Hardwiring 230VAC Compressor Question

Most new areas are seeing three phase power distribution. Single phase service is provided by tapping off one phase and neutral. The result is

208V.

Ted

Reply to
Ted Edwards
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On Sat, 06 Dec 2003 20:51:50 GMT, Ted Edwards put forth the notion that...

One phase and neutral gives you 120 volts.

Reply to
Checkmate

On 6 Dec 2003 11:12:08 -0800, Mike put forth the notion that...

I've been a licensed electrical contractor for 29 years... does that count?

Reply to
Checkmate

Your right. Sorry. Neutral to any phase is 120. As a result, from one phase to another is 208 volts.

Ted

Reply to
Ted Edwards

Nope. Considering some of the work I've seen that was done by "licensed electrical contractors", that alone doesn't mean much.

Ted

Reply to
Ted Edwards

On Sun, 07 Dec 2003 05:57:31 GMT, Ted Edwards put forth the notion that...

From my observations, that's usually because the guy with the license isn't actually doing the work... he's hiring a bunch of twenty-something year old's who think they're electricians, and he's not supervising the job properly. Sometimes incompetence is actually profitable. A good example is the electrical repair business. The guys that do troubleshooting and repair usually charge by the hour. Who makes more money, the guy who knows what he's doing and can find a problem in under half an hour, or the guy who hasn't got a clue, and spends the whole day pulling every outlet in the building apart until he stumbles upon the problem?

I know what you're saying... the advice you get on Usenet is often worth exactly what you paid for it. On the other hand, there are people out there who know what they're talking about and don't mind helping people. If you get enough answers from enough people, you can get a pretty good idea as to who knows what they're talking about and who's full of shit.

Reply to
Checkmate

On Wed, 3 Dec 2003 23:40:33 -0500, ATP wrote (in message ):

I spoke by phone with the electrical contractor who did the install and asked him exactly what kind of service I now have. He responded that I have two lines at 115VAC with a common nuetral. It is single phase 208VAC service. He mentioned that for 230VAC motor, I should do the install for a 220VAC line, with double breaker. I asked his opinion about running a 230 VAC 1-phase motor on 208 service...he said it wouldn't be a problem, at the very worst the motor may run a little slow.

-N.

Reply to
-n.

On Sat, 6 Dec 2003 14:37:52 -0500, Checkmate wrote (in message ):

Checkmate, can you elaborate on this boost transformer setup? Also, any idea what the equipment would cost (new or surplus) to gt it done? Thanks,

-N.

Reply to
-n.

On Sat, 6 Dec 2003 10:24:57 -0500, Rick wrote (in message ):

That is not news I want to hear. This goes for running 230VAC single phase on 208VAC single phase service? I had always thought there was some lattitude in voltage ratings of motors. Can anyone elaborate on why this would cause overheatng/shorting? Trying to learn something along the way.

-N.

Reply to
-n.

On Sat, 6 Dec 2003 15:12:08 -0500, Mike wrote (in message ):

Mike, I appreciate your admonition: with enough input I think the resolution(s) rise to the top of the heap eventually. It may take a little debate to get there, but often a few heads are better than one, and there are many knowledgeble folks on these newsgroups. In fact, a few years ago when I was first exploring electronics and wanted to design a digitalyl controlled circuit, an electrical engineer emailed me and it started a very intense email education...the man is brilliant and knew his stuff....one of the greatest learning experiences of my life. What you say can be true, but there are also some smart, informed , and experienced folks on here. I'll wait till I get the doubts ironed out and get consensus, then go forward. Cheers,

-N.

Reply to
-n.

On Wed, 3 Dec 2003 11:27:06 -0500, Anthony wrote (in message ):

To reiterate: I am clear on this above wiring scheme except for one detail. The panel I am starting from is a subpanel and it does not contain any grounding bar or grounding terminal. In this case, does my ground wire need to be bare, or will green insulated work just as well? I pull red, black, and green insulated ground (all 10awg). Red goes to one pole of 30A dual breaker, black goes to other pole of breaker, for ground I drill a hole into the subpanel box itself then scrape some paint away and screw in a copper grounding lug. I run these three wires through BX cable to a 30AMP rated disconnect box, making certain to affix the ground to the disconnect box via a screw and to acheive good continuity. THese 3 wires exit the disconnect box and are encased in liquid tite flexible metal conduit which goes to the compressor motor/switch. Again, black and red connect to respective lugs on the motor and the ground goes to ground lug on motor.

[If I find out a boost transformer is absolutely needed to boost my 208VAC service to run the 230VAC motor, I will research that and add it t the system]. If I wish to install a 120VAC recep box in the same room as the compressor, it must be on a seperate ciruit with a dedicated breaker, NOT off the hot wires used in the compressor circuit.

The only other modification I can forsee doing to this compressor circuit would be to install a 230VAC receptacle in the wall and a mating 230VAC plug/cordset [10awg, 3 wire: red, black, green] on the compressor .... and run that downstream from the disconnect box as an option instead of the hardwired liquid tite flexible metal conduit.

What do you think guys, does that sound like it confirms to code? Thanks.

-N.

Reply to
-n.

In article , -n. wrote: [snip]

This is extremely doubtful. 208V is THREE-phase, not single-phase. You might have two legs of a three-phase service. What voltage do you measure between the two 115V legs?

You need to get a _competent_ electrician, ASAP. This guy doesn't have a clue. Unless the motor was designed to be run on 208 3-phase, you'll burn it up in a hurry.

-- Doug Miller (alphageek at milmac dot com)

How come we choose from just two people to run for president and 50 for Miss America?

Reply to
Doug Miller

On Thu, 11 Dec 2003 08:46:14 GMT, -n. put forth the notion that...

Certainly. You take a 240 to 24 volt transformer, and wire the primary and secondary windings in series by connecting one end of the primary winding to one end of the secondary winding. Now, you have what's known as an autotransformer. Next, you connect your 208 volt input to the opposite end of the primary winding, and the common ends of the primary and secondary windings. Finally, you connect your output to the same end of the primary that you connected your input to, plus far end of the secondary winding, and you'll get 230 volts out. It's important that you connect the windings so that they're in phase. By that, I mean you don't want the windings in the primary to be connected to the windings in the secondary in such a way that one coil is wound clockwise, and the other is wound counterclockwise. Acme transformer has some excellent information on this, as well as the correct wiring diagrams for each of their transformers. Here's the link:

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Reply to
Checkmate

On Thu, 11 Dec 2003 08:41:49 GMT, -n. put forth the notion that...

What he should have said, is at the worst it may run a little HOT. AC induction motors turn at a certain speed because of the frequency of the sine wave, not the amount of voltage. If you give the motor less voltage than it was designed for, it won't slow down initially, it'll draw more current, and current equals heat. You really have to lower the voltage significantly to see a drop in speed, but you don't have to lower it a whole lot to see an increase in amperage and heat buildup. Some motors are designed to run on anything between 208 and 230, but not all of them. If the manufacturer says the motor is okay to run at 208, that's fine, but don't go by what the contractor is telling you.

Reply to
Checkmate

On Thu, 11 Dec 2003 09:22:04 GMT, -n. put forth the notion that...

You've got the right idea, and that's how I'd do it. Whether it conforms to code is another issue. Offhand, I'd say yes, but every jurisdiction has their own little quirks... what kind of conduit they want you to use, etc. I've seen some weird demands from even weirder inspectors. If you're going to get it permitted, it's best to run your plans by an inspector first and get his blessings, so to speak.

Reply to
Checkmate

There is some latitude. A nominal 230 motor will typically run fine on voltages between 220 and 240. But 208 is too low unless the motor is specially designed to handle it. Horsepower is voltage times current divided by 746. For a given horsepower demand, a lower supply voltage means the motor must draw more current to provide rated power.

Heat build up in the motor is a function of current squared times winding resistance. So the more current the motor has to draw to meet the load power demand, the hotter the motor will run. Notice that heat is a function of the square of current, so it doesn't take a whole lot of excess current to get the motor very hot.

Motors are designed for a specific maximum temperature. Exceeding that temperature will greatly shorten the life of the motor, ie you'll let out all the magic smoke if you persist in running a motor at its rated output power with lower than rated supply voltage.

Note that you could probably get away with running a 230 volt motor at lower than design voltage if you don't draw anything approaching its rated power output from it. In other words, if the motor is oversize for the load power demand, it'll tolerate running on a lower voltage. That's how motors rated to run on 208 to 240 are designed, ie they're bigger (larger windings and or more cooling air) than their nameplate hp would otherwise require them to be. That costs money, so equipment manufacturers typically don't use such motors unless you pay extra for them.

Gary

Reply to
Gary Coffman

Absolutely true.

If you want to operate a 230V motor on a 208V supply, you must add a buck-boost transformer to the system.

By definition, a motor must be able to operate at +/-10% of nameplate voltage.

For a 230V motor, that translates to 207V minimum which sounds like it might work on a 208V supply; however, the utility supply voltage is allowed to operate at +/-10% which translates into 187V minimum.

Therein lies the rub. You can't get there from here.

Add a buck-boost x'fmr.

They are small and relatively low cost.

HTH

Reply to
Lew Hodgett

He said 210 in a previous post.

His "electrician" is definitely clueless. There are plenty of 208/230 single phase motors however. Otherwise the vast number of commercial and institutional customers with 208Y service would not be able to use them.

Reply to
ATP

Typically motors that will operate at either 208V or 240V are more expensive than the equivalent motor wound to operate at 230V.

Over the years, have sold a lot of buck-boost transformers to solve this problem.

Get one, install it, then get a beer, and enjoy life.

Reply to
Lew Hodgett

On Thu, 11 Dec 2003 11:58:40 -0500, Checkmate wrote (in message ):

To reiterate, the supply voltage I measured is 209VAC between the two hots. The motor states 230VAC single phase, 1.25 service factor, and is thermally proteted with a reset button. Again, the electrical contractor who owns the bizz that did the install states I have single phase service @ 208VAC....although many of you are claiming that this latter data must be incorrect. I will need to bug that contractor again and discuss the doubts expressed here. I haven't spoken with the motor manufacturer to check if the motor can run on 208-230VAC, although it most certainly does not list 208-230VAC on the nameplate. Is there a way for me to check and confirm the phase of the service I have (1 or 3 phase) with only a voltage/amp meter? Lastly, going with only the data I now have, many of you have expressed concern that the motor would run HOT and possibly burn out, short, or experience a reduced servie life if it were connected as is without a boost transformer. Hypothetically, wouldn't the thermal protection/reset circuit in the motor protect it before damage was done by shutting off the motor in the event that it did experience an increase in temp?

Thanks again.

Reply to
-n.

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