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Thread: Two speed one winding motor troubleshooting advice

  1. #1
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    May 2018
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    Two speed one winding motor troubleshooting advice

    Hi,
    This is my first post here. I’ve read and gained lots of useful information from this forum over the years and for that I’m grateful. I’ve tried finding help for a current problem I’m having here and on other websites but can’t seem to nail down what I’m dealing with. I’m looking for some troubleshooting advice on a 3 phase 2S1W type motor problem. The motor is operated by three contactors. One contactor supplies line voltage to T1, T2, &T3 for Low speed while the other two contactors are open. For High speed another contactor supplies line voltage to T6, T4, & T5 and the third contactor shorts T1, T2, & T3. This motor is used to operate a cooling tower fan in a power plant type facility. It is a three cell cooling tower and generally all cells are in operation. In the warmer months all three cell fans will operate in high speed. My problem is that one fan continuously draws abnormally high current while operating in high speed. While in low speed the current is below FLA and appears normal. The motor nameplate information is below:

    High: 460VAC / 250HP / 290FLA / 1.15SF / 1790RPM / 2 SPD 1 WDG Variable Torque
    Low: 460VAC / 62HP / 100FLA / 1.15SF / 890RPM / 2 SPD 1 WDG Variable Torque

    Below are some of the tests I’ve performed thus far:
    High speed current to problem fan motor:
    T6 - 329.2A
    T4 - 327.6A
    T5 - 324.1A
    Low speed current to problem fan motor:
    T1 - 89.2A
    T2 - 91.2A
    T3 - 90A

    High speed current to other normally operating fan motor:
    T6 - 274.9A
    T4 - 287.1A
    T5 - 275.7A



    Output high speed voltage to problem motor at bottom of contactor:
    T6, T4, & T5 – All roughly 480VAC

    High speed voltage to problem motor at leads in junction box:
    T6, T4, & T5 – All roughly 476VAC

    High speed voltage to ground for problem motor at leads in motor junction box:
    274.4V
    274.2V
    273.4V

    Uncoupled motor from gear box and ran motor unloaded. Recorded motor current:
    High Speed:
    T6 - 191.1A
    T4 - 198.1A
    T5 - 191.9A
    Low Speed:
    T1 - 80.8A
    T2 - 81.8A
    T3 - 81.6A

    High speed shorted connection current to problem fan motor:
    T1 - 161.9A
    T2 - 154A
    T3 - 156.6A
    High speed shorted connection current to other normally operating fan motor:
    T1 - 135.5A
    T2 - 136.6A
    T3 - 139.4A

    Interestingly this motor was recently repaired having new bearings and rotor work done. An insulation resistance and HiPot test were done at the motor shop and both indicated good motor windings. The motor shop report indicated that in High speed unloaded the motor current was at an average of 152A and when operated unloaded at our site the High speed current is roughly 45A higher. All connections have been checked for looseness and corrosion and nothing found. The High speed motor conductors are individual sun resistant 600V, 500kcmil wires. There are two splices in two different phase 500kcmil wires that I have found. I have viewed both splices with a Fluke IR camera and saw nothing warm leading me to believe they are OK. The Low speed and High speed shorted conductors are a sun resistant direct burial 90C 600V, three conductor and ground 2AWG jacketed cable. When this motor is operating in high speed the three conductor cable is showing very warm with our IR camera, naturally since it has about 30A higher flow than the 2AWG is rated for. I have found some cracked insulation on the individual 2AWG conductors in the motor junction box and sealed these up with silicone high voltage tape and super 33. The cracked insulation could be due to the excessive heat that the motor is experiencing over an extended period of time as from what I’ve been told this has been and ongoing problem for years.
    We are not experiencing any breaker or overload trips just hot conductors and a motor which I believe are contributing to a reduced motor life. I have talked to our contact at the motor shop several times and he has run out of ideas and is now suggesting sending the motor in for winding replacement. I’m not convinced the motor is at fault and have my suspicions of the motor conductors since the motor shop unloaded high speed current differed significantly from what was measured at the plant site. If anyone on this board has any suggestions to what I should look at next it would be most appreciated. I apologize for the long winded description, I just wanted to give as much info up front as I could.

    Warm regards,
    Michael

    “What do electricians chant to meditate? Ohm….”

  2. #2
    Join Date
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    “Warm regards”... nice pun . Welcome to the forum.

    It’s entirely possible that since it is a fan, the actual fan load is higher on this unit than the other units. Motor load on a centrifugal machine is related to the flow. So if there is even a slight difference in air flow restriction, as in LESS restriction on this unit than the others, this fan will move more air so the motor will have to work harder, thus pulling more current. The lower air flow restriction could come from the inlet side or the outlet side, it works the same either way. Given the history of the fact that this motor had to be rebuilt but not the others, I’d hazard a guess that this one is the one that has more air flow than the others by virtue of its position in the system. With cooling towers, it might mean that this tower is positioned in such a way that wind is affecting it more compared to the other towers, something like that. Bottom line though, it’s not pulling more than FLA, so I would not try to fix something that isn’t broken. Keep an eye on it and watch for trending increases over time.
    __________________________________________________ ____________________________
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  3. #3
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    May 2018
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    Gaffney, SC
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    Quote Originally Posted by Jraef View Post
    “Warm regards”... nice pun . Welcome to the forum.

    It’s entirely possible that since it is a fan, the actual fan load is higher on this unit than the other units. Motor load on a centrifugal machine is related to the flow. So if there is even a slight difference in air flow restriction, as in LESS restriction on this unit than the others, this fan will move more air so the motor will have to work harder, thus pulling more current. The lower air flow restriction could come from the inlet side or the outlet side, it works the same either way. Given the history of the fact that this motor had to be rebuilt but not the others, I’d hazard a guess that this one is the one that has more air flow than the others by virtue of its position in the system. With cooling towers, it might mean that this tower is positioned in such a way that wind is affecting it more compared to the other towers, something like that. Bottom line though, it’s not pulling more than FLA, so I would not try to fix something that isn’t broken. Keep an eye on it and watch for trending increases over time.

    Thank you for the "warm" welcome.

    I have given some thought to the actual loading of the fan itself and I will look into this more closely. The thing that bothers me is that we are operating in high speed continuously at the upper end of the SF of the motor and this is making the motor HOT. I'm certain this is what contributed to the bearing failure in this motor. It had sealed bearings and when it died the grease had literally melted and was running out of the shaft seal.

  4. #4
    Join Date
    Apr 2009
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    Williamsburg, VA
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    5,438
    Welcome to The Forum. That this motor has already been replaced one time makes me think you have a operational or maintenance problem with your fan or gearbox, or an air damper setting causing an overload.
    Electricians do it until it Hertz!

  5. #5
    Join Date
    Sep 2008
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    UK
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    11,770
    Quote Originally Posted by phineascage View Post
    Hi,
    This is my first post here. I’ve read and gained lots of useful information from this forum over the years and for that I’m grateful. I’ve tried finding help for a current problem I’m having here and on other websites but can’t seem to nail down what I’m dealing with. I’m looking for some troubleshooting advice on a 3 phase 2S1W type motor problem. The motor is operated by three contactors. One contactor supplies line voltage to T1, T2, &T3 for Low speed while the other two contactors are open. For High speed another contactor supplies line voltage to T6, T4, & T5 and the third contactor shorts T1, T2, & T3. This motor is used to operate a cooling tower fan in a power plant type facility. It is a three cell cooling tower and generally all cells are in operation. In the warmer months all three cell fans will operate in high speed. My problem is that one fan continuously draws abnormally high current while operating in high speed. While in low speed the current is below FLA and appears normal. The motor nameplate information is below:

    High: 460VAC / 250HP / 290FLA / 1.15SF / 1790RPM / 2 SPD 1 WDG Variable Torque
    Low: 460VAC / 62HP / 100FLA / 1.15SF / 890RPM / 2 SPD 1 WDG Variable Torque

    Below are some of the tests I’ve performed thus far:
    High speed current to problem fan motor:
    T6 - 329.2A
    T4 - 327.6A
    T5 - 324.1A
    Low speed current to problem fan motor:
    T1 - 89.2A
    T2 - 91.2A
    T3 - 90A

    High speed current to other normally operating fan motor:
    T6 - 274.9A
    T4 - 287.1A
    T5 - 275.7A



    Output high speed voltage to problem motor at bottom of contactor:
    T6, T4, & T5 – All roughly 480VAC

    High speed voltage to problem motor at leads in junction box:
    T6, T4, & T5 – All roughly 476VAC

    High speed voltage to ground for problem motor at leads in motor junction box:
    274.4V
    274.2V
    273.4V

    Uncoupled motor from gear box and ran motor unloaded. Recorded motor current:
    High Speed:
    T6 - 191.1A
    T4 - 198.1A
    T5 - 191.9A
    Low Speed:
    T1 - 80.8A
    T2 - 81.8A
    T3 - 81.6A

    High speed shorted connection current to problem fan motor:
    T1 - 161.9A
    T2 - 154A
    T3 - 156.6A
    High speed shorted connection current to other normally operating fan motor:
    T1 - 135.5A
    T2 - 136.6A
    T3 - 139.4A

    Interestingly this motor was recently repaired having new bearings and rotor work done. An insulation resistance and HiPot test were done at the motor shop and both indicated good motor windings. The motor shop report indicated that in High speed unloaded the motor current was at an average of 152A and when operated unloaded at our site the High speed current is roughly 45A higher. All connections have been checked for looseness and corrosion and nothing found. The High speed motor conductors are individual sun resistant 600V, 500kcmil wires. There are two splices in two different phase 500kcmil wires that I have found. I have viewed both splices with a Fluke IR camera and saw nothing warm leading me to believe they are OK. The Low speed and High speed shorted conductors are a sun resistant direct burial 90C 600V, three conductor and ground 2AWG jacketed cable. When this motor is operating in high speed the three conductor cable is showing very warm with our IR camera, naturally since it has about 30A higher flow than the 2AWG is rated for. I have found some cracked insulation on the individual 2AWG conductors in the motor junction box and sealed these up with silicone high voltage tape and super 33. The cracked insulation could be due to the excessive heat that the motor is experiencing over an extended period of time as from what I’ve been told this has been and ongoing problem for years.
    We are not experiencing any breaker or overload trips just hot conductors and a motor which I believe are contributing to a reduced motor life. I have talked to our contact at the motor shop several times and he has run out of ideas and is now suggesting sending the motor in for winding replacement. I’m not convinced the motor is at fault and have my suspicions of the motor conductors since the motor shop unloaded high speed current differed significantly from what was measured at the plant site. If anyone on this board has any suggestions to what I should look at next it would be most appreciated. I apologize for the long winded description, I just wanted to give as much info up front as I could.

    Warm regards,
    Michael

    “What do electricians chant to meditate? Ohm….”
    Welcome from me also.

    An observation. The 152A on the unloaded motor with a FLC of 290A seems rather high. I would normally expect off-load current to be around 30% of FLA That, to me, could be indicative of a motor fault. Or, an outside possibility that it was connected incorrectly for the off-load test.

    What rotor work was done?
    Si hoc legere scis nimium eruditionis habes.

  6. #6
    Join Date
    May 2018
    Location
    Gaffney, SC
    Posts
    29
    Quote Originally Posted by JFletcher View Post
    Welcome to The Forum. That this motor has already been replaced one time makes me think you have a operational or maintenance problem with your fan or gearbox, or an air damper setting causing an overload.
    I would tend to agree. Yet the motor when uncoupled from the gearbox draws roughly 195A on each phase while placed in high speed vs. the 152A witnessed at the motor shop in high speed with no load attached.

  7. #7
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    May 2018
    Location
    Gaffney, SC
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    29
    Quote Originally Posted by Besoeker View Post
    Welcome from me also.

    An observation. The 152A on the unloaded motor with a FLC of 290A seems rather high. I would normally expect off-load current to be around 30% of FLA That, to me, could be indicative of a motor fault. Or, an outside possibility that it was connected incorrectly for the off-load test.

    What rotor work was done?
    I agree and thought the same thing about the 152A observed at the motor shop. Certainly our motor service specialists could have missed something but our contact mentioned he was very impressed with the results from the winding tests performed at the shop. They have mentioned rewinding the motor as a possibility but that's probably a last resort for us right now. I tend to believe the motor was connected correctly at the motor shop based on conversations I've had with our contact. I have checked our connections here on site and know that Line connects to T1, T2 and T3 with other connections open for Slow and that Line connects to T6, T4 and T5 with T1, T2, and T3 shorted for High. At this moment I can't guarantee that the phasing is per the drawings at the motor junction box other than knowing that the fan turns in the correct direction in both Low and High, which I would think would be immediately disastrous for the motor if both Low and High phased with opposite rotations. The rotor had damage from the bearing failure and I am told the repair consisted of it being built up, turned down and balanced. The most perplexing thing, to me anyway, is that the motor operates below FLA while in Low. To my limited knowledge I'd think if there were a winding or mechanical problem within the motor the current draw would be proportional in both Low and High.

    Thank you for all for the great responses. You're all helping me grease the rusty cogs in my old brain.

  8. #8
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    Sep 2008
    Location
    UK
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    Quote Originally Posted by phineascage View Post
    I would tend to agree. Yet the motor when uncoupled from the gearbox draws roughly 195A on each phase while placed in high speed vs. the 152A witnessed at the motor shop in high speed with no load attached.
    What current does the other motor draw when uncoupled?
    Si hoc legere scis nimium eruditionis habes.

  9. #9
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    May 2018
    Location
    Gaffney, SC
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    Quote Originally Posted by Besoeker View Post
    What current does the other motor draw when uncoupled?
    I have not checked this yet but have this on the list to do when the opportunity comes. Right now I do know that the our problem fan motor is drawing about 45A greater while coupled to it's gearbox than either one of our other fan motors in High that are both operating normally.

  10. #10
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    Quote Originally Posted by phineascage View Post
    I have not checked this yet but have this on the list to do when the opportunity comes. Right now I do know that the our problem fan motor is drawing about 45A greater while coupled to it's gearbox than either one of our other fan motors in High that are both operating normally.
    It might be instructive to see what they all do uncoupled to anything. That would quickly clue you into whether the issue is mechanical or electrical.
    I know it isn't always easy to replicate seemingly simple tests on site But, if you want to bottom out the current disparities, you/your customer may have to bite the bullet.
    Si hoc legere scis nimium eruditionis habes.

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