PTO generator
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PineRidge
Super Member
Just an update on surge amps; we tested the instantaneous surge and it came upto 270amps for just a fraction of a second, while running amps was only 39-40amps at each of three legs. the motor was rated at 63.5 amps/208volt/3phase, and the 270amp starting surge has never tripped the 100amp main breaker. so I am totally confused and back to the beginning. 39x208x1.73=14,000watts. at 3X overhead would I need a 42kW generator to start this elevator? My average running load is only 8kW with a 5 minute peak of 17.5kw- so this sounds like a very inefficient setup, as the specs show fuel consumption per kWhour to be 50% less efficient at half load. Worse yet, 8kW/42kW is closer to 19% of full load. Any thoughts?
DarkBlack
Elite Member
Fred, 270 amps won't trip most 100 amp breakers, unless it was for a longer time period( over 1 second ). I've skipped most of this thread, but I will offer that you probably used a meter that doesn't show real power( if you didn't have 4 clamp-ons and 4 voltage probes, then you didn't ). In this case your amp based power calculations will come out too high.
One trick used to bring up high inerta, high currnet draw 3ph motors is to connect them for Y-start, delta run. In your case you would probably need to spend a few buck on some realys to make it happen, otherwise you can put in a cheap VFD. In either case the mods woud be cheaper than a larger generator needed without them.
One trick used to bring up high inerta, high currnet draw 3ph motors is to connect them for Y-start, delta run. In your case you would probably need to spend a few buck on some realys to make it happen, otherwise you can put in a cheap VFD. In either case the mods woud be cheaper than a larger generator needed without them.
yelbike
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That's why systems are sized according to code table and not assumptions. Breakers are size according to their respective rating( whether they are %100 rated (rare) or 80%(most common type). Their are so many tripping charactistics that even the code requirement have built buffers. Typically breakers are rated at 125-150% for rated FLA current and td fuses are at 250%. Don't try to hard too understand as I still refer to charts, I'm a construction and power electrician with a back ground in marine systems with 16 years experience.
Thanks, you are correct. We used a digital ammeter and I've been told by the electrician that an instantaneous digital reading could be twice as high as an analogue reading. I will try to learn more about "Y-start, delta run", and a "VFD". wikipedia has been very helpful. thanks again.
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PineRidge
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SPYDERLK
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You have a momentary 100KW load when that motor starts to start. DarkBlacks point of no V measurement is true, but not material here because the motor wants those AMPS and when youre on mains you have a hard voltage source providing ample "pressure" to push those amps. The same would be true if your mains connection were replaced by a ~90 or 100KW gen. The question for choosing a gen as low as 40KW or lower is how sensitive is the rest of your system to a relatively long and deep brown out. That brings the question of whether there are significant compressors running when starting the elevator motor. They could stall and cause system current demand to avalance upward. Next is what kind of load is that elevator motor starting under. Im guessing that it is a load that presents only inertial load at 1st movement, but that a real pumping load develops as rpm increases. What bothers me is the high apparent load you figure indicates on the running motor. Is the elevator rising during that measurement.
If it is not I would expect to see a significantly lower "idle" current ... perhaps 10A.It is premature at this point but I anticipate the need for a 40KW gen driven in an underpowered mode by an engine in the neighborhod of 30+ HP [capable of supporting your sustained 17KW max loading]. Depending on the nature of your composite load, this value gen should not show debilitating voltage buckling under your peak load and "flywheel" inertia will ride thru the brief momentary overload to the engine.
larry.
You have a momentary 100KW load when that motor starts to start... What bothers me is the high apparent load you figure indicates on the running motor. Is the elevator rising during that measurement.
It's a hydraulic elevator, 39amp(x1.73 for three phase) max load only rising upward, downward is mostly a release of pressure, but we will remeasure and check the current in the down mode as well. I will also inquire about the costs of installing a VFD. Otherwise, there are no compressors running, just the hot water circulating pump which stays constant on for about seven months, and then the boiler burner fan, which does cycle, but it is a small 5amp 208v motor. Flourescent lighting ballasts are constant on, the only other starting loads would be an occaisional washing machine @ 120v. Power company has confirmed that 17.6kW "actual" as opposed to billed demand, has been the maximum in the past five years.
Thank you larry, that is a great idea. I hadn't considered a custom pairing, but it will be unlikely that I will find a pre-owned system like that. Do you have any advice regarding the VFD?
