DC to AC migration 1

I have seen many rebuilds where the DC system has been kept. Both Siemens and ABB have good solutions that are easily installed and very economical.

Both use an external control unit that produces gating pulses for your existing thyristor bridges as well as doing everything else needed from a modern control system - including HMI, communication, fault diagnosis, everything.

All existing power components, including cabinets, bus bars, breakers, fuses, thyristors etcetera can be kept. The capital cost is minimal and the work can be done in a few days. So, if your main reason is to make the drive system servicable again, this is the way to go.

If you go AC, the common DC bus is always preferred. No doubt about it. A front end for the common DC bus can be four quadrant and also have a decent PFC structure. Having that in every single "rectifier" in an AC/AC system is less favorable.

With a common DC bus, you will not need to regenerate as much power as with an AC/AC system. Mainly because those sections that do not brake will absorb power from those that do. But you should still make the front end four quadrant. Braking those dryer sections to standstill within prescribed time almost always needs regeneration.

DC motors are still being built. I see lots of ABBs BMI motors being installed in places where one wants to avoid EMI and other VFD effects. Sometimes, there are also space restrictions and the need to keep retraining of maintenance personnel to a minimum.

Re. the 5 - 7 % better efficiency: I couldn't say. It depends a lot on how the machine is run. DC motors are usually quite efficient. There is no slip to produce losses and there is about the same current heating similar amounts of copper. I really couldn't say. You have to do a calculation for each drive and with the typical running profile of your machine.

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

Gunnar

A tire manufacturing client of mine has many dc motors from 450 hp to 1500 hp for various processes like extruding, mixing etc. The motors have had a service life from 15 to 20 years with proven performance.

To eliminate the brush maintenance, the new General Manager wants to replace all the dc motors with vfd ac motors and some consultant have advised them to go in higher capacity ac motor to meet the "torque" requirement. So, now the client plans replace the 450 hp dc motor with 600 hp ac; 1500 dc motor with 2000 hp ac etc.

I find that this would rise their energy bill and lower the plant pf due to under utilization of these ac motors.

What is your take on this migration from dc to ac motors ?

I am going to visit a tyre manufacturer in Eastern Europe later this fall. They have installed new DC drives and motors for their calenders and other high-torque machines.

I am not at all against AC induction motors. They do a great job. But some applications do need torque (and overtorque) capability all the way. The new breed of DC machines have that capability.

Fans, paper machine drives, pumps, conveyors - all OK with AC. Some cranes (container ship-to-shore), rolling mills and other applications get better performance with DC. At least, that is what many users have experienced and they install DC drives in such applications.

Regarding maintenance. Brushes need maintenance. But so do machines with bearing EDM. Inspecting motors and brushes is not only bad. It makes maintenance people move around and look at other things as well. I think it is good to have a few guys "walking the floor". They notice little problems before they turn into BIG problems.

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

Yes, EDM failures are a maintenance/breakdown issue. Thanks.

Present day engineers/techs do not want to get their hands dirty. And then, there is the bane of bean counters cutting cost by cutting personnel.

A proposal like this should be based on life-cycle costing, not simply initial cost and individual maintenance issues. This is particularly true with DC/AC conversion since the performance can be accomplished with either motor type.

In my experience, the more comprehensive the analysis, the better AC looks. And, also in my experience, the 5-7% efficiency advantage of AC holds up pretty well too.

The biggest plus for DC, in my opinion, is somewhat simpler system design and, as a result, somewhat simpler maintenance training and troubleshooting. Of course, that can usually be overcome with more training.

I find it curious that EDM is cited as a negative for AC since it can and should be dealt with in the initial design and not as an afterthought, particularly in larger systems like paper machines.

I think the world-wide verdict is in favor of AC but not overwhelmingly so.

I couldn't agree more, DickDV.

But, the fact is, that mitigation techniques are poorly understood and applied. A recent article in IEEE Industry Applications Magazine (Dahl, Sosnowski, Schlegel, Kerkman & Pennings) July/August 2008 is among the best written in many years. But it still doesn't convey information in an easy enough manner for the practical machine builder/planner/designer to do the right thing.

Recommendations are usually very sketchy: "Ground well", "Use grounding brushes", "Isolate the bearings" are very common recommendations. With no regard to the actual installation. So, not many installatons I have seen are without EDM problems. Good for me - I make a comfortable living correcting those problems

A seminar on EDM, that I will run September 1st, has attracted more than 110 paying attendees from Sweden alone. They come from large (the largest) drive and motor companies, power stations, HVAC users, paper mills, steel mills, airports, bearing manufacturers, lubrication oil and grease manufacturers - just about any industry you can imagine. That alone shows that the problem is far from mastered and the "information vacuum" is very real.

What I was trying to say is that the AC/DC discussion isn't one that one shall expect to be settled once and for all. There are too many aspects. We have but touched a few here.



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

Thanks folks for sharing your ideas and experiences. EMD considerations can be taken care of during during design and installation. At my earlier assignment, not 1 EMD related failure in 10 years. Cable selection, laying and grounding is crucial. Why plant managers are worried about DC Motor maintenance is mainly due to non technical problems and will increasingly go against DC Motors as people want to stick to computer screens.
thanks again
Sharma

One thing not usually considered is that DC drives (thyristor based) are much more rugged than AC drives (IGBT based). If I was betting, I'd put my money on a longer service life and cheaper maintenance for the DC drive, especially when you're looking at the smaller drives - less than 300 or 400kW.

Hey, I thought DC was dead?