Replicating the environmental conditions in which hybrid vehicle electric motors operate brings a new dimension to accelerated durability testing.
The traditional way to test an electric motor involves subjecting an entire hybrid-electric vehicle to extreme climate conditions. "But this method lacks the control and repeatability of using dynamometers in the test lab. It is also quite expensive and can have a substantial environmental impact as many vehicles are needed to complete the testing," said Randy Palmer, Marketing Manager for Horiba.
Putting an electric motor and an internal-combustion engine in a test cell is another option. "However, the cost and environmental impact are considerable when an actual fueled engine is running. The fueled engine is also a huge producer of heat and that has to be compensated for by oversizing the specimen fluid conditioning systems. Liquid nitrogen or equivalent fluid could be used for cooling, but the gas is lost to the environment," said Norm Newberger, Manager of Driveline Technology for Horiba.
Horiba's Electric Motor Durability Test Stand is not an entirely new option. "The key elements of the testing system—meaning Horiba's high-speed, low-inertia, four-quadrant, liquid-cooled dynamometer; the STARS test automation system; and the Systems Platform Advanced Real Time Controller (SPARC) advanced dynamometer controller—have been available and proven for several years in engine and driveline testing applications. What is new with the Electric Motor Durability Test Stand is the capability to perform environmental fluid temperature simulation," said Newberger.
Since a hybrid-electric motor is power dense, the motor releases substantial heat as a consequence of energy losses in the electromagnetic steel and copper material. "Fluids are used to remove this heat—and to lubricate the bearings—so running an electric motor with the fluids conditioned to extreme cold and extreme hot a certain number of test cycles correlates to a specific equivalent service life. A speed-to-market advantage is achievable since this type of testing is done in a highly compressed time frame," said Newberger.
Adding environmental temperature simulation to the accelerated durability electric motor test proved to be challenging. "The temperature extremes needed by the fluids and the impact of those temperatures on the test stand necessitated a robust test stand design. The fluid conditioning system was another very significant factor, which is why Horiba retained specialized suppliers. Horiba and its suppliers also developed unique methods to meet the objectives of cryogenic and superheated extremes as well as the fast transitions in temperature," said Newberger.
Design considerations also meant delivering the fluid—whether hot or cold—to the specimen as well as to the chamber surrounding the specimen. "We had to be sure the headstock did not sink away in the temperature, and the torque meter had to be insulated from the temperature changes. The system also had to have a feature for safely handling the harmful liquids. In short, all of the previous standard practices for the plumbing of coolant and oil took on a completely different perspective with such broad and changing viscosity. We feel that our latest Electric Motor Durability Test Stand will increase the speed to market of many more hybrid electric vehicles," said Newberger.