Wiring a 120/240 motor to 240v, examined in detail. - Woodworking Talk - Woodworkers Forum
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post #1 of 41 Old 08-13-2016, 03:50 PM Thread Starter
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Wiring a 120/240 motor to 240v, examined in detail.

Regularly someone suggests that it's better to wire a 120/240 volt table saw motor to 240v. The responses generally fall into two groups, the first pointing out that motor performance is exactly the same running a 120/240 volt motor at whatever voltage it's configured for (this is correct, and tends to be the group I fall into) and the opposing group of people who've swapped saws from 120 to 240 and then observed a noticeable improvement in performance.

My theory:

That when operated at design voltage, there is no advantage to operating a 120/240 motor at 240v.

That any difference can be traced directly to voltage drop.

That, for motors of 1.5hp and above, it can be very difficult to reduce the voltage drop to an insignificant percentage when run at the lower voltage.

That existing 120v circuits are much more likely to have voltage reducing factors than 240v circuits.

That in actual use saw motors will tend to perform better when wired to 240v than when wired to 120v.

So, now lets test out some these theories, starting with "there is no advantage to operating a 120/240 motor at 240v".
We will use ordinary A lamps to illustrate what is going on in the motor windings. We take two 120v 43 watt light bulbs each producing, along with the 43 watts of heat, 680 lumens of visible light. They are connected in the conventional way, each lamp directly to 120v.

Untitled by [url=https://www.flickr.com/photos/[email protected]/]

The result isn't a surprise:
Untitled by [url=https://www.flickr.com/photos/[email protected]/]



note the amps, there are .69 amps producing a total of 86 watts of heat (and some light).

Now, lets plug the two lamps into the 240v circuit. To keep it from burning up both lamps we will rearrange the wires. Just like the saw if we don't connect the wires differently we will burn up the lamps (the motor) very quickly.

Here we are:
Untitled by [url=https://www.flickr.com/photos/[email protected]/]


Now the light is the same, the watts (heat) is the same and the amps in that wire have dropped to .34, but there are also .34 amps in the other wire, for a total of 6.8 which is the same as the 120v circuit (once you correct the rounding error). So the light, the heat, and the amps, all the same.

A motor will act exactly the same as these two lamps, it does not care what voltage circuit you ran into it's splice box. Each lamp (winding) get 120v either way.

What's the difference to a woodworker, and why the endless debate? The voltage drop.

Lets compare a pretty typical situation. You move into your new house, set up a shop in the garage, plug your 120v saw into a convenient 120v outlet. Are there other loads on that circuit? Maybe. Does that circuit run through other outlets, perhaps push-wire outlets that don't pass the current through as well as they should, adding to voltage drop? Probably. Aluminum wire? #14 wire somewhere in the circuit? Not usually, but sometimes.

Lets ignore those factors for now and focus on distance. Assuming 75 feet of distance from the saw to the breaker, a 1.5hp saw pulling 20 amps (that's the number the NEC says to use unless have the motor and can look at it) with #12 copper wire installed and perfect connections, the 120v at the panel will be reduced 4% to 115 volts at full rated load. Longer distances, less than perfect connections, any load sharing, it just gets worse.

Next, having decided to take that excellent advice you got from the forum at woodworkingtalk.com, you run a 240v circuit to your saw. Are there other loads on that circuit? No. Push-wire outlets? No. Perfect connections? Probably.
Again we ignore those factors, the big difference is that where before we had 6.8 amps traveling 75 feet in #12 wire now we have 3.4 amps. Those electrons slip through the wire much easier because there are only 1/2 as many of them, so what started out as 240v is now 1% less, 238 volts. Each saw winding gets 119 volts.

This is best-case and I would expect to see larger, sometimes much larger differences is actual installations.

Opinion:
In real world conditions wiring a saw motor to 240v will provide better performance in theory, while the same motor wired to 120v will also be more likely to encounter factors that tend to reduce the voltage and therefore the performance.

I welcome any thoughts especially from those who disagree. And, thanks for taking the time to read this.
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post #2 of 41 Old 08-13-2016, 04:29 PM
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Great!

Thanks for "shedding" some light, actually 2 lights, on this "heated" topic. Your circuit board is a great way to explain how the windings are wired. I was just about ready to post on voltage drop myself and so you saved me the trouble. Dedicated 240 volt circuits beat questionable 120 volt circuits every time. :smile3:

There is another equally important issue that involves shop wiring in my opinion and that is "in wall" wiring vs thinwall or conduit wiring. I have 2 shops, each wired a different way. The woodshop is "in the wall" and obviously I am stuck with the outlets and circuits as they were wired in place. The metal and welding shop is all in thinwall and I can add or change circuits as needed. Just pointing that out for anyone considering planning a shop or wiring an existing one.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #3 of 41 Old 08-14-2016, 12:56 AM
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In my situation, I had a 2hp saw 10' from the breaker box wired with 10 ga. wire because it's all I had at the time so there was no voltage drop. Wired to 120v, ripping red oak the saw would overheat and trip the thermal overload and I would have to stop and allow the saw to cool down before continuing. I made comment about it to the motor repair shop I use and the guy told me to wire it for 240v and it would solve the problem. I went back to the shop and changed the wiring to 240v and never had another problem with the saw overheating. In fact the saw had more power and I was able to rip red oak faster than I every could before. I don't believe I was achieving 2hp with the saw at 120v. I think it wasn't until the saw was wired to 240v it reached it's full potential.
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post #4 of 41 Old 08-14-2016, 01:19 AM
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Quote:
Originally Posted by Steve Neul View Post
In my situation, I had a 2hp saw 10' from the breaker box wired with 10 ga. wire because it's all I had at the time so there was no voltage drop. Wired to 120v, ripping red oak the saw would overheat and trip the thermal overload and I would have to stop and allow the saw to cool down before continuing. I made comment about it to the motor repair shop I use and the guy told me to wire it for 240v and it would solve the problem. I went back to the shop and changed the wiring to 240v and never had another problem with the saw overheating. In fact the saw had more power and I was able to rip red oak faster than I every could before. I don't believe I was achieving 2hp with the saw at 120v. I think it wasn't until the saw was wired to 240v it reached it's full potential.
Steve it doesn't matter how the saw actually performs, this is a virtual woodworking situation so it all depends how it works out on paper. :smile3:

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post #5 of 41 Old 08-14-2016, 07:54 AM
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Steve it doesn't matter how the saw actually performs, this is a virtual woodworking situation so it all depends how it works out on paper. :smile3:
I've never used a paper saw though.
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post #6 of 41 Old 08-14-2016, 11:27 AM
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"..tripped out on thermal overload..."
no surprises there, Steve.

if the saw is just running, doing no "work" i.e. not cutting anything, it draws very little power thru the wires.

as the "work load" increases an electric motor demands more power, i.e. it sucks more amps through the wires. this is why overloaded circuits get hot.....

back to basics:
V(volts) = I (amps) x R (resistance) - Ohms Law
P (power) = V x I

a little substitution shows:
P = I x I x R

so, the amperage demand goes up by the square of the power (R is a constant; the resistance of the windings does not change.)

using 240 volts, the saw only draws half the amps (vs. 120v) - increasing power demands on the saw draw fewer amps at 240 - and it's the amperage that causes overheating.
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post #7 of 41 Old 08-14-2016, 12:13 PM
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I thought that was the point of discussion how a motor applies to woodworking. I agree completely if you are going to just stand and watch a saw motor running it doesn't matter if it's wired to 120v or 240v.
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post #8 of 41 Old 08-14-2016, 01:35 PM
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Originally Posted by Steve Neul View Post
I thought that was the point of discussion how a motor applies to woodworking. I agree completely if you are going to just stand and watch a saw motor running it doesn't matter if it's wired to 120v or 240v.
That is when light bulbs really shine. :smile3:

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post #9 of 41 Old 08-14-2016, 02:21 PM
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Quote:
Originally Posted by Steve Neul View Post
In my situation, I had a 2hp saw 10' from the breaker box wired with 10 ga. wire because it's all I had at the time so there was no voltage drop. Wired to 120v, ripping red oak the saw would overheat and trip the thermal overload and I would have to stop and allow the saw to cool down before continuing. I made comment about it to the motor repair shop I use and the guy told me to wire it for 240v and it would solve the problem. I went back to the shop and changed the wiring to 240v and never had another problem with the saw overheating. In fact the saw had more power and I was able to rip red oak faster than I every could before. I don't believe I was achieving 2hp with the saw at 120v. I think it wasn't until the saw was wired to 240v it reached it's full potential.
What you are describing here is a product of the efficiency of the motors. Obviously the design of the motor as it was wired for 120 volt was different than the efficiency of the motor as it was wired for 240 volt.

All motors operate at different efficiency. There is no such thing as a 100 % efficient electric motor.

Many (most?) people forget this factor when they start discussing and comparing motors. It was nothing to do with whether the motor is wired for 110/120 volt or 220/240 volt.

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post #10 of 41 Old 08-14-2016, 02:39 PM
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Take an ordinary table saw, an ordinary 40 tooth blade and make a rip cut. As you feed the wood into the blade at a normal rate, it will cut efficiently, assuming a sharp blade. When you increase the feed rate, the blade will slow down and not cut as efficiently. Apply more feed pressure and you can stall the blade, assuming the motor is not 5 HP. So, a motor that develops it's maximum torque at startup and run speed will eventually run out of power if that torque level is exceeded, and stall out, and or trip the internal breaker because it's drawing excessive current.

So what? If you are having issues with a motor overheating or slowing down radically, it ain't enough motor. Or your feed rate is excessive, OR your blade is getting gummed up or dull, OR your blade is binding in the kerf, or the fence and blade are not parallel, OR your wiring is inadequate. Once all the controllable issues above have been dealt with, if you are still having a stalling or tripping issue, it ain't enough motor. The solution is get a motor with more HP.

I still say a 2 HP motor will "run" on 120 volts, but to eliminate wiring issues, wire it for 220 volts. All other things being equal, your motor should run more efficiently. A 1 HP motor is where the discussion becomes muddy, but I would still wire if for 220 volts if possible.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #11 of 41 Old 08-14-2016, 04:12 PM
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It's also worth mentioning that a 2hp saw running on 120v will be more prone to overheating because of the higher amp draw. At 120 the saw is pulling 20 amps, which leads to a lot of heat buildup. The same motor on 240v will pull half the amp load, leading to less heat for the same amount of power. The power doesn't change at all though

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post #12 of 41 Old 08-14-2016, 04:12 PM
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The illustration is very good, the discussion good as well. But in the realm of 'perception is reality' then when a motor 'runs' better on 240V than on 120V then the common belief is that 240V is better. I'm in the camp of understanding the difference and reasons why the motor should be the same at either voltage but in practice I've had too many saws and tools run better at 240V so I opt for that out of the gate whenever and wherever possible. No sense wasting time hoping 'this time 120V will work just as good' - just cut to the chase and go with 240V.

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post #13 of 41 Old 08-14-2016, 05:57 PM Thread Starter
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Quote:
Originally Posted by Steve Neul View Post
In my situation, I had a 2hp saw 10' from the breaker box wired with 10 ga. wire because it's all I had at the time so there was no voltage drop. Wired to 120v, ripping red oak the saw would overheat and trip the thermal overload and I would have to stop and allow the saw to cool down before continuing. I made comment about it to the motor repair shop I use and the guy told me to wire it for 240v and it would solve the problem. I went back to the shop and changed the wiring to 240v and never had another problem with the saw overheating. In fact the saw had more power and I was able to rip red oak faster than I every could before. I don't believe I was achieving 2hp with the saw at 120v. I think it wasn't until the saw was wired to 240v it reached it's full potential.
Hi Steve,
clearly the motor or the circuit had a problem you fixed when you went to the 240v circuit. I have a few ideas what that might have been but without having the motor and circuit it's just speculation.
Great that it worked out for you.
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post #14 of 41 Old 08-14-2016, 06:08 PM Thread Starter
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Quote:
Originally Posted by TomCT2 View Post
"
using 240 volts, the saw only draws half the amps (vs. 120v) - increasing power demands on the saw draw fewer amps at 240 - and it's the amperage that causes overheating.
Tom, increasing amps increases the quantity of electrons moving through the circuit, that can generate some heat in the circuit wires.

Inside the motor the heat is the same no mater the voltage at the same level of work. When you want to express the amount of work being done watts are really useful, a watt being 3.41 BTU's.

Two identical motors side by side, 120v next to 240v, will draw (using the previous examples) 20 amps and 10 amps but the watts (heat) from the motor will be the same.
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post #15 of 41 Old 08-14-2016, 06:29 PM
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Hi Steve,
clearly the motor or the circuit had a problem you fixed when you went to the 240v circuit. I have a few ideas what that might have been but without having the motor and circuit it's just speculation.
Great that it worked out for you.
It wasn't so much I had a problem with the motor, according to the repairman it's just the difference between wiring one to 120v rather than 240v. A motor just preforms better on 240v is why he made the suggestion without ever seeing the motor.
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post #16 of 41 Old 08-14-2016, 06:58 PM Thread Starter
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Originally Posted by woodnthings View Post
As you feed the wood into the blade at a normal rate, it will cut efficiently, assuming a sharp blade. When you increase the feed rate, the blade will slow down and not cut as efficiently. Apply more feed pressure and you can stall the blade, assuming the motor is not 5 HP. So, a motor that develops it's maximum torque at startup and run speed will eventually run out of power if that torque level is exceeded, and stall out, and or trip the internal breaker because it's drawing excessive current.
Explaining this another way, without using the concept of 'stall'- Take a 3,450 rpm motor, a common speed. In the US, we have 60 cycles/second 'field speed'. That (simplistically) means that the shaft of the generator, which is kind of just a big motor, is spinning at 60 cycles/second, or 60hz. It's spinning magnets attached to the rotor past magnets attached to the stator that don't move. If you swap the motor and the generator and spin the motor shaft the generator shaft will spin, now the generator is a motor, they are that similar.

Anyway, at 60hz you have 60 cycles/second which is 3,600 cycles/minute or 3,600 RPM. Apply power to the motor, the field is spinning at 3,600 rpm and the motor should spin at 3,600 rpm also, except for friction and other losses. In real life the field 'slips' by the magnets on the rotor, and every time it slips it draws power trying to keep up. The amount that this happens is called 'percent slip' and the more slip the power is being used. That's why the motor speed is 3,450, it needs the 150 rpm slip to spin.

As you push that block of oak through the saw blade the motor slows down and the percent slip increases, producing more horsepower and more heat. For every motor, there is a certain amount of heat the motor can handle before the insulation on the motor windings starts to melt. For the example we've been using the 120v saw at full load draws 20 amps, and 20 amps times 120 volts equals 2,400 watts. For the 240v example, 10 amps times 240 volts equals 2,400 watts, so once again, it's the same.

2,400 watts equals 8,184 btu's of heat. As you push the saw beyond it's 1.5hp rating to 2hp it will deliver the hp but not it's trying to get rid of about 11,000 btu's of heat and pretty soon it will burn up, as they say.
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post #17 of 41 Old 08-14-2016, 07:31 PM Thread Starter
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Quote:
Originally Posted by Steve Neul View Post
It wasn't so much I had a problem with the motor, according to the repairman it's just the difference between wiring one to 120v rather than 240v. A motor just preforms better on 240v is why he made the suggestion without ever seeing the motor.
Steve, I get you believe this, no problem. I'm not attached to changing your mind, and if you and everyone you advise runs their motors at 240 and not 120, no harm done for reasons stated in the OP. So there I agree with you.
The problem is trying to understand the why of it. You can see this happening in your own posts every time you try to explain why the motor 'runs better' at 240v:

" If only one field is used as in 110V it draws more amperage to get the job done than if it used two magnetic fields as in 220."
"there are two individual fields on a duo-voltage motor. You have the main winding and the auxiliary winding. When one is positive the other is negative when running at 220v."

I think for your purposes it's better to avoid explaining it.
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post #18 of 41 Old 08-14-2016, 08:25 PM
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but that's not how a thermal overload protector works.

thermal overload does not measure the temperature of the motor.
it is (typically) a bi-metal strip that opens the control circuit when too many amps cause it to heat up and warp.
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post #19 of 41 Old 08-14-2016, 11:45 PM
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Steve, I get you believe this, no problem. I'm not attached to changing your mind, and if you and everyone you advise runs their motors at 240 and not 120, no harm done for reasons stated in the OP. So there I agree with you.
The problem is trying to understand the why of it. You can see this happening in your own posts every time you try to explain why the motor 'runs better' at 240v:

" If only one field is used as in 110V it draws more amperage to get the job done than if it used two magnetic fields as in 220."
"there are two individual fields on a duo-voltage motor. You have the main winding and the auxiliary winding. When one is positive the other is negative when running at 220v."

I think for your purposes it's better to avoid explaining it.
I made an attempt to explain why in another thread and several folks argued the point and perhaps I didn't get the specifics quite right but basically my repair guy said it's because at 220v two fields are used which makes the difference. It's been 16-18 years since it was explained to me. The way I see it is an field is an electromagnet so it would be common sense a motor would work better with two than just one.
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post #20 of 41 Old 08-15-2016, 01:05 AM
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Speaking as a retired general contractor who believes electricity is a powerful force controlled by magicians called "electricians", there was always an electrician who, upon hearing that I only had a grip on the basics of electricity, was sure that he was the guy who could explain it so clearly that he could make it all suddenly clear to me. In spite of the fact that I had been a general for 24 years and he's 23 years old.

They'd all use the same analogy, the thing with the water in the hose, and how voltage is the number of gallons, and resistance is the size of the hose, current is the amount of water, so on and so forth.

At the point where they got to the amount of water in the tub, a plumber would jump in and point out that the amount of water in the tub doesn't affect the flow of water. If a laborer happened by, he'd simply (and wisely) roll his eyes and start sweeping the floor.

Somewhere in there, I'd always get bored and find Executive Matters to attend to in the trailer.

Don't hook a 120 motor up to 240. Hooking a 240 motor up to 120 won't work.

Be sure and have your wife take pictures and post them here if you work on live circuits.

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