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Thread: 3 phase voltage question

  1. #1
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    3 phase voltage question

    had to change out a 5hp blower motor in a 2 year old rtu this past week. one year warranty on parts so, needless to say, customer not happy shelling out the cash to buy a new one of those. feeling his pain, I went through the blower circuit pretty thoroughly looking for any week links that may have caused us to lose one if the windings. the only thing that really jumped out at me was varying voltage across the line legs. I don't remember the exact voltages, but on 208v service I had something like L1-L2 212v, L1-L3 214v, and L2-L3 215v. amp draw across the windings also varied a few tenths of an amp in the new motor.

    this is the first time I've lost such a young 3ph motor. in the past I feel like the voltage across each leg on other three phase service that I've checked voltage on have been pretty much identical. I never really had a reason to think about it before now. it has me wondering if this voltage difference, and subsequent current difference, would cause the windings to fire slightly out of phase. I've been trying to research this as much as I can on the web but can't seem to put all the info together to answer my own question. it also seems that, depending on the winding configuration, if the windings fire out of phase they could create a short. any grounds to this? I'm trying to take thus opportunity to learn more about 3ph power and motors while trying to protect my customer from another costly failure.

    thanks! in advance.

    Sent from my XT901 using Tapatalk 4

  2. #2
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    Sell him a phase monitor to protect his new investment.

  3. #3
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    The voltages are closer than 2% and should be OK. The phase loss protector would be a good idea to prevent single phase damage, however.
    It is possible that there are other problems with the line voltage/current, but in a non industrial application that is unlikly and it could easily be one of those things that happen occasionally.
    I assume a winding grounded or shorted phase to phase and not a bearing failure. Voltage spikes are possible from the incoming power. I also assume that this is not a variable speed application where the motor would have to be inverter duty.

  4. #4
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    yeah, it's a single speed application.

    Sent from my XT901 using Tapatalk 4

  5. #5
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    Look up the ICM 450 control. It will monitor the front and backside of the contactor. You can adjust your voltage range as well.

  6. #6
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    I am familiar with the product. we use them on all our large commercial 3ph equipment. not usually on smaller stuff like this but it is a good idea given the circumstances and I will certainly run it by my customer. I was kinda hoping for a more technical answer as to whether the conditions described above are harmful to the motor or not and why or why not. my electrical theory knowledge is admittedly weak.

    Sent from my XT901 using Tapatalk 4

  7. #7
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    Do a search in the electrical forum. I know most motors will allow for a certain percentage of voltage variations. Also if the breaker is in a panel with single phase breakers, you could be getting a imbalance there. I like seeing all 3 phase breakers in the same panel. Much better balancing of loads.

  8. #8
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    In any three phase electrical system that also serves single phase a voltage imbalance between phases can lead to a motor failure.

    Even though the voltage imbalance at the time you test it is bellow 2%
    and you have a premature motor failure it is a good procedure to check the panel, other items may not be utilizing voltage at the time of test.

  9. Likes VTP99 liked this post.
  10. #9
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    Thread Starter
    when you say check the panel you mean run other loads and see what kind of voltage drop you get on the load in question?

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  11. #10
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    Check amps at the panel to see if the other loads are energized?

  12. #11
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    If you use a clamp on ammeter to check each phase you can compare the phase currents.
    even more telling is a little trick I learned. You can clamp the meter over all three phases and the meter should read ZERO indicating phase balance. If all three phases in the clamp read a current this is the total imbalance.
    Measuring the current on each phase and the voltage for each phase you can determine the reason for the imbalance. If the drop on a phase is higher across the breaker is higher it might indicate a a bad breaker or a drop across any device could indicatge a problem. The incoming power might also not be balanced. This is not that a rare thing.
    When I wonder why a motor failed I do an autopsy. Usually you can get an idea as to what happened. Did the bearings fail, a phase short to ground?
    Sometimes the power quality can cause problems. Transients and high THD (total harmonic distortion) can cause otherwise good motors to fail.

  13. Likes Juan Madera liked this post.
  14. #12
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    is thd in electrical power the same idea as in power audio amps?

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  15. #13
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    THD is exactly the same The ac power should be a perfect sine wave. Three phase power should be three perfect sine waves shifted by 120 degrees.
    If there is distortion in the sine wave this would be distortion. If lumps and spikes are present then this is also distortion.
    AC drives are a causr of this distortion along with flourescent light ballasts etc. Any load that is non-linear in nature can cause this rise in THD.
    At about 3% THD you start to have problems. In out plant, before changes were made one feeder bank of 13.8 KV was at 7.5% and we had problems with motors and drives.
    There is little you can do about these problems if you don't control the source.
    If you think about the way a sine wave is generated you can understand the result.
    If you thought about a perfect circle (wheel) rolling along a perfectly straight line and a point on the circle rolling with the wheel that point's position plotted horizontally would trace a perfect sine wave. If the circle was not round it woild draw a distorted sine wave.
    That perfect circle is what the magnetic fields form to drive the rotor around. If not perfect the magnatic fields dont drive the rotor at a pertect angular speed and the field and currents are not balanced and are fighting each other driving up the heat produced and not producing the engineered power.

  16. #14
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    good info. thanks. what do you use to measure thd? or is it a calculation based on other measurements?

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  17. #15
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    The power meters we use measure THD and I don't know the method used.
    As in audio measurements the devices were very expensive. Now of the power management systems measure the THD as standard. I know our Square D power meters do. I also know when we have high THD we have problems. Our problems are (to a large extent) due to 6 pole high power VFDs instead of 12 pole.
    Siemens has made what they call an SVR that electronically adds voltage and power to the line to round out that sine wave and the power company says it works good.
    Basically I am glad I am not responsible for it's maintenance since it it serial number 1.

  18. #16
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    JMC- I notice you are in Wisconsin. Do these units have resistant heat?

  19. #17
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    Thread Starter
    Quote Originally Posted by Joe Harper View Post
    JMC- I notice you are in Wisconsin. Do these units have resistant heat?
    no. we don't use resistance heaters much up here. too expensive to run.

  20. #18
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    what V99 and JB4 say, !
    Process cooling: NO COMPRESSORS Earth-Coupled since 1996
    ... however, much still needs to be hybridized energy transfer.

    CLOSED LOOP 2015 listed EER's
    even 49+ now; and "blended from low to high variable speeds" for 32deg.F ~ E-Star

    Perhaps you need a 32F Chiller/HW-Heat: buy a GEO-T Heat Pump (GHP with Heat-Recovery)
    http://www.energystar.gov/index.cfm?...mal_heat_pumps

    http://www.hydro-temp.com/products.html and Bosch/Carrier and AquasystemsInc.com

  21. #19
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    The situation you describe with the voltage differences is well within the design parameters of a electric motor. There are a few things that can happen to a motor to cause a premature failure. Most of them were covered by Jbarron and are good info. There are lots of situations that are caused by the incoming trash on the sine wave. I have seen situations where an industrial neighbor will add some drive or something, fail to install the proper filters on the incoming power to the device and inject scr firing pulses back into the utility... thereby, screwing up other customer's incoming sine wave. A tattle tale to a motor winding failure is where it failed. Take a good look at the motor and see if you can determine where it failed... for example it the failure happened in the middle of the iron towards the middle of the motor case... odds are the failure was caused by overheating... probably an overload or protection failure of some kind. If the motor failed as the winding exited from the iron stack the failure can be from abnormal sine waves... usually firing spikes of SCR's generated by VFD's and other electronic switching equipment. Even big battery chargers can cause damage. Another thing that will cause slot failure are lots of starting and stopping. In an across the line starting electric fan motor there are tremendous stress put on the coils that are outside the iron stack. In fact if you looked at the coils in a slow motion film you will see them actually move. In really big motors and generators you will see bracing attached to the coils to help keep them from moving. Every time the motor starts it is the equivalent of an across the line short until the steal in the motor is completely magnetized... that is what causes the amperage inrush... creating magnetism requires lining up all the electrons and it takes power to initially make them move and it takes power to maintain them in the orientated position. Movement at the point where the coil enters the steal stack will cause rubbing of the insulation and finally a short to ground at that point ... I have seen coils deformed on generators that have been subjected to a fault on a particular phase. It's weird to see but a good understanding of what actually does happen to a motor when it starts tells you why they fail after a period of time... Hence why we all carry motors on the truck. If I were you I would investigate and see if there is any short cycling of the motor in it's application. In the two years the motor has been in service it might have made 15 years worth of starts..... And the compressor might not be far behind it...

  22. #20
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    Quote Originally Posted by jmcgreevey View Post
    had to change out a 5hp blower motor in a 2 year old rtu this past week. one year warranty on parts so, needless to say, customer not happy shelling out the cash to buy a new one of those. feeling his pain, I went through the blower circuit pretty thoroughly looking for any week links that may have caused us to lose one if the windings. the only thing that really jumped out at me was varying voltage across the line legs. I don't remember the exact voltages, but on 208v service I had something like L1-L2 212v, L1-L3 214v, and L2-L3 215v. amp draw across the windings also varied a few tenths of an amp in the new motor.

    this is the first time I've lost such a young 3ph motor. in the past I feel like the voltage across each leg on other three phase service that I've checked voltage on have been pretty much identical. I never really had a reason to think about it before now. it has me wondering if this voltage difference, and subsequent current difference, would cause the windings to fire slightly out of phase. I've been trying to research this as much as I can on the web but can't seem to put all the info together to answer my own question. it also seems that, depending on the winding configuration, if the windings fire out of phase they could create a short. any grounds to this? I'm trying to take thus opportunity to learn more about 3ph power and motors while trying to protect my customer from another costly failure.

    thanks! in advance.

    Sent from my XT901 using Tapatalk 4
    Does this unit have a power exhaust? I have had experiences where if the unit has a power exhaust that is running constantly do to bad commissioning or wrong static setpoint, the supply fan will run backwards between calls for heat / cool. And when it starts it has to first overcome the backward spinning at large current draw before the correct rotation is established.

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