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I Forge Iron

Dimmer switch for an electric motor?


Jim Poulmas

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Apologies if this has been discussed a thousand times. Will a dimmer switch be able to vary the speed on an electric motor? It's an old Westinghouse electric motor. (Attached to a blower.)

115 volts
5.1 Amps
1/4 horse power

I guess it wouldn't be the first time I've been electrocuted. It'd be a shame to fry the motor though.

Thanks!

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Jim Poulmas , i've been runnin' a 1500 w , 240 v ( OZ / Australian single phase ) vac motor , hooked to a light dimmer swith for the last 12 or so yrs , still workin' , NOISY as all get out , but works

Not bein' a sparkie i didn't know it shouldn't work , thanks Steve for lettin' me know it wont :-)


Dale Russell

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Some motors will take it and some won't. Typically you can't use a dimmer switch. Obviously some folks have found their motors will work with one. Usually they make noise, overheat and die. If its an AC/DC universal motor, ok for sure, but...

Your mileage may vary.

A gate is a for sure idea, and its the common way to control air flow. All of the blowers i have ever had run at a constant RPM and I vary the air flow with gates and bypasses.

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Steve Sells, TEFC means Totally Enclosed Fan Cooled. This is one of the best enclosure types for a blacksmith shop as it does not allow metals dust etc to enter the motor internals. One can buy TEFC motors that are designed to work with a VFD. I believe that the "Inverter duty" are the best for VFD.Variable Frequency Drives will work with standard motors, including TEFC, just the motors designed to work with a VFD last longer, run cooler and with less noise.
At the last forge shop, we ran hundreds of standard TEFC 3 Ph motors with VFDs. Granted in a forge shop one won't notice motor noise, and the heat will also not be noticed. The motor life is usually tragically cut short there by damage from dropped hot forgings or forklifts:)

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as many know, this isn't the first time I screwed up a name :) and most likely wont be the last :(

The point is one can go ahead and try, but it was stated he did not want to risk losing his motor, anytime we do things to a motor that it was not designed for, we take the risk of failure. Many times a cheap dinmmer lasts many years on a motor, my local IBA has a dimmer on a old Vac motor. it is still working fine. But most will fail.

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I started out with a blower and slide gate for control. I bought too much blower. Although I could control the flow with the slide gate, I could not stand the noise of the blower. First I put a light dimmer on it and it worked but I worried about burning out the motor and replaced it with a proper control. Works fine and now I can hear the music playing.

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  • 4 weeks later...

I bought a blower from a blacksmith supplier, don't remember which. Didn't know what size to buy, got the biggest, 400 cfm. That was a mistake, too much air. The blower had an inlet damper and I got a slide gate for the outlet. With this configuration I could control the flow, however the blower was always going full speed ahead and the noise was awful. I put a dimmer on it and it worked but I was afraid the motor would burn out. I put an expensive reostat on it and it has been running that way for a couple years. If it ever burns out I will replace it with a small model or just use one of the hand cranks I already have in reserve.

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Thanks all. I was planning on an air gate anyway so I'm going to shelve the dimmer idea. The motor is already mounted to the blower so I'm going to let it alone. It's enough work fabricating an entire coal forge and I want this thing done.

Jim


That is the right decision, IMHO. My factory Buffalo blower puts out way too much air at the lowest setting on the rheostat so the air gate is there for a reason. I have another Champion blower on a forge with a conventional dimmer switch (and no air gate) that can be turned down to a whisper but it takes almost constant fiddling to get the fire right. For my money, the right blower with a good air damper is much more productive that trying to control the fire in other ways.
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Hello, I'm new here, and appreciate such an informative and friendly forum. I have been banging on metal for a couple of years now, but I have done it in isolation, and I don't really know anything 'cep what I've taught myself. Kinda like being raised by wolves. Anyway, I look forward to learning a lot from you guys.

As for the forge blower... I scrounged a heater blower out of a junk truck, and hooked it up to the rheoststat on the headlight switch- scrounged from the same wreck- the one that dims your instrument panel lights. That plus an old plumbing valve for an air gate works great for regulating the flow of air into the tuyere. I run the blower either by hooking it directly to my car battery, or by an old 12v battery charger set on the high amp (12 amps?) setting. One-a these days, I'll get a solar panel, and run it that way. Cost for the whole setup? Free.

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just a thought, bathroom fans are pretty cheap or easy to get used, I think they might start out at 50 cfm. I actually use one for a wood fired forge out back. Actually its a bid bottom of a boiler that I use for lots of stuff including cooking. But when I am hanging around out there, I throw some steel in the fire and I have an anvil with no horn I got for $25.
Anyway the bathroom fan has been outside for many years now and has worked fine. I agree about the dimmer switch, could work or not. I believe the dimmer switch chops voltage , so it goes to say 90 volts and drops out to 0 and then back again. So its not a complete wave form. the speed control sends the full wave form with a 90 volt peak. So small shaded pole motors will probably work, but split phase won't as well as larger amp draw... Its been a while I looked into dimmers so I could be off on that.

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  • 1 month later...

I use a vacuumm cleaner on a rheostat... been good for a couple years... get more noise from the corrugated hose (harmonic noise) at high volume.. any serious motor needs a frequency modulator to reduce speed, as opposed to reducing volts and amps.. which leads to cooked motors.. I get my vacuum cleaners from lawnsales for silly cheap!!

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If you do use a air gate remember this, it is easier to control what gose in instead of what gose out. if you use one on the output part of your blower when you close it of your blower is going to run alot harder but if you close of the intake it dosent change the runing of the motor but decreeses the air flow. this is what i do. hope this helps ya out.

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  • 11 years later...
On ‎1‎/‎7‎/‎2009 at 3:47 PM, Pat Roy said:

I started out with a blower and slide gate for control. I bought too much blower. Although I could control the flow with the slide gate, I could not stand the noise of the blower. First I put a light dimmer on it and it worked but I worried about burning out the motor and replaced it with a proper control. Works fine and now I can hear the music playing.

xxxxx..... I should be hearing MUSIC?!?!  ...xxxx!

Edited by Mod30
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Well when I was using a ribbed radiator hose as the air hose on a forge of mine it would give me an organ note at particular blower speeds---using a universal motor blower so easy to vary the speed with a simple dimmer.  I had to add in the percussion with the hammer and anvil...(Please not the post you replied to is from 2009...)

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