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DC power 240 volts

YoungDen

Cast Iron
Joined
Feb 16, 2004
Location
Kiron IA USA
I was at the scrap yard yesterday to see if there was anything neat in the pile. The owner asked me about a generator for his magnet that they move steel with. My question, would a DC motor serve as a generator? Such as these:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=26261&item=5574504792&rd=1
He wants 240 volts at 10 kw. If those motors were driven at the speed to produce 240 volts, would there be adequate watts, maybe?
Otherwise this thing:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=58189&item=7508242419&rd=1
Take the motor off and drive it from the crane engine, somehow. Would the 10 kw needed verses the 75 kw available cause any problems?
 
Most any DC motor 15 HP or larger will generate 10 Kw of DC power if the field and armature are connected in parallel. It won't be a perfect generator but it will work well enough for a magnet. Be careful not to overspeed the motor when it's connected this way. The motor voltage will compound with field excitation and possibly cook the field. You need a voltage regulator in the field circuit to do the job right.

In crane service the magnet generator has to be run at a constant RPM. If the generator RPM drops with engine RPM the juice will peter out and weaken the magnet's attractive force. Flux density is a function of current and since the generator is self excited and driven by a variable RPM and since the magnetizing current varies with the voltage you have an exponential relationship between engine RPM and magnetic attraction.

For that reason most magnet generators are usually wound for constant current like a welding generator and they have a way to reverse the power to the magnet to weaken the residual field to release light items still adhering to the poles.

Talk to a smart DC motor guy. Larger compound wound motors can sometimes be internally reconnected so the series fields and interpole magnets oppose the shunt field to give a constant current output characteristic.

If you install a separate 20 HP engine to run the magnet generator all my cautionary remarks are made moot.
 
Most scrapyard cranes are older crawler cable rigs with torque converters. Generally the engine driving the converter is running slow, and it is varied to vary the winch speeds. The ones I've looked at which are handling scrap have a separate engine/generator unit to power the magnet for the most part.
 
An M-G is the best choice if an affordable rectifier isn't available.

At the power level stated, an M-G is probably the best option.

If the generator part is compound-wound, then the M-G's output is more tolerant of load conditions.
 
Forrest, that is sort of the problem. He knew a fellow that would wire DC motors to be suitable. The fellow died.

Metalmuncher, thes units are the track mounted hydraulic backhoe type units. The one the boss was in when he waved at me to come talk to him still had the plastic bag on the operators seat. Couple of the Japanese models. One older one had the sound of being an old Detroit.

Peter, I was wondering about making AC into DC. That would take some parts that I don't know where to find right off but could look around. Then, almost a homeowner generator would do.

As a side note, there is way less drops and cutoffs and general screwups coming out of factories in the last year. I suppose if the guy drills too many holes in the wrong place, he gets shown to the door.
 








 
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