Recent content by DFordPE

Can anyone tell me where to get a simple description of what is happening during the various parts of the induction motor speed-torque curve? I have found some info using equivalent circuits, but: a. I don't need anything that precise b. It's been forty years since I graduated and longer than...

I'm curious. What is the application or are you just playing in your "skunk works?" As was already said, VFDs are cheap these days.

It's not about what I "thought was being done;" I never thought about it at all. Adding PF capacitors is just something you do with motors, for overall plant PF improvement. It was not necessary to think about what was going on in the motor, you just did it. I've worked for a couple of motor...

Thanks for all the input. I found the simple answer to be that the nameplate current is "apparent" current not "real" current, so the OLR is simply adjusted to account for that.

I believe that what I've researched says that you subtract capacitor current from motor FLA and set (I prefer IEC OLR) that number. That's where I'm stuck. It appears that I'm de-rating the motor after I improved the system. Next, the chart Skogsgurra drew shows the PF worsening under an...

Sorry, I must have read it too fast. mea culpa

Yes, I read that article, but I still am somewhat confused. Using Squeeky's example: I have a motor with FLA of 81.6. I add PF correction and the current is now 69.13. How could going back to 81.6a be considered an overload? It seems to me that by reducing the imaginary current and...

To ScottyUK, It's obvious that it's insufficient because it doesn't work! Would you want to try to design a retrofit without having all to the system info? Are you that bold? That's why I referred him to a company the makes bypasses.

Why not just install a filter ahead of the VFD?

It's common to see compressors operate into the service factor of the motor for short periods of time. I have also seen motors relabeled to 15% higher HP with a 1.0 SF. There will always be unusual cases or poor operating practices that require special care.

I recently heard that the OLR must be "resized" if a PF capacitor is connected on the load side of the OLR. Ref: NEC 430.22 and 460.9 Has anyone done this? I can't find a way to calculate. However, I have always placed the capacitor ahead of the OLR.

If all you want to do is bring it up to speed, why not use an SSRV starter. They come with bypass contactors built in.

Wow, I didn't catch this originally, but 65 years old is pre-U-Frame. Not only that but some refrigeration compressors have specially built motors.

The affinity laws (cube/variable torque) are a nice approximation of load on fans and pumps, but they only work in a perfect world, not what we have to deal with. However, if they did always work a 10% speed increase would only yield 10% more flow but would require 33% more current. Assuming...

If it is an IEC motor circuit protector, it will typically be wired ahead of the contactor. These devices replace both the fuses/CB and the OLR.