Dynamic Braking for Induction Motor

This is not a machining related question, but there are a bunch of really smart folks here that might know.

Working on a chipper. Currently a 2750 hp sync motor. Has dynamic braking going into a large resistor bank. There is a separate field controller to control the braking. Works great - stops in about 10 seconds. Mechanical brakes were a reliability nightmare and took about 90 seconds to stop.

They want to replace the sync motor with (2) 1250 hp induction motors feeding a common dual input shafted gearbox with fluid couplings. They want dynamic braking on the induction motors.

I understand the concept - close the motor leads into a resistor bank and use capacitors to excite the induction motor so that it is an induction generator. Problem is that I don't know how to size the capacitors and have been unable to find anyone that has done it. By default I was thinking of sizing them as power factor correction capacitors to 0.95 pf but that is a pure guess.

Anyone ever done this, know how to size them or know someone that I can turn to?

Very interesting question. Definitely an engineering problem.

I've heard of DC injection braking before but never mention of using capacitors for self-excitation. Just taking a stab in the dark here, one might want them sized to form a resonant LCR tank circuit so that the residual magnetism within the rotors can build current until full excitation is reached? Got to be careful with LCR circuits though... they can be capable of developing some wickedly high voltages under the wrong (or right) conditions.

Also worth considering is that the frequency the motors will output will change as they slow down, so you're facing a dynamic problem. (no pun)

I would definitely consult a professional engineer for advice on this one, especially since you're probably dealing with vacuum contactors and 4160V, 13.8kV or some other medium-voltage system.

Just a Sparky said:

I would definitely consult a professional engineer for advice on this one, especially since you're probably dealing with vacuum contactors and 4160V, 13.8kV or some other medium-voltage system.

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I am one and working with several others. Its just that no one has done this.

DC injection can be brutal on a motor. Would never do that to stop a motor like this. Small 3 hp motor yes - but not a 1250 hp.

But I found my answer! Good ole Wikipedia! Very simple calculation...........

Induction generator - Wikipedia

Brutal in what way? Mechanically? Thermally?

If the DC is ramped up via PWM rather than 'stepped' by closing in a contactor then there would be no real mechanical shock and surely there is enough mass present to handle whatever heating effects the braking would introduce.

I'm not sure I like the induction generator as a brake. This is for a couple reasons.

One is that the generator action may or may not be instant.... it may need to "build up", depending on just how the switchover to braking is accomplished. If it fails to build up, there is no braking. Braking should be "stone hammer reliable", it needs to work "for sure", with no "yes but" involved.

Another is the fact that the braking will drop off rapidly as the motor slows, since both the excitation and the induced voltage drop off with rpm, meaning the braking will drop off faster. Probably will drop off similar to the square of rpm. Unlike DC braking, the induction braking has no ability to brake the motor at slow speeds. DC braking will not "hold" the rotor either, but it can oppose any movement quite reasonably if a significant percentage of the FLA is used as DC. That is far less true with the induction generator approach, because rpm provides both the excitation, and the generated voltage to the load. At low speed, there is essentially no excitation.

The capacitors provide the excitation nicely at the design rpm. But they pass less current as frequency drops, causing excitation to drop off. As rpm drops, the frequency drops in proportion. That will have the effect of cutting braking rapidly as rpm decreases unless more capacitors are cut in as rpm drops. That is added complexity to achieve what is never going to be the only brake you need.

Just a Sparky said:

Brutal in what way? Mechanically? Thermally?

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That would be "YES"..................... Mostly thermally, since all the energy stored in the rotating system is dumped into the rotor. The rotor has to get rid of it by radiation across the gap into the stator, since by definition, the rotor has been "braked", stopped, and fans in the motor are not working.

The rotor may not have time to get rid of the heat before another start. Large motors are usually limited in the number of starts per hour. A DC braked stop is "worth" more in heating than a start because of the cooling. A running motor has good cooling, not so for a stopped motor.

The mechanical brutality is normally no worse than an across-the-line start. But large motors may respond worse to the mechanical forces.

It is not unknown for a motor which has been subjected to too much DC braking to actually melt the aluminum bar conductors in the rotor. Rare, but has happened.

This is used on syc motors, will not work on induction motors...Phil

I guess it would help to understand exactly what context this braking system is to be used for.

Are we talking service braking or an E-stop? 60 second stop time or as fast as practically achievable? Maybe/maybe not full deceleration or 100% reliable dead stop right now? Something that has to dissipate the heat of frequent stopping or only once daily/almost never?

If we're talking frequent, rapid stopping then VFDs are going to be the best bet - though switchgear-sized drives will not be even remotely cheap.

On the other hand once or twice a day just to keep the thing from spinning on it's own for 15 minutes straight or to bring it to a rapid stop in an emergency, then use DC injection. DC heating is equal to I^2R, so softer braking will generate exponentially less heat.

For giggles, maybe produce a figure for the total rotational energy of the rotor and it's load. That will roughly equal the total heating that will be produced within the rotor. Add that to the full-load temperature rise of said rotor, accounting for it's approximate thermal mass. Stator heating as mentioned will be equal to I^2R times time, plus temperature rise and divided into thermal mass again. Simple concepts, but practical formulas which are way over my head.

I'm still skeptical that a bit of mild DC injection will create any sort of problem even on motors as large as you're talking unless we're dealing with tremendous forces. Granted a chipper rotor has a lot of rotational energy, but that can be accounted for by providing a longer, gentler braking period for heat to conduct from the copper/aluminum rotor bars into the rotor laminations. As JST suggested, you won't be able to start and stop like this with any frequency at all, but the alternative is blowing huge sums of money on a drive that can ramp the frequency down slowly and dissipate the generated power via an external resistor grid.

Depending on the gearbox and the nature of the coupling between motors, it might be possible to start and stop the chipper unloaded using only one motor and drive (cheaper), then cut one or both motors across the line for full speed 'run' configuration. Although this will increase the cost of the switchgear and wiring. A cost analysis would be in order. It would also provide you with the benefit of being able to precisely balance the load being shared by both motors with a little bit of creativity.

I helped install a similar system for a bank of 4,000 HP pipeline pump motors. Each was ramped up on the drive to 100% then dumped onto the 4160V line sequentially until desired discharge flow or max. manifold pressure were reached. 6 feet per second at 900 PSI through a 24" line I think? 350,000 barrels daily is the figure that sticks in my head. One drive for three motors & provisions for a fourth.

Phil in Montana said:

This is used on syc motors, will not work on induction motors...Phil

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If you are referring to induction generation, it most definitely works with induction motors. I've done it, and there are many papers written on the subject.

JST is correct - DC injection overheats a rotor and melting the aluminum is a distinct possibility. We are not going that route.

I am really hoping to find someone that has actually done it. I asked all my leading EPD vendors, and all said they had no experience with it. I understand the concept and how it works, but I have no idea on how to model it - I might miss some important details if I just wing it, but I would learn! I do know that you run out of excitation pretty quick and that is why its not used that much, but if we can slow it down to a reasonable rpm electrically then the mechanical brake can complete the stop and the mechanical brake won't be working so hard so it should be much more reliable.

I know that we can't match the 10 second stop that we get from the sync motor, but I think we would be happy with a 30 second stop. We are not going to use a drive.

My other concern is that we would be going in reverse design torque of the fluid couplings - the vendor says it won't be an issue, but I have my doubts.

My strong recommendation was to buy another sync motor, but for some reason they have it in their head that they need this dual induction motor setup. There are some advantages with the two motors, but definite disadvantages also.

Asa far as induction generators, my experience is with the large induction generators we had on the hydros we used to own in Maine. But they were not self-excited - they were just tied to the grid. That makes it easy.

Hi Mark!

Okay, so what you want to do, is not only possible... it is done every day, in exactly the magnitude which you're needing... You just need to look at the proper industry, and talk to the right people.

You need to speak to the manufacturers of traction inverter drives for passenger and freight railway.

ABB, Seimens and Mitsubishi being the units I'm most familiar with... but I'm certain there's many others.

1250hp is right in the 'meat' of a traction inverter's range... you could pull one right out of the belly of an AC drive locomotive, connect it to your machine, make appropriate control and adjustment programming, and have it do your job with dead-solid reliability, as they're totally proven.

DaveKamp said:

Hi Mark!

Okay, so what you want to do, is not only possible... it is done every day, in exactly the magnitude which you're needing... You just need to look at the proper industry, and talk to the right people.

You need to speak to the manufacturers of traction inverter drives for passenger and freight railway.

ABB, Seimens and Mitsubishi being the units I'm most familiar with... but I'm certain there's many others.

1250hp is right in the 'meat' of a traction inverter's range... you could pull one right out of the belly of an AC drive locomotive, connect it to your machine, make appropriate control and adjustment programming, and have it do your job with dead-solid reliability, as they're totally proven.

Click to expand...

I'm not sure why there would be any specific reason to use a traction VFD... mains-fed VFDs of comparable power are fairly available and widely used for things like ski lifts, pumps, and industrial machinery like this. They generally support braking fine though you might need to use a separate brake chopper. I expect traction VFDs use resistor grids off the DC link like everything else.

Would a double-fed induction motor be an option? Essentially a slip-ring motor but a regen VFD drives the rotor windings. This gives you most of the benefits of a VFD-driven motor but the VFD can be much lower capacity. Disadvantage is control is limited if you get too far from synchronous frequency - you can take the edge off, but won't be able to brake it down to stop.