Motor to motor load test with VFD

[cork1025]

I'm trying to understand something that I'm being asked to help supply motors for. The customer wants two 600HP motors. One motor to run off their VFD and the other motor to run off the incoming 3 phase power. That part doesn't really seem to be an issue.

They also want to regen the power from the second motor. Everything I have read says that you need to have two VFDs for this to work. Option A is to tie the two VFDs DC BUS together. Option B is to buy a regen VFD to put the regen power back on the grid. Without one of those options, I'm not sure how they are planning on using regen power.

Does anyone here have any experience or thoughts? The customer is pretty sure that they can just have two motors and one VFD and be able to regen power without the power company knowing...up to 80%. Everything I've read says the contrary. Other people I've talked to and what I've read online says they are not right. But just trying to get some more insight.

Thank you.

 

[M0rt]

Hi,
If the purpose of the test bed is to verify that the new VFD has been built correctly, and everything works as it should, then connecting the load motor to the mains is going to recover most of the energy back to the grid, and provides an easy-to-maintain solution. This is the method used in our factory to perform final test on our VFD's. I did some investigation a few years ago into using regen VFD's in the factory instead of a mains motor. The advantage is that you can run the pair at any speed, so if you run at rated current but say 30Hz, in theory you are using less power. In reality, the energy which is actually lost is primarily in the motors themselves (at least for our small systems <20kW), so the cost is roughly the same. Especially because you now have to add the electronic losses in the regen VFD (IGBT switching, power supplies, etc.) You also have a more complex system to maintain, so in the end we decided to continue with the mains motor loads.
In development, then the Regen VFD is used for loading, as this gives us all the flexibility we need to test a new product thoroughly (the load can be automated through load cycles, etc.)
Cheers,
Mort.

 

[mikekilroy]

corky, corky, let me try! let me try! (I wanted to say lend me your comb)

Your 23 Jan 18:01 post answer: your are correct.

You said nothing of the two motors SHAFTS being connected together. My answer assumes you couple both motor shafts together.

simple salesman type answer: the 600hp LINE CONNECTED motor does NOTHING for your customer.

Now if your object is to sell TWO 600hp motors instead of one, then by all means sell them two. But know the line connected one does NOTHING for testing the vfd running the 2nd motor.....

KISS: if you want to sell TWO motors, *and* ANOTHER vfd, then sell them a non line regen vfd for that first motor - don't hook it to the line direct... NOW you have TWO - count em - TWO dc buses - and you can hook them together!

So.

Your customer turns on motor 1 (not across the line anymore, but on a 2nd vfd you sold them) set to 60hz. They then hook up the vfd they want to test to the second 600hp motor, AND TIE ITS DC BUS TO THE FIRST ONE'S, and set it to your 2,3,4,5hz FASTER or 62,63,,64hz... it will be a generator and put power into the DC BUS. But drive 1 you sold them for their test stand will absorb that and use it to power the first motor you sold them! The power from the power company down the street will be the inefficiency of the stuff, so probably only 3-5 amps! No fancy line regen drives needed.

Make sense? If so, come back to discuss how to set the test motor/vfd speed command to work and not overvolt your "test stand."

http://www.KilroyWasHere<dot>com