Dynamic Braking vs. Regenerative Drive

[Beggar]

I have an application that was defined by people no longer with the company. Without going into too much detail, I've got two motors of different sizes that are driving rollers in opposite directions. The rollers are used to shear material passing between them. The slower (and smaller) motor is controlled by a regenerative drive and attempts to maintain a fixed slip ratio between the two rollers.

I can replace the regenerative drive with another drive that simply has dynamic braking and which costs less than half of the regen drive.

My question, then, is what kinds of factors would cause one to pick a regen drive over a drive with dynamic braking? (I realize that the regen drive also has dynamic braking but I'm not sure of the terminology to distinguish between the two options.)

I presume the dynamic braking drive dissipates its energy primarily as heat rather than as power back onto the line. I also presume that the dynamic braking drive has a higher power demand than does the regen drive.

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[jraef]

I'll assume you meant AC motors.
Dynamic braking involves putting the kinetic energy of the rotating mass into a resistor and burned off as heat. It is typically used where braking is only part of the cycle, so that when not used, the braking resistors have time to cool down. Continuous use of DB will shorten the life of the resistors, and when they fail it will cause your system to shut down unexpectedly and possibly cause damage. How much time they will take to fail depends on how you size them and how they are used, so it is difficult to quantify that. Most VFDs will give you some duty cycle guidelines that you can expect for reasonable performance and longevity.

Regenerative braking is a much better choice for any continuous braking application. Sure the drive costs more, but it may keep running longer, and your machine makes no money for you while you are shut down because the DB system died. Keep that in mind.

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[DickDV]

I'm sorry, jraef, but I can't agree with your description of the limitations of dynamic (snubber braking, more properly). Properly sized, dynamic braking resistors are fully reliable even when used in a continuous mode. Consider the braking resistors in locomotives easing trains down long mountain grades.

The key difference between the two braking schemes is economics. Simply, is the energy being saved by regeneration worth the extra cost of the regenerative drive?

If it is, then use regen. If not, use snubber braking.

The cost of energy and the duty cycle bear heavily on this analysis. You are correct that most continuous or near-continuous braking applications are worth the extra cost of regen but that's because of the economics, not reliability issues.

So, to beggar I say, figure out how much braking energy is there to save and then translate that into the dollar savings of regenerating it. Then apply that against the added cost of the regen drive and figure out how many years the payback is. Is the payback fast enough for your company's money people? They will tell you whether regen is worth it or not!

 

[itsmoked]

Don't forget the heat generated is a factor in there too!

If you have the heat dumping in a space that might now need cooling or forced ventilation that it didn't before then you must take that into consideration too.

 

[jraef]

DickDV,
I suppose that when properly engineered, a DB system could be considered to be reliable (your locomotive example made me change my tune). But to be honest, I must NEVER have seen a system that was designed correctly for continuous or high duty cycle applications because they have all been unreliable. My experience has been with lumber machinery (carriage drives) and downhill conveyor drives. At some point someone takes a scientific-wild-ass-guess at sizing the resistors, and eventually they smoke. So they increase the wattage capacity and thermal dissipation design so they last longer, but eventually smoke again or the DB transistor fries. After repeating this process 4 or 5 times, their downtime costs could have paid for the regen drive I had suggested. Carriage drive OEMs that I have worked with have all switched to regen AC drives or stuck with DC drives, not one of them that I know of has made a DB system work reliably.

Do you have a decent formula for correctly sizing DB systems for continuous duty? I am unaware of one and would prefer to know. Regen drives are expensive and alternatives are always welcom.

I have also been hearing a lot of buzz about Matrix drive technology becoming available, which are inherently regenerative. I have heard they are essentially the same cost as existing technology regen drives (inverse parallel inverters), but by design are much simple and therefore more reliable. I heard that Yaskawa has officially released one, and maybe Allen Bradley too.


Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[sreid]

Your application sounds like a "Flying Shear" application. Does one of the rolls have to absorb energy from some source? What is the size of the rolls and the motor HP ratings?

 

[DickDV]

jraef, the brake chopper transistors are usually sized for continuous amps which, if kept to will result in acceptable service life. As a safety factor, I try to stay 10% under the continuous amp rating if the application is continuous.

Regarding the resistors, the continuous wattage must not be exceeded in longterm braking applications. Again, I try to stay 10% under wattage for reliability.

That leaves the resistor sizing which has to come from the formula max DC bus voltage/derated continuous current

As a practical field matter, I would expect the resistors to glow dark red but not bright orange when working.

 

[Beggar]

Only one roll absorbs energy. That energy comes from momentary coupling with the other roll (due to product transiting through the shear). The absorbing roll is about 60 hp, the driving roll is about 250 hp.

Thanks for the ongoing discussion.

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Bring back the HP-15

http://www.hp15c.org

--------------------

 

[fsmyth]

I think I would have to ask why the braking is not done on
the larger motor, using an idler or brake. Is there some sort
of timing issue that requires a constant drag?

Q. How does a /roller/ shear metal? I could see it, maybe,
an issue with embedded cutters, but then you should have
gear coupling. Side shears should have no timing issues.
<als>

 

[itsmoked]

You would dream of wasting 44kWhrs per hour over some obscurely small initial cost difference?!?!?!? What planet are you on?

 

[rmw]

A comment on the dynamic braking. I have experience with electromagnetic retarders used in vehicular and other applications, and my comment is that eventually the resistors toast themselves (manner of speaking, no pun intended).

Speaking of trains, if you have ever been on a down hill grade along side a RR track in the mountains at night where the road elevation was higher than the RR track, and there was a train going downhill besides you where you could look down on the top of the engines, the resistor grids dissipating the braking energy from the traction motors are glowing red, and I do mean red hot.

Electromagnetic retarder flywheels operate red, and I do mean red hot.

You could probably count the down hills that an average train has to do in a days time on one or two hands. I have no idea what the longevity of those resistor grids is.

The flywheels on electromagnetic retarders are high maintenance items.

I'm through commenting now.

rmw

 

[davidbeach]

As for locomotive dynamic brakes - a substantial down hill run may occur only once or twice a day, but it will be long enough that the dynamic brakes are in a continuous mode of operation rather than intermittent with cooling time. In addition to mountain grades, dynamic brakes are used for most of the braking at higher speeds.

 

[Beggar]

itsmoked: I'm just trying to make an informed decision, that's all. I inherited the project and have no benefit of the reasoning that went into the selections and thought I should. I've been speaking to the manufacturer's folks and thought it worthwhile to seek the benefit of the knoweledgeable folks here. I'm not clear why you'd find that so surprising.

For what it's worth, the upfront cost is substantially different (nearly 3:1), my sales price is prenegotiated, and frankly, it's not my problem if the customer pays more for electricity. They've squeezed me three days from Tuesday so I don't feel too strong a need to put up my money to save theirs.

fsmyth: The reason that we don't brake the larger motor is that it is the one that's providing the "overdriving" force for the smaller one. That large one runs faster than the small one.

Mostly, I'm looking to design a robust, reliable system. It sounds like the regen is the way to go.

--------------------
Bring back the HP-15

http://www.hp15c.org

--------------------

 

[Skogsgurra]

Beggar,

There are two very important word in your last posting: ROBUST and RELIABLE.

I am sorry to say that the robustness of a regen system is very dependent of the mains. If you are using a thyristor controller for the regen part of the inverter, then little power glitches will cause fuses to blow in the regen bridge. If, on the other hand, you use a normal IGBT for regen, then the same glitches (breakes actually) may cause overvoltage in the DC link. It depends on speeds and inertias involved.

Before you change anything, make sure that these parameters have been thoroughly evaluated. It may be wiser to stay with the non-regen braking. Steel people are used to putting a lot of energy into their processes and willing to pay for it. But I would, of course, rather see regen - for environmental reasons.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

Beggar; Yah never said it was for someone else....

But to me, it is environmentally unethical for anyone to waste the equivalent of something like 5,000 gallons of fuel per year if there are reliable alternatives.

Can the customer not embrace a 1 year savings of something like $27,000 a year difference in operating costs?

If you are replacing a regenerative drive do you think they won't mind their operational costs suddenly jump?

By use of essentially new but inferior equipment it would seem to me that you are opening yourself to a lot of headaches.

 

[sreid]

One thing I would do is to see how much regen energy is actually being produced. The drive 60 HP sizing may be based on the required drive power and the regen power might be a smaller fraction. A clamp on three phase AC Ameter could be used to do this under operational conditions.

Regen ballast resistors have a well deserved reputation for being unreliable but it is my experience that this is due to designs that don't account for thermal cycling. I have seen quartz heating elements used quite successfully in smaller drives (30 HP decel regen).

 

[Farkel]

Why not use the regenerative drive, and share a common DC bus, and there by save energy on both drive systems. The savings perhaps would pay for the increased costs of the drive vs using dynamic braking. Also, U.S. Drives out of Niagara Falls N.Y. makes such an animal. See

http://www.usdrivesinc.com.