Anybody understand Variable Frequency Drives?

[G._Hoffman]

So, I've got my "Leslie" up and running - sort of; I haven't wired up the speaker yet, or installed the speaker switching stuff yet, but I've built the relay box, and the speaker is mounted in the box, and it spins so I only need to do a little more woodworking - but only at high speed.  I want to have a bunch of speeds on this thing, so I need to come up with a speed control.  Now, I get the concept behind a variable frequency drive, and since the Leslie motor is an induction motor, VFD is the thing to use. 

So, the VFD uses an oscillator to control a couple of thyristors, which gives you your variable frequency.  That part is simple enough to understand.  What I don't get is how do you vary the voltage so you can keep a constant current?  This motor is kind of old (at least 30 years), and while induction motors are pretty bullet-proof, I'd rather not have to find a new motor if I can avoid it, thank you very much!

Anybody have any experience with building these things.

I should mention, I have more time than money, and I've got a pretty deep parts drawer at the moment, so I want to make it myself if at all possible.  Plus, I just plan like building things.  I'm sure people around here will get that!


Gabriel

[tubesornothing]

Hope this helps:  http://www.kilowattclassroom.com/Archive/VFDarticle.pdf

http://www.cnczone.com/forums/archive/index.php/t-31369.html


I  have a VFD on my CNC as well as my electric car.  I really don't get into the VFD aspect other than sizing it,  hooking it up and using it.  You can get small, cheap, premade, Chinese VFDs that should be able to control your Leslie for cheap. 

[PRR]

> how do you vary the voltage so you can keep a constant current?

You don't.

Electric motors are not resistors. They are a small resistor plus a big generator.

When they run at FULL speed (no load, no drag, no friction), current goes to ZERO.

When working anywhere near full speed (most motors), current is essentially proportional to Torque.

Check it. That high-milage table-saw with broken-in bearings? Take off the belt, blow the sawdust (added air drag) out of the windings, connect an ammeter. Sure it pulls big current while pulling its inertia up to speed. But when the speed settles, the current goes very low, less than 1/10th the nameplate amperage. The friction and drag is less than 1/10th the total work available. A tablesaw motor probably has a fan; bust the blades off and current will drop even lower.

Locked-rotor conditions are different. So is full torque at low-low RPM. You know how that tablesaw will dim the lights when it starts, and when it hits a tough knot. But this is not Leslie turf so I'll skip it.

Feed a common "60Hz 3600RPM" motor 60Hz and it tries to reach 3600 RPM (60 cycles per second times 60 seconds per minute). An induction motor must lag to work, so maybe it gets to 3,550RPM idle, 3,400 RPM hard work. Feed it 30Hz and it aims for 1,800RPM. At 1Hz it wants to do 60RPM (altho a thousands-RPM motor probably won't like that).

What is the current? Assuming you have enough motor to approach target speed, current is only enough to pull the load plus drag. If your load is the same at any speed (pure friction), the current is the same at any speed. If your load is a propeller, torque and current rise with RPM. Since a raw motor has air-drag, and a Leslie's load may have a significant air-drag component, you don't want to over-speed a Leslie's motor much. (Anyway you can fling-off the windings.)

Low Hz and low RPM with a light load, the current will stay in check. Probably reduce due to lower air-drag losses.

There is no "maximum" current. All motors pull short-term surges 2X to 10X their nominal current. The nominal current can be based on many things, but is essentially the current that flows when doing the most real work it can do safely. The same motor frame can be rated 2HP 20A for intermittent (shop-saw) duty but 1HP 10A for 8 hour/day constant (sawmill) work. Or even 20HP 200A for short bursts (electric car spends 90% of time at <25% power, and the battery goes flat before a big motor gets fully over-heated).

So why do AC motors have voltage numbers? That's to cover the high torque low-RPM situation. A "115V" motor will probably run on 24V. Or it may need a flip to get going against static friction. And it will accelerate poorly, and stall at very slight load. You want 100V-150V to get good starting and short-term torque. The 115V motor will run on 240V too, and at nominal load it may run just fine. But the start-up surge will be brutal, and if you hit a tough knot it may overheat quite quickly.

Oh, many DC motors, the no-load speed is proportional to voltage. They run twice as fast at 12V as at 6V.

[G._Hoffman]

When I came in here to look at this, I saw who the last poster was and inside my head I went, "YES!!"


The funny thing is, I've read basically what you just said, and I totally didn't get it.  Now I do.  


Thanks a bunch.  That makes life much easier.



Gabriel

[G._Hoffman]

Hey, PRR, can you tell me where the extra screw goes for my digital camera that i just tried to fix?  (Tried only because the soldered in place battery I need to replace is not one I'm going to find - I really hate planned obsolescence!)



Gabriel

[jed]

Sorry, I don't understand VFD's, I'm just thinking out loud, why could'nt you use a variac as a speed controller?

[G._Hoffman]

Sorry, I don't understand VFD's, I'm just thinking out loud, why could'nt you use a variac as a speed controller?

Because an induction motor's speed is set by the frequency of the AC, not the voltage.



Gabriel

[bnwitt]

http://www.st.com/stonline/products/literature/an/11796.pdf

http://www.aaroncake.net/Circuits/acmotcon.asp

[PRR]

Both are for "universal" motors. Sewing machines, power hand drills, electric chainsaws.

I assure you that when you try to slow an induction motor this way, you get a lot of smoke.

[bnwitt]

Both are for "universal" motors. Sewing machines, power hand drills, electric chainsaws.

I assure you that when you try to slow an induction motor this way, you get a lot of smoke.

Understood, I was offering an alternative to the leslie method using a different motor type for better and cheaper.

[G._Hoffman]

Both are for "universal" motors. Sewing machines, power hand drills, electric chainsaws.

I assure you that when you try to slow an induction motor this way, you get a lot of smoke.

Understood, I was offering an alternative to the leslie method using a different motor type for better and cheaper.

You ever listen to a universal motor in action?  My guitar amp isn't nearly loud enough.


Gabriel

[bnwitt]

Well if I were building a revolving speaker effect unit I would abandon old Leslie technology and use quiet DC motors along with modern materials.  The Rotary wave is one example of how to do that  cheaply and still get the effect.

[G._Hoffman]

Well if I were building a revolving speaker effect unit I would abandon old Leslie technology and use quiet DC motors along with modern materials.  The Rotary wave is one example of how to do that  cheaply and still get the effect.

I did look into that, but the mechanics of it were a bit much for me.  I'm excellent with wood, and I'm OK with electronics, but gears and pulleys, metal and electronics are not really in my area of competence.  By buying the existing mechanical bits from a Leslie, I saved my self a lot of time and irritation.



Gabriel

[bnwitt]

I did look into that, but the mechanics of it were a bit much for me.  I'm excellent with wood, and I'm OK with electronics, but gears and pulleys, metal and electronics are not really in my area of competence.  By buying the existing mechanical bits from a Leslie, I saved my self a lot of time and irritation.
Gabriel

I don't understand your post.  The Rotary Wave has no gears, pulleys, metal or complicated electronics.

It's about as simple as it gets.  A DC motor, a wall wart power supply with a potentiometer control and a styrofoam rotor.

http://www.mahaffayamps.com/rotary-wave.htm

http://www.mahaffayamps.com/resources/DIY_Rotary_Wave_Assembly_Instructions_3_7_04.pdf

[G._Hoffman]

Most motors run efficiently in at a fairly high speed.  The high speed of a Leslie is about 440 rpms.  You use a pulley to get the speed of the motor right.  When I tried to do this from the ground up, I ran it to problems with the pulleys, and the rotor I built was difficult to balance (and its hard to mic up a cabinet when it is walking across the room!)  By using the guts of an existing Leslie unit (taken from a home organ, and bought on eBay for $25), I didn't have to think about any of that - but its a single speed unit, and I want more speed options.  Trying to make another motor work with the mechanical drive train of the existing unit is something I don't want to deal with.  Electronics is easier for me.

All of this is taking a back seat to other projects at the moment, though, not least of which is finishing some guitars so I can hopefully get paid for them!


Gabriel

[bnwitt]

Boy, you're really working yourself hard for this one.  I've built a couple of rotary waves and they're about the simplest thing I 've ever put together.  One doesn't need electronics experience to build one.  Speed is completely variable from zero to full out with just a little pot.  But at least you're learning as you go.

[stingray_65]

Well if I were building a revolving speaker effect unit I would abandon old Leslie technology and use quiet DC motors along with modern materials.  The Rotary wave is one example of how to do that  cheaply and still get the effect.

I agree!

I put a 90VDC motor on to a 2 speed leslie motor plate and a simple controller  right on the wood chassis. I then ran the pot via some 16 ga hook up wire (I'll replace soon with 16ga sjow and a strain relief) to an expression pedal  removed from the same organ as the Leslie unit.

that high torque DC motor is amazing on how fast it makes that big wooden rotor react!

I believe thogh that I will switch it into one of the styrofoam rotor chassis for a quieter and smoother operation. the wood rotor isn't balanced for the speeds this can achieve.