Why 208??

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Re: Why 208??

This is why I used the example of a capacitor on a single phase circuit to demonstrate RMS values in single vs. 3 phase power circuits.
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With all due respect, I must continue to differ.

A capacitor connected in a single phase AC circuit does nothing to raise the RMS value.

The RMS value of single phase AC is the Instantaneous Maximum value multiplied by .707 with or without the capacitor.

what it does is expand the time that the near-peak voltage is present at the load. As the sine wave decays, the capacitor discharges thereby keeping the voltage at a relative higher level-
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What you have described is the effect that a filter capacitor has on the DC output of a rectifier circuit.

Ed

Rect7.gif
 
Re: Why 208??

Charlie, again I totally agree. As in the service stations the neutral is taken to the service by the utility and grounded per the NEC. It is the feeders to the sub panels where the neutral is only taken to the single-phase panel and not the three-phase panel. In my much younger years when working for DWP in LA, we would serve buildings with more than one service. One was single-phase the other three-phase. I think those days are gone and most utilities, at least in my area of the country only serve a building with a single service.
 
Re: Why 208??

Ed,
Take the rectifier out of your drawing and apply capacitance to an AC only circuit. The effect will be voltage leading current (just as it does in your diagram)- the stored current discharged goes back into the load as voltage. This stored energy is having an effect on the RMS voltage, raising it.
Your rule of thumb -.707 only applies to circuits of 60 cycle purely sinusoidal voltage and current. Reactive equipment (capacitors and inductors) changes the voltage and currrent RMS value because it is the mean value that is being manipulated by these devices.

Do we need a new string to carry on this stream? We are way over "Why 208?" question. I sure hope the fella got his answer!
later
 
Re: Why 208??

I got my answer.....I think. 120/208 (3 ph) will balance the system better than 120/240 (3 ph). I'm still a little confused over the 'go ahead and feed 208 to a 240 motor load though. The newer machines are 208-240/460. That tells me the motor is saying "I don't care, give me 208 or 240 (tapped low voltage) But the older machines don't give the 208-240....they are only stamped 230/460...which tells me NO 208, I'll roast at that low of a voltage.

By the way, I'm just a friend of the guy starting the shop who knows a little but you guys are like....WOW....I think I might have accidentally surfed into an E.E. site.
 
Re: Why 208??

I don't agree that you will roast a motor that says 240 if you run it on 208. When I worked for a control panel shop, we had the manufacturer re-nameplate motors all the time. We would buy standard 240/480 motors and they would re-nameplate them for us (of course they also changed the HP rating on the nameplate).
 
Re: Why 208??

My understand of three phase is slight at best, so bear with me. With that in mind, here are my assumptions based on what I think I know:

A delta with the midpoint of one phase winding grounded has a highleg on the B phase of the transformer, measured from B to ground. Measuring voltage from any two ungrounded conductors will show 240 volts, but the voltage between B and neutral is higher, the high leg. This high leg voltage is 208 Volts?

Can a 2-wire 208 appliance that was designed to operate on a 120/208Y system use this power, even though this voltage is line-neutral instead of line-line?
 
Re: Why 208??

25watter,

At abebooks.com you might buy an old Electrical Engineering text for cheap (new texts are more than $100 now). Basic Electrical Engineering is a well-worn book on my shelf, even though I only supervise electricians and help with their designs. The three-phase discussions and exercises are interesting. For example in a reply above, the constant 1.73 is noted, and this is an approximation of the exact number which is the square root of 3, so a 480 volt industrial 3-phase system can be wired to include 3 single phase circuits (that we try to balance the loads on) with a voltage of 480 divided by the square root of 3 (277 volts). Since 120 volt single phase is needed so much, but with 3-phase for some motors also needed (3-phase motors are elegant inventions, and you can get into calculations of non-slip RPM using the numbers of wound poles, etc., more fun exercises in texts). So with 120 volt single phase needed and 3 phase also needed, then 120 volts times the square root of three is 208 volts for your three phase service that will also have 3 120-volt circuits that should also be balanced. I'm rambling, and a text would explain this much better. Remember to have a qualified master electrician do your wiring, especially if you wind up with a service like 480/277 and a dry transformer for your single phase 120/240 needs. I've accidentally felt 240 and don't want to ever do it again, and my master electrician couldn't believe he touched a 480, which really shook him up for a few days. And I understand that 277 can be especially nasty. This forum shows that good electrical work and planning is done with a team of experts, etc. After you read the book, you can start a discussion on power factors, inductive loads, etc., and you and I will get even more confused (but fascinated).
 
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