Quote:
|
Originally Posted by Gary Dillard
OK, the instrument you are looking for is a dynamometer.
|
Exactly, but I am trying to figure out how to make one for the least cost.
So forget about the "torque sensor" idea. I guess what I am really looking for is a way to apply a known (but variable) rotational load/torque/resistance to a test motor and measure the power (voltage and current) it takes to overcome the rotational load/torque/resistance and keep the motor spinning. Like a mechanical brake but with a clutch that can be set to slip at a known torque. Something that can be clamped to a motor output shaft in some sort of a test stand.
Since I don't know where to get small mechanical rotary brake like thing with a user settable infinitely variable slip clutch but I do have a bunch of small DC motors laying around to play with what about this idea. First let me state that my assumptions are that for any DC motor the output speed is related to input voltage and the output torque is related to the input current. Someone please correct me if I am incorrect on this.
Lets say I hook two DC motors together end to end by the drive shaft using a coupler and secure the motors to some sort of test stand. I'll call motor "A" the TEST motor and motor "B" the LOAD motor. For this example, I am thinking a Fisher Price or drill motor as the test motor A and the well known CIM motor as the load motor B. If I apply power to the test motor A such that it gets both motors spinning I should be able to measure a voltage of some sort on the leads of the load motor B because motor B will act as a generator right? In fact, from what I understand you can actually use a small DC motor as a tachometer using a multimeter if you know the specs of the motor. Now if I were to apply power to the load motor B such that it wants to turn in the opposite direction as test motor A then the test motor A would have to start working harder to counteract the torque produced by the load motor B and thus the test motor A would draw more current from the power source. It seems to me that in an ideal world if you knew all the power curves (torque vs. current & speed vs. voltage) of the load motor B and could measure all the power (voltage & current and calculate watts) going into the two motors then you could solve for the power curves of the test motor A. Although the measurements could be taken with a multimeter the Medusa "Power Analyzers" I mentioned above seem to be a nice purpose built reasonably priced product to do all the voltage, current, watt, RPM, and even temperature measurements for the "dyno" system and even connect to a PC for graphing, etc. What seems to me to be the unknowns in the test "dyno" setup are:
1) What happens to the energy created by the "generator effect" when you then apply a reverse voltage to the load motor B? How does this effect the measurement setup and calculations?
2) Of course, there will be lots of energy lost to heat, friction, etc. How do you account for this when creating data points on the curves?
3) Would this motor to motor setup even be a good idea since you would basically not only be stalling the load motor B at all times but stalling it while it is being forced to turn in the opposite direction than it wants to turn. Seems like the whole setup might not last too long. Given this wouldn't it always be better to have a much higher power motor as the load motor B and barely drive it with a small amount of power relative to the test motor A so that it pulls a known amount of "drag" or "reverse torque" on the test motor A but minimizes the losses to heat, etc. from the stall?
4) The whole motor to motor test configuration is just an idea to be able to produce a known, measurable, and infinitely variable amount of reverse torque on the test motor. If the whole motor to motor setup is a really bad idea (I kinda think it might be) where would one find the small mechanical brake with infinitely variable torque calibrated slip clutch thingy that can be clamp mounted on the test motor output shaft? I guess it would have to be some sort of magnetic clutch type device just not a motor.
... N) What else have I not considered?
Now, given all of the above what about throwing a gearbox between the motors? Wouldn't a "dyno" setup of some sort like this be useful for team testing gearboxes? I am just throwing around some ideas I have floating around in my head right now and trying to an academic discussion on the topic so I might learn something. So does anyone have any thoughts?
Thanks,
Chuck