| Author |
Message |
niles steckbauer (Niles500)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, May 02, 2006 - 5:30 pm: | |
Was talking with some young Engineers I know - and the subject was raised by me as to numerous discussions on this board about electrical system design capacity and the consideration for load spikes that were required for AC's and other motors (and then the discussion went on to the Grid in general) - but on a micro level - why would it not be possible to use a torque converter to overcome the load spike at motor start and therfore lessen the max capacity of system to provide for these momentary power needs, and to bring it on topic allow multiple AC's to start and operate within a closer parameter of the system capacity?
Motor starts = engages torque converter = at wind up engages compressor = converter locks at predetermined load (say 80% or so) = drives compressor direct
Any comments? |
Marc Bourget
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, May 02, 2006 - 6:59 pm: | |
IIRC, (deferring to the smart guys)
The total inrush current requirement arises from the power necesary to develop not only forward rotation - accellerating the armature to speed and the load the motor must address, but it also must combat "back electromotive force" generated by the magnetic field created by the current flow.
This takes approx 600% of the full load amps. This is indepenent of the load and accelleration requirements.
If I understand your point correctly, the torque converter would reduce the startup load, but this would appear of minimal benefit as it would help by reducing a portion of the energy required by the motor - reducing a component of the energy requirement that composes only 16% of what the motor has to deal with until it's up to speed. |
Richard Bowyer (Drivingmisslazy)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, May 02, 2006 - 7:08 pm: | |
In layman's terms, the power factor (pr) of a motor just energized is in the neighborhood of .20 (lagging) which accounts for the extremely high inrush current. Basically this is just magnetizing current and really does not change whether the motor is at no load or at full load starting. After the motor comes up to speed the pf is in the .90 area (lagging), depending on the motor design.
Richard |
herman
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, May 02, 2006 - 8:31 pm: | |
If you can decouple the load from the motor, e.g. a compression release on an A/C compressor, then you could also add electronic motor controls that could knock off much of the current inrush, trading it for a longer spinup time.
The starting surge lasts only a fraction of the second; in many respects, the mark of a good inverter or genset, vs. the cheaper stuff, is the ability to take such a surge in stride. Weigh the difference in cost between the cheap and the good stuff against the cost of widgets to tame the load, and you're likely in the same ballpark.
I suspect that variable speed brushless DC motors are going to become the hot ticket in the near future. |
Ron Walker (Prevost82)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, May 02, 2006 - 8:45 pm: | |
The way to get around high starting current is to use a VFD (variable frequency drives)or Soft Starts |
Richard Bowyer (Drivingmisslazy)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, May 02, 2006 - 9:03 pm: | |
Thats right, but they sure cost a bunch of money!
Wye-Delta or Wye-Wye soft starts reduce the inrush signifcantly but again the starters required are quite pricy. Especially when you get up in the 500-750 hp range. LOL
Ricard |
niles steckbauer (Niles500)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, May 03, 2006 - 5:26 am: | |
Marc - its more like 400% inrush current for normal AC - but no consequence - there would be no net gain/loss of energy - just transference of energy from spiked inrush current to continuous input current until the torque converter engages the compressor - ie. no spike - not arguing a gain in efficiency, but just a continuous stable loading of the motor/compressor -
Richard - exactly, I'm talking about a Power Factor of .80+ - the motor is free wheeling until engaged by the torque converter - the wind up is more variable than the LRA and does not diminish the total value of the starting capacity - but merely accumulates and transfers the energy from the torque converter to the compressor - by taking the spike from the instantanious energy demand you have deleted the necessity for excessive load capacity -
Herman - I think you answered the ???? - staged/ variable compression of the compressor is a much simpler answer to the queery - a 6 cylinder compressor loadng one cylinder at a time would accomplish the same thing - just the answer that awaited me, but too obvious for me to ascertain - thanks
Ron - I think Herman answered that question for a much less $$$$$$ - Ditto Richard -
Thanks all - Niles |
niles steckbauer (Niles500)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, May 03, 2006 - 5:38 am: | |
Herman - I also concur with the fact of a "good" inverter that scavenges enough energy off the batts to overcome the spike of inrush current to start the AC's - thats what I have - I was just considering the many posters working directly off gensets w/o these types of inverters - FWIW - Niles |
FAST FRED
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, May 03, 2006 - 6:04 am: | |
The most common "cure" seems to be a huge condenser , or two stock ones working together.
Dont know if it helps much , but its common in long term sites where the volts seldome even gets to 110.
FAST FRED |
David Hartley (Drdave)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, May 03, 2006 - 6:56 am: | |
Many ice makers and other compressor based items have been updated with what is called "easy start" which is a heavier duty start capacitor which allows a lower current draw when starting refrigeration compressors on limited voltage/current.
The capacitor charges up and sends the larger current to the start windings rather than relying on the power line to supply all the stall current.
The kits have been available for rooftop a/c units and rv and marine icemakers for years.
The last one I installed cost about $21 .... |
Richard Bowyer (Drivingmisslazy)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, May 03, 2006 - 10:22 am: | |
A little more trivia. Locked Rotor KVA (starting current) varies on motor design and whether single phase or three phase.
For code F motors the LRA is about 5 KVA per hp.
For code J motors the LRA is about 8 KVA per hp.
NEMA Standards MG 1-10.36
As a rule of thumb I used to guesstimate inrush current at 600% of full load running amps.
Richard |
Marc Bourget
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, May 03, 2006 - 11:06 am: | |
Niles said:
"there would be no net gain/loss of energy - just transference of energy from spiked inrush current to continuous input current until the torque converter engages the compressor - ie. no spike - not arguing a gain in efficiency, but just a continuous stable loading of the motor/compressor "
I always understood the "excess" energy attributed to overcoming "BEF" went to heat. Are you saying the magnetic energy is re-absorbed and converted into rotational energy? (I'm not challenging you, rather seeking information! i.e. learning) |
herman
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, May 03, 2006 - 11:51 am: | |
Niles, I actually was thinking of a VFD to ramp up the speed.
As for gensets (of the traditional fixed speed design, vs. the inverter bases ones such as the Honda EU series and the Onan QuietDiesel), one can get generator ends that are designed for load profiles where motor starts are are more prominent. I think that the way these work is that certain aspects of the design that pertain to motor starts (e.g. the armature and/or field wire gauge and perhaps the capacity of the magnetics) are beefed up in order to handle starting surges with less voltage sag, while other aspects more pertaining to continuous duty (e.g. winding density, insulation temperature rating, cooling, etc.) are not.
There's also the mechanical inertia of the entire genset's rotating mass, and the responsiveness of the engine governor, that help to provide frequency stability during starts. Gensets not intended for mobile use are less concerned with weight issues, and thus can pour on the mass in these areas.
David, the soft start kits work, but not really as you describe. It's not like there's an AC to DC conversion that then charges a capacitor; it's still an all AC design. The capacitors are for power factor correction: cheap motors have none, better motors have one for either starting or running, and yet better motors have two, one each for starting and running. Use of run capacitors presumes a given load on the motor; for variable load profiles the benefit is not as great.
If I understand correctly, the LRA value is actually provided for the purpose of sizing breakers as regards their magnetic trip mechanisms; it describes the worst case peak current at the time of connection of the motor to the line, if that time should be disadvantageously phased to the AC cycle (and wherever, with respect to the field poles, the armature had previously come to rest at). This related to, but somewhat different from, what is generally referred to as the starting surge, which is more closely related to the area under the curve from the time of attachment to the time at which steady state is achieved. |
niles steckbauer (Niles500)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, May 03, 2006 - 3:41 pm: | |
Marc - for the sake of simplified example - lets assume 10 amp operating load, 400 % inrush current for 1 second is the same as - 4 second delay that provides 10 amps continuous to motor enabling the 'starting cycle' of the engagement and wind up of the torque converter - both scenerios comsume the same amp hrs
Herman - yes, my understanding of VFD's or AFD's gets it done at 150 % inrush - a lot better than 400 % - do you have any idea of the cost of these controls? I don't? just wondering |
Richard Bowyer (Drivingmisslazy)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, May 03, 2006 - 7:05 pm: | |
Many of the VFD's that I used actually limited inrush current to less than full load running current. Relatively easy to do with the PWM technique.
Richard |
John Jewett (Jayjay)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, May 03, 2006 - 11:40 pm: | |
A downside to slowing the inrush current is that the longer time period allows more time for heat buld-up in the feeder wiring, and in the armature windings. Heat-resistance-heat etc. Not a major portion of the equation, but nonetheless it can't be discounted. Slow blow fuses begin to approach their limits more quickly under this situation, and thermal-magnetic or adjustable trip magnetic breakers can alleviate only a portion of it. Field experience has shown that fusing has to be about 600% of FLA to achieve reliability. (i.e. to reduce premature drop-out) In a bus conversion, where we strive for compactness, where do we put the clutches and extra controls, and at what point do they become cost effective? If we settle these issues, I suppose you could all become quite wealthy! ...JJ |