MOTOR specification
MOTOR specification
(OP)
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
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





RE: MOTOR specification
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.
RE: MOTOR specification
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.
RE: MOTOR specification
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?*
RE: MOTOR specification
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.
RE: MOTOR specification
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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RE: MOTOR specification
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
RE: MOTOR specification
RE: MOTOR specification
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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RE: MOTOR specification
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RE: MOTOR specification
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
RE: MOTOR specification
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RE: MOTOR specification
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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RE: MOTOR specification
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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RE: MOTOR specification
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RE: MOTOR specification