3-Phase voltage loss

Discussion in 'General Electronics Chat' started by dalebob65, Oct 13, 2010.

  1. dalebob65

    Thread Starter New Member

    Oct 13, 2010
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    First the history: The industrial plant I'm in has recently installed about 30 flourescent lights to replace Mercury vapor lights. In the process a problem was noted that some were dimming down intermittantly. In checking voltages on our 277/480 volt Wye supply, one phase to ground would drop from 240 volts to about 198 volts, at which point the lights would dim. The other 2 phases, when checked to ground would increase in voltage to 300 - 305. This would happen regardless of plant power demands, heavy or light.
    To resolve the voltage drop issue, we found a bad fuse on some infrared heaters that we have that use 480 volt single phase through an SCR pack to vary the heat. Now, our voltages from phase to phase are 475-485 volts, and from phase to ground, we have 268, 269, and 252. The heater fuse for the SCR pack was 40 amps. Our incoming supply to the plant is a 1200 Amp buss system, from a Wye transformer.
    Now the question: What could an SCR pack do to drop buss voltage on one phase to 198 volts, and the other phases to increase? With 20+ years in the business, I've never seen this.
    If anyone has any thoughts, I would appreciate it.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Sounds like the loading is mismatched. When it is doing this have you measured the currents through each leg? It can be done with a clamp meter.

    This is not a physics issue through, it probably should have been in the electronics chat forum. A moderator might move it shortly.
     
  3. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Something's still wrong amid your neutral distribution unless you're pushing your source to the max. Another thing that can cause this (and go unnoticed) is a 480: 208Y/120 or 240 transformer that has a fuse blown on one of the three phase input legs.

    I may not get anywhere near the 4,000 and 2,500 amp sources I have coming in but mine is rock solid everywhere except for one time in a corner area of a warehouse, there was a blown fuse on one leg of a stepdown that slightly upset the balance.

    First thing of course is to look into anything that actually involves the neutral, especially that all the connections are very, very tight.
     
  4. DonQ

    Active Member

    May 6, 2009
    320
    11
    The center lead may have been sized significantly smaller than the other three by assuming balanced loads. For balanced loads, the current in this lead would ideally be zero. If the loads are unbalanced, the voltage on the center lead would move toward the voltage on the highly loaded lead, and away from the other two leads. This would give a low voltage on that lead, and a higher voltage on the other two. If you do the 3-phase math on the voltages you have, it would probably work out right.
     
  5. dalebob65

    Thread Starter New Member

    Oct 13, 2010
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    Thanks guys! The information gives direction and explanation. Due to scheduling, it may take some time to do the checking we need to, but I'll let you know how it turns out.
     
  6. zgozvrm

    Member

    Oct 24, 2009
    115
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    Good point! this would result in an "Open Delta" configuration and your available power would be reduced to 86.7% of full power.
     
  7. timrobbins

    Active Member

    Aug 29, 2009
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    The strange thing in your description dalebob is that you describe an scr heater powered from one phase to another - ie. no neutral connection, and not balanced across the three phases - and that it had a bad 'fuse'. I don't understand how a bad fuse could increase or change the current in the heater such that it would influence the distribution system. Perhaps it changed operation from full-cycle control of the heater elements to chopped phase operation.

    Tim
     
  8. PackratKing

    Well-Known Member

    Jul 13, 2008
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    215
    Is it within electrical codes to do this ?

    Three-phase is intended to run items that uses all 3 branches equally.

    Other motor circuits in the plant, depend on all three being equal, and it follows that when any motor on the same threephase supply starts, it will dim any lights that are hijacking its feed..........

    Same token, if one fuse on a 3 system goes, and a motor has to start with one missing branch, either delta or wye, it will cause it to burn up in relatively short order.
     
  9. timrobbins

    Active Member

    Aug 29, 2009
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    It is not uncommon for heater loads to be across just 2 phases (line-to-line) - no issue with regulations per se.
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    or even small motors. Buy a 480V condensor unit for a split system AC and the fan will just be a simple 480V motor across two of the phases.
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
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    I worked for a large aircraft manufacturer that had many 3 phase motors driving machine tools. We once lost a fuse on one of the power busses and the area ran fine until all the machines shut down for the weekend. One row of overhead lighting was slightly more dim than the others. Lighting was 277 single phase from line to neutral. What we found was that all the running motors were actually generating the missing phase. Enough power was being fed from the multiple motors to actually permit the start of other motors. The weekend shutdown created the real problem since the phase with the blown fuse went to zero volts.
     
  12. zgozvrm

    Member

    Oct 24, 2009
    115
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    This is called "single phasing."

    Motors can run on only 2 phases, but getting them started is difficult. If a phase drops out while it is running, you have an open delta situation. The 3rd phase is "generated" by the other 2 phases, not by the motor.

    Had the motor been running at, or near capacity, it would probably have either burnt up, or blown an additional fuse.

    Google: "open delta" and "single phasing"
     
  13. BillB3857

    Senior Member

    Feb 28, 2009
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    When running a delta, I would agree. Our plant was exclusively Y configuration. 440V line to line, 277V line to neutral. Lights ran off the 277V. If you take a three phase motor which is running and disconnect one phase (pull one of the fuses) and measure voltage between the three motor leads, you will see one pair at full line voltage and the other two pairs at slightly lower voltage or measuring to ground, 277V on two legs and slightly less than 277 on the one with the fuse out.. This voltage is actually what would be called the Back EMF if the line were still under power with a good fuse.

    As to your other point about the motor running at near full load, if you only have one motor on the line, I would agree that a good size load would cause major problems. When you have close to a hundred running and a fair portion of them being lightly loaded, the "generator effect" is quite significant. Only when most of them shut off do you run into really big problems.

    Also check out http://www.youtube.com/watch?v=17zTzpvh3rw where a shop uses a 5 HP 3 phase motor with a timed capacitor start as a rotary converter. That 5 hp motor, after starting with the caps, generates the 3rd phase to run a 3 phase lathe motor. All kinds of info on rotary phase converters is available on the internet.
     
    Last edited: Oct 31, 2010
  14. zgozvrm

    Member

    Oct 24, 2009
    115
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    Not exactly. Back EMF is the current produced by a collapsing magnetic field and is why you'll see diodes across the coils of relays that are powered by solid state outputs; they prevent the back EMF from damaging the solid state device when the output is turned off and the relay is de-energized.

    The fact that the line-to-line voltage appears between all three phases (A to B, B to C, and C to A) when one line is dead has to do with the vector sum of the remaining 2 phases (being 120 degrees out of phase). This comes at a reduction of available power though ... only 57.7% of the power delivered when all 3 phases are being supplied.
     
  15. BillB3857

    Senior Member

    Feb 28, 2009
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    If that were the case, the single phase motor wouldn't have to be running at all. Back EMF in motor terminology, is the self generated voltage that tends to oppose the incoming voltage, therefore reducing the overall current for a given load.

    http://en.wikipedia.org/wiki/Counter-electromotive_force

    For motors that contain permanent magnets, (brushed and brushless) a term Ke which represents velocity constant, typically represents the voltage required to spin the motor at 1000 RPM. Likewise, if the motor is back-fed (spin the motor from an external mechanical source with no power applied to the motor), it will generate the Ke voltage, or very close to it, when spinning at 1000 rpm.

    Did you look up the theory on rotary phase converters? A lot of info out there. In my original example, (post 11 in this thread) there were a lot of motors running with each one acting as a rotary phase converter contributing power to the line with the blown bus duct fuse..
     
    Last edited: Oct 31, 2010
  16. zgozvrm

    Member

    Oct 24, 2009
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    Although you are correct, regeneration is the more appropriate term.

    The point remains that the 3rd, or missing phase, of a 3 phase power source isn't "generated" by a motor (or motors) connected to it. In fact, with no load connected at all, you will still be able to measure the full line-to-line voltage. As I stated before, this is due to the phase angle between the remaining 2 phases.
     
  17. BillB3857

    Senior Member

    Feb 28, 2009
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    If the motor were replaced with heating elements, would your results be the same?
     
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