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Indiana | I am putting 3 phase power in to the farm for grain system. I plan to use low voltage and now utility company is asking me if I want 120/240 phase to phase or 120/120/208. I have not order any motors yet. So is there a advance one over the other? The way electrician explain it most motors can be wire either way. I may buy a used dyer and some motors. Would this make a differents on the choice I should make? I have done my own wiring in the past (single phase) but have not quite rapped my head a round 3 phase yet. The electrician I talk to suppose to be good but I am not so sure he know a lot more than I do. I am just trying to make the best decision in my situation. Any suggestion would be appreciated. Thank you! |
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| 3-phase 240v. |
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| For motor loads, higher phase to phase voltage is always better, however 4 wire 120/208/3 panels can be used for all your electrical needs and are not limited motors alone. U may already have single phase, so adding just 3 wire 3 phase is the best choice.....Large buildings use the 4 wire 208 the same way households use single phase..Their 3 phase 480 is used for liting, A/C and motor loads.
Edited by dave7745 7/15/2012 10:10
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Caraway, AR | Why not get 480v 3ph? John |
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| I think the high leg or wild leg(can't remember correct name today) is best for
farm single systems, but it is not allowed any more in Saskatchewan. I use it
as it gives my 240 single phase for welder, old motors, and house. The
problem is one leg of the three phase is 208 volts to ground not 120. Last
time I had an inspection the inspector hit the roof when he saw all the
"Ws" in the panel. Total my I had to change the system. I got him cool down
because of stove, welder and a couple of old motors before he left with a
warning that it should be changed. As for newer motors the don't care
whether 208 or 240.
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Agent Orange: Friendly fire that keeps on burning. | For me it would depend on how much 120V you plan to use. Compare that to how much 240V single phase and 3-phase loads. Most motors will work on 208V or 240V but 240V is better and they will run cooler. Here is a poor photo of my panel in the machine shed. Notice the three hot wires in the box: The two blacks are 120V to ground and the one marked with yellow is poorly regulated 208V (properly called the 'high leg'). Any of the three hot wires hooked to any other hot wire will be 240V. Use all three and you have 240V 3-phase. If you look down the left row of breakers, the first gang is 3-phase, the next single breaker is 120V, skip the next space because that's 208, and then the next breaker is another 120V, and the last is another 240V 3-phase. Going down the right row, we have 240V 3-phase, 240V 3-phase, 120V, 240V single phase, 240V single phase, 120V, and then 240V 3-phase.
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| I've got the bastard phase as well. We are adding a 600 amp service and keeping it 240v for all the welders and other shop items that may not like the 208v as much. |
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| 480 isn't more efficient in electricity use, just in copper wire use. Along the way it increases the cost of panels and switches, won't support 120 volt loads without an added transformer that draws power all the time. Arcs in 480 volt circuits tend to burn down the whole panel, while 240 volt arcs tend to burn a gap and quit. Some experienced electricians injured by 480 some time in the past simply refuse to work on 480 for any reason.
Gerald J. |
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Faunsdale, AL | When you're using mostly 3phase with a little single phase for a few lights, operating coils on starters, maybe an office AC or two etc, go with 240. It is usually supplied with an open delta transformer bank and the phase balance will depend on getting the transformers sized correctly. I believe most of the problems which are historically associated with these services are because they were originally set up for mostly single phase with a single or very limited 3 phase load. Phase voltage balance was not very good when the transformer bank was built to the minimum size.
120/208-Y 3 phase is good for commercial buildings with lots of 120 volt lighting and receptacle circuits to supply because these circuits can be applied to all 3 phases and keep the load balanced as opposed to the 120/240 3 phase where these loads can only be applied to 2 out of the 3 phases and cause voltage balance problems.
Running a 3 phase motor, if there is a voltage difference between any of the 3 phases, there is a resulting amperage imbalance about 3 times as great. So anything more than a small voltage difference results in derating of the motors. I know on our farm, a motor which is fully/over loaded has a shortened life compared to one which has a little room.
Motors sold in North America typically have a service factor rating on the tag above their rated full load amps. The motor should handle the load as long as the service factor amp rating is not exceeded, but life of the motor that operates full time in the service factor area is shortened somewhat. Operating on imbalanced voltages makes the motor operate hotter and decreases the safe amperage rating, I would recommend not operating in the service factor when on 120/240 3 phase because the voltages are not typically balanced as well as the 120/208Y services.
You can still have imbalance on the Y services from line voltage imbalance on the utility high voltage lines as well. When everyone comes home from work and turns on the AC, the washer and dryer, and the kitchen stove, sometimes the load isn't too well balanced and incoming voltage will go change more on one phase than the others. Usually have to put on a data logger to document this, but it's worth considering if you have a problem with motors lasting.
Of course, phase loss protection is a very good investment for any 3 phase motors. (loss of a phase is the worst voltage imbalance possible, so it results in rapid overheating of all but the most lightly loaded motors) Phase loss/reversal protection is available in some starters, or can be an add-on to perhaps shut down a whole system at once if a phase is lost. Modern motor starters with a properly adjusted overload relay offer some phase loss protection, but will not always save a motor that is overloaded or getting imbalanced voltage. Best to go with true phase loss/reversal detection in addition to the overload unless it is incorporated into the design like some are.
Edited by ccjersey 7/15/2012 11:55
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Agent Orange: Friendly fire that keeps on burning. | The open delta must be a regional thing. Here's my transformer setup that they pull 4-wire delta from. One of the transformers is center-tapped to ground to make the 120V legs. All the transformers are just hooked as a Delta but notice the blue tape on the left most wire. That's the high leg wire.
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Faunsdale, AL | From what I understand, the problem with a 3 transformer Y/Delta bank is that it will backfeed a dead line (with lower than normal voltage!) causing all sorts of havoc. The solution to this is that the fuses have to be sized correctly. The day the linemen don't have the right one on the truck and put in one too large is when this situation gets started. An old guy told me one time that if the fuses are the right size, when one phase goes down, it will blow and the transformer bank will then operate as an open Y/open Delta bank until someone notices the fuse hanging down.
If a line has a 3 phase breaker feeding it, so it goes off all at once, then no problem. Out here, typically there will be a 3 phase breaker at the sub, but out along the lines there will be breakers on individual phases so the whole thing doesn't trip off for one tree limb on the line. |
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Caraway, AR | What kind of imbalance (between phases) would there be on a grounded phase. (480) Other comments on that system welcomed. John |
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Agent Orange: Friendly fire that keeps on burning. | Loss of a phase can cause interesting things to happen to the voltages.
The line that feeds me kind of wanders along the river's edge for 15 miles or so. It was originally installed for irrigation from the river but has not been heavily used for that. Only a few residences powered from it and loosing a phase isn't all that unusual and will often go un-noticed for several hours or even a day or two. Since I only use it in the storage shed/summer shop it isn't a big deal and hasn't really caused any equipment malfunctions. Of course, the 3-phase motors don't run when a phase is lost. Depending on which phase is lost, the 120V lights may be slightly dimmer or lots dimmer. Compact fluorescents may not even light if the right phase is gone. Switching power supply in the shop computer keeps right on working but some instances will cause the CRT monitor to have a pretty small picture. The drill press with the VFD seems to work as long as power is coming down the wires -- regardless of which phase is missing. |
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Faunsdale, AL | If one phase of an ungrounded 480 delta is grounded, then nothing happens, it simply becomes a "corner grounded delta". Actually in some ways it's a better situation than opeating ungrounded. Voltage is stabiized and referenced to ground and phase to phase voltages remain the same. 3 phase equipment operates normally.
Ungrounded services can experience arcing faults which result in overvoltage damage to a lot of equipment all at the same time. I've seen a diagram of how the overvoltage comes about, but don't ask me to explain it! It's only on ungrounded service and with arcing faults. Ungrounded service has an advantage that a fault to ground (a "short") doesn't create a problem, everything operates as before except one phase voltage to ground will become 0. The problem comes when a second phase also has a short to ground! Then current will flow between the two shorts on the two different phases. These systems are usually only used where downtime is really expensive and where there are qualified electricians on staff to chase down any faults before they become a problem. |
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Caraway, AR | Yes, it is made that way. There is even a copper pipe put in the center lugs of the fuse box.........said they didn't want to take a chance on blowing a fuse on the grounded leg. John |
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| From the user engineering side, the open delta 3 phase service is the poorest way to supply three phase. Way too many power company engineers don't understand its limitations that were pointed out by Charles Proteus Steinmetz in his 1912 electrical engineering text book. He was chief electrical engineer at General Electric back then and also taught electrical engineering courses.
The first failure is figuring that the main transformer (240 volts with mid ground) is supplying 2/3 the 3 phase power and the wild leg transformer is supplying 1/3 the power so can be 1/2 the rating of the main transformer. The problem is that the phase angle of the current in the transformer for that power is about 60 degrees from the line current giving the transformer a very poor power factor. That can make for short transformer and motor life. Short life for the transformer because the volt amps are quite a bit larger than the three phase current would indicate, and hard on the motor loads because of impedance unbalance and load induced shifts in the phase angle of the wild leg. A three transformer bank improves the impedance and voltage balance and the phase angle. The windings in a motor are built for 120 degrees phase angle, that angle isn't accurate in the open delta. The wild leg transformer in the open delta needs to be the same size as the main transformer when the loads are primarily 3 phase motors.
A three phase motor typically draws 6 times running current for starting. Turns out in the motor analysis, that 6 times (or stalled impedance 1/6th the running impedance) also applies to unbalance, that the percentage current unbalance will be 6 times the voltage unbalance. So if the voltage unbalance is 4% the current unbalance will be 24%. That can be deadly to the motor and when examined shows up as one phase winding burned to a crisp. I've seen several with that condition. Some were really hard to get into, like a submersed 50 hp pump motor encased in concrete and stainless steel.
The source impedance of the open delta is guaranteed to be unbalanced so even if the motor drew balanced currents the voltages at the motor terminals would be unbalanced. Worse if the wild leg transformer is the typically undersized.
I had a case where that submersed well pump kept frying. Protection was two phase (another problem) and I moved the thermal elements closer to the motor rating (yet another problem) but the destruction continued until the wild leg transformer fried. The power company put in a bigger transformer because they had it on hand. It was still too small. After that the motor windings survived, but the unbalance makes the rotor vibrate as if it was a single phase motor and that was too intense for the water lubricated graphite bearings and regularly destroyed those bearing. When the pump is 50 hp and 350 feet down a well, pulling it to change the pump gets to be as costly as a new motor.
Many a motor controller has been built with only two current overload elements ignoring the possibility of losing a phase or having unbalanced phase impedances leading to excessive current in only one phase. If that's the phase without a heater, that motor is toast. Expensive toast from saving a few bucks for three phase thermal protection.
Because of nuisance trips, I've found most electric supply houses when asked to pick the thermal elements will look up the motor rated current and deliver heaters rated one or two steps higher than the motor current rating. That allows enough current unbalance to fry the motor without tripping the thermal overloads at the contactor.
When I was seeing the current unbalance in that well pump power feed, the local utility type tried to tell me seeing unbalance was because my clamp on meter was out of calibration. I soon informed him that it was showing the percent difference between phases and even if it was way out of calibration it would still show the same percent differences, and he shut up.
Seeing current unbalance can come from variations in the clamp on meter core and probably a slight influence of currents outside the core.
One other source of motor current unbalance I've seen can come from dirty connections on the transformer secondary or any secondary wiring connections, like the meter socket, the service drop connections, the panels, and the motor leads. Had one open delta start taking out motors and it proved to be a bad connection where aluminum wire was connected to the bronze lug on the transformer up on the pole. The coop fixed it with a copper pigtail crimp spliced to the aluminum service drop cable. And it gave no more problems.
I suppose the delta winding could induce voltage in an open Y primary but I don't think that is a serious problem because with one primary supply line open, the transformer bank sees only single phase power and so the delta connected voltages cancel at the open leg. I'll have to think about that and draw some vector diagrams.
Most major and minor substation transformers are delta- Y connected because the delta connection shorts out the third harmonic currents that come from transformer and motor iron cores and keeps the primary current less distorted by those third harmonic currents. And these days computer and solid state light ballast loads contribute much of the harmonic load currents that the delta winding keeps from propagating back to the power source.
Gerald J. |
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Warburg, AB | Anyone have any books or websites they recommend for getting up to date on this stuff? I get the concept of 3 phase, obviously understand single phase, but looking to get up to speed on stuff like the delta configurations, etc. I understand how you come up with 208V, etc. but wanting to get a little deeper into it.
Bruce |
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Princeton,IN | Here's one option. Not sure how much you will understand, but it will show you how to build all the powerbanks you'll ever need.
http://www.etgiftstore.com/items/all-products/gifts-for-electrical-... |
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Faunsdale, AL | Probably they know a lot more about it than me, but I don't agree with not having the grounded phase fused. Now it it were truely a neutral (a tap in the center of one delta winding which is grounded, like the 120/240 3 phase 4 wire) and was used in Line-Neutral connected single phase circuits, then it should not be fused. That is not the situation with a corner grounded delta. If it is overloaded or shorts to another conductor, it needs to be able to blow a fuse. The ground connection is a reference point, not that current is going to flow into/through the ground. It flows through the wires from the source transformer not the ground. |
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Which electric power should I use?
