4160v motor autotransformer starting

[jbouverette]

newbie here
recently got a new reduced voltage starter for our 4160V 400HP motor at our plant. doesn't work.

single phase transformers, 167,167,&250 kva. i can "hear" a two stage start(reduced voltage and full voltage). the motor is a rebuild from corporate stock in North Carolina (we are in Michigan).

contracted engineer for the contractor we hired for this says current waveform imbalance from mismached transformers(OK, i'll buy that), undersized transformers, autotransformer to big(he designed), and no damper on fan to reduce load. oh, he also says motor is not a 400hp but could be a 500hp.(autotransformer to big?)

i myself could not reduce timer to get autotransformer completely out of circuit, still comes on for two seconds (crawling around inside by the buss makes me nervous). after that the motor takes another 14.9 seconds to come up to full speed. draws about 250A, nameplate is 48.

the wierd thing...if i start it with the autotransformer in the circuit with (his) recommended start of 30s. then A phase will draw about 80A (250kva trans), B & C draw about 67A (167kva trans) then the run contactor pulls in.... and then the multilin then will shut everything down on excessive acceleration time

kinda new to this whole voltage class, any suggestions would be appreciated...thanx

 

[rbulsara]

Morale of the story: Hire experienced consulting engineer to design and size starter, starting method and protection of such important installation in first place and then hire a qualified contractor to install it. Engineering fees usually cost less than 10-15% of total construction cost. You can do the math as to how much you saved.

Now that is out of my system....

1. The transoformers are OK, although little odd to use units of mismatched ratings, but it is not cause of the problem.

2. It is true that having a damper that is closed during starting (acceleratin period) will help a great deal.

3. What I gather that the motor starts ok in 14.5s when started direct on line. 250A is about 5X FLA. Both the acceleration time and current is very normal.

4. If there are no other adverse impact on rest of the system like excessive voltage dip or complaint by utiltiy co etc..then I would eliminate the autotranformer and start the motor direct on-line.

5. If you want to keep/ need reduced voltage starting, install dampers in supply side.

6. What is the tap set at? 85%, 65%, 50%? Use higest tap, but by then its as good as DOL start...

Good luck..

 

[ScottyUK]

A possible addition to rbulsara's item 1).

The 250kVA transformer almost certainly is a stiffer source than the smaller 167kVA transformers, so under load the voltage drop on the larger transformer is lower than for the other two. This will account for the higher current seen on the line fed from this transformer.

If you have one of the newer Multilin's, the Multilin may not like the current imbalance as it has a large negative sequence component which will overheat the rotor if allowed to persist. The Multilin may be protecting your motor by tripping during acceleration while the rotor is within its thermal damage curve, or the NPS protection may be a bit too sensitive. Get assistance from someone who knows how to interpret the motor data to set the relay.


What do you mean by [blue](crawling around inside by the buss makes me nervous)[/blue]? I hope you do not mean that you have the front of the starter open so you can make adjustments while the starter is in service. Have you ever seen an MV switchgear fault?



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[jraef]

To further ScottyUK's concern...
Low Voltage is like a poisonous snake. You have to do something wrong or accidental to make contact and get hurt.
Medium Voltage is like a hungry lion. All you have to do is open the door!

"Venditori de oleum-vipera non vigere excordis populi"

 

[BJC]

jbouverette
There's not enough information. Where are the tranformers? What are the voltages?
They sound like diatribution transformers. Most auto transfromer starters I have seen have the auto-transformer build into the starter, it's usually a three phase transfromer.
Are the 3 transformers upstream of the motor starter? How are they conneced? If ther connecet in parallel and one is different you could be over loading two of them.
Do you have a schematic of the starter circuit?

 

[aolalde]

Please clarify the following.
167,167,250 kVA, are single phase transformers intended to provide 500 kVA three-phase to feed the motor? from ? volts to 4160 V?
The starter has it own autotransformer with one step reduced voltage ?% and full voltage after ? seconds.
The motor was rebuilt from? HP,? poles, ? Hz, ?volts to ?HP, ?poles,? Hz, ?volts, manufacturer?.
The load is a Fan that spins at ? rpm and the BHP at ? rpm is?. If the dampers are closed the BHP at full speed are reduced to?.

 

[jbouverette]

Sorry it took so long for me to get back, thank you for all of your valuable posts, especially rbulsara, thanx

let me clarify and answer some things
1. i would never, ever, enter an energized MV cabinet. even with the disconnect down, ground stips contacted and hot sticks used; i still will not get my hands in there....there are people to do that for me. thank you!

2. motor is a rebuild, no info on thermal damage curves or anything like that. i would like to see motor sent out to shop and inventoried, tested, etc... to get this info but the contractor never mentioned it at turnaround so it will be at least this summer before that can be done (there are no reliable motor backups. yes, yes, i know, not my decision) motor is plated at 400HP,4160V,48A,class F insulation,don't remember the frame off hand and then that's it.

3.in response to DOL starting. i like it a lot,but, we paid 50g for a softstart, electric company is hitting us with the peak charge. bean counters want to know "why".

4.transformers are single phase with 14.4 at primary.

5. the reason i included the info about 80-67-67 on softstart is because the "contracted engineer of the contractor we hired" says this motor is in the thermal damage curve if we continue to start more than 30s. ( i don't know where he got this from, is this an industry standard or something?) he says to get below 30s. we need a supply of 150A on startup. autotransformers are at their 80% tap, highest one avail on these. these are single phase autotransformers. i personally suspect that the weird amp draw has something to do with the saturation points of the autotransformers; but, like i said i am new to this voltage class. i have heard of techs using autotransformers on 120V lines for use of current limiting, i am still looking into this.

please, this starter is "built" by the contractor. do not assume this was "ordered".

i do realize that there are a lot of weird and mismatched parts here, trying to identify the cause of the problem, if there is one.

thanx a million for all your responses, i really appreciate it.

 

[rbulsara]

jbouverette:

Thanks for getting back. Also thanks to ScottyUK.

I do tend to agree with ScottyUK that your Multilin settings may be too sensitive and may be tripping on unbalanced startnig current. What flag does it give when it trips?

Firstly you have right to ask for identical tranformers (preferably all 250kVA) from the contractor. Do insist on that.

Meanwhile, see if you can add delay to override starting period or set the negative seq current setting to alram only temporarily to see if the motor starts OK on reduced voltage start. This may involve some risk of motor overheating, I can't say without much info,how risky it is.

I still standby with my original post with one addtional advice based on what ScottyUK suggested, ask for identical transfomers.

 

[jraef]

Something is still not right here. You mention that the transformers are 14.4kV primary? That sounds as if they are stepdown primary transformers in front of the starter, not the autotransformers inside the starter. If so, I would venture to say that the 167kVA ones are too small for this application, even with an RVAT.

One other disturbing issue is that you mentioned the "electric company is hitting us with the peak charge". Did your "consultant" explain to you how the utility charges for peak demand? Chances are very high that the Michigan utility in question uses the most common method of assessing peak demand charges here in the US, a sliding 15 minute demand window. This means that no matter how you start the motor, the demand meter does not even see it, it only records the total power consumed within the highest 15 minute period. Starting ATL (DOL), RVAT, RVSS, VFD, etc. will have no real effect on your demand charges!

Sounds like you need to ask you "consultant" some more questions!

"Venditori de oleum-vipera non vigere excordis populi"