Three phase power

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ronaldrc

Senior Member
Location
Tennessee
Re: Three phase power

This is going no where like I said I would agree to disagree.

CS whats your point? :)

[ March 27, 2004, 12:29 AM: Message edited by: ronaldrc ]
 

cs409

Senior Member
Re: Three phase power

yep,,,its going in a large circle...... i have no point..but as good as this forum is,,,the use of UGLY'S can help solve/understand a lot of problems !!! if any of the people on this site do not have one,,,,invest the 10 or so bucks to buy one,,,heck, Big Orange store has them now....
 

cs409

Senior Member
Re: Three phase power

another good book to invest in......AMERICAN ELECTRICIANS' HANDBOOK......like mentioned on this forum,,,,sometimes it takes some longer to get the answer,,,but if u reread,,,it will come to u.....if not,,,then u have the good people on this forum.....thats the point :cool:
 

ronaldrc

Senior Member
Location
Tennessee
Re: Three phase power

John I have no earthely idea I no, I'm ugly but I've never posted my picture here so he or they could not no that.So I no its nothing personal.

Ronald :)
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
Re: Three phase power

Hey Ronald and John, CS could have saved his $10.00 and just gone here :D

Roger
 

cs409

Senior Member
Re: Three phase power

now Roger, i did not say I spent 10 bucks.....i used my 10bucks to buy burger/ff/drink and 3bucks of gas for the day and used someone elses. :D

online uglys is good,,,
 

Ed MacLaren

Senior Member
Re: Three phase power

The UGLYS book is great if all you want to do is look up a formula, but it doesn't do much to help those who want to learn basic principles.

Another way to calculate three phase power, if you have access to the values of voltage and current for each phase of the load, is to calculate the power in each phase and add them.

Here are two examples of how a 4.32 kilowatt, 208 volt, 12 amp, 3 phase heater could be constructed.
One has the three elements wye connected, and the other one is delta connected.

In each case, the power per phase is (approximately) 1440 watts.
1440 X 3 = 4320 watts.

Ed

 

ronaldrc

Senior Member
Location
Tennessee
Re: Three phase power

Thanks Ed

I really enjoy your easy to understand graphics and teaching.I'm glad you applied that with a resistive load instead of using a unknown Power factor. :)
 

leeschen

Member
Re: Three phase power

I'm a new forum member. My background is in electronic equipment and computer engineering. I have worked some with power wiring and motor control circuitry (DC motors run from 3-phase and 1-phase using SCR's fed by speed control circuitry). I have even accomplished my own home and shop wiring, but 1-phase only.

I'm not an expert on 3-phase though I have worked with it a bit. A question has come up on whether it is allowable to directly connect a 240v 1-phase motor accross 2 phases of a 3-phase supply. Does it matter what the 3-phase configuration is (delta, wye, etc)?

Might it be allowable in a large plant where the phase-phase load will balance over time, but not in a small shop where only one motor will run at a time?

Would it be allowable/preferable to use a Scott transformer to create 2 separate 1-phase circuits?

I hope the assembled wizards here can provide me with a proper answer.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Re: Three phase power

Originally posted by leeschen: A question has come up on whether it is allowable to directly connect a 240v 1-phase motor across 2 phases of a 3-phase supply. Does it matter what the 3-phase configuration is (delta, wye, etc)?
It does matter what the 3-phase configuration, and it is not so much an issue of ?allowable? as it is an issue of ?advisable.? The actual answer for your actual installation will most likely have to come from the manufacturer of the 1-phase 240 volt motor.

Most of the transformers that I have seen in industrial and large commercial facilities, the ones that provide power to the 120 volt equipment, have a 480 volt Delta primary and a 120/208 volt WYE secondary. That means that the voltage between any phase and neutral is 120 volts, and the voltage between any two phases is 208 volts. Yes, you can legally connect some 1-phase 240 volt motors across two phases of this type of power source. But you need to understand that the motor will not be getting 240 volts. So if the manufacturer has rated the motor to operate with either a 240 volt or 208 volt supply, then there is not problem. If the motor is only rated for 240 volts, then the motor will draw excess current, and will likely fail.

As to the balancing of loads, that is a design issue, not a code issue. Design issues are not addressed in the NEC.
 

bennie

Esteemed Member
Re: Three phase power

Lee: Welcome, from a former Grants Pass resident.

It is not a good idea to operate a 240 volt motor on 208 volts. Get a motor with a 200 volt nameplate rating.
 

leeschen

Member
Re: Three phase power

Hi Bennie,

Pleased to meet you. When did you live in GP and where do you live now?

I've been in CJ since Feb, 1991. I feel like an Oregon Native now. I sure couldn't imagine moving back to the BIG CITY (SF Bay area).
 

jschultz

Member
Re: Three phase power

Why couldn't the service be a 240v 3 phase?

Anyways, it is a design issue. If you have multiple motors you would want to put them on different phases to balance out the load. On 240V 3 phase systems you have to be very careful when calculating the service size if you have 120v loads. Because you can't put 120V loads on the high leg phase which makes the load not balanced across all three phases. So you end up calculating the load on each phase as a 120V load. Which ever phase, usually A or C has the highest load, ends up determining the size of the service. One way to get around this is to provide a 240-208/120 three phase transformer for the 120v loads.
 
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