# AC Motor and PWM - does it work?



## GCWyatt (Aug 30, 2012)

I've scoured the forum looking for my answer and can't find it. In short, I'm building an Axworthy and I've got my basic rig setup with a sewing machine motor. As expected, it's a bit too fast for my liking and I'm afraid using the included pedal will kill my torque when I set it up with the line and ghost. I've been looking into PWMs but I can't get a straight answer from the documentation - lines like "Can support AC and DC input. Non-existent power supply polarity problems" don't help much.

Has anyone built a prop (Axworthy especially) with an AC motor and PWM and had success?


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## niblique71 (Dec 2, 2009)

Hi GCW. I am far from an Expert on this topic but I do have some experience being frustrated just like you.

My Axworthy uses a laundry "Dryer" motor and is exceptionally reliable EXCEPT in higher winds and Rainy conditions. I have been wanting to have speed control over my setup for several years now. My type of motor differs from yours (I think) because it is an Induction motor, and your sewing machine motor is most likely a "Brush" type motor. 

My Gut instinct is that the "Pedal" type speed controller that came with the sewing machine is your absolute best option. It's made for that particular motor and you should not suffer any torque or performance issues with regard to having speed control.

I don't think you need a PWM unit to control this setup. The PWM is designed to digitally alter the current in pulses to allow a "Fix speed Induction motor" to have some amount of speed control.

In Short, I am DYING to anty up the $$$ to buy a cheap PWM unit to see if it works...

Anyone else with more knowledge... PLEASE step in... I'm dying to have better info on this too.


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## hedg12 (Jul 6, 2008)

You _can_ use pwm to control a/c, but it's not as simple as d/c. The issue is where/when the "pulse" occurs in relation to the a/c waveform. If it occurs too far from the zero cross point the power to the motor is greatly diminished. PWM a/c motor speed controls have a circuit to detect the zero cross point in the a/c cycle and regulate the pulse generation accordingly. Don't ask me how - I'm only vaguely familiar with the concept. PWM a/c motor speed controls are commercially available - they're used in woodworking to control routers fairly often (cnc routers, too.)


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## CDW (Jun 26, 2012)

Actually, pretty much any household dimmer these days is doing PWM on AC. There are a few different ways of doing it, some more efficient and versatile than others, but it all comes down to the same basic concept as PWM. The other methods of limiting power to an AC load aren't really all that practical - variable resistive loads give off lots of heat and waste energy, and variacs (variable transformers) tend to be quite bulky and expensive. As to your situation, I would say it depends on the pedal. If the donor machine was an old machine, the pedal likely uses a big variable resistor, and you're right that that will compromise torque at low speeds. However, an old sewing machine motor will probably respond quite well to a regular household dimmer. If, on the other hand, the donor was a new machine, then it probably has an electronic pedal that's already doing PWM, just like a dimmer switch, so in that case you should just go ahead and use the pedal it came with, since it's built to work together - though I imagine spending all Halloween with your foot on a sewing machine pedal might get tedious :jol:.


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## corey872 (Jan 10, 2010)

I think the reason you can't find a simple answer is - there isn't one. There are a wide array of motors types out there ... is it an induction motor, is it shaded pole, permanent magnet, is it a universal motor? - all these respond differently and generally need different styles of speed control. Likewise, there are many different types of speed control - is it a simple PWM, is it a variable frequency drive, can it detect zero crossing on the AC waveform.

My hunch is this - trying to use PWM on AC may not yield the best results. Even if you find the correct zero-crossing PWM, I think you will loose a lot of torque at low speeds, simply because the voltage on the AC wave is dropping as well. You will quickly get into a situation where you slow the speed, the rig stalls out, you slightly increase the speed and it takes off flying and revs way up.

On the light-dimmer. I don't know how well that will work - especially for any length of time. Dimmers are made for a purely resistive load...incandescent light bulbs...which are a simple resistor as far as the controller is concerned. Controlling a motor is going to give an 'inductive kickback' in the power lines which will put stress on the dimmer. Every time I have tried, it's resulted in a burned out dimmer. Though you may be lucky.

In the same hardware store section, you can sometimes find true motor speed controllers, but those are generally designed for ceiling fan motors which are most likely a different style from your sewing machine motor. Again - the motor type mismatch might kill the effort. 

The last thing - 'most' sewing machine motors I know of are universal brushed motors. If you can see inside the motor while it's running and see carbon brushes and little sparks flying, this is a universal brushed motor. These 'can' run on either AC or DC. They run on AC for simplicity in the sewing machine, but if a person were to build a rectifier, or come up with a 120V DC source, the motor should run the same. Now that you are on DC, a standard PWM speed controller would also work, AND being that it's a DC speed controller, you would have nearly full torque down to very low speeds.

So my advice would be - if you see sparks and brushes in the motor, it's likely universal. Try a standard PWM running 120V DC to the motor. It would likely work and give you the speed control you need.

**Standard disclaimer 120VDC is just as likely to shock as 120VAC so take equivalent safety precautions.


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## CDW (Jun 26, 2012)

Good point, corey, I should clarify: I would only suggest using a dimmer on a sewing machine motor, because I know people have had success with that specifically. It may damage other kinds of motors, and it definitely won't work well with them.


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## GCWyatt (Aug 30, 2012)

Thank you all for your advice. Looks like I have some homework to do. I'll report back my findings.


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## GCWyatt (Aug 30, 2012)

Good news and bad news. Good news is I did find carbon brushes under plastic caps near the back of the motor. This is the motor I have (though I bought it on eBay) and you can see the plastic cap:
http://www.sewingpartsonline.com/motor-alphasew-na35l.aspx

Bad news I don't have any DC power supplies large enough to drive this thing. I tried my modified computer PS but 12v is just not enough.

Any idea on where to get a big DC PS? If I have to spend a bundle on this I might scrap the sewing machine motor in favor of a little DC screwdriver.


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## corey872 (Jan 10, 2010)

It would be crude, but radioshack sells a full wave bridge rectifier which would easily handle this motor, for about $3

http://www.radioshack.com/product/index.jsp?productId=2062583

Really, they have a whole series of them. I would think even one as low as 3-4 amps would probably run the sewing machine motor. So that gets you a rectified AC wave. If you could find a large electrolytic capacitor of ~200+ volts, that would help smooth out the wave even more and would be pretty close to a DC supply.


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## GCWyatt (Aug 30, 2012)

An interesting idea, Corey. You wouldn't have a schematic for that, would you?


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## corey872 (Jan 10, 2010)

The overall circuit is pretty simple:










Instead of the transformer (T) shown in this diagram, you would just be plugging into the wall. The full wave rectifier is the diamond shaped component and the capacitor 'C' No real exact specifications for the capacitor...it just needs to withstand the voltage - so should probably be rated at 220 volts or higher, and the bigger the better - or you could add several smaller capacitors in parallel.


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## HomeyDaClown (Oct 3, 2009)

That's an AC motor and you'll have to supply it with AC to get it to spin, diodes and rectifiers are for DC circuits or conversion to DC from AC.

A PWM (DC) supply would have to be pretty hefty to get that motor cranking at all since the motor is expecting to be driven from 60HZ AC and PWM supplies are best at high frequencies (16KHZ and UP).

It appears that you had it working on 110v AC but it was just too fast, correct? The current rating on the motor is .9 amps and would be below the average maximum of an AC lamp dimmer (leviton dimmers are $5 at Lowes/Home Depot). I have several contolling the speed of AC motors and a couple hot foam cutters with transformers to set the temperature. Both cases are AC in and AC out. One is hooked up to a gutted palm sander for a vibrating table. In any case they all work fine and give decent speed control and the dimmers all have the OFF positiion and mount in a simple AC outlet box which is a bonus. I really don't think you'll be hurting an AC motor.

Two other option are to get a Harbor Freight AC speed control ($20) or try various AC step down transformers (that are rated at least 2 amps) until you hit the voltage level that gives you the desired speed.

Not sure what Variacs (variable AC trasnformers) are running these days but you could also check Ebay for one.

UPDATE: http://www.edaboard.com/thread280268.html


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## GCWyatt (Aug 30, 2012)

Thanks Corey and Homey. Corey's rectifier solution, if I understand it correctly, would require a very high power DC supply. I can find 48v, but above that they start getting very expensive, and this project has gotten expensive enough already.

Honey's solution, however, calls for an AC plug in dimmer; I have two, also used for foam cutting. I'll give them a try, but failing that I might just go with a DC motor salvaged from a Black&Decker 7.2v hand saw. A 6v power supply should work just fine with it, and I could use a PWM with it if I needed to slow it down further, correct? The real downside with the DC motor is that I don't have the wheel on the drive shaft to pull the belt. Any ideas there? And what about overheating?


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## HomeyDaClown (Oct 3, 2009)

Lot's of people use wiper motors for axwothy setups so a DC motor with some torque will work.

You will laugh at the pully on my old axworthy (12 years and still cranking).
It's a k-nex pulley from my son's old set with a sweeper belt tying it to the drive wheel.

The saw shaft should accept a bolt and lock washer I would think if it's the circular type.


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## corey872 (Jan 10, 2010)

Just to clarify a few things:

- My original suggestion/post was based on the 'sewing machine' motor likely being a 'universal' motor. This type of motor can operate on AC or DC power. As such, it can be controlled easily by a PWM controller in DC mode. This might not be the absolute most efficient, but for one month a year on a sub 1000W motor, it should not be any big loss.

- There is no 'large DC supply' needed. The full wave bridge rectifier and capacitor(s) are the 'DC supply'. Admittedly crude, but cheap. The rectifier ($4) can be fed directly from an extension cord plugged into the wall. The capacitor(s) can be scrounged or picked up off ebay fairly cheaply. DC+ and DC- then drive your PWM and on to the motor.

- Again, standard warnings apply...120 volts present so insulate/isolate/fuse where appropriate. Capacitors can hold a charge for a long time...discharge before handling, etc.

Though the additional suggestions may be worth consideration. If you have to sink $15-20 into the rectifier or $15-20 into a 12V motor, you might as well go with the 12V motor - it would likely be less hassle, safer and longer lived when all is said and done.


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## GCWyatt (Aug 30, 2012)

First, I tried the 7.2v B&D motor:
Axworthy B&D Motor Test 

It works, but it's noisy as hell.

Then I checked these posts and Homey mentioned a wiper motor. I had completely forgotten it, but I bought a wiper motor from Monster Guts a couple of years ago for another prop I still haven't built. I dug it out and it's perfect! Well, as long as I can find a 12v power supply that can handle the load and not break the bank.

But I'm still interested in Corey's idea of using the full wave rectifier on an AC circuit to drive the sewing machine motor at a lower RPM. If I understand it correctly, the complete schematic would look like:









Which only leaves the rating on the cap - maybe .02uF @ 125V?


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## corey872 (Jan 10, 2010)

Yes, that is a better schematic than the one I dug up. The rectified AC will approach 170V, so the caps need to be rated for at least that much. Also, it's a smoothing cap, so the more microfarads (uF) the better. 220 volts, 2,000 uF would be a good start. 10,000 uF would be better. But either way, you want thousands of uF, not 0.0X uF.


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## Cirvinfox (May 2, 2013)

I used a sewing machine motor for an Axworthy setup years ago, I just wired it up to a dimmer channel on the dimmer rack I was using. They're universal motors that work just as well on DC as AC due to how their stator and rotor field windings are wired up to each other. They've always got an opposing magnetic field and thus it turns, no matter what the polarity of the input voltage. 

Using a line connected rectifier is a bit dangerous but so long as your circuitry is encased and properly grounded and fused, you should be ok. Be sure to use a GFCI properly wired when you put it outdoors, and the best thing to do is get a 1:1 mains isolation transformer. That will up your safety by a big margin.

Either way, using the phase angle control of a household dimmer, or the PWM of a dedicated device, you'll still have similar problems regulating the speed. The speed of universal motors is dictated by their load, so any changes in the load will affect the speed. You'll find that you'll have to crank up the duty cycle to get the motor stated, then back it off to slow it down and balance your forces. Probably won't be an issue if you turn it on and let it run all night, but I had a whole pain of a time with it with my haunt since I turned it on and off depending on the positions of the people in the driveway where I had it.

On a side note, I used one of the K'nex pulleys for my rig, and a taught string wrapped around a few times to couple it to the bike wheel, be careful with this though, high starting speeds and slipping WILL wear a groove into the soft plastic!

AC motors from fans and dedicated gear motors are nice because their speeds are predictable and dependent more on the line frequency, so long as you use them within their limits. You can actually make a variable speed transmission by using a rubber wheel on your motor and aligning it perpendicular to a larger disk, such that the motors wheel drives the bigger disk. The further away form the center of the disk the motor's wheel contacts, the higher your gear ratio. Near the center you can get 1:1 coupling, at the edge, it can be more like 1:10. I've seen this on variable speed drives on gas powered snowthrowers


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## HomeyDaClown (Oct 3, 2009)

Only problem I see is that the label says 120v AC, not 120v AC-DC as would a universal. Brother's (the manufacturer) spec for this motor also say it is AC with no reference to universal. This would be a UL/NEC motor labeling requirement.


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## GCWyatt (Aug 30, 2012)

I can now see why wiper motors are so popular. That was really far too easy.





I'm still going to make the circuit for the sewing machine motor. I bought the full wave rectifier and a cap (the only one Radio Schlock had in stock that could handle the voltage was 0.01uF - I'll see if I can find one rated at a higher capacitance) but I don't have the PWM yet. Once I get it I'll build that rig and see how it performs vs the other two motors.

In the mean time, I'm very happy with the performance of my rig. It's running at about 5.3in/s and since I was hoping for about 6in/s I'm very pleased. Emma Spector's ghost will float slowly and silently this Halloween!


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## HomeyDaClown (Oct 3, 2009)

Looking good GCWyatt and the low voltage will keep from frying any tots...hahaha

One thing that will help when you start stringing your line that I've discovered is to attach a plate to the bottom of the drive wheel to keep the line from falling off. This extends the bottom lip and keeps the line in place. I use the black crabbing cord that Scott Axworthy reccomended in his original design and found it worked much better than fishing line cause it does not stretch much at all.


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## GCWyatt (Aug 30, 2012)

AAARG!

I ran a 3-hour burn in test on the rig and it nearly burned! I have two problems: the motor was really hot, and the power supply melted and nearly burned out. The PS is still operable, but I fear if I push it again it will fail.

I'd love some suggestions, but I figure I need a more powerful PS and possibly some sort of heat sink on the motor.


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## HomeyDaClown (Oct 3, 2009)

Sounds like the supply was too small.

The motor should not overheat....I have beat wiper motors to death and have yet to have one fail.

I would use a regulated supply. Wiper motors can draw up to 6 amps under a load. Computer (atx) power supplies are the cheapest option and give you 12 and 5 volt options so you'll have two speed options besides any the motor offers from wiring.

Scary Terry has a good reference for most wiper motors and supplies here:

http://www.scary-terry.com/wipmtr/wipmtr.htm


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## GCWyatt (Aug 30, 2012)

I've have an old computer PSU that I hacked into a bench PS and every time I try to hook the motor up to the 12v the breaker pops and I have to wait a few minutes for it to reset. I use it to drive my low voltage lighting for my lawn display, and t worked perfectly on Halloween. I'm going to have to pen it up again and see what's wrong anyway.


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## Cirvinfox (May 2, 2013)

ATX supplies are sensitive to overcurrent, they'll trip out with the starting current of the motor. A low value, high power resistor in series with the motor will limit inrush current, then it can be switched out once the motor is moving to create a sort of soft start.


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## HomeyDaClown (Oct 3, 2009)

There are minimum and maximum load limits on most switching power supplies and associated load sensing to protect the supply by shutting it down.

the only effect you get from a resistor in series with the load (the motor) is a voltage drop across the resistor, it has no effect on the current. 
This is why motors use starting capacitors, try placing a large 16 volt electrolytic cap (say 10000 uf or higher) in parallel with the motor.
The capacitor will charge quickly and supplement the current of the power supply much like a battery would.








​


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## GCWyatt (Aug 30, 2012)

Oh! That's a great idea! Something like 1kOhm?


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## GCWyatt (Aug 30, 2012)

HomeyDaClown said:


> try placing a large 16 volt electrolytic cap (say 10000 uf or higher) in parallel with the motor.


I will try that today.


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## HomeyDaClown (Oct 3, 2009)

It all comes down to ohms law V = I * R

Assuming your motor draws only 2 amps (probably much more at a dead start)
A 1k (1000 ohm) resistor would have a voltage drop across it of:

V = 2 Amps * 1000 Ohms

V = 2000 volts, but since your supply is only capable of 12 volts the motor sees pretty much zero volts.

A 1 ohm resistor would have a volltage drop across it of:

V = 2 amps * 1 ohm

V = 2 volts

As you can see, the only effect of adding a resistor in series with the load
is a decrease in voltage supplied to the load. 

You would also have to determine the maximum wattage rating of the resistor (I have discussed this in detail in a another thread here somewhere) since it has to disspate heat in the process without burning up.


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## corey872 (Jan 10, 2010)

HomeyDaClown said:


> ...
> the only effect you get from a resistor in series with the load (the motor) is a voltage drop across the resistor, it has no effect on the current. ...


While the capacitor would work as good, I'd have to disagree on the resistor. They don't call them 'current limiting resistors' for nothing.

At 0 rpm, the motor appears as a dead short to the power supply, so it might trip the ATX current limit protection in the first fraction of a second due to the inrush current. It will be even worse starting under load as the very low RPM period will be prolonged.

If one were to put a resistor in series, even given the dead short, the current is limited to 'V=IR' or to put it in terms of current, I=V/R.

I only throw this out because it might be easier/cheaper to find a low ohm power resistor - or even use a 12V light bulb, than to find a 10,000 uF capacitor.


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## HomeyDaClown (Oct 3, 2009)

To put this in perspective, when I say current I am talking of the current in context of this circuit. The current sensing of the power supply will see little difference between an added series resistor and just the motor by itself whereas the capacitor will act as a buffer and offer a little boost to get the the motor cranking. It is the same exact motor start solution used in electric and hybrid cars only smaller scale. A capacitor can supply a lot of current very quickly on demand that in turn supplements what the power supply is capable of. Once you get the motor turning the current sense will be satisfied and the supply will work normally.


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## GCWyatt (Aug 30, 2012)

I just tested a 3300uF/16V cap and it did nothing. The motor did not move. Voltage in front of the cap was 12v, behind the cap was .5v. I had the caps's positive side hooked up the the 12v terminal on my PSU, the negative side to the motor. 

So I put a load on my PSU by wiring up an old Molex harness and adding an old DVD drive to one outlet and the motor to the other. That worked perfectly. Once I got it running I unplugged the DVD and the motor is still running. I'll let it burn in for a few hours and see how things are going. If it all works out I think I have my rig, I just need to find another computer PSU to hack.


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## GCWyatt (Aug 30, 2012)

Just checked the rig - the PSU is cool and working perfectly. The motor was over 140° and I could smell the lubricant cooking. I just fired off an email to MonsterGuts asking what I can do, or if this is OK for the motor. Any ideas here?


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## GCWyatt (Aug 30, 2012)

Here's the latest.

I've been emailing back and forth with Dean from MonsterGuts and he's been great. He informs me that the wiper motor normally runs about 140° and it can handle that temperature.

Being crazy, I decided to try to get that themp down a bit by making a motor heat sink out of a piece of copper coupling and a pair of old AMP heat sinks. This is what I ended up with:





It only brought it down a bout 20°, but that's enough for it to stop smelling like burning motor grease. I figure if I tweak it a bit and get some more contact with the copper pipe, maybe add some thermal grease, I might be able to get it down even further. There is a product called Gap Filler by Bergquist - it's thermally conductive and can fill gaps like caulking. It's also not available at Lowes, so, yeah. Maybe I can order a sample.

Still waiting for my PWM to arrive and then I'll test the sewing machine motor with the full wave bridge rectifier.


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## scarybill (Oct 7, 2012)

I would go to harbor freight and get a speed controller that they have for a router. It controls speed and does not reduce tourque(you need it to cut the wood with) They are pretty cheap too.


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## Hippofeet (Nov 26, 2012)

GCWyatt said:


> So I put a load on my PSU by wiring up an old Molex harness and adding an old DVD drive to one outlet and the motor to the other. That worked perfectly. .


+1 lol. I love this stuff.


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## GCWyatt (Aug 30, 2012)

scarybill said:


> I would go to harbor freight and get a speed controller that they have for a router. It controls speed and does not reduce tourque(you need it to cut the wood with) They are pretty cheap too.


We weren't sure a router controller would work with this motor. And building it is way more fun, plus I learn more this way.


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## GCWyatt (Aug 30, 2012)

The PWM arrived today and I tested it with the full wave rectifier and the sewing machine motor and I am terribly disappointed. Even with all this electronics helping I was unable to get the motor to the low RMP I want while still retaining the torque the wiper motor has. Wiper motor wins hands down. I'll find some other use for the sewing machine motor, but my Axworthy will be run off of a 12v DC gear motor. 

Thank you all for your help and your interest. At the very least I hope this thread answers this question for other haunters.


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## typoagain (Feb 27, 2012)

I use a monster-guts wiper motor on my FCG. 
But my grip is that the PWM I am using makes the motor a bit loud. Running full speed I have no noise issues, but turn on the PWM and here comes the noise!

What PWM are you using and do you have an issue with noise?


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## GCWyatt (Aug 30, 2012)

I bought the Mach3 off of eBay:
Mach3

I didn't notice it running any louder then straight off of AC with it's foot pedal.


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## corey872 (Jan 10, 2010)

Just curious, you said you tried the PWM with 'the full wave rectifier'. Did you ever find a suitable cap? Last I knew you had talked about a 0.01uF cap and I had mentioned something like 10,000uF would help smooth out the AC. So you had a cap which was a million times smaller.

With a small or no capacitor, the rectified AC drops to nearly 0 volts 60x per second. 








This may cause the PWM to also reset or otherwise act weird - ie give very low torque at low speed.

RE: PWM noise - is this a high pitched whine? Some PWM's operate at 12-16kHz which is down in the human / audible hearing range - better PWMs will be 20kHz or higher which is ultrasonic and can't be heard by humans. Hard to say without seeing the circuit, but if it uses a 555 for timing, you might be able to tweak capacitor/resistor values to increase the frequency and eliminate hearing the noise.


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## Aquayne (Sep 1, 2008)

Love the idea of a variable speed rig. Gonna give it some thought. Thanks for the post


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