Gear dynamometers from G-Force Transmissions measure drag torque in NASCAR Nextel Cup car differentials.

Gear dynamometers from G-Force Transmissions measure drag torque in NASCAR Nextel Cup car differentials.


Flux-vector drives from Yaskawa Electric America Inc., Waukegan, Ill., go in highly accurate, self-cooling gear dynamometers used to test differentials for NASCAR Nextel Cup cars. The dynamometer maker, G-Force Transmissions, Cleona, Pa., chose electric motors and the Yaskawa drives over conventional "tractor" dynamometer designs that rely on throttled hydraulic motors and pumps for power and load. Such tractor dynos lack control sensitivity needed for the work.

A differential break-in test, for example, runs 2 hr, during which technicians plot differential temperature versus time, a measure of drag torque. The tests also heat the hydraulic oil, changing its viscosity and, hence, load torque. Such unwanted load-torque variations hurt test accuracy. Complex oil-to-water heat exchangers and cooling towers help but don't eliminate the heating problem.

Also considered was an eddy-current drive and load absorber. But this arrangement also needs a cooling tower to dissipate heat, and performance is only marginally better than that of conventional dynamometers.

The G-Force gear dynamometer, in contrast, does away with liquid cooling systems. It uses two ac motors hooked to a common power supply: one motor at the differential input and another at the output (absorber). A separate Yaskawa F7 adjustable-frequency, flux-vector drive controls each motor. A technique called common busing joins the drive's dc buses. Power from the absorber motor goes to the input-drive motor with little waste heat, thereby eliminating the need for cooling towers. About 92% of absorber motor power returns to the input motor.

The drives' closed-loop flux vector control lets the dynamometer supply the consistent 128 lb-ft of torque at 2,800 rpm needed for break-in tests. In practice, technicians using the equipment plot motor current and temperature of the differential under test. They reject units found to consume more than a prescribed amount of power.

NASCAR team Severn Motor sports uses the Yaskawa-equipped gear dyno to break in differentials on-site rather than outsource the work to an engine shop, saving the company about $600/day.

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