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  1. #14
    Join Date
    Nov 2008
    Location
    Spring Green, WI
    Posts
    69
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    Thread Starter
    good info. thanks. what do you use to measure thd? or is it a calculation based on other measurements?

    Sent from my XT901 using Tapatalk 4

  2. #15
    Join Date
    Apr 2009
    Location
    Mobile, AL.
    Posts
    82
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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.

  3. #16
    Join Date
    Aug 2007
    Location
    Winter Haven, FL
    Posts
    4,383
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    JMC- I notice you are in Wisconsin. Do these units have resistant heat?

  4. #17
    Join Date
    Nov 2008
    Location
    Spring Green, WI
    Posts
    69
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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.

  5. #18
    Join Date
    Jun 2013
    Location
    NORTHERN
    Posts
    1,178
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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

  6. #19
    Join Date
    Nov 2004
    Location
    Anchorage Alaska / Seattle WA
    Posts
    205
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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...

  7. #20
    Join Date
    Feb 2014
    Location
    California
    Posts
    9
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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.

  8. #21
    Join Date
    May 2014
    Posts
    5
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    Three phase motors have the same resistance or ohm reading in all three windings so if you have a voltage unbalance or difference in the applied voltage to the three windings the electrical unbalance will cause the windings to over heat. A voltage unbalance of more than 2% can cause damage. I ran your numbers and your unbalance was about .8% so I don't think that is your problem. The formula for voltage unbalance is voltage deviation / voltage average. In your case you said L1-L2=212, L1-L3=214, and L2-L3=215 just add 212+214+215=641, then divide. 641/3=213.6 this is your voltage average. Your voltage deviation would be 213.6-212=1.6, 214-213.6=0.4, and 215-213.6=1.4 your greatest deviation is 1.6. Voltage deviation 1.6 divided by voltage average 213.6 = 0.00749064 x 100= .8%. Current unbalance is calculated the same way, the max current unbalance is 10%. Hope this helps.

  9. #22
    Join Date
    Jun 2014
    Posts
    1
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    Three Phase motors

    Im a student currently enrolled at Penco tech and we are currently learning about three phase motors, and you equation gave me a better understanding of finding the resistance. thanks

  10. #23
    Join Date
    Mar 2014
    Posts
    1
    Post Likes

    Voltages

    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 I 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
    The voltages seem ok. Just out of curiosity, do you know if the power source is coming from a open delta transformer?

  11. #24
    Join Date
    May 2001
    Location
    Kansas City
    Posts
    3,419
    Post Likes
    Check your contactor and all the electrical connections. I have seen motor failures where once the 3ph motor starts one of the phases drops out, the motor will stay running in a single phase condition and eventually burn out the new motor from running hot. A bad (new) motor will either not work out of the box or fail soon after installing it.

  12. #25
    Join Date
    Apr 2016
    Location
    Parts unknown
    Posts
    130
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    Put a vfd on it. Great motor protection, reduce stress on motor (belt?) because of soft start. Also can fix free-wheeling issues (if that's an issue) Maybe there are incentives from the power company.
    You don't even have to change to an inverter-duty motor just use inductive chokes. They are relatively cheap, easy to install and they will clean the voltage spikes up before they hit the motor.

    If you are really concerned about it happening again you should at the very least install a phase monitor.

  13. #26
    Join Date
    Apr 2016
    Location
    Parts unknown
    Posts
    130
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    Just realized that the OP was 3 years old.
    I wonder if he heeded our advice or if harmonic distortion and burned contacts did him in again.
    Or that wicked .8% voltage imbalance....

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