AutoTransformer Motor Starting

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mull982

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hdpeng said:
How's it going out there? Has anyone heard from 'mull982' in the last 2 days?
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I apoligize for not responding sooner, I have been in the field trying to get this new crushing plant up and running.

With that said, I was finally able to get this motor started. What I ended up having to do, was raise the overload curve that was set in the motor protection relay. For those who are familiar with the relay, it is a Multilin 369 relay, and the overload curve was raised form a (4) to a (6). The curve setting of (4) was giving us problemns and was causing the thermal capacity to build to 100% during starting. Raising this curve setting to (6) allowed the motor to start, and only brought the thermal capacitty to 73% during the start. I spoke with the engineer who designed the settings for this relay before I changed them, and he verified that it was o.k. to raise the overload curve to a (6).

In speaking with him, I also found out that the reason for the AutoTransformer start of the 300hp motor was due to the fact that the manufacturer of the motor application (a crusher) recommend that we brought the motor up to speed slowly. The autotransformer starting had nothing to do with the avaliable system capacity. With that said, I found out that the design was to use 80% taps on the autotransformer. I have now switched these taps from 50% & 65% respectively to the 80% taps on the autotransformer.

When starting the motor with the new 80% taps, adjusted overload curve, and autotransformer switching time of 5 sec I am seeing an acceleration time of 8.2 sec. I am still seeing 314A during the start which has me a bit confused b/c as I stated earlier LRC of this motor is only 256A. With reduced voltage I would expect to see less than LRC when starting, and and confused to why I am seeing more at 314A. Any ideas??

Anyway the motor is finally starting o.k. but I'm sure it will require some fine tuning. I am still curious, to what is the best time to switch from reduced voltage to full voltage during the start. I think Jaref mentioned 80% before but I'm sure this is something I'll have to play with.

Thanks for all the help.
 
Jraef

Jraef

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Electrical Engineer
Delving deeper then;

The LRC is 256A according to the nameplate, or are you assuming it is 600% of FLC? Because the 600% is a general rule of thumb, not an absolute. It depends upon your motor's design. (Edit: Must be nameplate, the math doesn't work out right after reading the original post)

Are you sure you don't have an issue with a CT ratio or something?

Are you using a Peak Hold feature on a meter to read that 314A value or the 369 display?
 
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mull982

Senior Member
Jraef said:
Delving deeper then;

The LRC is 256A according to the nameplate, or are you assuming it is 600% of FLC? Because the 600% is a general rule of thumb, not an absolute. It depends upon your motor's design. (Edit: Must be nameplate, the math doesn't work out right after reading the original post)

Are you sure you don't have an issue with a CT ratio or something?

Are you using a Peak Hold feature on a meter to read that 314A value or the 369 display?
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The LRC value of 256A is from the motor nameplate data. I will double check the CT's but Im pretty sure that they are correct based upon the fact that the current readings on the Multilin match those on a seperate analog meter.

I am using the 369 metering function on the Multilin in order to read this current value of 314A.
 
weressl

weressl

Esteemed Member
Location
Texas by the Gulf, geat crab, great shrimp
frenchelectrican said:
one thing it kinda bother me why 9 seconds delay between the autotransformer and swiched to the DOL [ direct on line ] connections but i have a hunch but not sure it may ring in here right.

with the long delay the motor speed do drop down a bit and get the phase connect out of sych. by time the DOL contractors get on line and it draw just much current as plain jane DOL set up.

i will just suggest try to closen up the gap time from 9 seconds to something like 5 seconds or so. the reason i suggest that because it will really reduce the amout of the time the motor loose the speed when switching over.

Merci,Marc
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Marc, it is not DOL it is FVNR(Full Voltage Non-Reversing) and it would include SS(Solid State) contactors that are not reduced voltage starters.
 
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