220v conversion question

[Tom Veatch]

At 240v there's slightly more power available to the motor because of less power loss in the circuit wiring due to the higher current at

120v. This is especially evident under high current conditions like startup when the motor is temporarily drawing several times it's normal current draw.

If the 120v circuit had half the resistance of the 240v circuit, there would likely be little or no difference in the behavior of the motor. But that's typically not the case. A 20 amp 120v circuit would typically be wired with 12ga wire. A 20 amp 240v circuit would also typically be wired with 12ga wire. Therefore, all other things being the same, the 120v and the 240v circuits would have roughly the same resistance. I'd expect the motor running on 240 under those conditions to have faster startup characteristics as well as being slightly harder to stall.

Tom Veatch Wichita, KS USA

[dpb]

[e-mail] wrote: ...

I'm not "telling" anybody to do anything--I'm simply pointing out it's been common practice until quite recently.

As for your experience, there has always been an external safety ground supplied on driers; whether one bothered to hook them up is another matter.

I stand by my contention the 3-wire split circuit existed in such numbers for so long that if it were truly a dangerous practice it would have taken far less time than it did for it to finally percolate its way to the radar screen.

I'll also reassert given my experience on Standards committees there is that aforementioned need to find _something_ to modify...they finally got to the bottom of the barrel where this became one thing they could find to change.

again, imo, ymmv, etc., etc., etc., ...

And, yes, Code now mandates 4-wire connections; others make your own determination, I'm not advocating you violate Code willy-nilly but the subject of a work light on a drill press...

[dpb]

Doug Miller wrote: ...

You misread what I was talking about--I was talking of the split volatage circuit...

...

Again, that "all" was specifically the "all" I was speaking of.

[Chris Friesen]

I hope that's supposed to be a joke.

Chris

[Chris Friesen]

You forgot the whole "divided by resistance" part. and Potential divided by resistance is current.

P=(V^2)/R = V*I

As Tom Veatch indicated, in the typical case when you rewire the motor for 240V you are also increasing the resistance so that the final power ends up the same.

Again...why would this be the case? The available electrical power of a motor drawing 6A at 240V is exactly the same as one drawing 12A at 120V.

Chris

[Chris Friesen]

Technically true. But on a properly designed circuit the NEC requires at most 5% voltage loss. Even assuming that we got perfect efficiency at 240V I defy you to notice a tablesaw coming up to speed 5% faster.

This is not a "wow, start up is almost instant" sort of difference.

Chris

[Nova]

Depending on the total wire resistance the voltage drop to the motor would vary. The power loss formula P = I² R shows that if you double the voltage you reduce the power loss by a factor of 4. On motors above 2 HP the difference between 120V and 240V is quite noticable.

[Doug Miller]

But you were responding to a question that did not involve dual-voltage circuits.

Sounds like we're on the same page, though, just neither of us completely clear on who was talking about what.

[Lew Hodgett]

The above is known as "Chris' Therom" which is false.

The total voltage drop across a motor consists of the voltage drop across the motor itself plus the voltage drop across the cable feeding the motor.

The voltage drop across the cable is as follows:

VC = I^2*R

For purposes of explanation, assume:

Case 1: (120V service) R = 1 Ohm I = 10 Amps

VC = 10^2*1 = 100 volts

Case 2: (240V service) R = 1 Ohm I = 5 Amps

VC = 5^2*1 = 25 volts

Increasing the supply voltage from 120V to 240V, all other things being equal, reduces the line losses by a factor of 4:1 (100/25) which results in a higher voltage being delivered to the motor.

All other things being equal , the higher the voltage, the more efficient the connected motor will be.

Lew

[Dan Coby]

No. The voltage drop in the cable is VC = I * R

The power lost in the cable is Pc = I^2 * R

No. The voltage drop is 10 * 1 = 10 volts. This means that each motor winding will see 120 - 10 = 110 volts.

No. VC = 5 * 1 = 5 volts. This means that each motor winding will see (240 - 5)/2 = 117.5 volts.

This means that the 240 volt motor will have 6.8% more available power due to the lower cable losses (assuming the same 10 amp current).

The differences are relatively small (6.8%).

A few more things to consider:

To get your 1 ohm resistance, you would need 314 feet of 12 awg wire. That is longer than the typical wiring run in home shops. A shorter run would also make the differences smaller.

The motor starting current may be several times the running current that makes the voltage drop in the wire during start up much worse. This will favor the 240 volt system with if the wire size is the same as the 120 volt system.

The above calculations also have a built in assumption that the 240 volt system was wired with the same gauge as the 120 volt. That may or may not be true. If the 240 volt system was wired with smaller wire then it may have worse voltage drops.

Dan

[Leon]

Ok, again, I am no electrical expert but in my coloring book world of looking at electricity, You have less resistance up to the motor using 240 vs 120. Each of the 2 wires carrying 120 volts is carrying 1/2 the load up to the motor than the single wire in a 120 volt application.

[Allen98]

iesen

5ma on the heart will kill you.

[Scott Lurndal]

There has not always been a grounded conductor on driers, only a grounding conductor. Unfortunately, in the olden days, the grounding conductor has been used as a grounded conductor as well for 120 volt loads, which can cause the frame (which is attached to the grounding conductor) to have a potential difference relative to earth. (if they didn't bother to hook up the "external safety ground" as you call it (NEC calls it the grounding conductor), the light in the dryer wouldn't work at all, so it's quite likely that they _did_ bother to hook them up in almost all cases).

Modern NEC requires both a grounding and a grounded conductor on 240v/120v split loads.

scott

[dpb]

Tom Veatch wrote: ...

...

Yes, it was you, Tom; that's the description I recalled, just couldn't recall who...

I'm willing to bet that's the same scenario OP has--a machine built before the change and documentation of same removed to avoid the issue from their end.

--

[dpb]

Not the way I read it--the whole thread was about split voltage circuits and the question was about when needed a neutral as well as ground (or at least that's what I thought (and still think) the question was)...

--

[dpb]

...

How large _are_ the 110 loads? To keep the one particular individual off my back note I'm _NOT_ telling you to do this but there still _is_ the unmentionable solution... :)

--

[Lew Hodgett]

"Duh, Brain Fart Dummy", he says to self.

Thank you for the correction.

Below is the corrected version.

The total voltage drop across a motor consists of the voltage drop across the motor itself plus the voltage drop across the cable feeding the motor.

The voltage drop across the cable is as follows:

VC = I*R

For purposes of explanation, assume:

Case 1: (120V service) R = 1 Ohm I = 10 Amps

VC = 10*1 = 10 volts

Case 2: (240V service) R = 1 Ohm I = 5 Amps

VC = 5*1 = 5 volts

Increasing the supply voltage from 120V to 240V, all other things being equal, reduces the line losses by a factor of 2:1 (10/5) which results in a higher voltage being delivered to the motor.

All other things being equal , the higher the voltage, the more efficient the connected motor will be.

Note:

The resistance value used is totally immaterial since mathematically, it cancels out in the calculations.

Absolutely.

Being able to handle inrush with out blinking is a great advantage.

I long ago standardized on #10AWG for motor conductors.

Simplifies the distribution and any increased costs go away if you buy

15-25 ft, molded cord sets and chop off the receptacle end, then wire to motor for 240V service.

Lew

[dpb]

Scott Lurndal wrote: ...

Read what I wrote instead of looking for an axe to grind and to try to show how shumart thou art... :(

I didn't write "grounded conductor" I wrote "external safety ground"--two different things but the latter solves a large portion of the problems (but they were often never connected or to an adequate ground in practice)...

--

[Doug Miller]

OK, maybe I got mixed up, but I thought you were responding to the guy who asked if 240V circuits needed a neutral, as a general question, not in reference to any specific equipment.

Anyway, we agree about the wiring requirements.

[krw]

The resistance of the wire is the same. ...unless you're wiring your house with 16AWG.

No, you're not. The house wiring resistance is the same. Any drop in the house wiring will shop up in the motor power (squared).

Wire resistance.