Case Study on failing motors due to high neutral current

[2banee]

I remember browsing a thread that talked about a case study detailing the existence of high neutral current within an industrial building that caused multiple motors to subsequently fail. Does anyone remember the thread or could provide me with that study? I think it was started by a member from Australia. Please advise.
Thanks

 

[jbartos]

Suggestion: The high current in the neutral conductor often represents unbalanced load conditions. These conditions can cause harmonics that also may flow through the neutral. If such conditions exists; especially, those conditions that also have an unbalanced three phase power supply voltage, the motors will fail due to overheat caused by the voltage unbalance, current unbalance and harmonics.

 

[alehman]

I can't find the referenced thread. I agree with jbartos. The high neutral current in and of itself will not effect motors. Most motors have no neutral connection. Assuming we are talking about a solidly grounded system, harmonic distortion and/or voltage unbalance due to other causes are the most likely culprits. How high is the neutral current (compared to phase currents)?

 

[2banee]

Thanks for your response. The case study that I was looking for was just for my own edification and nothing else. It wasn’t related to a specific problem.

I do have a question regarding a commercial building though. The building has a 480/277, 4-wire service with a 1200 amp panel. He has 7 AC compressors on the top of the roof that are 480 3-phase, 3-wire service. He has lost all 7 of these compressors within the last 3 months due to a short to ground, so they say. Everything looks fine from the utility side. Monitoring equipment was installed and there is no phase imbalance (within 1.5 %), no abnormal voltage deviations or large inrush currents to indicate anything is wrong. Since there is no load imbalance and no neutral wire could harmonics play a part in his problem? It seems ironic that he would lose all of these units within a short amount of time. The one thing that is kind of odd is there is a case ground on the roof-top subpanel that is #6 Cu. Any thoughts?

 

[ScottyUK]

Cliffordball,

If you have high triplen harmonics, they end up in a delta winding somewhere. Do you know if the failing motors were delta connected? Harmonic current has a more severe eddy heating effect in the iron.

Hang a clamp-on current probe on the motor cables and display the output on a 'scope. Should give you a quick yes/no if any heavy harmonic current is flowing, because the sinusoid will be distorted.

Gut feeling says something else is killing your motors. Rooftops are a wet environment in the UK (where are you?) - I'd be guessing at some environmental problem initially.

 

[2banee]

We're in the central valley in California. Hot dry climate. I don't suspect that it is environment related. They are delta connected and our Rustrak monitoring equipment only shows some short duration peturbrations in the current wave form, but of high magnitude. We have a cap bank close, but is set on time & temp operation. Doesn't operate during the time periods that the equipment is failing. I'm still at a loss.

 

[ScottyUK]

What duration are the perturbations? Multiple cycles, or sub-cycle? What else shares your supply (computers? fluorescent lighting?), and is your utility transformer a star-delta or star-star?

 

[2banee]

It is a Delta-Wye grounded. The perurbations are subcycle of less than a 1/4 wave form. The building houses a lot of computers and single phase fluorescent lights that can add to the 3rd harmonic (non linear loads).

 

[GerH]

An aspect about 3rd harmonics is that the 3rd harmonic components of each phase in a 3-phase circuit are exactly in phase. You will see this if you draw out a sine wave diagram for fundamental frequency and superimpose the 3rd harmonics for each phase. It shows all three 3rd harmonic waveforms are in phase. This means that in a delta winding the 3rd harmonic components circulate in the delta rather than cancel each other or giving a residual neutral current. The windings carry higher current than what measured line currents would predict. Check if there is voltage waveform distortion.

 

[jbartos]

Suggestion to Cliffordball (Electrical) Aug 14, 2003 marked ///\\Thanks for your response. The case study that I was looking for was just for my own edification and nothing else. It wasn’t related to a specific problem.

I do have a question regarding a commercial building though. The building has a 480/277, 4-wire service with a 1200 amp panel. He has 7 AC compressors on the top of the roof that are 480 3-phase, 3-wire service. He has lost all 7 of these compressors within the last 3 months due to a short to ground, so they say. Everything looks fine from the utility side. Monitoring equipment was installed and there is no phase imbalance (within 1.5 %), no abnormal voltage deviations or large inrush currents to indicate anything is wrong. Since there is no load imbalance and no neutral wire could harmonics play a part in his problem? It seems ironic that he would lose all of these units within a short amount of time. The one thing that is kind of odd is there is a case ground on the roof-top subpanel that is #6 Cu. Any thoughts?
///The ground wire size should be related to the rating or setting of Automatic Overcurrent Device in Circuit ahead of Equipment, Conduit, etc. not exceeding 1200A for example, and related minimum size equipment grounding conductors for grounding raceway and equipment shall be #3/0 AWG copper according to NFPA 70-1996 Table 250-95.\\\

 

[DanDel]

Cliffordball, was the 1.5% phase imbalance you mention in the utility voltage or current?

 

[alehman]

And was the 1.5% measured at the utility service or near the motors? Can you describe the #6 "case ground on the roof-top subpanel?" Exactly what is connected to what?

 

[2banee]

The 1.5% volt phase imbalance was measured at the equipment subpanel (near the motors) on the roof top. The subpanel has 7- 40 amp breakers that feed each separate motor. The case ground is only connected to the subpanel and then to the building. No neutral wire to the motors. Does that answer your questions???

 

[DanDel]

Check the current imbalance at the motors while running. With a 1.5% voltage imbalance, you will probably have 10 - 15% current imbalance. This will definitely cause the motors to fail.

 

[busbar]

In NorCal, a typical voltage-unbalance limit is 2½% at the utility connection point. [PG&E ER2 §C.1.d.] 1½% is likely acceptable.

The 6AWG equipment-grounding conductor would be acceptable for a 200-amp feeder, and not required in a metallic rigid conduit or EMT system.

Does the HVAC equipment use electromechanical [starter/contactor] compressor control or electronic means, like VFD or SCR control?

Has the feeder been checked for excess harmonic content or fast transient voltage that could cause thermal damage or insulation puncture in the compressor stators?

For the repeat failures, is stator insulation resistance being tracked?

As implied by others, motor-winding failure and neutral current are likely unrelated.

 

[ScottyUK]

So your distribution transformer has no delta winding, you have loads of harmonic-generating loads, and your motors are delta connected.

I think the triplen harmonics are probably ending up in your motor windings, rather than the neutral.

Can you arrange a test where all the motors are disconnected and you can measure the neutral current, then start once of the motors? The motor shouldn't affect the neutral current, but if there is a change (reduction) there is a fair chance that the harmonics are circulating in the motor winding. Ideally, use a current probe for a 'scope to watch the neutral current during the test - third harmonic is quite distinctive when present in large quantity so you would see a significant change in wave shape.

 

[2banee]

Busbar you are correct. We're not PG&E, but our Electric Service Rules state that voltage balance between the phases will be maintained as close to practible to 2.5% maximum deviation. The HVAC equipment is electromechanical and I have no knowledge of any pulse width modulation circuitry within the building. At the utility transformer that feeds the building in question there is no evidence of problems. This feeder has multiple customers on it and it seems to be isolated to just this building. They are the only customer reporting problems.

The compressor company is in the process of performing an autopsy on the latest failed unit. I’ve already requested their findings when available.

 

[alehman]

About that capacitor bank - you say it doesn't operate at the times when the motors are failing. Any time a capacitor switches on or off the circuit you will get a significant transient. This may damage winding insulation which doesn't show up until some time later. Are the motors running when the capacitors switch on or off? Do you have TVSS protection near the motors?

That combined with possible excessive heating from harmonics, voltage imbalance and the hot environment it all may be too much for the motors. Also, do you know the motors are sized correctly for the load?

 

[RAMConsult]

I concur with alehman, there may be some unexpected interaction with the capacitor bank. What is its purpose, how is it connected and who owns/controls it? It is possible to have an unintended resonant condition exist between a utility supplied capacitor bank and customer premise equipment. The transients generated during switching can easily destroy the insulation of the motor if the conditions are just right. You might consider installing surge suppressors as close to the motors as practible.

 

[2banee]

Gents, good point. Possibly a series of events slowly degrade the motors enough that they eventually fail.

The cap bank is on time and temp. It switches on at 8 AM and off at 8 PM or at an 80 degrees F threshold.

I just analyzed our most recent data from our monitoring equipment. I had it set on subcycle capture. All of the data was collected at the subpanel where the 7 compressors are fed from. With respect to voltage, the 3rd is only .3 % of the fundamental, the 5th is 1.6%, 7th .2%, 9th .1%, 11th .4%. The rest are negligible. There is a THD of 1.6%. Fairly low. With respect to current; 1.7%, 3.7%, 1.3%, .2%, .6% respectively, with THD of 4.9%. IEEE Standard 519-1992 states acceptable levels for harmonic voltage distortions of 3% for an individual and 5% for THD on the utility system. This is at the building so I don’t think this is too far out of line. What are your thoughts?

The subcycle wave forms are of an interesting shape though. I see a high frequency transient on the current wave form that eventually pulls it out of phase and spikes up to 25 times higher in magnitude than the operating current. Over an 8 day period this happened 10 times. Not always. It has a short duration of .2 miliseconds. This coincides with the turning off of one of the compressors. Any other revelations? Could this be an indication that another one will subsequently fail?