Three phase voltage query.

[Lofty]

Hi Guys,

Don'tdo that much with 3 phase but have an issue at the moment.

Had a 3p motor burn out on a farm elevator. Anyway motor was returned to the reconditioner as it was only about 7 months old and it's NOT in constant use.
Reconditioner said 'It's a right mess inside looks like you've lost a phase'.
The elevator is controlled via the control panel of a potatoe grader and this runs 3 or 4 elevators depending on what they want.
Another elevator was connected to the panel on another outlet ( the original had tripped) and runs fine.

So new motor arrived and before running it I got the following readings from the 'faulty' outlet....
Phase - Phase ( L1-L2 , L2-L3 , etc.) 393v or 394v and all showing a righthand rotation. The query is this though....

L1 - Earth 255v
L2 - Earth 212v and
L3 - Earth 212v

Did the test several times and always L1 to Earth was 255v

Is this the norm for one to be that much higher ?

Thanks in advance.

Lofty.

 

[topquark]

Something amiss here. Your PD between phases seems consistent from what you've indicated. I'd suggest checking your Earth->Neutral PD as it appears that you may have a voltage present. What type of earthing system is this on?

 

[Lofty]

Hi Topquark,

Thanks for the reply. I'll have a look at the Earth - Neutral PD. Have to try to work around the guys as they're grading about 200 tons of onions a day at the moment !! If I have to keep going into the panel they get hacked off, well the farmer does !!. No neutral at the elevator motor just the 3 phases and earth, hence getting into the panel which is of course part odf the grader.
Not sure about the earthing system, again I'll investigate. This is a huge farm which power all over the place.

Cheers,
Lofty.

 

[Knobhead]

394V should give a balanced 227V to earth. It looks like a floating neutral problem. That on it’s own wouldn’t have caused the motor burn out. If you can, do the same tests with the motor running, also test neutral earth.

 

[vaughant]

If it's losing a phase to cause burnout it should really trip out the breaker,that's always been my experience?
Is it fed from a contractor somewhere,motors usually are,there could be a good chance if there's a poor contact on the contactor your occasionally losing a phase.
The spurious voltage sounds like leakage somewhere between neutral/earth and as Tony stated shouldn't give you much bother on this.
I'd like to put a clamp meter (ammeter) on each phase when running to see what it's drawing under load and compare to the motor rating plate,check everything's correct.
Also,if it's not used much is there anyway water could ingress if it's not used regularly?

 

[Lofty]

Hi Vaughant,

It is fed via a contactor in the contoller which is part of the grader machine. Thought about water ingress as it's stored either outside or in an open sided barn, but all was dry surprisingly.
Called in today but they haven't finished with the crop yet, still another 700-1000 tons of onions to process over the week-end so hopefully either Mon or Tues the grader should be free for me to play with.

I too want to test it under load conditions.

Cheers,
Lofty.

 

[Darkwood]

Check your incomming supply to building this may be a suppliers issue and if i remember should be within 10% of nominal voltage to ground per phase (230v), feeding a motor with one phase 30volts higher than the other two will create a 2 fields the same and a weaker 3rd field and depending on the motor size to load ratio could drop torque ability substancially, enough to have the motor overheating in the weak field winding, this may not trip an overload device as the motor is only running weak on one field and been driven by 2 strong fields so may not be enough to trip overload but enough to overheat one field windng, ask the motor repairs company if the damage was mainly to one winding.?
As mentioned in previous post you should clamp each phase when running motor under load and check current demand in each phase is more or less balanced and within the constraints of the motor rating.

 

[Knobhead]

It doesn’t work like that with a balanced load such as a motor. So long as the line voltage is right the motor will run OK. The voltage and phase angle remain at 120°. Phase voltage is a different mater, the voltages can vary but the vector sums always equal the line voltage.
As I said this is most likely a floating neutral that’s been brought to light by the motor burning out.

Balanced

View attachment 7950


Un balanced

View attachment 7951

 

[Darkwood]

hi tony long time no speak, yes agree with your diagrams, but that is relative to frequency thus speed and irespective of voltage within certain parameters of course this is true but if you drop voltage in all phases it will effectively reduce the motors handling torque, vsd's do this and also to the frequency too in step with the voltage to control speed(ignoring any boosts for high torque starting). What im getting at is that when you have 3 fields present you will establish a rotation as you've shown above but this isnt voltage conscious to a degree but when you need the motor to do a duty and require torque then the voltage becomes an important factor and having low comparative voltage in one field can overheat that winding.
P.S. not in disagreement with your theory about floating neutral as it a good place to look, but my suggestion is just an alternative answer to given problem seen as you had covered the most plausable one.

 

[i=p/u]

The neutral of a generator should never get into a condition where is is floating. All generators have the neutral tied to the frame of the generator set. It comes this way from the manufacturer. If it is floating then someone has disconnected the neutral wire from the frame. Just reconnect the neutral to the generator frame again to stop it from floating. Larger generators should also have a ground rod connected to the generator frame for grounding purposes.


​

 

[Knobhead]

Where did the generator come from?

Lofty, is the motor star or delta connected?

 

[Lofty]

Hi Tony.
Apologies for the delay in replying.

The motor was obtained by the farmer from a local company who recondition them. They've put another elevator onto the same grading machine and this has worked OK since the motor burnt out.

Motor is 'STAR' config.

Cheers,
Lofty.