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  1. #1
    Join Date
    Jul 2011
    Location
    Columbus, Ohio
    Posts
    136

    Toshiba ground fault

    This is a new drive model Q9440IER3 toshiba. The customer reports it trips out on ground fault though i have never seen it do it. All the line side and load side wiring has been meggered and is good. The job is two hours away and it is getting frustrating. The drive tech has been of little help but says the problem is not in the drive and that it is a wiring problem. I have double checked wiring and the only thing weird I can find is the ground wire going from the out door disconect at the cooling tower to the motor when isolated from the disconect reads 3000 ohms to the conduit and tower its self. The conduit is isolated at the peker head by a rubber gasket thus causing the resistance. the motor also has some resistance to the tower frame due to the paint on the base. However the ground wire coming from the drive to the out door disconect reads zero ohms to the conduit as you would expect. So when you junction the two together in the disconect the motor now has zero resistance to ground. I think I am barking up the wrong tree but am running out of ideas. Any ideas would be appreciated.

  2. #2
    Join Date
    Oct 2005
    Location
    Las Vegas
    Posts
    45
    When using a VFD a good ground is important.

    I have been fighting a similar problem, customer reports a ground fault, I show up & the drive is running. There is a ground fault in the drive history, meggered the motors & wires & could find no issue. The tower is mounted 10 feet above ground level on block walls. What I finally discovered is the control system doesn't cycle the valve properly & the tower overflows. The disconnects are mounted to the same block wall that the towers sit on. We finally found the water trail from the mounting holes in the back of the box to L1.

    I would clear up the ground issue, re-check all terminations, ensure there is enough rubber tape on all connections in the motor junction box,

  3. #3
    Join Date
    Oct 2002
    Location
    Lady Lake, Florida
    Posts
    799
    I have come across simular ground fault trips at a past customer's that had all ABB drives on their equipment, and after intermittent faults found all it takes is a spot where the wiring insulation had started rubbing thru but not exposing the wire.

  4. #4
    Join Date
    Jul 2007
    Location
    The Hot South
    Posts
    1,294
    Quote Originally Posted by Trublshter View Post
    I have come across simular ground fault trips at a past customer's that had all ABB drives on their equipment, and after intermittent faults found all it takes is a spot where the wiring insulation had started rubbing thru but not exposing the wire.
    I have found this on many jobs also. On AHU's, i've found this where the flex conduit meets the rigid conduit due to the motor and blower on being on spring isolators.

  5. #5
    Join Date
    May 2011
    Location
    Fort Worth, Texas
    Posts
    49
    Considering you've meggered the motor and have good resistance and I'm sure you've already traced all your wiring for shorts, could humidity be building up inside your junction boxes or disconnect. Maybe the moisture from the cooling tower is creating humidity problems inside the box or disconnect creating conduction causing your ground faults.

  6. #6
    Join Date
    Jul 2011
    Location
    Columbus, Ohio
    Posts
    136
    Going back this week customer said it tripped three times then ran for three days straight with out a problem. Thought maybe moisture was an issue, took conduints apart in some places looked for moisture, found nothing. The disconect is new and no trace of water. The customer can not relate rainy or humid weather to the cause. I am going to look further into conduint pinch points and such, but you would sure think a megger would show up that kind of problem. I will fix that funky ground thing at the tower but have my doughts that is the problem. I hope it catches on fire soon so I can find the problem. Thanks for the advise.

  7. #7
    Join Date
    Jul 2007
    Location
    The Hot South
    Posts
    1,294
    Quote Originally Posted by synergy View Post
    Going back this week customer said it tripped three times then ran for three days straight with out a problem. Thought maybe moisture was an issue, took conduints apart in some places looked for moisture, found nothing. The disconect is new and no trace of water. The customer can not relate rainy or humid weather to the cause. I am going to look further into conduint pinch points and such, but you would sure think a megger would show up that kind of problem. I will fix that funky ground thing at the tower but have my doughts that is the problem. I hope it catches on fire soon so I can find the problem. Thanks for the advise.
    Not necessarly. When the insulation rubs away and the wire just barely starts to short, it will be intermittant due to the vibration. I've seen this many times on drives. Also, moisture could be the problem.

  8. #8
    Join Date
    May 2011
    Location
    Fort Worth, Texas
    Posts
    49

    I've been there!!

    Quote Originally Posted by synergy View Post
    Going back this week customer said it tripped three times then ran for three days straight with out a problem. Thought maybe moisture was an issue, took conduints apart in some places looked for moisture, found nothing. The disconect is new and no trace of water. The customer can not relate rainy or humid weather to the cause. I am going to look further into conduint pinch points and such, but you would sure think a megger would show up that kind of problem. I will fix that funky ground thing at the tower but have my doughts that is the problem. I hope it catches on fire soon so I can find the problem. Thanks for the advise.
    LOL!!!

  9. #9
    Join Date
    Oct 2005
    Location
    Las Vegas
    Posts
    45
    We did have another issue related to what Trublshter said. Drive was failing on ground fault on a seemingly random basis. We ended up pulling new wire through an underground conduit & the problem went away.

    IF there is redundancy with cooling towers and your customer is willing you could test it. First by pulling the leads off the drive that feed to motor, place the drive in hand at 60hz for a couple of days. Then re-attach those leads & turn the disconnect off (if there is one between the drive & the motor) & let it run for a couple of days. Hopefully the problem will show itself by this time. If not, next separate the leads from the motor in the motor junction box & switch the disconnect back on & let it run for a couple of days. I know you stated this location has some drive time involved, and this is time consuming, but if you can't find the issue at emt/flex to box fittings it will be the next step.

    If the drive runs without fault with the leads off (U,V & W) then the problem is not the drive. By running the drive separating the wire where it makes sense you isolate where a potential problem is. But here comes the tricky part. At some point it will be cheaper for your customer to just replace the wire (due to the drive time) if that's where the problem is.

    Just looked at the manual, Link is here, page 201 of the Q9 ASD Install & operation manual. It states causes for a ground fault" "Mis-wired ground" and "Loose ground connection" as a possible causes.

    Good luck

  10. #10
    Join Date
    Jul 2011
    Location
    Columbus, Ohio
    Posts
    136

    update and a question

    So I finally make it back to look into this ground fault problem. Tore everything apart, meggered, wiggled, pulled ect. Put it all back together and ran the drive. As I am checking the amps it trips on ground fault. Crap.So I am searching the manual and find out that if the I/O board in the drive is connected to earth ground, there will be problems. I discover that the I/O board common is indeed going to earth ground. Traced the problem to the 4 to 20ma speed control signal, that was going to ground with zero resistance on the black return wire.This in turn grounded the I/O board. Pulled the signal wire off and running in invertor manual mode for two days now with out a problem. This has been warrenty so far but now the boss of course wants to bill for all the time. My question is are not all or most 4 to 20 ma signals going to earth ground on the return wire. Cant say that I ever noticed before. I am just thinking it is not really there fault if it is common pratice.

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