On Thursday, March 10, 2011 09:27:37 AM andy pugh did opine: > On 10 March 2011 01:24, gene heskett <[email protected]> wrote: > > But that was a 20HP motor, running on 235V 3 phase, and drew 39 > > amps/phase at rated power output. �So a 100 HP motor would have > > needed only 195 amps, not 330/phase. > > 195A per phase on 3-phase input. Wouldn't it be 50% more on > single-phase input? (Or would it be 3x as much?)
I don't have a mathematical basis, but it seems to me that to get that same 100 horse out of a single phase, one would have to factor in two things, first is the higher operating slip angle of the single phase, which is commonly run at 1725-1750, or 3425-3450 revs, which alone doubles the heat losses in the rotor, compared to the 3 phase, which often runs at 1775 or 3550 rpms, so its "magnetic slip angle" is less. That, common sense says, has to translate into reduced heat losses in the rotor. And secondarily, the 3 phase operates at the vector voltage per winding, which on modern 127 volt per leg circuits, is around 238-239 volts across each winding when wired delta. The single phase would normally be operated on a balanced circuit for 254 volts, 2 x the std 127. All of which make a direct comparison a wee bit difficult, so about all that can be said is that the 3 phase, which normally operates at the lower slip angle with its more even magnetic field rotation, will be a few percentage points more efficient. Couple that with the 3 phase is generally being operated at a bit less intensity of the peak field, heating from saturation effects on the iron is reduced. All of this can be swamped by poor choices on the part of the maker, like using a wire gauge smaller to make it less costly for the copper, which allows a few pounds less iron to be used because the wire is physically smaller. Carried to what I would call extremes and you wind up with that Ajax pump motor, still a 15 horse, but its cooling fan raises its output noise level by 50-60 db. Sitting on the floor, plumbed in parallel to the much larger GE powered pump, switchable by enabling its disconnect, opening the 2" gate valves that isolated it, closing the main pumps gate valves and opening its disconnect. Then we could break the unions and remove the off lined pump for service. All while remaining on the air. The relay/breaker that controlled them was rated to handle both, but the individual disconnects were 50A/phase Heinman circuit breakers which turned out to be an expensive, very expensive, choice. One of them failed open on just one phase, so the pump was single phased and within a second stopped & locked. I was at the console at the time, recognized the sound of a single phased motor, and my hand had made it about halfway to the beam power off button before all hell broke loose and it got very dark, very fast. No windows. The sudden stoppage of the coolant allowed the electron beam to burn through the collector bucket on the bottom of a $150,000 klystron. That crowbarred the 20 kilovolt supply when the tube filled with steam & copper vapor, taking down the buildings 1200 amp 3 phase entrance breaker, and I expect that bump was seen at least 100 miles away. The cure for that was a pair of 50 kilovolt vacuum relays, whose coils were wired across the 3 phases with the contacts in series with the 19.7 kv beam voltage, so that if any phase even drew a breath to sneeze, the klystrons were turned off, while an aux contact also sent an open to the GE AK-225 main breaker for the high voltage. That put a hell of a strain on the filter capacitors as the metering showed a pulse from the normal 20kv, up to nearly 35kv, but they held, and even if they failed shorted, it was a heck of a lot cheaper than another $150,000 4KM100LA klystron blown. Yeah, I've 'played' with some moderately sized iron. And I still envy Stuart. ;-) -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) <http://tinyurl.com/ddg5bz> Is this TERMINAL fun? ------------------------------------------------------------------------------ Colocation vs. Managed Hosting A question and answer guide to determining the best fit for your organization - today and in the future. http://p.sf.net/sfu/internap-sfd2d _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
