Running motors at reduced current ?

[Mark Williams]

Looks like a VFD can set max current output, so why couldn't I run a 3 hp motor that I have with the 2 hp VFD. Wouldn't I still get close to 2 hp from the motor . Let's say the VFD puts out 7.5 amps and the 3hp motor draws 9.6 amps when available. Would the magic smoke be released or would it just shut down the VFD. Phil, ?

 

[pso]

Hello Mark

We're in luck. I was able to catch the last Applications Engineer before he went home. He confirmed that what you are doing should be OK since the 3hp motor is not that much larger than the 2hp rating of the VFD. You will still only be able to get 2hp out of the motor because that is the limit on the amount of power that the VFD is able to transfer to the motor without damage to itself. Just don't try to start the grinder while it is loaded ie you are trying to grind as the motor is starting.

He suggested that you refer to the VFD's manual and set the "motor current rating" parameter of the drive to the motor's name plate current rating of 9.6A. The drive probably has some feature to protect the motor that uses this value. The VFD's current limit should still be the 7.5A limit of the VFD.

If your 3hp motor is rated at 9.6A, that means that when it has a 3hp load applied to it, it will draw 9.6A. With no load, it will draw some small, say 0.5A or less. I would guess that your motor would probably want to draw something like 6.4-6.5A when it has a 2hp load applied to it.

Good luck!

Phil

 

[DC KNIVES]

Mark, good thing we got Phil around and yes he is right.I have used many VFD's in different applications usually enough to drive the application engineers crazy.Fortunately VFD's are somewhat self-protecting and will trip out before showing the magic smoke.I have run some up in the 80-90 hz range before I was surprised how well they did, even though it is not reccommended.If you do experince any trip just cut the power and wait for the power light to go out then reapply the power.It should correct itself unless a problem still exists.If the problem is still there, check the manual and there are some trouble codes that may lead you in the right direction.Dave

 

[Mark Williams]

Cool, I was hoping you would see this Phil. This will just be for a short while. I've ordered the proper motors. Just wanted to play while waiting on them. Thanks Dave. I never realized these drives could do so much. Amazing little package.

 

[pso]

Give thanks to the rapid advancement in microprocessor technology. They are the "brains" behind all of these drives now days. Microprocessors keep getting more powerfull and cheaper every year. To differentiate their products, the manufacturers add more and more features in their software. Once they have the microprocessor in the drive, the only cost to add many of these features is the development time. No extra components are needed. They don't pay us engineers that well, by the way.

The technology in the power circuit, the part of the drive that is directly connected to your supply and motor advances at a much slower pace. If you were to compare the schematic diagrams of several competing drives, you would find that the power circuits to be pretty much the same in all of them.

The self protection features are pretty much mandatory now days. Many of the larger industrial users will keep extra drives on the shelf. If one does blow up, they can get back up and running within hours instead of waiting for the drive to be sent out and repaired.

I am glad to help out however I can, in return for all the stuff that I am learning from the rest of you. I am looking forward to getting the kitchen renovations done and the garage cleaned up so that I can set up a bit of a shop and apply what I have learned.

Phil