Talk to me about VFDs.

[soundman]

I think I have to start with a 4 pole 1200 ish RPM motor to get a reasonable rev range with the limited pully options I have.

If I was to run the 2 pole motor I'd have to introduce a jack shaft and more pullies and that will get expensive.

what I realy need to know is it reasonable to power limit a 4hp motor using a facility in the VF drive or am I better off looking for a 3hp unit.

cheers

[RustyArc]

what I really need to know is it reasonable to power limit a 4hp motor using a facility in the VF drive or am I better off looking for a 3hp unit.

I think it depends on the VFD. The two units I've had most experience with have different options - the ABB ACS500 - a very common drive - will throw a fault and shut down on overcurrent with an option to restart after a delay. The Danfoss VLT drives offer more - they'll do the above, or they can estimate the heat in the motor and drop the speed by a specified percentage.

If you want the drive to smoothly moderate the speed of the motor once it reaches a specific current, then you could use a current sensor and use it as a reference for a PID loop.

[BobL]

. . . .what I realy need to know is it reasonable to power limit a 4hp motor using a facility in the VF drive or am I better off looking for a 3hp unit.

It depends how you are going to use it.
Most 3HP motors should be able to deliver 4.5HP for a brief time so I wouldn't normally worry about current limiting a 3HP motor on a 4HP VFD.

[pjt]

Just a few more thaughts and questions.

Do these VFDs have the ability to current limit.


cheers

The Zener's I have certainly do, I am currently setting up a VFD controlled blower https://www.woodworkforums.com/f200/vfd-controlled-blower-189507 I did a graph of my first test running, I plotted amps that the motor draws and plotted the VFD current limit/s, there are 3 user settable limits, (motor protection sub menu) in my case I2t max =3.6, (from motor lable) I2t start =2.5, I2t cnr Hz = 35Hz I2t (IxIxtime) is an algorithim that the VFD uses to determine load on the motor, with these three settings I can set the current limit closer and to follow the actual curve of the motor current.
My initial start current was too low (1.75) which did trip the VFD I adjusted to 2.5 so now all of the limit is above the actual, I had inadvertantly derated the motor at start to about 15Hz, (got to start somewhere) in this particular istallation I am not worried about the motor being oversized for the connected load (fan) so the limits stay as they are.



Pete

[pjt]

Just a few more thaughts and questions.

So going the other way would it be possible to run a 4 Hp motor off a 3HP VFD buy applying some sort of electronic limitation.


cheers

In conversations with the Zener guys the general thought is that this is not a good idea, not sure on the why exactly, during some of my early bench tests I had a smaller VFD (2.2Kw) connected to a 3kw motor and the motor didn't like it, it did run upto about half speed (no attached load) and it didn't sound right, like a missing engine almost, I changed the VFD to a larger and it was all good.



Pete