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In Reply to: RE: Attenuating the generator. posted by twystd on July 25, 2016 at 10:09:28
twystd-
You have to think about the motor as both a motor and a generator (it is in fact, constructed very similar to an AC generator with a permanent magnet spinning inside of field coils). As the motor spins, it generates a back EMF (voltage) that opposes the drive voltage, effectively increasing the motor winding impedance. If you keep the voltage constant, at some speed the back EMF will equal the applied driving signal and the net effect will be zero current flowing in the motor (actually, it would stall before they became equal, but you get the idea). 78 RPM is 2.34 x 33 RPM so the back EMF will be similarly higher. On my 3 phase prototype, I monitor this and change the output voltage for any change in frequency so the power remains nearly constant. The drive voltage at 5 RPM platter speed is 2-3VPP, and at 78 RPM, nearly 24VPP.
Follow Ups:
Thanks for that. Since I'm seeing an increase in voltage when I change from 60hz to 81hz that is due to back EMF, and when I decrease the generator voltage to get 120V on the motor at 81hz I am actually decreasing the ratio of driving EMF to back EMF. I assume this would decrease torque at 81hz @120v, when compared to 60hz @ 120v. Do I have this right?
Now I'm thinking that maybe I should machine the capstan to run @50hz @ 33 1/3, and 67.5hz for 45. My transformer cores are good to 50hz. Do you think that would be a good idea?
twystd
The increase in voltage you are seeing is most likely a reduction in the voltage sag of your output. You can't really directly measure the back EMF, but it will manifest itself as an increase in effective impedance of the motor coils as the motor spins faster. Decreasing the voltage output at 81Hz will reduce the torque, but the torque will decrease with the higher speed even with the voltage held constant as there is less current flowing in the coils as the effective impedance increases.
I believe Mark Kelly posted somewhere that the approximate impedance of the motor coils = (coil resistance x forward voltage)/(forward voltage-back EMF). At zero speed, the output of the amp sees the DC resistance of the coils; as the motor increases in speed, the effective impedance will increase. If you want the motor to consume the same power at all speeds (and maintain torque) you will have to increase voltage with speed to compensate.
Going to 50Hz will require more power than at 60Hz. There's really no reason to do this unless you have problems with higher voltage at 81Hz.
Just when I thought I had a grasp on all this, going to have to re-think. I'm pretty sure this VFD will meet my needs, but a little more experimenting and testing is in order. Thanks again for the heads up, don't hesitate to let me know if I'm barking up the wrong tree. As you can probably tell, I'm completely self taught, so any help is greatly appreciated.
twystd
I have what I think is the same Chinese 3phase signal generator, and I've found that if you hold down the far left key after making changes the unit saves all the settings.
I'm in the fortunate position of building my setup for a Papst motor driving a Thorens TD124, so I don't need variable frequencies. Unfortunately my Papst motor isn't the one designed specifically for the TD124, and has a different shaft diameter, so I will have to machine my own drive pulley, not too much of an issue as I anticipate completing the finish after attaching the pulley to the shaft.
Thanks for the tip about the generator. There are other benefits with VFD other than changing speeds. I've discussed the accuracy required for machining a capstan, and there is time and trouble (basicly trial and error), which can be expensive in a machine shop. The 1/10hz adjustability theorectically gets you the same accuracy as .00015 on the capstan so especially helpful with capstan changes. There are the other benifits such as lower noise, higher torque, and lower temperatures, when running on true 3 phase as well.
Not saying you need to do it, but thought I'd point it out.
Twystd
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