AWA, Neil Heslington, Managing Director, was keen to point out how versatile it was. Other testers were designed for long production runs. The AWA is a flexible instrument equally at home in the factory's various test situations as well as on our much shorter batch runs. The software is particularly straightforward and good for PC interpretation.

The smart fortwo ev sets a new benchmark in the electric vehicle sector, the 55kW drivetrain produces a top speed of 100km/h. The 13 kilowatt hours per 100 kilometres gives it a range of 107 kilometres, ideal for short urban journeys. Battery charging takes three and a half hours to charge from 30 to 80 percent of capacity. An empty battery can be fully charged in no more than eight hours, for example overnight.