Simple, Accurate Motor Testing

Testing motor-driven consumer hand tools doesn’t have to be complicated or expensive and low cost doesn’t mean low accuracy. Wagner Spray Tech tests their new PaintEater™ paint stripping tool using a hysteresis brake and dc power supply to accurately and repeatedly produce the required torque loads. A PLC with a 0-10V output signal varies the power supply output current, to simulate various user loadings.

To effectively test the high-speed, rotary motor used in the product, the torque load is applied to simulate a person stripping paint. The load is applied and cut repetitively during a total testing time of 40 hours. The unloaded shaft turns at 2,500 rpm but when the system is loaded down with 110 oz-inch torque, it slows to 2,000 rpm. The brake is applied for 45 sec, off for 15 sec and the cycle is repeated for 10 min. The test stops for 1 min and then is repeated until the actual loaded time reaches the 40 hour total.

“The biggest challenge with the application is the high rpm required and the heat generated by the high speeds,” says Jeff Pedu, president of Placid Industries. He says a non-standard modification to the hysteresis brake added a port where compressed air could be put inside the brake to cool it.

By selecting the right type of brake for the application, the actual design and integration process of the test system went smoothly. Pedu says a magnetic particle brake wouldn’t have worked in the application. While magnetic particle brakes offer more torque and a much more compact package, the size of the hysteresis brake dissipates larger amounts of heat while still offering the precise load torque needed for low torque testing.

He says with any device, including a hysteresis brake, materials inside can be damaged by excessive temperatures. The grease in the bearings, if it rises above the recommended temperature specifications, can deteriorate very quickly and turn from grease to carbon. The wire for the brake’s coil windings is rated to withstand high temperature, for this special, high-heat application. It is important for the temperature of the brake to stay below the maximum temperature of the materials selected.

The hysteresis brake handles the relatively high speed and low torque reliably and accurately. Hysteresis brakes, unlike friction type, produce torque solely with a magnetic field. Since torque is produced without contact between moving parts, there is no wear or decrease in torque with usage. Closed loop torque feedback is unnecessary, saving cost and torque is proportional to input current and independent of speed.

Pedu says the system runs reliably because torque values aren’t changing over time, as long as the grease in the bearings or wires don’t overheat.

“You have the same reliable test year in and year out, without the need for a torque transducer to prove the actual load torque since the torque applied by the brake is so repeatable,” he says. “The torque transducer and electronics to go with it would be five to 10 times the price of the brake itself and provides a very good test for a smaller amount of money.”