3phase induction motors

[jennifer]

    Im looking @ getting a new fan for the booth...I would like to control the speed of this fan, If a 3/4hp motor is run on a 110 line
using a VFD,
     #1 Is it possible using the 110/60hz?
     #2 Would a starter have to be included in this design?
     #3 Can a Vfd be set to start again @ 60hz on start up (every time its turned on)?
     #4 Is there a better way?
     #5 Is controlling a single phase with a rheostat any safer?

[MonMotha]

3 phase motors do not require start/run capacitors as you may know.  The VFD should be able to implement soft-start for the motor, and it'll do so in a way much nicer than pretty much any other solution.  As to whether it can be set to remember speed upon power on, that'll depend on the VFD but it should be a common feature.  Many of them have programmable startup patterns.

Do you actually have 3 phase service?  If not, you'll need to find a VFD that's single phase in / 3 phase out.  That's electrically quite doable, and I'm guessing they do exist.  Most three phase motors are 208/240 delta, though.  120V would only be for wye, which I don't see on motors very often.  To run a 208/240 motor off a 120 single phase circuit, you also need voltage boost.  Again, electrically quite doable, especially at only 3/4HP, but it will limit your choice of VFD.

At 3/4HP, you can probably get by with a conventional chopper style speed controller like are used to control power tool speeds and a universal (brushed) motor.  Not the nicest thing for the motor, but it'll work.  Won't work with an induction machine, though.

[jennifer]

       Quite informative, void or RPM formulas and written so as even a girl can understand.... Thank you for that.
 No I dont have 3phse service, but after looking into what you said a inverter style 120v to 220v VFD (apparently there is such a thing)
 Is what I need. Horsepower ratings seem to be a thing, So in typical Jennifer overbuilding fashion the 2hp is overkill But should allow
 for more headroom (bulletproof). Im feeling much better about this project, and no longer scared of it.

[Ed_McCarron]

a inverter style 120v to 220v VFD (apparently there is such a thing)

There is such a thing -- I just set up two for a customer, driving chemical feed pumps.  They were spendy.  I'll see if I can get part #'s for you.

[lilshawn]

http://www.bardac.com/pages/products/ac_optidrive-i55.html

[DaOld Man]

Those are going to cost the baby.
Jennifer, cant you just use a single phase motor for the fan? Single phase speed controllers are a lot cheaper than smart VFD single to 3 phase converters.

[jennifer]

    Option this, ratings that, So confused...The Rpm formulas do seem to be a part of the overall picture, Even with a simple fan.
The faster the fan turns the harder it has to work, Tourqe than becomes issue...OMG... This isnt as simple as I first thought.
    The Soft start of the 3phse and the variable control is kinda what Im thinking here, even though there may be cheaper options.
After looking (and getting lost navigating the site) This Bardac VFD co. seems to have what I need.

[lilshawn]

Even with a simple fan. The faster the fan turns the harder it has to work, Torque than becomes issue

not necessarily true though. motors are funny things. you'd think it would be a straight line...faster speed means more torque and more power needed. But in reality its more of a logarithmic curve shape.

(orange line) a huge surge of power is required to start the motor turning but as your speed increases, the power required to maintain that speed drops significantly. this is because the motor has rotational inertia that wants to keep going. it's easier to keep something moving.

BUT it changes again once you put a static load on it (like a fan blade)

the load (PINK LINE) will start out high but stay higher throughout the rpm range. much like you'd expect... it takes more power to achieve the same RPM, but it doesn't really vary much. with it's lowest power consumption right at it's rated and designed RPM speed.

also i messed up my labels on the graph... up and down should be power and left to right should be time. oops.

[jennifer]

       Oddly enough I understand what your saying.... Your graph is Awsome and pleases me, Unfortunally this creates a problem,
You guys (all of you) seem quite knowledgeable on this subject and I will treasure this thread....The problem lies with me, the fact
That I am In a mans world learning guy things...Secrets if you will...Thank you all for your help.

[lilshawn]

nothing secret, just foreign. I spread my interests out over a wide variety of subjects. If you work everyday doing one thing you know everything about that one thing. I know a little bit about everything.

I'm sure even ditch diggers know more about ditches than I could possibly know. 

[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. 

[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]

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.

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).
 

[lilshawn]

     Im so going to cry myself to sleep, Omg.... That is quite a rash of information, And again I understand most of it,

don't worry, you needn't worry much about 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....

yes, exactly. your going to hit a wall where you can't move any more air with the fan/or the room will not accept any more. ideally you want them to be the same.

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).

good thing we aren't engineers, otherwise you'd really be lost XD

[MonMotha]

good thing we aren't engineers, otherwise you'd really be lost XD

Speak for yourself...

I guess that may explain why I confuse people so frequently

Where the power needed is determined by its load, not the amount of work.

A key thing to realize is that power is just work over time.  You may be doing work to create heat, light, move air, turn a conveyor belt, etc., but it's just work over time.  The concepts are interchangeable, but it's sometimes convenient to think of things in one or the other.  e.g. when you want to know how much power you need to move a certain volume of air, you can work out the rotational speed an torque required on the fan blade to get the mechanical power.  Then if you want to also plug your arcade cabinet in to that circuit, you can just add the powers up, not caring about how the "work" gets done inside the cabinet.

You also cannot rely on an induction motor to power limit itself by slowing down in a fan application.  An induction motor will want to run close to (but shy of) the synchronous line frequency regardless of how much torque it has to develop to do that.  If it has to develop more torque than rated, thus exceeding its rated power, to attain that speed, it will draw more current than it's designed for and burn out.  In this case, your only options are:
1) Use a different fan blade that requires less power to turn at that speed (and therefore probably moves less air)
2) Slow down the motor e.g. using a VFD (moving less air)
3) Use a bigger motor that's rated for more power (and therefore it draws more power)

Note that this is somewhat different than the high speed "universal" motors commonly found in e.g. vacuum cleaners.  These will, to some degree, power limit themselves by adjusting their speed based on torque requirements.

[DaOld Man]

I think Jennifer should forget this silly induction motor idea and hire Geisha girls with paddle fans.
Why back in my day you could hire them for peanuts, and most of them aint too hard on the eyes either.

(Ok DaOld Man will go back to sleep now.)