Floating or grounded neutrals on motors and supplies
Floating or grounded neutrals on motors and supplies
(OP)
Hello
I am interested in learning how industrial motors (1000HP to 50,000 HP MV to HV)and their supplies are connected. More specifically I would like to know if the neutral of the motors and the newtral of the supply side are grounded. Also how does the neutral connection/grounding approaches differ from Asia to Europe and North/South America ? Your help and opinion is much appreciated
Best regards
Karim
I am interested in learning how industrial motors (1000HP to 50,000 HP MV to HV)and their supplies are connected. More specifically I would like to know if the neutral of the motors and the newtral of the supply side are grounded. Also how does the neutral connection/grounding approaches differ from Asia to Europe and North/South America ? Your help and opinion is much appreciated
Best regards
Karim





RE: Floating or grounded neutrals on motors and supplies
Supply grounding and grounding methods depend both on protection philosophy and the type of supply.
Wye or delta/ primary or secondary/ solid ground, impedance ground, corner ground, no ground, zig-zag grounding or wye:delta grounding.
Bill
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"Why not the best?"
Jimmy Carter
RE: Floating or grounded neutrals on motors and supplies
An interesting thing about motor dynamic analysis (such as Krause), it seems to assume the motor neutral is connected back to the supply system. This seems a very wide assumption for motor simulation but I haven't ever heard of matching practice.
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RE: Floating or grounded neutrals on motors and supplies
Many thanks to Bill and Pete. Your answers are most helpful. If one could develop a 1000 times more sensitive ground fault or stator insulation leakage current differential CT, on what motors could one use it ? I was askd this question at a recent IEEE on Motors and Gens Conf.
Thanks
Karim
RE: Floating or grounded neutrals on motors and supplies
On the other hand, using a more sensitive trip function wouldn't benefit most applications - certainly where loss of motor impacts the process severely. Also with existing ground fault trip the damage is limited to the winding, not the core. The improvement I guess would be to be able to save the winding...I'm not sure if this benefit would offset the disadvantage of more trips.
Going back to the benefit of early warning, we had two instances of intermittent ground fault trips, after which motor tested fine and ran fine. These were warnings that something was going on.
One turned out to be a quarter inch hole in the groundwall insulation at the very bottom of the knuckle on bottom end of vertical motor. It never went hard ground and didn't show up on insulation tests, but when we opened it up we saw it. There had been water in the motor at some time and apparently dripping from knuckle to ground plane below created a fault that blew open that portion.
Another turned out to be a cable splice fault on 15kv cable for 13.8kv system. Actually the EPR cable tested 30 megaohms at 10kv, which we have come to accept as normal for these runs of cable 2000', sometimes wet. But between 10kv and 15kv it broke over to zero ohms.
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Floating or grounded neutrals on motors and supplies
Very intersting response, I was not aware of the two points you mentioned.
Can you quickly explain how grounding neutral point of motor helps balance unbalanced line voltages and how doing this incerases thermal heat in the motor?
RE: Floating or grounded neutrals on motors and supplies
If the line voltage is unbalanced the mismatch will cause excess current. This crrent will be limited by the system impedance and the motor impedance. This is why a small voltage unbalance causes a large current unbalance.
The unbalanced current is in a direction to correct the voltage unbalance however the system impedance is often so low in relation to the motor impedance that there is little actual correction of the applied voltages.
In an instance where a large motor is supplied by a transformer with not much more capacity than the motor requires and with a relatively high impedance the correction of a voltage unbalance may be noticeable.
My comment concerning an "attempt" to correct unbalanced voltages was somewhat tongue in cheek. The attempt will usually yield so little effect as to be futile. The extra current will result in more motor heating and rotor heating may be be an issue also.
I have seen a "soft" distribution line with a unequal currents, voltages and the resulting displaced neutral corrected to a large degree by the action of a wye:delta transformer bank with the primary neutral connected. The KVA size of the transformer bank was significant compared to the KVA of downstream unbalance. The action has some similarities to the action of a motor fed by unequal voltages. The relative sizes of motors compared to transformer banks results in the corrective action of motors to usually be much less than the possible corrective action of a large transformer bank.
Again, yes there is some correction, but no, don't plan on it, it is usually not enough to be useful.
Bill
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"Why not the best?"
Jimmy Carter
RE: Floating or grounded neutrals on motors and supplies
Thanks for the info
Regards
Karim
RE: Floating or grounded neutrals on motors and supplies
Also I am pretty sure the grounded motor neutral is very rare if it exists at all. I wonder what NEC would say about it? (I don't know).
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Floating or grounded neutrals on motors and supplies
"Also I am pretty sure the grounded motor neutral is very rare if it exists at all. "
should read
"Also I am pretty sure the externally-connected motor neutral is very rare if it exists at all. "
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Floating or grounded neutrals on motors and supplies
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Floating or grounded neutrals on motors and supplies
If a motor is large enough that differential protection is desired, it will be ordered with all winding ends accessible and suitable for installation of CTs.
Bill
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"Why not the best?"
Jimmy Carter
RE: Floating or grounded neutrals on motors and supplies
Anyone else in IEEE DEIS... you've got until 9/29 to vote.
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(2B)+(2B)' ?
RE: Floating or grounded neutrals on motors and supplies
Bill
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"Why not the best?"
Jimmy Carter
RE: Floating or grounded neutrals on motors and supplies
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(2B)+(2B)' ?
RE: Floating or grounded neutrals on motors and supplies
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Floating or grounded neutrals on motors and supplies
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(2B)+(2B)' ?
RE: Floating or grounded neutrals on motors and supplies
"cap current"
should have been
"capacitive current"
(I am assuming no capacitors connected to ground of course)
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(2B)+(2B)' ?
RE: Floating or grounded neutrals on motors and supplies
I was responding to this particular post:
Bill
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"Why not the best?"
Jimmy Carter
RE: Floating or grounded neutrals on motors and supplies
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(2B)+(2B)' ?
RE: Floating or grounded neutrals on motors and supplies
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(2B)+(2B)' ?
RE: Floating or grounded neutrals on motors and supplies
Many thanks for coming back to this topic and thread again and take time to help. I did obtain a lot of information from our field, service, installation and marketing guys on this. There is no standard way of connecting these motors even though this may sound very risky in some situations. Grounding of MV to HV neutral supplies is not always there (suprising). The more we learn about this the more surprized we are at how diverse approaches and practices are.
Thanks Pete for voting for me on the IEEE DEIS run. I am a rotating machine guy and I agree, focus is on other topics even if WE produce the MVAs ! All assests are important though of course.
The discussion on the differential CT sensitivity, capacitive and resitive insulation current values is the heart of the thread. I would just say that if we have a diff CT precise enough to give C and DF online then we would have IEEE286 ONLINE possible. A big deal I think.
Cheers
Karim
RE: Floating or grounded neutrals on motors and supplies
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(2B)+(2B)' ?
RE: Floating or grounded neutrals on motors and supplies
Muthu
www.edison.co.in
RE: Floating or grounded neutrals on motors and supplies
A typical GFI receptacle in North America will be rated at 15 or 20 Amps but will trip at 15 milli-amps. There is your 1000:1 ratio.
I don't suggest using a GFI but the measuring methods may be scalable.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Floating or grounded neutrals on motors and supplies
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(2B)+(2B)' ?
RE: Floating or grounded neutrals on motors and supplies
=====================================
(2B)+(2B)' ?
RE: Floating or grounded neutrals on motors and supplies
=====================================
(2B)+(2B)' ?