adamasd

Figureing the speed of a motor
« on: January 20, 2005, 12:35:31 AM »
I am in the process of building a leslie speaker and I am wondering if there is any way to figure out the RPM of an unknown motor. Like dropping the voltage given to it to 1.2 VAC so you can count the RPMs and then just multiply it by 100 to find out what it runs at from the walll voltage?

adam
so there


CJ

Figureing the speed of a motor
« Reply #1 on: January 20, 2005, 12:46:20 AM »
You don't want to do that!!!!
If I can't fix it, I can fix it so nobody else can!
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adamasd

Figureing the speed of a motor
« Reply #2 on: January 20, 2005, 01:13:13 AM »
Talk about your elaborate responses. I just want a way to figure the RPM other then makeing a counter and useing and LED/LDR to count the rotations. I know nothing of motors other then that they spin and the basic idea behind how the work.

adam
so there

PRR

Figureing the speed of a motor
« Reply #3 on: January 20, 2005, 01:43:58 AM »
> Like dropping the voltage given to it to 1.2 VAC so you can count the RPMs and then just multiply it by 100 to find out what it runs at from the walll voltage?

In a DC motor (and some AC/DC Universal motors), if there were ZERO mechanical friction, that would work. No-load speed is proportional to voltage.

BUT: most motors spin far too fast to count, and if you slow them to countable speed, friction becomes a very large factor. In fact you will probably find something like 0-10V no-spin, 10V it starts and spins up too fast to count.

Also most common AC motors are "line-locked". They spin AT power line frequency or they don't spin at all. (Actually they spin 1% slow no-load, 5% slow full-load.) Changing the voltage does not change their speed, just the maximum load they can carry.

Nearly all common AC motors spin near 3600 RPM (3000RPM in 50Hz lands). The rated loaded speed is often like 3450RPM.

1800RPM motors exist but mostly for very heavy low-speed loads like air-compressors, or for sizes above 5HP-10HP where the heavy rotor isn't safe for 3600RPM. You don't want to lug a 10HP motor around!

It's been years since I examined a Leslie but I do think it uses a non-standard motor, both to get the RPM and also to get a choice of RPM.

If you can still buy sewing machine motors, they can cover a very large range of RPM and have reasonable power.

PRR

Figureing the speed of a motor
« Reply #4 on: January 20, 2005, 02:01:33 AM »
> a way to figure the RPM other then makeing a counter and useing and LED/LDR to count the rotations.

Make a 6-point star(*) out of cardboard. Jam it on the shaft. Spin-up. Use a drinking straw to blow through the star teeth. This is a "siren", same as a fire siren (except they use a much bigger blower than a straw). A typical 3600RPM motor spins at 60 turns per second. The 6-point star will give a 6*60= 360Hz scream, somewhere below A=440. Hear the musical pitch and look up the Hz. Divide by 6 to get turns per second, or multiply by 10 to get RPM.

(*) A six-point star is used by some religious groups, but I'm not grinding that axe. It is about the right number for many motors (gives a good pitch), is easy to draw and cut, and reduces base-60 math to an easy base-10 problem. If you prefer, you can use 5 or 13 teeth; 8-tooth is also easy to draw and cut (but harder to calculate Hz-to-RPM on your fingers).

If you find a very odd-speed motor, use more or less teeth on the star and run the numbers again. A 180RPM motor (almost always a gear-motor) might need 30 teeth to get a 90Hz tone you can hear a clear pitch from. Our pitch accuracy falls off below 200Hz or so, but "bad pitch accuracy" is still pretty good accuracy for motor-RPM. We complain about 0.5% pitch errors above 400Hz, but we often don't need even 10% accuracy in motor-speed.

Note that a typical AC motor, powered-up loose on the table, will twist and jump, rip the power leads or dance off the table. Don't let it get away from you. Don't wear loose shirt or tie around rotating machinery. Don't grab a dancing motor by the electrical terminals: it's really no fun at all.

PRR

Figureing the speed of a motor
« Reply #5 on: January 20, 2005, 02:28:44 AM »
> What about washing machine motors? They can spin slowly..

These are usually 3600RPM motors, plus a heavy gearbox, plus some complicated wiring. Top speed may actually be in the range of a Leslie, but weight and size and noise and wiring may make it a bad idea.

And washers are often retired because the motor/gearbox died, and new motor/gearbox assemblies cost more than a new on-sale washing machine.

Of course if you have a rusted-bucket washing machine behind the woodshed, try it!

> I love that method.

Counters and tachs are for non-musicians. A musician's ear is a fabulous instrument. But musicians sometimes don't have the learning to convert an RPM problem that they can't answer into a pitch question that they can solve with a beer in each hand.

adamasd

Figureing the speed of a motor
« Reply #6 on: January 20, 2005, 02:32:14 AM »
Ahhh, great idea, I knew there was a simple way to do this, maybe I can dig out that that old gear from a hammond I have somewhere so I can make a monophonic 2 pitch tone wheel organ too. Now I can figure my pulley sizes to get the speeds I want. It took a long time to find a decent motor for the job but I finaly found a 2 speed one that has enough tourque, and is very quiet. I can let it sit loose on the table while running and it barely moves. Almost no Vibration at all. I used to have a tube amp whoose power transformer vibrated more then this motor.

Thanks,
adam
so there

PRR

Figureing the speed of a motor
« Reply #7 on: January 20, 2005, 03:04:15 AM »
> Now I can figure my pulley sizes to get the speeds I want. {found} a decent motor

If you have a likely-looking motor: mount it, cobble-up any likely size pulleys, and tie a transistor radio to the slow pulley. You actually don't want an RPM, you want a Leslie-wah, and when you spin the tranny radio you will instantly know too-fast/slow and know what size pulley to try next.

I suggest you start with the largest slow-pulley that you can fit. At low speeds like this, big pulleys always work better. Better belt-grip, more tolerant of belt-stretch, a less-tight bend over the small pulley. Your upper limit is the width of the horns (since they barely clear the cabinet), and I would not go smaller than half of that. Then I might try a "small pulley" that was just the motor-shaft: that's probably too slow but sure is cheap, and you'll quickly estimate how much bigger it needs to go to approach Leslie-speed.

(In high-speed drives, a belt on a big pulley moves so fast it lifts off the pulley. This is a real problem in engine cam-chains and a small problem in engine fan-belts, but I don't think a Leslie has either the speed or the space to have that problem.)

Actually-- this is another musical problem, only in beats not pitch. Listen to the test-rig while you tap a Leslie-beat. If you tap 7 times while the test-rig spins twice, then you need to spin 7/2 or 3.5 times faster and need a pulley 3.5 times bigger. If you used the 0.25" motor shaft, you need a 0.25"*3.5= 0.875" (7/8") pulley.

Though, for pulleys not a LOT bigger than the belt thickness, you should add the belt thickness (count half the thickness on each side; assume the middle of the belt moves at average belt speed). 0.25" shaft with a 0.1" thick belt acts like a 0.35" shaft with infinitely thin belt. So in this case you need 0.35"*3.5= 1.225" effective pulley, minus the belt thickness is a 1.025" real pulley. This isn't normally a big deal unless you start with a too-small pulley. (0.1" belt is really too thick to run happy on a 0.25" pulley.)

If you use V-belts, measure the pulley at the midline of the belt, not the rim or valley. V-belts run fine on large flat pulleys, you only need the V-groove on small pulleys.

tmbg

Figureing the speed of a motor
« Reply #8 on: January 20, 2005, 02:25:12 PM »
the chopper idea is a neat application of a concept I learned a few months ago, when I was researching 40's and 50's chrysler air raid sirens.

The things are awesome, they're a ~300cid hemi v8 that drives an air compressor, a chopper plate, and a slow-rotating turntable base.  The chopper plate has 6 holes, and so the tone it screams is the rpm of the motor / 10, just like the 6 pointed star example above.

The things were reportedly so loud that they could start fires.  :shock:

Test Point

Figureing the speed of a motor
« Reply #9 on: January 20, 2005, 02:25:38 PM »
Quote from: What about washing machine motors? They can spin slowly..[/quote


The problem with using a washing machine motor is you have to close the lid for it to start spinning  :grin:  :grin:

Sorry, I couldn't resist :oops:
TP
"People of Earth, your attention please, a voice said, and it was wonderful.  Wonderful perfect quadrophonic sound with distortion levels so low as to make a brave man weep."

 "Prostetnic Vogon Jeltz


PRR

Figureing the speed of a motor
« Reply #10 on: January 20, 2005, 05:41:59 PM »
> I was researching 40's and 50's chrysler air raid sirens.

How did you get roped into THAT odd topic?

> a ~300cid hemi v8

IIRC (my book is at home) the Chrysler Hemi V-8 replaced the Straight Eight around 1951, and was introduced at 331CID. The 331 was used in english and french hot-rods, as well as a million Chryslers. Plymouth got a semi-hemi: my Mom had one in pink. The Hemi was actually a terrible engine for post-WWII US fuels: it would run good on low-octane but we had leaded gasoline. Cheaper/lighter wedge-heads made the same power as The Hemi. Chrysler took the Hemi up to 392CID. To make the most of the Hemi, you had to run a GMC truck blower and low-explosive Nitro fuel; then it made 1,600HP for 7 seconds and dominated unlimited drag racing. But as a street cruiser, it was just heavy and wide and costly for no good reason. Around 1959 they abandoned it for the B-series Wedge: lighter, cheaper, and was taken as high as 440CID when power was needed. And then NASCAR opened an opportunity for a "stock" engine that could turn 6,000RPM without gasping: Chrysler re-invented the Hemi heads on the B-series block for the 1960s 426 Hemi. Great racer, but the big Wedge engines were much better street and street/strip engines.

> The things were reportedly so loud that they could start fires.

There is an old Scientific American article where you build a super-whistle and a horn to focus it down to a very small area. A scrap of cotton at that place will burn.

Acoustic power is real power. But we never have so much of it that we feel heat. A 1,000 watt amplifier averages 100 Watts of music electrical power, speakers are 1% efficient so we have 1 Watt of Acoustic power in the room. 1 Watt of heat spread all over a room isn't going to keep you warm (and the 99 Watts of electric heat in the speaker give more warmth). You will tear your ears before you warm your skin.

But let's see. The Hemi 331 could be rated 250 HP short-term. A siren is around 20% efficient, so we have 50 Acoustic Horsepower. This is over 35,000 Acoustic Watts. Yeah, if you aimed the siren horn right into a pile of wool blankets it would ignite real quick. If a tree grows in front of the siren you might set it on fire.


 

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