Main > Everything Else
3phase induction motors
jennifer:
I dont think there are many women digging ditches either, But I see your point... Anyway back to this graph, Im not real sure what
static pressure is yet,( fan ratings) But It would seem to me that would relate to the density (or amount) of air available in the booth and
ducts. This (thoeretically) could change the levels of your lines, They would then stay the same only @ higer levels? , Even in a vacum?...
The vacum would be created as the motor got up to speed making it work harder thoroghout its start to stablize cycle. A smaller motor
then would what than, get to a point to where it wouldnt effectivly move the air while keeping the pink line low?...That would be my guess.
This would bring me to hp ratings of the motors... (and again Im guessing) But the Hp would be a product of Power/speed/time?
The booth Im building is 800cf (big enough for a full size cab) The fan is a 3/4hp and 1500cfm...Overkill I know, but a clean air manifold
bypass and a VFD will allow adjustment, While retaining a good push out the stack.
lilshawn:
At least if you where simply driving a conveyor belt...you know the load requirements...it's easy to pick a motor/driver combination because the power requirements are steady...air is a very dynamic load. it's going to vary according to RPM and other factors (including where the air is comming from...where it's going...but also what's restricting the output/input ... is where the static pressure is going to come into play.)
basically speaking static pressure is how much air leaks from a given area/room.
see the thing is, the motor has only the weight of the fan to turn at first (no air is actively trying to resist the motor (yet). if you hammered the blades flat as a pancake, the motor would spin quite merrily along.
but at the start, low speed air movement is going to start applying a load to the motor...a small amount of air will begin to move...and a small amount of drag down on the motor is going to be applied. in your airflow direction, the pressure is going to increase. This air needs to go somewhere (open windows/vents/around the doors)since this isn't a sealed box how much resistance there is determines how much static pressure the room has.
as you increase the RPM's you also change the load because you must move more air...air resists high speed movement. The load on the motor goes up, The air flow increases, the air still needs somewhere to go...if the air input to the room is greater than the static pressure (IE: airflow out) the air will begin resisting even further (actually a whole bunch of things happen which I won't get into here) basically you get to a point where no mater how fast you push air in, no more will be accepted and you end up with air getting pushed back through the fan in the opposite direction. (around the tiny spaces between the fan blades and housing) the opposite can happen too as well...you can't "suck" in enough air and you end up with the "vacuum cleaner effect" (where you cover over the hose and the motor spins faster and faster but can't move any more air.)
fluid dynamics is it's own ball of caca that has years and years of horrible mind numbing calculations and theories behind it that entire universities have dedicated their existence to it.
oh...and hp. Ugh...erm...the only things about horsepower I remember is:
1hp is 746 watts
and
force X distance/time
and
it has nothing to do with horses and it was James Watt who said something about a pony doing a bunch of work and not a horse. :dunno
MonMotha:
Fun fact, if you ask an electrical engineer, then 1HP is EXACTLY 746W - end of story. If you ask a North American mechanical engineer, then 1HP is 745.69987W. If you ask a European mechanical engineer, you may get either 745.69987W or 735.49875W. A fluid power engineer (specifically working with moving water as a power source) may say that 1 horsepower is 746.043W. Fortunately, these are all pretty close. And yes, at least the first two really do show up quite often. The 0.04% difference between the EE and ME groups can be a fun one to figure out sometimes.
Of course, if you ask a boiler technician, they may answer that one horsepower is 9809.5W. I have no idea where they got this one from.
Regarding power in a rotary fan application, you may want to know that {P = \Tau x \omega}. That is, power (as a scalar) is the product of rotational speed (e.g. in RPM) and torque (e.g. in foot*pounds, if you're in the US). That would be the mechanical power developed by the motor. If you want to know the electrical requirements, you need to factor in motor efficiency, of course.
Also, FWIW, unlike the universal motors used in vacuum cleaners, an induction motor will NEVER exceed so-called "synchronous speed" which is determined by the AC line frequency and the number of motor poles. In the US, this would generally be 3600RPM or 1800RPM. Under no-load conditions, you can get arbitrarily close, but you'll never exceed it. Under load, you'll have what's called "slip", which is required for an induction machine to develop torque, and the speed will be somewhat less. At rated load and steady state conditions, you should end up pretty close to the speed rating on the motor nameplate, assuming you indeed put in the rated AC frequency (a VFD of course lets you vary this).
lilshawn:
--- Quote from: MonMotha on November 23, 2012, 06:32:55 pm ---Of course, if you ask a boiler technician, they may answer that one horsepower is 9809.5W. I have no idea where they got this one from.
--- End quote ---
that is BHP "boiler horsepower" - it was X amount of water needed Y amount of energy to heat it to 100 Celsius/212 Fahrenheit in one hour. that came from steam engines way back when. ( i don't have any idea what the numbers where)
jennifer:
Im so going to cry myself to sleep, Omg.... That is quite a rash of information, And again I understand most of it, So Basically a smaller
motor would use less energy and consequently doing less work, In the design of a booth, or pump station, all these factors would have to be
taken into account to make it efficient... If say a 1/8 hp motor was used, in the same set-up, besides being underpowered, would have to
work harder to spin the same load to its rated point then stabilize to a less torque situation.... I can see from an engineering standpoint where
in the design the ratings of the motor were taken into account, not just upgraded until it worked... This is a whole different concept than
say, building a cab, Where the power needed is determined by its load, not the amount of work... Whoever thought to rate BHP, was
pretty much on the cutting edge for the day, Jennifer may look into that a little, just to satisfy my curiosity (Not to build a steam powered spray booth).
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version