120v to 240v Clarification Please

[scott spencer]

This subject comes up frequently (I'm aware of another current thread), and the responses are typically split about the benefits. I understand that the voltage across the coils is always the same whether wired for 120v or 240v.

My question is specifically regarding voltage loss from the supply circuit. I read often that there is no benefit by switching to 240v if the 120v circuit is wired properly according to code. Do we agree that there is higher potential for voltage loss at 120v than 240v? ...Especially on start up when peak amperage demand is many times the nominal demand? Or are there split opinions on that aspect as well?

My logic could be flawed, but if the saw comes up to speed faster on startup when wired for 240v, it seems logical that it also comes back to speed faster during heavy load when it slows...or at least that the way it seems in my experience. Some slowing of the blade occurs fairly frequently with a 1-1/2 to 2hp motor when cutting materials like dense 2"+ hardwood, and the feed rate needs to be adjusted accordingly. If the motor recovers faster on 240v due it's ability to more adequately supply the needed amperage, why isn't that considered a bonafide uncontested benefit over 120v?

TIA...

[Chris Schumann]

It all depends on whether the premise is true. The premise: Motors wired for 240 spin up faster. Is it true?

In theory, it shouldn't matter, but in most shops, 120v circuits power all kinds of things: lights, heat, fans, radio, maybe parts of other rooms. So if some current is already going somewhere else, and you start a 120v motor on that same circuit, there could be less current available for it, making it slower to start.

If you have a 240v circuit in the shop, a motor on it is likely to be the only load on the circuit, and if that motor starts quicker, it will also recover quicker.

As for voltage loss, V=IR, so in the same gauge wire, halving the current (which happens when you wire for 240v) halves your losses along the wire (meaning the supply circuit, not the windings).

But torque inside the motor is generated by current, not voltage, but with the AC and inrush at rest, it's a very complex system... and I didn't do well in my two semesters of electromagnetic field theory.

[scott spencer]

It all depends on whether the premise is true. The premise: Motors wired for 240 spin up faster. Is it true?

In theory, it shouldn't matter, but in most shops, 120v circuits power all kinds of things: lights, heat, fans, radio, maybe parts of other rooms. So if some current is already going somewhere else, and you start a 120v motor on that same circuit, there could be less current available for it, making it slower to start.

If you have a 240v circuit in the shop, a motor on it is likely to be the only load on the circuit, and if that motor starts quicker, it will also recover quicker.

As for voltage loss, V=IR, so in the same gauge wire, halving the current (which happens when you wire for 240v) halves your losses along the wire (meaning the supply circuit, not the windings)....


Thanks for the input Chris. I'm assuming a dedicated 20 amp 120v circuit, or whatever configuration offers a fair comparison.

[Howard Acheson]

>> My logic could be flawed, but if the saw comes up to speed faster on startup when wired for 240v,

Let's think about why that is. If you have a properly sized circuit for 120 volts and lets say 18 amps and you change the motor to 240 volts you halve the amperage which is now 9 amps. The wire size required for 240 volt operation is a couple of sizes smaller. But, most folks do not change the 18 amp wire size when the go to 240 volts. The wire size for the 240 volt operation is now an order of magnitude too large.

So, the reason that a motor comes up to speed faster on 240 volts has nothing to do with the motor itself. It's just that there is less of voltage drop because of the oversized wire. No electricity is "lost" no matter how the motor is wired as long as the motor recieves adequate electricity, it doesn't care. Motors are designed to deal with the higher startup voltage with it affecting the longevity of the motor.

[Mike Henderson]

If the circuit wires are the right size, the motor will not come up to speed faster on 240 volts - the motor will perform exactly the same on either voltage. That includes startup time, heat, recovery from load, etc.

If the wires in your 120 volt circuit are too small, and you put in a 240 volt circuit with the proper sized wire, the motor will perform better. But there is not a "higher potential for voltage loss" in a proper 120 volt circuit than in a proper 240 volt circuit. The circuit (either one) will be designed to provide rated voltage under all conditions of normal operation.

I think what you're saying is that many existing 120 volt circuits are not sized properly* for the load of a table saw, and therefore putting in a properly designed circuit (of either voltage) will improve the performance of the saw.

Mike

*or may be load sharing with other devices

[Tom Veatch]

... Do we agree that there is higher potential for voltage loss at 120v than 240v? ...Especially on start up when peak amperage demand is many times the nominal demand? Or are there split opinions on that aspect as well?...

Yes, we are in agreement. There may be split opinions, but a wise man once said that the universe does not respect anyone's right to their own opinion save the opinion be based on fact. The fact is, for the same power delivered, higher voltage means lower amperage, and lower amperage means less voltage drop in the circuit wiring. Of course, that assumes the same circuit wiring in both cases.


...If the motor recovers faster on 240v due it's ability to more adequately supply the needed amperage, why isn't that considered a bonafide uncontested benefit over 120v?...

That is the only advantage to using 240v. In high current draw conditions such as starting, stalled conditions, or recovering from a stall (effectively the same as startup), the percentage of line voltage delivered to the motor will always be higher with a higher voltage. For a given current, there will always be some voltage lost to line resistance. For half that current, the voltage lost will also be half. So, double the supply voltage and half the amperage means the drop as a percentage of the supply for 240v will be 1/4 that for 120. That's true regardless of the "goodness" of the circuit.

That does not mean however that the voltage lost is significant in either case. If you lose 1 volt to line resistance with 120v, it'll only be 1/2 volt with 240. Significant? Not likely anything you'd ever notice. On the other hand, if you lose 10 volts to line resistance with 120, it'll only be 5 volts for 240. You may very wll see a differnce in performance between a 120v motor being fed 110 and a 240v motor being fed 235v.

Whether or not you see a difference in any given scenario, is entirely dependent on the ability of the circuit to supply the desired amperage without significant voltage drop.

[Mike Henderson]

I have to slightly disagree with Tom's analysis. The percentage voltage drop does not have to always be greater on a 120 volt circuit - it depends on the resistance of the wire in the feeder circuit.

In the 240 volt circuit, the voltage is twice that of 120, and the current required will be half. If the resistance ratio of the two circuits (240 compared to 120) is four, the circuits will perform exactly the same.

You can check this by assuming some currents for the two circuits (make the 120 volt currents twice the 240 volt currents), then assign a resistance to the two circuits in a 4:1 ratio, and calculate the voltage drop for each of the currents. Then do a resulting percent of nominal voltage for each. You'll see that the voltage supplied in each case is exactly the same percentage.

I'll do one current as an example. Assume 4 Ohms for the 240 volt circuit and 1 Ohm for the 120 volt circuit (4:1 ratio). Then assume 20 amps for the 120 volt circuit and 10 amps for the 240 volt circuit. The voltage drop for the 120 volt circuit is 20 volts, while the voltage drop for the 240 volt circuit is 40 volts. The resulting supply voltage is 83% in both cases.

If your supply circuits are "equivalent", the motor will perform exactly the same. But many times, the 120 circuit is not "equivalent" to the 240 volt circuit and people see an improvement when they go to 240 volts.

Mike

[edit for Rick C. - yeah, I just picked some numbers that were easy to calculate with. Your example is more realistic.]

[Rick Christopherson]

What typically happens, but is often forgotten, when people change their tool from 120 volts to 240 volts, is that they change many, many other factors in the process, but attribute all of the performance changes solely to the voltage change.

Typically, the original tool started out on a 15 amp circuit (which is too small for a 1-1/2 hp motor at full load). When they wire a new circuit, it is almost guaranteed from the human nature of a woodworker, to install either a 20 amp circuit, or even a 30 amp circuit. They went from taxing the circuit 100% to taxing it 37% or even 25%. They have oversized the circuit when they make the conversion.

If you compared the same change on a properly sized circuit (20A @ 120V Vs. 15A @ 240V) then the change in circuit loading is a change from 75% to 50%, which is a less dramatic improvement, but an improvement nevertheless.

additionally, the original 15 amp circuit was typically not a dedicated circuit, so the tool was not the sole load taxing that circuit. It is also not uncommon that the original tool was operated from an extension cord, but in the new circuit, the extension cord is eliminated. This alone is the greatest improvement.

The reason a motor takes longer to come up to speed on any circuit is due to the voltage drop when high amperage is flowing. At startup, this amperage is 5 times larger than the maximum amperage you can draw from the motor at maximum rated load.

However, on a properly designed circuit, the voltage drop is far less significant than many people think. Consider a 50-foot circuit (100 feet of copper). The voltage drop for 15 amps in #12 wire is 2.25 volts (117.75 volts available). The voltage drop for 7.5 amps in #14 wire is 1.5 volts (238.5 volts available). The voltage of the power grid fluctuates more than this small amount throughout a normal day.

Because most cases of rewiring a motor also involve adding a new circuit, the proper comparison would be to see how the saw performance increases when you install a proper 120 volt circuit compared to the original circuit. People would see nearly the same change in the performance of the tool.

Edit: I didn't see Mike's posting while I was making mine, but in light of that, I wanted to point out that the numbers I used above are actual numbers of resistance and amperage for a 1-1/2 hp motor and American Wire Gauge (AWG).

Another Edit: By the way, in my opinion on the topic, a 2 hp motor should be converted whenever feasible. A 1-1/2 hp motor is borderline, and a proper circuit would fix the problem. Anything less than 1-1/2 hp is just a waste of time to convert, and there will be no gain.

[Chip Lindley]

Many have tried, many have failed to convey a thorough synopsis of the 120/240V controversy. Rick makes the most sense I have read so far.

Many variables DO change when a motor is switched to 240V. Usually a dedicated circuit. The same benefit would, no doubt, be seen from installing a 20A, 120V circuit with 10awg wiring for your 1.5hp table saw.

Some building codes allow use of 14awg house wiring on a 15a 120v circuit. Not Nice for an induction motor--especially if shared with other loads (lighting, fans, battery chargers,etc.) Shared loads on a 15A circuit create enough problem with 12awg, much less 14awg.

As long as a dedicated circuit is seen as *fixing* this problem of slow starting (or much worse--circuit tripping) why not just go for a new 240V circuit?? Any machine 2hp or larger demands 240V, and you may just acquire one in your woodworking future. In a small shop only one tool is used at a time, plus running a dust collector (usually 240V, if of any adequate size.) So, installing a 30a 240v circuit with wire sized to your code, would probably support both running together.

Motors smaller than 1hp (12a) can function nicely on 120V. In fact I used a Sears TS (13a) on 120v for years without problems. But a Rockwell CS with 14a motor tripped the same breaker during heavy cuts in oak. Switching to 240V cured its problem and it ran like a champ for as long as I had it.

If your TS runs great on 120V, and performs as you expect, don't mess with it. But, on the other hand, if its performance is sluggish or breakers trip, 240V is very much indicated to improve your attitude toward that particular TS, and it's attitude toward performing the work you demand of it!

[Dan Friedrichs]

Several very good comments, here - a few not entirely accurate.

Here's the simple answer:

-A TS plugged into either a correctly-sized 240V or 120V circuit will operate EXACTLY THE SAME.

-"Correctly-sized" for a 120V circuit would mean ~20A dedicated circuit with large (12 or 10ga, dependant on length of run) wire.

-People see reduced performance on 120V circuits vs. 240V circuits because the 120V circuits are typically undersized for a TS and/or are being shared by other loads.

-If you're going to install a dedicated circuit for a TS, you might as well run 240V because you can get away with thinner (less expensive) wire for the same load. There is no other reason to run 240 vs. 120.

[Tom Veatch]

...In the 240 volt circuit, the voltage is twice that of 120, and the current required will be half. If the resistance ratio of the two circuits (240 compared to 120) is four, the circuits will perform exactly the same...

Of course you're correct Mike, but we're talking about two different things. If you'll notice in my post, I said "assumes the same circuit wiring in both cases."

If you have a 120v circuit running a 120v machine, you'll have a given voltage drop. If you change that circuit to 240v (same wiring) and rewire the motor to 240v, the result will be 1/2 the voltage drop and 1/4 the percentage voltage drop. Guaranteed!

[Mike Henderson]

Of course you're correct Mike, but we're talking about two different things. If you'll notice in my post, I said "assumes the same circuit wiring in both cases."

If you have a 120v circuit running a 120v machine, you'll have a given voltage drop. If you change that circuit to 240v (same wiring) and rewire the motor to 240v, the result will be 1/2 the voltage drop and 1/4 the percentage voltage drop. Guaranteed!
You're right, Tom. Sorry, I missed that.

Mike

[Rick Christopherson]

If you're going to install a dedicated circuit for a TS, you might as well run 240V because you can get away with thinner (less expensive) wire for the same load. http://www.talkfestool.com/vb/images/smilies/newsmilies/laughing1.gifhttp://www.talkfestool.com/vb/images/smilies/newsmilies/laughing1.gifOh that's priceless!! Can I nominate you for the most unrealistic statement of the month? :D (Please don't get mad, I am just teasing, and I do agree with what you wrote.) The statement that you can use smaller wire is often repeated as justification in these discussions, but then in the discussions about adding a new circuit, there is not a single person (except myself sometimes) that ever suggests using anything smaller than 20A/#12 wire, and more commonly, to use 30A/#10 wire. So your statement is kind of ironic, and I always get a chuckle out of when I see it.

Sorry about my little diversion there. :) On a more serious note, you reminded me of another important aspect of these discussions that also gets overlooked. A person could go ahead and run a 20A/120V dedicated circuit to see if it corrected their problem, and this could be done without altering the motor. Then, if they discover that they still have a problem, they could simply change the single-pole breaker for a double-pole breaker, change the motor, and be off and running with 240 volts without changing the wiring in the wall.

[Jeff Bratt]

The last three posts by Rick, Chip, and Dan seem (IMHO) to summarize this issue pretty well... Maybe this will put it to rest - for awhile.

[Dan Friedrichs]

http://www.talkfestool.com/vb/images/smilies/newsmilies/laughing1.gifhttp://www.talkfestool.com/vb/images/smilies/newsmilies/laughing1.gifOh that's priceless!! Can I nominate you for the most unrealistic statement of the month? :D

Ah, you're right, Rick. I guess I should have said that you CAN run smaller wire, even though you actually won't. Although, if you're planning to run garden-hose sized conductors no matter what, then the voltage drop difference between 120 and 240 is negligible, and there will be no difference (performance-wise or cost-wise).

[Rick Christopherson]

Although, if you're planning to run garden-hose sized conductors no matter what.....Garden Hose? You're thinking way to small. You don't want to know what we call the big stuff, but it is along the "equine" line of nicknames. :eek: http://www.talkfestool.com/vb/images/smilies/poking.gif

[scott spencer]

Thanks all for the great explanations!

[Dan Friedrichs]

Garden Hose? You're thinking way to small. You don't want to know what we call the big stuff, but it is along the "equine" line of nicknames. :eek: http://www.talkfestool.com/vb/images/smilies/poking.gif

Yeah, I've heard that expression, too :eek:. I used to work in an engineering office where I had to talk with construction guys in the field somewhat often. Their "equine" name for the flex conduit that connects between a motor and JB is what taught me not to use the speakerphone when other clients were milling about! :eek:

[Chris Padilla]

Now we know why power companies run their lines at hundreds of kilovolts (kV) and step-it down to feed our houses:

higher voltage means lower current (to deliver the same power)
lower current means smaller wires
smaller wires are cheaper than larger wires

[Jason Whitaker]

well, not trying to beat a dead horse... but

I was having issues running my table saw and making the lights dim a bit on start-up/under heavy load. It was a straight 120v run from box to outlet, no more than 60ft (total circuit) with 12G wire. This was a dedicated table saw outlet that I installed, no other load on this line whatsoever. But I was still getting the lights dimming.......figured two way to solve this.... run heavier gauge wire to reduce resistance..OR switch it to 240....I opted for the later and after about 15 minutes total time and no more than $15 was finished. turned the saw on (after rewiring of course:-)) and VOILA... not a dim light in the house on startup or while cutting through a few pieces of 8/4 hard maple.

Not sure what this proves ...but just thought it was an interesting observation. Problem solved thanks to reading this thread and finally getting a better understanding on volts, amp, ohms, etc.. So THANKS!

[Mike Henderson]

well, not trying to beat a dead horse... but

I was having issues running my table saw and making the lights dim a bit on start-up/under heavy load. It was a straight 120v run from box to outlet, no more than 60ft (total circuit) with 12G wire. This was a dedicated table saw outlet that I installed, no other load on this line whatsoever. But I was still getting the lights dimming.......figured two way to solve this.... run heavier gauge wire to reduce resistance..OR switch it to 240....I opted for the later and after about 15 minutes total time and no more than $15 was finished. turned the saw on (after rewiring of course:-)) and VOILA... not a dim light in the house on startup or while cutting through a few pieces of 8/4 hard maple.

Not sure what this proves ...but just thought it was an interesting observation. Problem solved thanks to reading this thread and finally getting a better understanding on volts, amp, ohms, etc.. So THANKS!
If you were going direct from the main breaker box to your saw with 12 gauge wire and you were dimming the lights in the house, you have some other problems. The breakers in the main box are attached to buss bars that are connected to the main feeder wires (through the meter and a main breaker). If a load on 12 gauge wire will pull down the voltage on that buss, there's a problem other than whether you're using 120 or 240 to power your saw.

Mike

[Sonny Edmonds]

..because I would easily go way over most heads here. :confused:

Suffices to say to look at it from a very simple point of view: (U.S.A.)
Single phase is 120 VAC, typically. A single sine wave.
Three Phase is typically 208, 240, 480 volts. Three sine waves, 120 degrees apart.
Nobody ever mentions 240 volt as being two phase. But it has an L1 and L2 designation. As does the primary connections at the transformer stepping down the utillities power.
You can get two single phase 120 VAC circuits from 240 VAC by having a neutral leg for either one to referance to, a grounded center-tap of the transformers secondary winding.
But when using 240 volts to your motor (keeping this rudimentary with the post), there is no neutral used by the motors windings. Only L1 and L2 directly from the transformer (and of course the circuit breakers for amperage control).
The ground is merely for personell protection so if a failure of the insulation occures, it will (hopefully) carry the current back to the panel and give the current a path besides you to ground.

So look at it from that rudimentary perspective of 240 volt power giving you "2 phase power" over 120 volts single phase power.
Then it is much easier to grasp why 240 volts is far better for your motor driving your equipment. (Be it a TS, BS, Shaper, Planer, Jointer, Etc.) :rolleyes:

[Mike Henderson]

..because I would easily go way over most heads here. :confused:

Suffices to say to look at it from a very simple point of view: (U.S.A.)
Single phase is 120 VAC, typically. A single sine wave.
Three Phase is typically 208, 240, 480 volts. Three sine waves, 120 degrees apart.
Nobody ever mentions 240 volt as being two phase. But it has an L1 and L2 designation. As does the primary connections at the transformer stepping down the utillities power.
You can get two single phase 120 VAC circuits from 240 VAC by having a neutral leg for either one to referance to, a grounded center-tap of the transformers secondary winding.
But when using 240 volts to your motor (keeping this rudimentary with the post), there is no neutral used by the motors windings. Only L1 and L2 directly from the transformer (and of course the circuit breakers for amperage control).
The ground is merely for personell protection so if a failure of the insulation occures, it will (hopefully) carry the current back to the panel and give the current a path besides you to ground.

So look at it from that rudimentary perspective of 240 volt power giving you "2 phase power" over 120 volts single phase power.
Then it is much easier to grasp why 240 volts is far better for your motor driving your equipment. (Be it a TS, BS, Shaper, Planer, Jointer, Etc.) :rolleyes:
That is ABSOLUTELY not correct. Both 120 and 240 (in a residential setting) are single phase power. The idea that you're getting two phases is a common misconception that people have, but it is completely wrong.

Mike

[Long ago, before three phase, there was a two phase power distribution system with the two phase 90* apart. But that system went away early in the 20th Century. See here (http://en.wikipedia.org/wiki/Two_phase)for more.]

[Sonny Edmonds]

That is ABSOLUTELY not correct. Both 120 and 240 (in a residential setting) are single phase power. The idea that you're getting two phases is a common misconception that people have, but it is completely wrong.


Mike

[Long ago, before three phase, there was a two phase power distribution system with the two phase 90* apart. But that system went away early in the 20th Century.]

240 volts is 180 degrees apart, one phase is positive while the other is negitive.
Single phase 120 volts has only a single sine wave.

[Mike Henderson]

240 volts is 180 degrees apart, one phase is positive while the other is negitive.
Single phase 120 volts has only a single sine wave.
Both 120 and 240 are supplied via two wires. It's impossible to have a poly phase system on two wires.

Check wikipedia for "two phase" and "three phase" and you'll see what they are and how they're different from single phase.

Mike

[Sonny Edmonds]

Here Mike, check out where I worked (http://en.wikipedia.org/wiki/Pacific_DC_Intertie)for 17.5 years of my 40+ year and counting career in the electrical utility and industrial electrical industry.
I've probably forgotten more about power and systems than you will ever know. But from the trenches, not a boardroom or, (shudder), wikopedia.
But hey, go ahead and be an expert.
(Definition of an expert: A Drip, under pressure.) :D

In order to repair it, first one must understand it. :rolleyes:
Do you know anything about Wye or Delta power systems? Phase rotation or phase shift?
And think about this: If what you purport to be true was true, then all those guys (and gals) using phase converters in their shops to run 3 phase machines from 240 volt (L1 and L2) to 3 phase (L1, L2, L3) converters... couldn't.
Hummm? ;) (I could continue, but it is a rather moot point)

[Mike Henderson]

Here Mike, check out where I worked (http://en.wikipedia.org/wiki/Pacific_DC_Intertie)for 17.5 years of my 40+ year and counting career in the electrical utility and industrial electrical industry.
I've probably forgotten more about power and systems than you will ever know. But from the trenches, not a boardroom or, (shudder), wikopedia.
But hey, go ahead and be an expert.
(Definition of an expert: A Drip, under pressure.) :D

In order to repair it, first one must understand it. :rolleyes:
Do you know anything about Wye or Delta power systems? Phase rotation or phase shift?
And think about this: If what you purport to be true was true, then all those guys (and gals) using phase converters in their shops to run 3 phase machines from 240 volt (L1 and L2) to 3 phase (L1, L2, L3) converters... couldn't.
Hummm? ;) (I could continue, but it is a rather moot point)
I'll stand my by knowledge of electrical systems. You can see more about my professional experience and qualifications here (http://members.cox.net/michael.henderson/).

However, I might point out that throwing "qualifications" back and forth is meaningless. If you cannot support your position with facts and reason, all of the "qualifications" in the world are useless.

Mike

[It is said that "Patriotism is the last refuge of a scoundrel". Right before patriotism is "qualifications".]

[Tom Veatch]

240 volts is 180 degrees apart, one phase is positive while the other is negitive....


Both 120 and 240 are supplied via two wires. It's impossible to have a poly phase system on two wires...


In an attempt to find some common ground here, can we agree that on the load side of the center tapped transformer residential 240v service can be mathmatically modeled as two 120v phases 180 degrees apart when L1 and L2 are referenced to ground or neutral?

Can we also agree that when only two wires are used to supply the load, the load cannot distinguish a phase relationship between the two leads in a 240v service any more than it can distinguish a phase relationship between the two leads in a 120v service.

If you feed a center tapped transformer with residential 120v service do you not then have the identical situation that you have with the 240v residential service. In this case, L1 and L2 referenced to the center tap can be modeled as two 60v phases 180 degrees apart while on the input side the model is of a single 120v phase.

Personally, I think that you are both correct within your own respective viewpoints and the difference is simply one of viewpoint.

[Chris Padilla]

Okay, the horse is dead.

Phase gets tossed around too much and has many meanings but I agree with Tom, you're both correct.