Flux-Vector Variable Speed Motor

[zdas04]

It has been 34 years since I took my only Sparks and Magic course. I just came across a pump for Oil & Gas wells with a Flux-Vector Variable Speed Motor which the pump write-up says can stroke 25 times per minute (dead stop to moving in the other direction 50 times per minute) without a problem.

The motor is driving a pinion gear that rides in a rack attached to a downhole pump. On the upstroke the motor is lifting a bunch of weight (thousands of feet of steel and a bunch of water. On the downstroke it is trying to control the rate that the rods are falling. Every direction change, the motor has to stop and then start under load. This sounds really difficult to me.

Can anyone explain this Flux-Vector motor (if that is a real term and not just a made-up marketing thing) and give me an idea if starting under load 50 times per minute times 5 years is even possible. It is a fairly new pump that seems to be working well, but I'm trying to get my head around possible life expectancy.

Thanks for your help.

David

 

[jraef]

It's technically not the motor that is flux vector, it's the drive, but they are likely selling them together as a unit, a wise decision. Assuming they knew what they were doing, a Flux Vector drive and a properly applied Inverter Duty motor designed for this type of application can reap a lot of benefits with little expectation of reduced life. The drive magic is that it knows exactly where the motor shaft is at any moment in time so it can tweak the output of the drive to be exactly and only what the motor needs to provide maximum performance for the task with minimal stress; think servo motion control from a large HP machine. The potential pitfalls are that the motor must be designed to not need the cooling it might otherwise get from spinning fast, and it must be able to handle the complex waveform of the power going to it from the drive. The drive will actually provide as soft of a start and stop as possible, minimizing the mechanical strain on the pump components.

This kind of technology is fairly mature now, if it's a reputable company with a history of success in this kind of application, it's likely fine.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[zdas04]

Going back to the manufacturer's brochure, they are saying "Flux-Vector Variable Speed Drive". I was reading it as "Driver".

Thanks for the information. The company has been around for a while so it sounds like this is more reasonable that I expected.

David

 

[Skogsgurra]

Adding to Jeff's comments. There are servos that use standard induction motors and (flux) vector drives. They respond in milliseconds and reverse (1500 RPM CW to 1500 RPM CCW) in around fifty ms or less. That is, it will use 50+50 ms for the actual reversing and 1100 ms at full speed, if needed. Your application is probably less demanding with reverse times a little longer.
As Jeff says: Go for it!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[zdas04]

Thank you both. I've been out of the Navy for a really long time, but I still think about reversing course in terms of an aircraft carrier going from flank speed to full reverse--takes all day. The load on the motor is on the order of 4 tons, but there is a pretty significant reduction gear before the pinion to turn that 1500 rpm into linear movement.

David

 

[Compositepro]

The issue I would be concerned with is energy recovery or efficiency. Normal pump jacks are counter-balanced so the motor only has to lift the liquid and not the dead weight of the sucker rods. Without a counter weight the motor must be much larger. I would expect that the drive is acting as a generator while lowering the pump string so energy is being stored in the electric grid rather than a counter weight. It seems to me that the energy losses will be significantly higher. I guess the advantage is a physically smaller, and less visible, package.

 

[zdas04]

I've asked the manufacturer about motor sizes, he hasn't gotten back yet. I would be surprised if they generate power back into the "grid" (I'm planning on a genset 200 km from the nearest power line), but they may. I did find that the motor is a pretty standard induction motor so it could easily act as a generator (and the fancy drive could probably sync it back into the grid I guess).

Between rod stretch, seal leakage, valve leakage, and gas in the pump; energy effeciency has never been a real driver of rod pump driver selection. Mostly we calculate volume lifted over the distance lifted and come up with (say) 5 hp, then we install a 15 hp driver to make sure we have enough. Folks that try to get too close to the edge of the envelope find that failure to grease a bearing pushes them over max hp (or some other seemingly trivial screw up), most of us just put in a huge safety factor and accept the inefficiencies of part-load operation.

David

 

[davidbeach]

Running it from a genset means that you absolutely must have one that does NOT regenerate beck into the source. It will need to regenerate back into a braking resistor or a batter. That highly cyclical load is not going to do your genset any favors at all. The best arrangement would probably be to power three of the units from one genset, all run from a common controller, each a third of the way through the cycle separated from each other. That way you'll have a relatively constant load on the prime mover of the genset.

 

[zdas04]

Not sure I'm that far along yet. The entire pilot will be 5 wells spaced over a square mile (about 1300 ft from one to another, the manufacturer wants the drivers less than 100 ft away), a man camp (about 45 kW load), and 5 small air compressors. The rod pumps I'm talking about here are probably 3-5 hp five times so each one will be about 7% of the total peak load (but maybe 20% of the late night load). Is that big enough to cause a problem with the genset? The genset will be located next to an evaporation pond, so I'd probably put in resistive heaters to dump the extra power into the pond to speed up evaporation instead of batteries.

You guys are giving me a long list of great questions to take to the manufacturer. Thanks a lot.

David

 

[davidbeach]

If the motors all load and unload together, and they regen back into the rest of the load, that is a 50 hp oscillation in the power required of the prime mover 25 times a minute. The fuel rack will be continuously hunting and never in the right place. You wouldn't want to read by lights powered from that generator.

 

[zdas04]

I've got a meeting with the pump manufacturer next week. Hopefully I'll get the details of the driver operation and whether or not I need to provide a place to dump any regen power.

David

 

[Skogsgurra]

Dump it in an empty well. Together with the CO2. ;-)

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[zdas04]

I was really thinking more in terms of dumping it into the evaporation pond through a resistance heater. Not many empty wells at this site.

David

 

[Skogsgurra]

That is a nice electric way of doing it. You haven't forgotten it all!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

From the generators point of view of this horrendous cycling load.

Maybe you should consider a really large motor running just a flywheel. This would be low maintenance, reliable kinetic storage. Then the drives could regenerate and the energy would have somewhere to be stored for return to doing useful work. Much more useful than heating pond water!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zdas04]

The purpose of the pond is to evaporate the non-potable produced water. Anything that speeds that up is a good thing.

David

 

[itsmoked]

Not at the price of diesel it isn't. Aerate the pond or pump the water thru a few rain birds. Don't create a large bacterial nightmare by warming the water up a little. And heating it up until it's too hot for growing nasties means it will be dangerous to animals.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Waiting for Bill's comments. He has loads of experience with gensets.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[zdas04]

The price of diesel is a good point. This is a new field so we don't know the water quality yet. Based on other similar fields, the water will most likely start out harmful to animals so we are going to have to keep them out anyway.

We are really at the brainstorming phase of project development at this point. The pump manufacturer may say that the drive already has the means to store/reuse the breaking power. I should find out next week.

David

 

[jraef]

VFDs are NOT inherently Line Regenerative, you have to pay a lot extra for that because you essentially have two back-to-back inverters instead of one inverter and one rectifier. See if they can give you a make and model of the VFD they are going to provide.

A single stand alone drive cannot store energy (short of a very massive battery system). But if you have multiple drives, you can set up what is called a "Common DC Bus" arrangement and have them all tap off of and regenerate back into it. With a decent coordination system, you could have one regenerating while another is motoring, that would SIGNIFICANTLY reduce the fuel consumption on the generator. But the drives have to all be in the same cabinet for that to work.



"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376