Case Study on failing motors due to high neutral current
Case Study on failing motors due to high neutral current
(OP)
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
Thanks






RE: Case Study on failing motors due to high neutral current
RE: Case Study on failing motors due to high neutral current
RE: Case Study on failing motors due to high neutral current
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?
RE: Case Study on failing motors due to high neutral current
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.
RE: Case Study on failing motors due to high neutral current
RE: Case Study on failing motors due to high neutral current
RE: Case Study on failing motors due to high neutral current
RE: Case Study on failing motors due to high neutral current
RE: Case Study on failing motors due to high neutral current
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.\\\
RE: Case Study on failing motors due to high neutral current
RE: Case Study on failing motors due to high neutral current
RE: Case Study on failing motors due to high neutral current
RE: Case Study on failing motors due to high neutral current
RE: Case Study on failing motors due to high neutral current
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.
RE: Case Study on failing motors due to high neutral current
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.
RE: Case Study on failing motors due to high neutral current
The compressor company is in the process of performing an autopsy on the latest failed unit. I’ve already requested their findings when available.
RE: Case Study on failing motors due to high neutral current
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?
RE: Case Study on failing motors due to high neutral current
RE: Case Study on failing motors due to high neutral current
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?
RE: Case Study on failing motors due to high neutral current
Looks like you've ruled out harmonic content.
The impulse description sounds like a chattering contactor, but that doesn’t answer why all have periodic problems.
I’d move the logger directly to the subject compressor’s terminals and watch.
Simple AC millivolt-drop tests within the equipment may be in order.
RE: Case Study on failing motors due to high neutral current
RE: Case Study on failing motors due to high neutral current
RE: Case Study on failing motors due to high neutral current
The 25x transient is a big worry and sounds like some kind of restriking problem at first glance.
Is it possible for you to measure the recovery voltage transient across the contactor at quite high speed?
RE: Case Study on failing motors due to high neutral current