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Q about DC motors Voltage and Amps

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Forum topic by Thuzmund posted 01-19-2015 10:51 PM 3578 views 2 times favorited 38 replies Add to Favorites Watch
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Thuzmund

142 posts in 1092 days


01-19-2015 10:51 PM

Topic tags/keywords: dc treadmill lathe

Hi folks,

After days and days of research, I’m tossing out a couple questions that I just couldn’t find answers to. I’m researching the DC Treadmill motor lathe conversion. Looks like it’s worth the 150 bucks or so to give it a try for my needs—safely turning out of balance pieces and coring with a little more power than my current 3/4hp motor has. I have seen at least four different threads on this topic here at LJ…but no answers to the following questions.

1) Lots of motors on ebay are listed at amps or volts beyond what I think I can supply. My residential service is 110 (or 115/120…haven’t actually measured the darn thing) VAC, 15 amps. I’m aware that 15 amp circuits can supply more than 15 for short periods of time, but I am naturally afraid of running, say, a 18 amp motor on my 15 amp line for several hours. Beyond this, many motors only give volt ratings, like “1.5hp at 95VDC.” More confusion for me.

Now, to supply DC voltage I must use a controller box that converts to DC and supplies (and therefore limits?) juice. Therefore, am I correct in assuming that I can avoid overloading my house wiring by using a a controller that supplies a safe amperage?

2) What happens when the motor runs at lower amps than it is rated for (let’s say an 18 amp motor)? I’ve read that it will heat up and die prematurely, but after shopping around for treadmills, I see that many many treadmills with 2.5 or 3hp motors have normal 120V power cords and say nothing in the manual about using 20amp service. They do say you must use a power strip (circuit breaker) for safety. This means that thousands of treadmills are running the exact same setup as I’m imagining—a motor running under amperage but prevented from drawing an unsafe amount of juice. These treadmills must not be burning up left and right. Is it safe to assume that I can run a motor for many years under the rated amps on the nameplate?

3) Last question: Will my house 110VAC actually be able to provide 110VDC after the conversion, or is voltage lost during the rectification process? I’ve seen sources saying both—some say I can only get 75% of the voltage at DC and others saying it will be a 1-to-1 conversion. Any ideas on what type of DC voltage I can expect from a DC controller plugged into my wall?

Thanks in advance. I’m trying to decide between motors listed at 1 or 1.5hp around 10-15amps, or 2 to 2.25 hp rated around 16-18 amps. All of these are “peak figures” and the nameplates say that they will run continuous duty around 90VDC, with a loss of about 30-50% of hp.

-- Here to learn


38 replies so far

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MrUnix

4221 posts in 1662 days


#1 posted 01-19-2015 11:59 PM

First off, get off e-bay and start looking on CL for a free or dirt cheap treadmill to get.. or even put out a wanted ad asking for one. Or look around at local salvation army, hospice or similar outlets for one cheap. It can be butt ugly, missing bits and pieces and smashed up.. as long as it runs when you plug it in the wall, you are usually good to go.

For your questions: #1 – Don’t confuse AC house current with DC supply current to the motor.. different beasties. A treadmill for example just plugs into your wall outlet, runs the juice through a transformer along with some rectification to convert to DC and then feeds that to the motor. The amperage rating on the DC motor is not the amperage the treadmill will be using from your wall outlet. Most treadmills only need 10-12 amps from the wall (or less), so a 15 amp circuit is fine, regardless of the rating on the motor. Just look at the amperage rating on the treadmill to see what your wall outlet needs to supply.

#2 see #1

#3 Current is always lost in the form of heat when used to power a device. Be it in the motor, the electronics or even the line supplying the current. As for changing voltages, there is no one answer. You can convert 12v to 120v through an inverter and run corded (ie: 120v) power tools off your car battery. Or you can plug a transformer into your wall outlet (120vac) and power your 5vdc cell phone. Power output of an AC to DC (and visa-versa) converter is entirely dependent upon the design, so there is no way to tell you what it may be.. you need to read the specs.

As an example, I currently have a treadmill that I got for free off CL. It says it wants 12 amps from the wall outlet. The motor is a 2.25hp 90vdc thing that says it will suck up to 18 amps max. I don’t give a rats arse about what the motor wants, I just look at the current draw from the wall and make sure the outlet can supply that. If there was a problem, I’m sure the manufacturer of the treadmill and controller wouldn’t have designed it that way.

Cheers,
Brad

-- Brad in FL - To be old and wise, you must first be young and stupid

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runswithscissors

2187 posts in 1488 days


#2 posted 01-20-2015 12:23 AM

Why don’t you use the controller from the treadmill? I don’t see why you would need to do anything except maybe shorten the wires or reroute them.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

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TheFridge

5765 posts in 949 days


#3 posted 01-20-2015 12:28 AM

dc motors need an inverter to run off ac.

You never plug in something that can draw more than the rating of a receptacle. You really want it to be around 80% max.

Hp is hp. Higher for less amperage.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View Thuzmund's profile

Thuzmund

142 posts in 1092 days


#4 posted 01-20-2015 01:11 AM

Thanks for the quick replies. Yes, I’m aware that DC and AC are different and that there will be some sort of transformer in between the two. Otherwise, this is great info.

Scissors, I am looking at motors for sale on ebay, most of which don’t come with the controller. There are some popular options out there though to control the motor, like the MC-60 unit that many have written about.

It sounds like some of us have gone down this road before :)

-- Here to learn

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REO

889 posts in 1537 days


#5 posted 01-20-2015 01:19 AM

unless it is fully loaded the motor will not draw peak amps. the nameplate amperage is based on running at peak amperage. the breaker or fuse in your box will restrict the amperage in the line to an acceptable draw that is what they are for! overloaded they will trip. an Inverter actually is for getting AC from DC input. A rectifier converts AC to DC. The major problem with running DC at slow speeds is uniform loading. during roughing or intermittent cuts you may experience variations in speed. for extended periods of slow speed running you may want to supplement cooling with an auxiliary fan.

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tomd

2027 posts in 3233 days


#6 posted 01-20-2015 01:24 AM

I agree with Brad, go to craiglist buy a used treadmill I got mine for $25. Make sure it runs, everything you need is in there.

-- Tom D

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TheFridge

5765 posts in 949 days


#7 posted 01-20-2015 02:06 AM



unless it is fully loaded the motor will not draw peak amps. the nameplate amperage is based on running at peak amperage. the breaker or fuse in your box will restrict the amperage in the line to an acceptable draw that is what they are for! overloaded they will trip. an Inverter actually is for getting AC from DC input. A rectifier converts AC to DC. The major problem with running DC at slow speeds is uniform loading.
- REO

I hope you don’t size wiring by what “you think” it might draw. Hence. fla.

Breakers and fuses don’t restrict anything but an amperage above what they are rated for. It’s not like a dimmer.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

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Thuzmund

142 posts in 1092 days


#8 posted 01-20-2015 03:30 AM

I was “shopping” for treadmills to see if 2 and 3hp units were restricted to certain electrical supplies. I found out through my sleuthing that very few units ask for high voltage or amps; they just ask that you not plug anything else into the same outlet. Of course, salespeople don’t know anything about electricity or house wiring so I read a lot of manuals.

I’ll keep an eye on CL but it looks pretty empty near me and I’ll probably end up paying 100 bucks or so for a setup. It’s fun for me—as long as I can be somewhat confident I’m not being a bonehead. I tend to be more ambitious than I am informed. Forums like this one are great for my health!

-- Here to learn

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REO

889 posts in 1537 days


#9 posted 01-21-2015 05:07 AM

I was hoping one of the guys that has worked with the treadmill motors might get involved. Rick M would have applicable input because he has done a treadmill motor install on a lathe.

Rectifiers come in two main varieties. Half wave and full wave. a half wave rectifier is only about 42 percent efficient. a full wave rectifier is about 81 percent efficient. using this you can determine what it will take on the AC line input to drive your motor at 90 volts 18 amps.

Apparently I was not clear as to what I meant in my earlier post. If your home has been wired to electrical code the breaker or fuse at the panel limits the current to the circuit to an acceptable level for example 15 amps for 14 gage wire. or 20 amps on 12 gage wire. During an electric motor start the breaker will actually pass a much higher current for a short time. the amount of time that it takes to trip the breaker depends on the amount of over current. You will not be able to run in excess of the breaker rating for extended periods of time.

The amperage draw on a motor nameplate is at full HP rating.A motor draws much less just sitting there idling and some where between when the tool is actually doing something. Most equipment even in industrial shops don’t see full loads even when in use. about the only thing that most people have in the shop that is designed around full load is a dust collector which draws he most when it is sitting there just sucking air with no sawdust in the stream and uses less current the less air it has to move.

ok back to the amps volts thing… Ohms law relates Resistance, Voltage, Power, and Current

Amps multiplied by Volts gives you Watts. so…90 volts X 18 amps gives you 1620 watts but with a full wave rectifier giving you 82 percent efficiency you actually use 1976 watts on the AC side.

1976 divided by your 120 ac voltage gives you about 16.6 amps or if your AC is 110 almost 18 amps. This is when the motor is FULLY loaded working its very hardest.
I have not heard of a motor burning out because it was drawing Less amps. A motor running at slow speeds for extended periods will overheat because the cooling fan becomes inefficient at slow speeds. cooling can be augmented by a separate fan like a muffin fan mounted to the end of the motor running full speed all the time. AC motors running at slow speeds take higher current to maintain torque so they generate more heat at slow speeds. you will find this on AC motors with speed controls running at less than 36 HZ. DC is much more forgiving of slow speeds. On intermittent cuts that go from max available torque to minimum they will speed up and slow down or surge.

The large majority of DC motors that you will find suitable for your intended use will be either 90 or 180 volt DC. 110 AC in you will get 90V DC out. there are 110V 180DC rectifiers but they have a built in transformer. Most of the time the 180 VDC rectifiers are 220 AC input.

I have done several DC drives but as I mentioned none with a treadmill motor.

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Thuzmund

142 posts in 1092 days


#10 posted 01-21-2015 05:27 AM

REO, thanks so much for such a detailed response. The startup current has always been nagging at me, since I know many electrical components ask for more juice as they start (or when there’s strain on them, like a deep cut with a lathe tool).

It sounds like I can be fairly safe by using a circuit breaker (protecting my house) and not have to worry about lower power hurting the motor.

I’m really excited to give this a try soon. I hope other folks can find this thread; I found it very difficult to get answers when researching. Numbers and theory are one thing, but an experienced voice who can explain the practical realities is very valuable. Thanks again!

-- Here to learn

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Rick M

7913 posts in 1843 days


#11 posted 01-21-2015 06:08 AM

You are over thinking it. The PWM will take care of voltage and AC to DC. Buying a working treadmill will be much simpler in the long run because it will come with every part you need and be wired correctly. Don’t be too afraid if they tell you the treadmill doesn’t work, 99% of broken treadmills will have working motors. It’s often something simple like the safety switch that breaks. The older the treadmill, the simpler the electronics. This is a good time of year to watch for them. If you get one, take pictures while disassembling and sketch the wiring; doesn’t hurt to label the wires too. They will often have daughter boards that you won’t need but it isn’t too hard to trace the wiring and lose the unneeded boards.

-- http://thewoodknack.blogspot.com/

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MrUnix

4221 posts in 1662 days


#12 posted 01-21-2015 06:19 AM

What Rick sez.. unless you want to design the control and rectification circuits yourself, keep it simple. No need to worry about current draw from the motor, the existing stuff on the treadmill already does that for you. Basically plug and play. I just checked, and there were dozens of treadmills on CL in my area ranging from around $10 and up, and I’ve snagged two for free in the last year (both in perfect condition).

Cheers,
Brad

-- Brad in FL - To be old and wise, you must first be young and stupid

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JoeinGa

7481 posts in 1470 days


#13 posted 01-22-2015 03:18 PM

You guys that find free (or $10 ) treadmills on CL better thank your lucky stars. I’ve been watching CL for a year (the closest two to me are an hour away in opposite directions) and have found NOTHING, ZIP, NADA, NONE ….

-- Perform A Random Act Of Kindness Today ... Pay It Forward

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Rick M

7913 posts in 1843 days


#14 posted 01-22-2015 07:18 PM

Yeah it can be hit or miss around here too but it helps to be near a city. This is the time of year that many show up and then again come spring.

-- http://thewoodknack.blogspot.com/

View crank49's profile

crank49

3981 posts in 2434 days


#15 posted 01-22-2015 07:29 PM



dc motors need an inverter to run off ac.

You never plug in something that can draw more than the rating of a receptacle. You really want it to be around 80% max.

Hp is hp. Higher for less amperage.

- TheFridge

Huh??
An AC motor needs an inverter to run on DC, not the other way around.
A DC motor running on AC would need a rectifier to convert the AC to DC first.
AC can go through a simple transformer to increase the voltage, DC cannot.

Now HP is HP I’ll agree, but higher is NOT less amperage, it’s more.
Now if you meant to say higher voltage is less amperage that’s true. But that’s not what you wrote.

We need to be perfectly clear when giving electrical advice. This can be dangerous stuff.

-- Michael: Hillary has a long list of accomplishments, though most DAs would refer to them as felonies.

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