Way Way Ot

The head of the construction division of the company I work for has gotten himself into a bit of a pickle. A printing plant with 4 - 60HP D.C. drives wanted to have a genset capable of running the plant. An undereducated sales rep wandered in took the nameplate voltage and fuse size info off of the drives and came up with a selection of gensets suitable for the project. No one, not the sales rep, or construction DIV head made any comment about non-linear loads. The genset and transfer switch were installed, and one of my fellow service techs was sent down to perform the startup, well surprise, surprise the unit worked fine until they hit the jog button on the press. The voltage regulator on the genset will handle a maximum of 4.5% Harmonic distortion. The drives are mid 80's era Fincor drives all fed from a common xfmr, no load or line reactors. We hired someone to come in to determine THD (28-35% depending on load), and he suggested throwing an unloaded 75HP induction motor onto the AC bus to absorb some of the 5th and 7th harmonics. My question is: Has any ever heard of this technique for harmonic mitigation??

WOW!.....

I think someone will be looking for a new job very soon

So have they thought about buying the correct one? I think this will be an issue forever, but why would they need a genset? I know its not cheap but I also think it would be better in the long run... or hook their air compressor to it, that will be the other constant load.

At the plant I work for now, we used to loose power a lot.. then they supplied us with two feeds, now if we loose one then other is there to pick it up and we did not have to pay...

Yeah new job sounds right to me, however he's one of the owners. This plant is not located in an area with more than one grid nearby. If we lose main power the next nearest tie point is about 3/4 mile away (right through a residential suburb).

geniusintraining said:

WOW!.....

I think someone will be looking for a new job very soon

Click to expand...

At least one person is looking for a job maybe several!

Sizing generators to drive industrial loads is almost a disipline in itself. The supplier of the gen-set should have been involved with sizing it in the first place.

Adding an unloaded AC motor will provide some inductive load to the system but I can't see how that is the best way to help with harmonics? Are you sure it is the harmonic content that is causing the problem? "Harmonics" is a real problem, but it tends to cause people to blame it falsely on a lot of electrical problems because there are few that really understand it (salesmen come to mind).

Your best resource is the gen-set people, this is what they do for a living and should be able to steer you in the right direction.

Good luck with the new promotion.

I have no way of knowing if this would be a complete solution but I am quite sure it would be a large piece of the solution. Install reactors ahead of the DC drives and increase the job accel time to the longest that the application can tolerate. And, that would be individual reactors for each drive, not a single set in a common feed to all the drives.

I meant "jog accel time", not "job accel time". Sorry!

Reactor might help, but sizing becomes an issue when you consider that most genset impendence levels are usually an order of magnitude higher than most grid sources, we'll probably look at individual tuned harmonic trap filters, with additional reactors thrown in when the transfer switch activates, alternatively we may have to go to an active filter. My question remains does anyone have experience with a running unloaded induction motor acting as a harmonic filter? I have been doing this long enough to remember when MG sets were the primary souce of DC for Motor drives, and have never once heard of this. I'm beginning to think the fellow who said this may have had his tongue stuck on a power factor correction capacitor too long.