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Originally Posted by Tim Skloss
...measuring the torque properties is completely different as you need to have a way to measure and control torque.Any idea out there on how to do this?
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One of the guys in the National Instruments Motion group has a very crude dynamometer setup on his desk. All it is composed of is 2 motors (in his example they were the same type of motor) with their shafts connected, we'll call them the "load motor" (LM) and the "motor under test" (MUT). The MUT is powered in the usual way (for FIRST it would be via a Victor) and a simple current sensor measures the current draw of the motor. The MUT drives the LM, causing the LM to generate a
Back-EMF voltage. The Back-EMF voltage is a constant voltage generated by the motor per thousand RPM of the motor, and is linear across the RPM range of the motor; with the Back-EMF voltage it is possible to get a really good idea of the speed of the motor by measuring the voltage alone! The LM is then connected to a high-watt 10 Ohm resistor (like 300 Watts, it's almost the size of the motors!) purchased from
Ohmite. The resistor allows the voltage from the LM to produce a current, which then powers the LM in the opposite direction. By measuring the current coming off the resistor you can calculate the torque of the LM by using a torque constant (which gives you however much torque per Amp) found in the spec sheet of the motor. So, by measuring the currents and voltages on the LM and MUT you can generate some fairly accurate torque curves with a minimal hardware set.
One thing to note, however, is that in doing this you need to make sure the LM and MUT are aligned PERFECTLY so that there is no stress on the shaft coupling (and placing them in an enclosure would be wise, too). From what I hear the "first run" of the simple dynamometer wasn't lined up as well as it should have been, and the snapping of the shaft coupler wasn't a pretty sound (could have damaged the motors too!).
-Danny
PS: LOL, I guess this is the same setup as Jeff's above!