Can a difference in phase voltages cause a motor to fail?

[RogerMorris]

Here is the background:
A new Grundfos 3 phase sump pump was installed in a bore hole to pump groundwater to a holding tank.
After 6 weeks the motor failed, the installer measured the voltage of the 3 phases which at the time were
P1-237v
P2-250v
P3-233v
and suggested this was the likely cause of the motor failure and I should install a 3 phase voltage stabilizer.

Do you think he is correct?

 

[Darkwood]

Yes voltage imbalance can be a very bad thing for a motor, even just a few percent but you have about about 5% difference there and I suspect that has led to the demise of the motor without question, a 2% different can lead to a 10% increase in winding temp' and a 5% can be a 50% increase... you have a voltage difference way beyond these figures.

I would have concerns myself if we see 2-3% variation and I believe NEMA limits require to be less than 5%, it get complicated because one has to factor, load, duty and so much more when seeing what a small variation will actually do to any given motor but given you voltage values I can only agree with the installer although I question if this variation was present when installed which opens a whole different direction to this thread although it is hard to prove unless documented, voltage variations can also appear at certain times of the day or night depending on network loading and balance.

PS - I can only go on the information you give, have you had the motor looked at by a motor service company, more often than not the characteristics of the damage in the motor can determine the likely cause of its demise, although it might be a small investigative charge to look it may save you money in the long run but I wouldn't be at all surprised if they confirm the story.

 

[Darkwood]

Here is a very informative link which is written in simple to understand format and should help you understand the issue.
The graphs are in particularly helpful especially the temp' rise and the motor derating tables for voltage variation.
If you were in doubt beforehand I suspect you won't be after reading the link.

https://library.e.abb.com/public/0c4186f3e7354f6982124decf92742d8/AVC-20 white paper (low res).pdf

You seem to be located to Spain, I do not know the supply standards of the network providers but over here in the UK we could raise this as a issue with the DNO with hope of a solution on their part.

 

[pc1966]

This on-line calculator gives 34.72% motor temperature increase. No idea how accurate that is!

3 Phase Imbalance Calculator

Calculate voltage imbalance.

www.hvacrschool.com

 

[pc1966]

Just realised the above are L-N but the calculator assumes L-L, however it helpfully tells you in red:

Shut down the equipment and rectify imbalance immediately​

 

[Darkwood]

Actually @pc1966 you have probably pulled me up on an error I made, I calculated the difference between the highest and lowest voltages and not the mean average of the 3ph ... either way it doesn't change the facts of the matter - Ill edit that 8% value to 5%

 

[pc1966]

I was wondering how easy it would be to correct the voltage imbalance, as going down the route of a 3-phase UPS seems like an expensive path?

 

[Lucien Nunes]

Does the pump have a starter with an overload relay, a thermostat or any specific protection other than fuses / MCBs? Because, while voltage imbalance can cause overheating that is often reflected in the line currents and ideally would be picked up by the protection rather than leading straight to motor failure.

If the imbalance is caused by large single-phase loads being haphazardly assigned to phases within the installation, it might be sufficient to rearrange them to bring the voltages at the pump within tolerance.

 

[Darkwood]

You raise a point I missed @Lucien Nunes, why no protective devices tripped here to prevent the failure.

As this is Spain going from the profile I cannot comment of network reliability and whether this is rural or urban which may factor in to the voltage irregularities but a good point raise about balance within the property.

 

[pc1966]

Another thing not mentioned is what power this motor is.

 

[RogerMorris]

Thanks very much for your comments and knowledge, it is rural Spain and I have since discovered that phase imbalance is not unusual.
The motor is a Grundfos 3VC 2.2kw and it is connected via a PROBD21P Digital Pro Pump Relay which as I understand it will cut the motor if the current is either too high of too low but not react to phase imbalance

 

[RogerMorris]

MS 402

Submersible motors

product-selection.grundfos.com

Avel - electronica,lda

Electric control boxes and electronic relays for water pumping systems

www.avel.eu

 

[pc1966]

Can you (safely) measure the phase-phase voltages? They ought to be around 415V here.

Most likely the motor is connected only to the 3 phases, and not neutral, so an offset/imbalance in the neutral is much less of an issue.

Worth having as much information as possible in case of any dispute over the motor problem, though it still seems likely the supply is a problem.

 

[RogerMorris]

Yes the motor is only connected to the 3 phases not neutral and the voltages are now

L1 247v
L2 241v
L3 236v

L1-L2 421v
L2-L3 415v
L1-L3 422v

 

[Darkwood]

I would be looking at setting up a system that give extra monitoring, protection from voltage irregularities and redundancy.
I did a 2 pump system that usually runs each pump alternatively for drains and sewage but under exceptional circumstances (heavy rain) the level sensor will trigger both pumps for fast evac' and is monitored for overtemp, overcurrent, short circuit and earth leakage on each motor, a PLC program will recognise any condition raised, swap the program to 1 pump operation and raise the alarm, it has been running 3 yrs now no issues thus far, the previous system cost them 5k in call outs and tank emptyings one yr so already the investment has paid.
The above would probably push you for 2k to 3k (£) or if you really want to pull out all the stops then fitting VSD's and giving extra protection from clogging and stalling with anti-rag software adding another 1k to the pot...
Depends on your pocket, the size of the holding tank and the cost of failure to inconvenience and business if applicable.

As you have voltage issues I didn't have then your scenario would need extra to ensure you can reduce or eliminate the effects of voltage imbalance.

 

[RogerMorris]

Thanks for the advice darkwood.

It has been suggested that I cut my losses with the 3 phase approach and install a single phase motor.
What are the downsides of that idea?

 

[stevethesparks]

Would a single phase to 3 phase vsd not be feasible?

 

[pc1966]

Yes the motor is only connected to the 3 phases not neutral and the voltages are now

L1 247v
L2 241v
L3 236v

L1-L2 421v
L2-L3 415v
L1-L3 422v

That on-line calculator is now saying:

• Average Voltage: 419.33
• Voltage Imbalance: 1.03%
• Motor Temp Increase: 2.14%

It is advisable to decrease imbalance to below 1% if possible​

 

[RogerMorris]

Do you mean something like this

Single phase input, Three phase output VFD | ATO.com

www.ato.com

 

[pc1966]

So that particular measurement is not really looking a very likely reason for failure, but your L-N voltages look more balanced now as well so there might be some significant fluctuations in the supply balance.