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MOTOR specification

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power2020

Electrical
Aug 18, 2005
127
Hi Friends,
We have a motor of 3.3kv, 450KW. This motor is going to get supply from 3.3kv switchgear, which some times operated by grid supply and some times by Generator supply.

Grid system is having solid ground neutral system.
Generator is Resistive ground.

My Question: What should i specify in my motor purchase datasheet and specification?
Shall it be designed for resistive grounding or solid grounding?

Thanks in advance
 
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Normal electric motors have internal neutrals and no external connection is done; except for those for differential phase protection with external open neutal leads.
The generator neutral if grounded through a resistance will limit or reduce the current intensity of a ground fault. Your concern should be directed toward the ground fault protection relays rather than the motor specification.
 
Hi aolalde (Electrical),
I am not looking for protections, rather i am looking for design constrints of motors.
For star connected motors, i guess they will insulate the motor winding for phase voltages, i am not sure whether they will design it for line to line voltages.

I understood, If in case of resistive grounding system, during one phase to ground fault in power system, the voltage between other phase to ground will build up to line to line values. i have to work out for exact values..

My concern is : What should i specify in my motor purchase datasheet and specification?
Shall it be designed for resistive grounding or solid grounding?

or

There is no need of specifying grounding system in motor specs or datasheets.

Please discuss..

Thanks.


 
The motor /generator winding insulation is designed based on line-to-line voltage and not on phase voltage (for the simple reason the standards specify the final ac hv test based on the rated nameplate voltage and not on whether they are connected in wye or delta).

As aolalde says, it is more of protection issue than the motor issue.

*Why a man thinks he outrun a chasing dog when it has twice as many legs?*
 
I agree - the motor design will be the same. But if there is a motor winding ground fault while operating from the solidly-grounded grid, the damage to the stator core will be much greater than for a ground fault into a resistance-grounded system.

IEEE recommendation is that all medium-voltage systems serving medium-voltage motors be resistance grounded. Operating motors directly from utility grid voltage is generally not a preferred method.

Also, if you connect motor directly to grid voltage, I'd strongly recommend surge arresters and surge capacitors at the motor terminals.
 
I agree with the others.

Large transformers may be designed with graded insulation systems (more at the line terminal than the neutral), but motors are not.

What you tell them about the electrical system likely won't enter into how they design the motor. But what you require for electrical testing may. The ac hi-pot is pretty standard 2E+1. But the surge testing has some options in both NEMA MG-1 and IEEE522 (2.5 pu or 3.5 pu) which can affect the design.

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Thanks guys,
I have got good information from all of you.
However, my main question is.

1) Do we need to mention power system grounding in motor purchase order/specification?

I am looking / expecting answers like..

Yes it is required
or
No It is not rquired (to mention grounding Solid or Resistive).

Thanks again
 
Whether you mention grounding or not it won't change the motor, but if you do mention it you may have a bunch of questions to answer from potential suppliers. I wouldn't mention it.
 
It doesn't matter whether you mention it or not. It won't affect the way the motor is supplied.

We ran into an interesting parallel type of question. At our power plant, we have ungrounded 460 vac electrical systems with many motors. Our motor purchase and rewind specs don't say anything about the power supply being ungrounded. We had an independent engineering assessment of our electrical system practices and reliability. One of the outside engineers commented that we should specify the ungrounded 460vac system in our motor purchase/rewind spec, since (in the outside engineer's mind), the motor supplier would provide a different insulation system to cope with the intermittent arcing fault phenomenon associated with ungrounded systems (that we have never experienced).

In response to that assessment finding, I called several motor suppliers and rewind shops to ask what they would do different if I told them it was an ungrounded system. The universal answer was: "nothing"

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correction - the system is 480vac, the motors of course are 460vac

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I understand that the arcing ground fault (or as it is sometimes called, a discontinuous ground fault) may develop a voltage in excess of any standard test voltage or insulation rating for a 480 volt motor.
One of my instructors related a first hand incident.
The enclosure was damaged on a compensator. (manual autotransformer starter.) The grounded metal case was deformed so as to contact the autotransformer winding near the wye point. When the starter was operated, the arcing ground developed a high frequency potential that was then transformed up to a high voltage by the autotransformer winding. This high voltage RF was impressed on one phase of the power supply and was high enough to cause random motor burn-outs throughout the plant. Always a failure of the first few turns of the same phase. The starting time was short enough that no problems were noticed with the starter.
When the cause was discovered an attempt was made to measure the RF voltage. The meter used was rated for 2000 volts and it failed from excess voltage. This was a fairly remote plant and repairs were effected before another meter could be obtained.
The point is:
This is not a condition that a supplier could reasonably be expected to anticipate or insulate for. Such failures are part of the cost of doing business with an ungrounded system.
respectfully
 
Also of note, NEMA MG-1 doesn't make any mention of the type of system the motor is to be hooked up to (grounded, ungrounded, high-R grounded etc). Another confirmation that it is not an important element of the motor specification.

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As far as the intermittent arcing fault generating wild voltages on ungrounded systmes, I agree it has been reported the way waross described it. But I believe it is a very rare phenomenon and a very few unlucky systems have the exact right combination of parameters (something like ratio of capacitive impedance to ground as compared to load impedances) that make it possible (and those plants will modify their system to eliminate it). The vast majority of ungrounded systems will never experience this.

At our two power plants we have a total of more than 1000 motors on 480volt ungrounded systems operating for 27 years. I have been the motor guy for the last 7 years and never heard of simultaneous motor failures on the 460v ungroudned systems (we have had simultaneuous failures of two higher voltage motors on resistance grounded 4.16kv or 13.8kv systems on two different occasions... once was a very nearby lightning strike, the other we think the generator action of one motor feeding the faulted motor caused the generating motor to fault due to end-turn movement).

The US Navy has ungrounded electrical systems as well. I have never heard reports of simultaneous multiple motor failures shipboard (and earlier in my career, my job allowed me to be informed of unusual failures of electrical equipment on approximately 50 S5W submarines for a period of about 6 years).


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I will mention that even in the absence of the freak transient behavior discussed above, it is to be expected that motors on ungrounded systems may be exposed to higher line to ground voltage on system faults. If there is a line-to-ground fault, the system doesn't trip (only alarms), and the motor sees a voltage to ground equal to the full line-to-line voltage (more than a motor on a grounded system would ever see on a sustained basis).

In spite of this, the motor suppliers and shops will give you the same motors for ungrounded system as for grounded system. I don't see it as a problem.... there is margin built into the insulation systems (ac hi-pot at 2E+1). If I did want a better insulation system, (and I don't), then I would specify a higher ac hi-pot voltage. Telling the motor supplier about your power supply system does nothing to tell him to change the insulation.

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Just wondering about something: does the output of a vfd resemble a grounded system or an ungrounded system?

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VFD output resembles the VFD power source grounding type unless there is no isolation transformer in VFD drive.
 
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