Neutral grounding of Large Motors 3

[prc]

Neutral of large Ac generators are grounded through Neutral grounding resistors to limit the zero sequence currents. How the neutral of motors is grounded? Same way or through inductors? If later is the case, reason for the same.

 

[waross]

Three phase motors do not have a neutral connection, so, no grounding.
But then I haven't seen everything yet. The largest motors that i have worked on have been 3000 HP. Both DOL and wound rotor.

Bill
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"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I've never seen a wye-connected 3-phase motor with a ground.

 

[waross]

We are familiar with the high currents and heating caused by supplying a motor with power with phase-angle and/or magnitude unbalances in the supply voltage. If grounding the wye point implies a connection to the supply system neutral, it will in most cases make the reaction to an unbalanced supply worse.
The only possible reason to ground the wye point may be to correct unbalances on the supply line. This would happen at the expense of some or all of the motor HP capacity.
The correction to the supply balances and the resulting higher currents may be similar to the action of a four-wire-wye/delta transformer bank.
I have never seen a wye/delta bank intentionally used to stabilize the supply.
I did consider using a wye/delta bank to correct a serious unbalance problem at a large sawmill but the customer solved the problem by going out of business.
I did see it happen by accident once.
A three phase distribution line extended about 15 or 20 miles. At the end of the three phase section there was a customer with a wye/delta service. From that point two single phase lines extended for 10 or 15 miles in two directions.
This resulted in a fairly substantial neutral current and both phase angle and voltage unbalances on the three phase line.
Instrumentation at the substation always showed the distribution line to be well balanced.
The three phase customer went out of business.
The utility disconnected the wye/delta bank.
Then the instrumentation at the substation always showed the distribution line to be poorly balanced.
Bottom line; Under normal circumstances don't consider grounding the wye point of a motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Ive never seen the earthing systems typically found on generators applied to motors, regardless of size.

Earth fault limitation is typically by resistor at the transformer neutral point on the industrial systems which supply the majority of large motors. Most resistors are metallic, but there are plenty liquid types still in service. Reactor earthing is - at least in the UK- fairly unusual on industrial systems, and I can't think of any example of a Petersen Coil arrangement on a non-utility network. Someone somewhere will probably have one on their industrial network just to prove me wrong, but it would sufficiently rare to be worthy of comment.

 

[jraef]

Remember, a generator is FEEDING power to various loads, the distribution of which is not in the purview of the generator mfr. resulting in the possibility that the generator must be capable of feeding an unbalanced load.

In the same machine used as a MOTOR, the only load is the motor windings themselves. So how is the load current going to be unbalance if it is not a problem leading to shut down? In other words, you don't WANT a motor running continuously in an unbalanced condition, so there is no need to ground the eye point anyway.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ScottyUK]

Hi jraef,

Big utility generators don't supply any neutral current to the system by virtue of the delta-star GSU transformer connecting them to the system. Any line-neutral loads on the systemare seen by the generator as imbalanced line currents on the delta side, although in honesty the machine line currents are pretty well balanced on most machines connected to the transmission system. The neutral-earth connection only passes a handful of amps under fault conditions.

 

[waross]

I prefer to see a generator feeding into a delta/wye.
With a generator feeding into a delta/wye any unbalanced currents are fed by all three windings of the delta and all three windings of the generator. For an unbalanced current in one phase the current in all three windings is equal to 1/2 of the unbalanced current.
For a unity power factor unbalanced current, the current in one winding will be in phase, in the second winding 50% leading and for the third winding 50% lagging.
Generator kW = Load kW. Generator KVA = 150% of load KVA.
Look at the double delta connection used to convert a three phase generator to true single phase and the allowable currents for single phase and three phase.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

Thank you all for the great explanation.