Large motors and soft start

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Harold_V
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Large motors and soft start

Post by Harold_V » Sat Jun 08, 2019 4:26 pm

My project for the past couple years has been to get an old motor/generator era power supply, for an induction furnace, in operation. It's a monster of a thing, tipping the scales at about 6,000 pounds. The unit is rated @ 50 kW, with the motor and generator on a common shaft, mounted vertically.

I have started the unit about ten times or so. On three occasions, it has blown the fuse on one leg on the primary side of the power supply (the fuses are between the transformers and the primary lines, which is the high voltage supply provided by PUD, our power source).

I requested them to try to determine what the problem might be. I was not thrilled with what I learned.

I have a three phase Delta service, rated @ 400 amps, 240 volts. That's more than enough to operate the power supply, which demands 190 amps at full load. However, I was shocked to see how high the starting amperage was. I knew it would exceed the service rating, but didn't expect it to be so high. Instrumentation disclosed an instant spike of 1,492 amps as the start button was depressed, which fell to about 50 amps after seven seconds. Once up to speed, the drop in demand was nearly as fast as the spike occurred when starting. The generator, at this point, is not excited, so there is no load.

I have a couple options. I can pay PUD to install yet larger transformers, which would then allow for larger fusing on their end, or I can pursue a soft starter for the MG set. I may have a third option, which I will explore, and that being the use of slow blow fuses by PUD, as the panel I use was fused with 350 amp fuses, which held until the last time I took out one of those on the primary. The same leg in my panel also failed, so I upped them to 400 amps (I am wired accordingly). There may be the slight chance that the time delay fuse will hold long enough without failing.

Mean time, if any of you have anything to suggest, I am open to discussion. I would also be interested in a soft start unit, rated for 75 horse, and 240 volts three phase.

I should note that I have also considered spooling up the MG set before hitting the start button, but access to the armature is pretty much impossible, so that option is likely not one I can entertain.

H
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armscor 1
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Re: Large motors and soft start

Post by armscor 1 » Sat Jun 08, 2019 5:05 pm

Hi Harold, I assume the motor is DOL, Direct On Line?
If the motor has 6 terminals the cheapest option is to install a Star Delta starter and reduce your starting current by root 3.
1492 Amps divided by root 3 equates to 861 amps, almost half the starting current.
A much cheaper option than a soft starter.

John Hasler
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Re: Large motors and soft start

Post by John Hasler » Sat Jun 08, 2019 5:27 pm

I second the star-delta starter suggestion. If that option is unavailable slower fuses have a good chance of working.

Harold_V
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Re: Large motors and soft start

Post by Harold_V » Sun Jun 09, 2019 12:57 am

Thanks for the idea! Now if only I understood what it meant. I know just enough about electricity to get in trouble.
I understand the difference between Star and Delta. I'm not sure I understand the DOL comment. Power leaves the 400 amp disconnect and is hooked to the large motor starter, which, in turn, is activated by a push button on the control panel. Could you please help me understand if that is DOL? If not, perhaps a description of what is?
I'm keenly interested in the proposed idea. I can't help but think that if I can cut the starting amperage by even 10%, it will spell a huge difference, as it starts without taking out a fuse most of the time.

H
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armscor 1
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Re: Large motors and soft start

Post by armscor 1 » Sun Jun 09, 2019 2:26 am

Dol simply means there is one contactor, three sets of contacts, one for each phase which close when pressing start and feed directly to the motor windings, brutal considering an electric motor needs at least 5 times the full load capacity amperage on the nameplate of the motor.
If you look inside your control panel and only has one large contactor, it is Direct On Line.
A Star Delta has 3 contactors, Main, Star(Wye) and Delta.
On starting Star Delta starters the main and star contactors close and after a predetermined time set by a timer usually around 3 seconds the star contacts open and immediately the delta contacts close, the star and delta contactors are interlocked electrically or mechanically to prevent both being closed.

Harold_V
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Re: Large motors and soft start

Post by Harold_V » Sun Jun 09, 2019 4:45 am

Yep! It's as you describe.
My one concern is if I simply prolong the overcurrent, that it will still heat the primary fuse to the point of failure. If memory serves, I was told that the fuse is adequate to deliver about 485 amps in the secondary, and I suspect it's a slo-blow. If the starting amperage is reduced to, say, 800 amps, I'll still be drawing beyond its capability---although not as much---and the prolonged demand may still blow the fuse. Your thoughts on that would be appreciated.

H
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Bentworker
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Re: Large motors and soft start

Post by Bentworker » Sun Jun 09, 2019 9:52 am

Harold-
For clarification which type of starter do you have now?

Direct on line or Star-Delta?

I couldn’t make out what your current configuration is from your last post.
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Patio
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Re: Large motors and soft start

Post by Patio » Sun Jun 09, 2019 10:20 am

Bentworker, it is a direct line. Power comes in from the meter, through a disconnect then directly to the unit. Inside the unit there is a contactor for starting. I don't believe is has any overload heaters, but I could be wrong, it has been awhile since I looked at the schematics. It is fused in the disconnect.
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spro
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Re: Large motors and soft start

Post by spro » Sun Jun 09, 2019 1:49 pm

I feel for Harold in this. I'm too simple to proscribe the other actions that were suggested but they make sense. The idea of a "pony" gear head motor comes to mind. There is flat out inertia in that MG, drawing huge amps to move it. A gear head motor with its own feed and disconnect clutch could get it rolling..? Yes, Harold already thought about that. He also said that the generator wasn't excited at start up which is fine, providing there is other switch gear. All the stuff I used to be involved with, required the turbine or engine to get up to speed first. Then the load was load placed upon it. immense drag at start up because a generator draws so much current when even turning.
This is different than small MG units .

rrnut-2
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Re: Large motors and soft start

Post by rrnut-2 » Sun Jun 09, 2019 3:06 pm

Harold, look at this one. There are others, but this is a pretty good company to deal with.

https://www.automationdirect.com/adc/sh ... )/sr55-302

Jim B

rrnut-2
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Re: Large motors and soft start

Post by rrnut-2 » Sun Jun 09, 2019 3:12 pm

Here is another one. I use a lot a WEG VFD's and they are good. Not made in China.

https://www.wolfautomation.com/soft-sta ... -200-amps/

Jim B

choprboy
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Re: Large motors and soft start

Post by choprboy » Sun Jun 09, 2019 3:14 pm

Harold,

I agree with the others, a star-delta (or wye-delta) starter would seem to fit the bill. The star connection reduces the effective voltage across each motor winding by ~40%, which reduces the starting current by ~66%.

Do you know the actual fuse installed in the service? Thermal fuse blow characteristics follow a time-current curve, which most manufacturers publish. A time-delay fuse like the Bussmann FRN-R400 will hold roughly 5500A for 1sec, 2000A for 10sec, 1000A for 100sec, and take several hours before blowing at a bit over 400A (may be a bit optimistic, but that is what Bussman says in their charts). It sounds like your current fuse may be a standard blow, not a slow/time-delay blow.

NOTE: Ignore the above... it applies to customer service (meter/disconnect), not the utility service. I misunderstood that Harold was blowing his service fuses, not that the utility was blowing the pole fuses.

Note: You said the motor spins up and current drops to 50A, with no generator load. Is that right? Your burning 20kW/27HP just turning the motor? Are you sure there isn't something wrong with the windings? Or does the current continue to drop?

https://electrical-engineering-portal.c ... or-starter
https://literature.rockwellautomation.c ... _-en-p.pdf
Last edited by choprboy on Sun Jun 09, 2019 5:29 pm, edited 1 time in total.

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