3 phase motor in star configuration - any benefit of grounding the star point? - Page 1
Forum section: electronics / beginners

author: magic
author_rank: Super Contributor
timestamp: September 09, 2022, 10:05:03 am
content: I hope I am not starting a religious war with thisMost induction motor wiring diagrams I have seen show the shorted ends of the windings simply left floating, on the grounds that phase voltages and motor windings ought to be symmetric, so the center point voltage is approximately zero anyway.On the other hand, there appears to be a minority that insists that the star point should be connected to neutral in order to equalize small current imbalances that do occur.Is this ever a serious concern?

author: Gyro
author_rank: Super Contributor
timestamp: September 09, 2022, 10:26:38 am
content: A good question. Possible factors I can think of...- Does leaving the star point floating assist in 'balancing' the motor drive in the case of unequal phase voltages?- Does it reduce the risk of motor damage if one phase drops out?- How solid is the Neutral in a particular installation, sometimes it is weak and high resistance.- On the other hand, err...

author: capt bullshot
author_rank: Super Contributor
timestamp: September 09, 2022, 01:34:28 pm
content: Never seen the motors star point connected to anywhere.It might work (meaning you should get away with "no magic smoke escape") if you do so with a regular 3ph mains, but never ever do this with a VFD (inverter) supplying the motor.In a very special case, when one phase is open circuit, the motor might still be able to run if the start point is connected to neutral.

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 09, 2022, 02:05:53 pm
content: A title edit is suggested because "grounding" a circuit galvanically conected to a live source would be a code violation in most countries, however Magic correctly refers to connecting star (Y) toneutralin text of o.p.Anyway:A 3 wire connection is usually preferred because the 3 sources can handle the Negative sequence currents due to imbalance in the motor impedances or torque fluctuatiuons.A 4th wire is not needed to do this balancing.A 4th wire would/could have Zero sequence current in the neutral which is undesirable and might be higher if there is harmonic voltage distortion on the 3 phases which there usually is.An industrial 3 phase distribution board will usually have a neutral bar to get single phase supplies , and it is desirable to reduce neutral currents to be as low as possible in the interest of reducing noise and balancing the loads on the incoming transformer.With all the small phase supplies often having capacitor input DC filters, the neutral currents have a 3rd harmonic component in which the contributions from the 3 phases add instead of cancel.This is a reason why the Power Factor Correction type of small power supplies are needed.( Like the one posted by Tim yesterday : )https://www.eevblog.com/forum/projects/power-factor-correction-discrete-control-circuit-(for-bcm)/

author: Zero999
author_rank: Super Contributor
timestamp: September 09, 2022, 02:16:40 pm
content: Connecting to neutral wouldn't hurt, but it won't make any difference either. Do not connect it to safety earth/ground. That's a bad idea and is against the safety regulations in most jurisdictions.

author: coppice
author_rank: Super Contributor
timestamp: September 09, 2022, 02:20:24 pm
content: It depends. There is no point of grounding the star point of the actual motor. However, you often find there is something associated with the motor's wiring that is running from just one phase. For example the lighting on a three phase machine, or some electronic controller box.

author: bdunham7
author_rank: Super Contributor
timestamp: September 09, 2022, 03:12:43 pm
content: At best it won't matter and at worst you'll get significant startup surges and 5th harmonics on the neutral which may disturb your control circuitry or even stuff in the rest of the building. Or, if this connection is made inside of a unit which is later moved and connected to a corner-grounded delta service (a US thing only AFAIK, so no worries in Poland) you might have a bit of sparkles depending on what you do with the neutral.

author: magic
author_rank: Super Contributor
timestamp: September 10, 2022, 04:46:30 pm
content: Quote from: mag_therm on September 09, 2022, 02:05:53 pmWell, I meant grounding in the circuit theory sense. I had a character limit to fit this title inObviously, no one in his right mind advocates for connecting motor windings to safety earth.But once in a while I run into handwavy arguments in favor of connecting it to neutral. For example here, and I frankly have no idea what they are talking about.https://www.allaboutcircuits.com/textbook/alternating-current/chpt-10/three-phase-y-delta-configurations/QuoteAnd there are others, often internet forum users with low post countwho use language like "balance" without further elaboration on what exactly should go wrong if said "balance" is lacking.I smell BS because wiring diagrams from manufacturers of motors and Y/Δ switches show the floating configuration, but I'm no expert on motors so maybe I'm missing something.

author: bdunham7
author_rank: Super Contributor
timestamp: September 10, 2022, 04:53:32 pm
content: QuoteI can tell you with some certainty that in a system that has a neutral, having your large three-phase motors neutral connected will not prevent a flaming catastrophe if you lose a phase and your breakers don't promptly trip. Your single-phase loads that use the neutral for the lower voltage ((208/120 or 380/230 or similar) will be subject to huge surges as the neutral voltage goes crazy. And yes, they're typically bonded to the ground at the service entrance in the US, but there is no breaker on the neutral!

author: elekorsi
author_rank: Regular Contributor
timestamp: September 10, 2022, 05:31:36 pm
content: On 3ph motors you never connect the neutral. Never.It's a balanced loadI am not saying that i had never saw one, but those cases were the result of "electricians" who got their eletrical degree from youtube and diy forums...

author: fordem
author_rank: Regular Contributor
timestamp: September 11, 2022, 02:26:18 am
content: Quote from: Zero999 on September 09, 2022, 02:16:40 pmI'll just point out that many "jurisdictions" REQUIRE the neutral to be bonded to "safety earth/ground" at some point in the installation, typically the building entrance.

author: langwadt
author_rank: Super Contributor
timestamp: September 11, 2022, 02:29:23 am
content: Quote from: fordem on September 11, 2022, 02:26:18 amand _only_ at that point

author: elekorsi
author_rank: Regular Contributor
timestamp: September 11, 2022, 07:01:05 am
content: Quote from: fordem on September 11, 2022, 02:26:18 amNot really, combining Earth and neutral is only at the source, which in our case is the distribution transformer.In TN-C system you get earth (PE) and neutral (N) combined, designated as PEN conductor from the distributor. Per IEC, PEN conductor is green/yellow. If you need neutral, you seperate them in such way that the PEN is directly connected to the PE bar and then linked to the N bar. At this point you make the TN-C-S type of system.After the point that PEN is seperated into PE and N, they should NEVER be combined again.Minimim allowed cross section of the PEN conductor is 10mm2 for Cu.TN-S: you get seperate PE and N, no joining allowed. You join your own PE with the distributions PE.TT: you get only N from the distribution, PE is yours, no joining allowedIT: only 3 phases from distribution, no neutral, your own PE

author: geggi1
author_rank: Frequent Contributor
timestamp: September 11, 2022, 07:56:00 am
content: First of all regarding the cross section of the PEN conductor.The cross section of is different in different countries depending on the regulations used.Here in Norway the regulations basically is that the PEN must be able to handle the same current as the phase conductor.The reason for this is that with a single phase earth fault on a TNC or TNC-S system the full current will run through the N and PEN conductor back to the source.This will ensure the quickest disconnect time at the protective device.Second regarding connecting connecting the star point on a 3PH motor to ground or neutral.Connecting to ground will most likely give you a earth fault trip or alarm depending on how your system is set up.Connecting to neutral is not needed because when looking at the currents on the phases the vector sum is zero. If the protective device for your motor is a smart electronic one it might also nor work properly.

author: capt bullshot
author_rank: Super Contributor
timestamp: September 12, 2022, 05:34:28 am
content: Quote from: magic on September 10, 2022, 04:46:30 pmOK, these circuits look like the stuff you've got to see in "Energietechnik" at university. The left hand side star isn't a motor at all, but rather imagined as a generator or secondary of a transformer. And they're talking about having a neutral line or not in transmission systems of electric energy. The three phase motor is a load (a balanced one, as others pointed out too) that never gets connected to the neutral.

author: magic
author_rank: Super Contributor
timestamp: September 12, 2022, 08:09:10 am
content: I guess you're rightAlthough I still don't get their remark about neutral connection "alleviating problems should one element of a 3-phase load fail".Maybe a reference to something in the earlier articles, but I'm not sure if I want to bother reading them all.

author: capt bullshot
author_rank: Super Contributor
timestamp: September 12, 2022, 08:43:17 am
content: "Should one element fail"I guess this is e.g. about a load composed of three independent resistors or whatever. If one fails without neutral, you'll see a series connection of two resistors between two left over phases. In case neutral is connected, two resistors from each left over phase to neutral would remain.In case your load is resistive indeed, this would result in quite different power consumption of the remaining circuit. Pointless for loads that rely on the three phase system, like three-legged transformers or motors (or electrical machines in general).

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 12, 2022, 09:00:09 am
content: Yes, that article could be in relation to residential service distribution in country like USA ( note the archaic units) where each user only has access to a single phase.In that case the neutral is rated so that 2/3 of users stay on in event of one line failing.Trees or lightning.Another application is shipboard where some services stay on with partial damage like ground fault on one phase.Industrial services with 3 Phase loads like motors and transformers would want all 3 phases to trip immediately to avoid damage like overloaded motors or cables.Ordinary industrial overcurrent protection devices trip all 3. And some have trip on imbalance.