MV 400 HP VFd alternatives 2

[Tmoose]

We need to speed up a mill for some testing for a few weeks. Contemplating we may wish to try speed increased up to 20 or even 30%. It's a "medium voltage" 4000 V 350 HP induction motor. The manufacturer says it is suitable for VFD use electrically, with modest speed increases being OK but limited because mechanical rotor characteristics are unknown. 350 or 400 HP 460 Volt VFDs are readily available. 4000 V are not, and expensive too. Some 460 VFD manufacturers publish info about installing a step down transformer before the 460 drive, then a step up transformer between the drive and motor.

We are looking at any and all options to be able to speed up the motor and thus the mill for testing. Are there outfits that rent or lease transformers, or VFD/transformer combos? My internet searches have come up empty so far.

thanks,

Dan T

 

[dpc]

Never heard of VFD rentals. Just keep in mind that even if you increase the speed, it's still only a 350 hp motor.

 

[ScottyUK]

Is it? You'll get the same torque, but if you can maintain the V/f ratio then you can get proportionally more power out of it as you increase the speed.


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

 

[jraef]

Avoiding the tangent just created (we are a tangential bunch around here), you cannot flippantly use a "standard" transformer on the output side of the VFD, because typically a transformer's design is assuming a fixed frequency. Having a variable frequency means the core and coils must (should) be designed with that in mind. If your "test" is relatively short term, as in a few hours, then you may be able to get away with it, or you may be able to deal with it by seriously over sizing the transformer (it won't change the problem, it will just have more mass to allow you to run longer without damage).

The step-down-step-up arrangement was used a lot in large down-hole submersible pump systems, especially around Texas and Oklahoma. There were / are pump suppliers, Reda and Centrilift come to mind, who used to offer a skid mounted system that was this exact arrangement; a step down transformer, 480V VFD and a custom designed step up transformer all in one package. Most likely there are a few of those floating around the surplus market now.


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[jraef]

Sorry, meant to add this link to a forum specific to that industry, where you can peruse used equipment for sale or post your needs to a place where people in that industry may read it.

http://www.espdrives.com/

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[LionelHutz]

Why would a standard transformer on the output of a VFD fail? You just have to maintain the V/Hz ratio fed to the transformer and de-rate it because of the PWM input voltage. There would likely be issues near zero speed but that's not the case for this application.

I though a lot of the down-hole applications increased the voltage to say the 700-1000V range to compensate for the voltage drop in the motor leads...

 

[waross]

I was working near several down hole bitumen pump supply skids last week. All appeared to use VFD drives. The supply voltage is 480V. Two skids used filters on the VDF outputs. One skid used a 480V:3000V transformer but no filter.
I suspect that as well as reducing the voltage drop and the line loss the transformer mitigated a lot of harmonics. I wondered if the transformer was a standard transformer or one specifically designed to reduce harmonics. Does anyone out there have first hand experience with these installations? Comments welcome.

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

 

[jraef]

I'm no transformer expert by far, but this is how it was explained to me. Standard MV transformers are not usually that good at handling the extra harmonics on the output side of the VFD. Eddy current and magnetic hysteresis losses in a transformer increase with frequency. On the surface you would think then that the transformer losses at lower fundamental frequencies would decrease when you lower the speed using a VFD. But the opposite ends up being true on the output of a VFD because the high harmonic content represents high multiples of the fundamental frequencies and that translates to HIGHER eddy current and hysteresis losses. If the transformer core is not designed for it, the higher losses cause the temperature to rise very rapidly.

So what they tend to do it design the transformer as if it were being used in high frequencies, i.e. more laminations, low hysteresis higher quality steel (in addition to more mass). The winding insulation is also subject to the same issues as the motor windings as well, and although most motor manufacturers have addressed this, most transformer manufacturers see no need to on standard designs. So transformers designed up front for use on the output of the VFD will have higher voltage insulation material as well.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[jraef]

By the way, the advent of good filters may very well have changed this concept now. I learned this 20 years ago when filter technology for VFD outputs was just beginning to come into existence. If you can effectively eliminate the harmonics before they get into the set-up transformer, I suppose you can possibly use a standard design now.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[waross]

Hi jraef. Thanks for the information. On the skids that I was looking at either a filter or a transformer was used, but not both. The transformer was rated for VFD duty. For what it's worth, the filters were about the same physical size as the transformers.

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

 

[ozmosis]

From our perspective, we would always supply an output sinewave filter in a "hi-Lo-Hi" configuration when using a LV VFD onto a MV supply controlling an MV motor.
I work for this company (so please note), but the application was carried out at a site I visited recently in S.Korea. This is just for info:

http://www.danfoss.com/NewsAndEvent...ion/0875018E-0431-43F9-9E4B-8F21FD64D5ED.html

However, I am quite sure these systems are not rented out due to the case-case engineering work required.

 

[jraef]

So Patrick, were the transformers made for that application or off-the-shelf? (As if MV transformers are ever off-the-shelf).


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[ozmosis]

I am quite sure the transformers were not specific design for VVVF operation but, as you say, not quite 'off the shelf'. The key is the dv/dt effect and this is overcome with sinewave filter. As long as the kva rating is within overload requriements, it is not too special.
I know is some cases the feed transformers are phase shift to reduce the harmonic distortion and this is quite an effective (cost effective) solution due to the fact you would need a phase shift transformer anyway on a LV supply for 12/18 pulse systems and this will incur nearly the same cost. Where we have supplied hi-lo-hi solutions it is usually typical where the end user has been 'burnt' with MV Drives (lack of support) and wants the standard LV VFD offer to minimise risk of downtime awaiting for spares/service etc on MV drives where LV drives are a little more common.

 

[Skogsgurra]

My experience is from starting Asplund refiner motors in the 10+ MVA, 6 kV range.

There was an old system with transformers from around 1980 which needed to be run from Siemens standard VFDs fed from a 500 V grid and stepping the voltage up with a transformer. The modernization was needed for several reasons - lack of spare parts for the old system was one.

We used the old, existing transformers, no filters whatsoever and were quite lucky with that. Only problem was that we had difficulties going through the full speed range without getting heavy oscillations in the sensorless inverter/motor system. It was solved by going from frequency mode to torque mode and limiting the braking torque. This system has been running for around two years now.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[CJCPE]

The transformer needs to be designed for the highest V/Hz that it is going to see. For a fan or centrifugal pump load, that could be the rated V/Hz of the motor. A constant torque load will need voltage boost at low frequencies even if it only operates at low frequencies when the motor starts. That may mean that the transformer needs to be designed as if it were going to operate at 25 Hz or as if it were going to operate at twice the rated primary and secondary voltages.

With a simple V/Hz drive, it would be relatively easy to limit the voltage boost adjustment to avoid saturating the transformer. With a sensorless vector drive which should be used for a constant torque application, it may be more difficult to figure out whay it takes to avoid saturating the transformer.

CJC

http://www.vfdriveinfo.com

 

[LionelHutz]

Jeff - I find that a little hard to believe. Almost any motor can be used on a VFD assuming the motor will withstand the dV/dt. It seems odd to me that transformer steel would be so much different than motor steel. Still, the transformer would need to be derated, similar to how an older motor should be derated from 1.15 SF to 1.0 SF when connected to a VFD because it is basically operating as a tranformer and a VFD output filter.

Waross - The transformer should make a very good output filter. I'd expect to see close to a sinewave voltage out of the transformer - better than any other filter besides a sinewave filter.

 

[Tmoose]

"...... this link to a forum specific to that industry, where you can peruse used equipment for sale or post your needs to a place where people in that industry may read it.

http://www.espdrives.com/

........"

The espdrives moderator has still not approved my registration.

Got a few eager calls from salefolks offering a new MV drive for 150 to 200 K$ . They also insist that (their!) modern MV drives are more economical than hi-lo-hi method
$150 K will be a hard sell for this R and D project.

First cut at a 400 hp 460 volt motor and VFD is more like 60,000 bucks

I still expect there are some lease, rental, or good used ones for sale out there somewhere.