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rmarotta (Electrical) (OP)
28 Aug 10 15:58
I'd like to use batteries to power the DC bus, and modify a small VFD to control a larger HP AC induction motor.
Is there any DIY info available about how to "marry" a small VFD to larger capacity output devices?  (ie: IGBTs rated for higher voltage/amperage.)
Any help appreciated.
Regards,
rmarotta
Skogsgurra (Electrical)
28 Aug 10 17:27
Interesting hack. But be prepared to change more than the IGBTs - gate drivers for one thing.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

waross (Electrical)
28 Aug 10 19:54
If you buy a larger VFD you will have most of the parts that you need for the hack. It may be cheaper than buying individual IGBTs retail.

Bill
--------------------
"Why not the best?"
Jimmy Carter

jraef (Electrical)
28 Aug 10 21:14
Why bother "hacking" a small VFD to run a larger motor? Is that not the same, except a lot more steps, as using a larger VFD? The expensive part of a VFD is the inverter section, the part you would need to "hack" to make it work on a larger motor.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

Skogsgurra (Electrical)
29 Aug 10 0:52
Guys! It is a H*A*C*K!

Don't spoil the F*U*N!!

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

ScottyUK (Electrical)
29 Aug 10 3:47
Depending on the relative sizes you might find that the IGBTs of the larger drive can't operate at the carrier frequency used by the smaller drive.

Seems like a slightly crazy thing to be doing but, like Gunnar says, it could be educational. Gotta ask though: why?
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

rmarotta (Electrical) (OP)
29 Aug 10 5:08
Thank you, I appreciate your responses.
I know this must be an odd request because I can't seem to find much info about it.  In reading service manuals from various VFD manufacturers about their models and features, it seems they all do basically the same things with the controller end.  Since I have several drives in various condition that I would like to make use of, what I'm after is assembling the controls from a small drive with the components suitable for driving a larger motor.
I have some basic electronics experience, so I wouldn't be uncomfortable with fabricating some sort of interface between these components.  My interest is personal understanding rather than for commercial gain.  I'm not trying to re-invent the wheel.
Again, thanks for anything you gentlemen might be able to provide.
Regards

 
ScottyUK (Electrical)
29 Aug 10 7:57
Watch for the 'extras' such as voltage and current sensing elements which may or may not be isolated.

Be careful and use the right test equipment. No crazy stunts like disconnecting the scope chassis from earth and floating it at line voltage.

Bear in mind that the DC link contains enough energy to kill you several hundred times over, and just once is enough to ruin your day. wink
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

jraef (Electrical)
30 Aug 10 0:52
I think it will depend on what size you start with and what size you want to go to. The really small drives, up to maybe 20HP or so*, are now using Integrated Power Modules (IPMs) that have the inverter power devices and gate driver circuits all in one monolithic device with a direct TTL interface. In other words you can't separate the control and the power devices without some extrordinary interface magic. So you would have a lot of trouble trying to make the control board for a 2HP drive work to fire transistors for a 50HP motor.

* I've been out of the drive design end of things for a while now, IPMs might already be used on much larger sizes, maybe someone else can comment.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

LionelHutz (Electrical)
30 Aug 10 8:43
We really can't be much help considering the fact that you have provided no real details of what parts are available.

In a broad sense, you would have to take the IGBT's and the gate driving circuits from the large VFD's and connect the controller from a smaller one.

Quote (Skogsgurra):

Don't spoil the F*U*N!!

Ya, I like explosions.
rmarotta (Electrical) (OP)
30 Aug 10 10:26
Okay, to be more specific:

In my current project I want to run a 10hp 230/460v 3ph motor with an Allen Bradley 1336e series drive. The drive is rated 20hp but it's main control board is bad.  It appears that all necessary signals are communicated via a ribbon cable between the main control board and the gate driver board.  Anyone have info or a pin-out of these signals?

I also have a number of smaller (1-5hp) drives that are functional.  The smaller drives are older AB models 1305 and 160 series.  My collection has been obtained from ebay and, as you might imagine, very inexpensive.  A new control board for the 20hp drive would be out of the question on my budget.

As stated ealier, I intend to run the drive from batteries.  Twenty, 12v, series-connected to the DC bus.
 
It is my hope to later build a mains powered charger using the rectifier from the 20hp drive.

If this works out I'll try the same thing on a much larger Seimens 125hp drive.

Thanks for your interest if you have followed this far.
 
jraef (Electrical)
30 Aug 10 13:35
If I remember correctly though, the 1305 was built for A-B by Koyo or some other Asian supplier whereas the 1336 was built by A-B themselves, which means the control systems would be completely different.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

rmarotta (Electrical) (OP)
31 Aug 10 6:00
I guess there's no way to determine the requirements of a particular drive without an intimate knowledge of it's design.  What would be helpful is a component-level schematic diagram instead of simple block diagrams used for assembly/dis-assembly.
Does anyone here know if such schematics are available?

Now I'm not an engineer, so I don't intend to try to re-design any circuits.  I'm simply looking for a way to bridge between components that have already been designed for the job.

To simplify, as I understand it, the controller circuit of a drive tells it's output devices what to do in order to power the motor.  When a drive is built, the limiting factor determining motor size is simply the capacity of it's output components.
Unless the driver signal values of a small drive are wildly incompatible with those of a larger drive, I can't see why an interface between the two should be difficult.  After all, the principles used to control an induction motor should be the same, regardless of its size.

Thanks again for listening.
Regards,
Ralph Marotta
Skogsgurra (Electrical)
31 Aug 10 7:58
The problem, if you excuse me saying so, is that you do not have the insight that may be necessary to understand the problems waiting for you.

I suddenly don't feel like you should do this. Sorry.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

LionelHutz (Electrical)
31 Aug 10 9:00
I bet if you called up AB they would be more than happy to send you schematics.....not.

This is something you'll have to figure out on your own. It goes well beyond the "engineering tips" nature of this forum.
 
rmarotta (Electrical) (OP)
31 Aug 10 9:56
Again, I do appreciate and understand your comments.  I guess I will just have to figure it out on my own.....

It still seems to me that conversion of existing analog & digital signals by an order of magnitude of 10 (2-20hp) cannot be that big of a deal.

Thanks to all for your trouble.
 
rasevskii (Electrical)
31 Aug 10 12:16
rmarotta:-

Practically everything in todays electronics is microprocessor driven and completely proprietary property of the manufacturer. No way will they send you schematics and even if you had them what could you do wiih them? Can you repair or modify something done with SMD components so small that they have to be looked at with a magnifying glass?

Just accept what you have and do not try to modify anything.

Respect thr dangers in power equipment. One mistake and you are dead or someone else...

It seems that not much money is around for your efforts if you have to get used (junk) from Ebay..

Buy everything new with the manufacturers guarantees and let them commission the equipment. It is also possibly a tax writeoff to do so.

rasevskii
ozmosis (Electrical)
31 Aug 10 21:13
I'm a little puzzled why you want to use a whole stack of batteries to power the DC bus and yet use the mains power supply to recharge the batteries via the rectifier.
It seems an incredibly complex method and innefficient way of powering a very old AB drive. On top of this you want to use a control card from another drive because the original one is defunct.
Quote: "... seems to me that conversion of existing analog & digital signals by an order of magnitude of 10 (2-20hp) cannot be that big of a deal"  
hmm, I'm sure there might be many R&D folk and a few billion $'s thinking differently!
If it is just an experiment, then fair enough. Stand back, wear your welding goggles, make sure you have some form of protection to your batteries, don't operate it alone, carry a long dry wooden stick...
I think you get the message
MikeHalloran (Mechanical)
31 Aug 10 21:31
The problem with hacking power electronics, in addition to the real dangers already stated and implied, is that there's usually not much evidence of forensic value left after the, uh, experiment.

You were warned.

 

Mike Halloran
Pembroke Pines, FL, USA

LionelHutz (Electrical)
1 Sep 10 9:09
Don't forget to wear hearing protection too.
jraef (Electrical)
1 Sep 10 18:02
This might be a cheaper lesson...


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

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