DC Generator

DC gensets should be relatively easy to restore to full operation. The brushes and/or the commutator need to be replaced and serviced. A knowledgable alternator guy should be able to work this.

Jim Vrzal Holiday,Fla.

Building electric engine is a hobbie of mine...

It's a bit dangerous to accuse another poster of too much ignorance when you are not too sure of your own facts.

Jim Stewart has already dealt with your comment on power rating. Your comment on reverse diodes is equally misleading.

In an earlier post I recommended that the reverse diode be rated for at least the nominal coil voltage and current.

This is because, at the instant of disconnection, the current that was flowing in the inductor is diverted to and flows through the forward direction of the reverse diode. This current then decays at a rate determined by L/R of the magnet coil. The stored energy (1/2 L x I squared) is dissipated in the resistance of the magnet coil - not the diode. The reverse rating is determined by the voltage that appears across the magnet in normal operation.

Jim

Hey DoN,

Owwww....what about the AC hum when he's got ahold of a 1/2 ton of scrap pieces? Ouch!!

Take care.

Brian Lawson, Bothwell,, Ontario.

Seems to me to be some confusion between the term Kilowatts and heat. KVA or Kilowatts in the case of motors and machinery is the energy used or available, and not just as heat. Properly designed and operated, a 25 Kilowatt MG set gets no "hotter" than a 50 KW machine, or a 1KVA transformer won't get any "hotter" than a 10KVA, nor should a properly designed and used magnet. They just use that much energy, and not necessarily as heat. If they were just heat dissipators like a grid, then yes, one would get significantly hotter than the other.

And I don't know, but I doubt that anywhere in this system will there be a single simple across-the-line contactor to operate the magnet, DC or not. It would scare the shit out of the operator if he just had that. Pieces of metal would be flying everywhere instead of being properly and slowly attracted and dumped to/from the magnet.

Take care.

Brian Laws>Richard J K>>

Wrong DoN. I'm "DoN. (Donald Nichols)", not Don Foreman, who wrote what you were responding to.

Probably not too bad. Based on my mag chuck for my surface grinder, there is some residual magnetism which lasts for some while after the chuck is turned off, and this could control the hum rather well. As a matter of fact, it is wired up to feed raw AC to it to demagnetize it so you can get the workpiece off without having to fight for it.

Enjoy, DoN.

Nope. Read the magnet specs someone else posted for the DC resistance of the coil and the DC current. I*I*R ends up as heat in the magnet coil.

Seems to me to be some confusion between the term energy and power.

Right Ken - but there are other models - IR does make them this size and much larger. I see them on construction sites and always wished I had one to fire up after an earthquake.

Maybe someday.

Mart>>Gosh - with this one - you can have coffee also!

You don't know how they turn it on or off. On something like that there could be several coils a main one and a stepper. The stepper is the extra you need to grab and take .

The windings are likely browned down and up - not on/off. You want to get it below pickup to drop and above the safety level to grab.

Martin

Dump current can be no greater than excitation current, so diodes than can deliver excitation can handle it no problem. 35 amp 600 volt diode bridges cost less than $2. The energy from the collapsing field is mostly dissipated in the resistance of the load just as excitation power is. Diode dissipation is fwd drop * current whether exciting or dumping.

10 KW is about 13.4 HP. A 25% efficient 5 HP lawnmower engine dissipates more heat than that, with far less surface area. Gae thee to a scrapyard, behold the size of the electromagnet....

The original post asked for a replacement generator of specified power for a system that has worked in service. Declaring him ignorant is neither helpful nor defensible by technical premise you present.

Indeed it was the wrong term, I apologize.

Perhaps, but it has the advantage of blowing most of that heat out the exhaust, instead of raising itself to a temperature that blackbody radiates it.

Good point. The engine also has forced-air cooling on the fins. Still, those magnets are pretty big so there's lots of area for convection air cooling.

To your first point, we don't know that the magnet draws 10KW, only that it was driven by a generator capable of 10KW. How much power the magnet really draws is indeed a relevant question.

Yes, and as such, I know now that I need to try to obtain more info concerning his particular situation. As I mentioned in another post, I'm getting all of this "second hand" and basically was told he was looking for a 10KW DC generator.... Hell, for all I know, he may be using an AC generator :-) Next time I see this "second hand guy", I will let him know that I could use a bit more into. Thanks for all the help and suggestions. Ken.