REALLY big motors


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I may be wrong, but I believe he is intending to run the motor with a single phase feed to the VFD. Then you need to oversize. Greg
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Greg O wrote:

I think I would if I could, but the VFDs I checked out on the web all seem to be 3 phase INPUT.
I'm starting to wonder. This is for a motor that most certainly does not need to be variable frequency. Sounds like I just need a real beefy motor start capacitor to accomplish the startup. Isn't that where the starting current in a VFD comes from ?
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I would guess that a majority of the three phase VFD's on the market are capable of running with a single phase input. You just de-rate the drive rating by approximately 1/3, to avoid overloading the diodes that are used to charge up the DC bus, since you will be missing one of the phase lines, and the diodes in the current paths of the single phase supplied to the drive have to pass all the current that the drive uses. I use 230 and 460 VAC three phase VFD's at home to get three phase for some of my machinery, and have had no problems. You do want to use the drive to start and stop the motor though, because you stand a good chance of damaging the drive if you break the circuit between the motor and the drive with the drive running the motor. Also, as a rule, you should keep your wire lengths between the motor and drive to a minimum, because you can get reflected wave voltages between the motor and drive that can damage the motor wire insulation and reduce the life of the motor when you have really long wire runs (typically 100 feet + ), unless you have inverter duty motors with a higher voltage insulation rating.

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Scott Moore wrote:

Yes, but they will ALL run on single phase. The rectifier and capacitors run a bit hotter on single phase, that's the reason to derate them.
No, a VFD can slow-start the motor. How slow do you want it? Most can go to several hundred seconds. Allowing the motor to gradually wind up from zero speed to rated speed allows the line current to be hardly more than normal running. I have my 7.5 Hp lathe set up for a one second acceleration, and the lights don't blink at all. I know for sure starting a 7.5 Hp motor directly off the line would make them blink plenty. My 2 Hp air compressor blinks the lights quite visibly.
I had a friend who tried to start up a 17 Hp motor-generator set, and all he could do was blow breakers. He even managed to trip the breaker on the pole transformer, once! We did eventually manage to spin it up with a delta-wye conversion, but we never mamged to get it to run on the line in the proper configuration.
Jon
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Jon Elson wrote:

Well, it seems the consensus is that it is possible to run motors in this range. That was what I was looking for, before I went off to purchase a beast this big. Thanks for all the good answers in this thread. Glad also to know I am not the biggest equipment nut on the block :-)
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Scott wrote:

I'm dissapointed. The thread title was "REALLY big motors", and it turns out we're talking about 10HP ;-)
Big motors are 700HP like the ones in our lab - they use 5HP motors just to move the cooling air. REALLY big are like the ones in the steel rolling mill downtown. They are rated at 3000HP each (3 on one shaft, 6 shafts, 54,000HP total in one big room). They use several hundred HP each just for the cooling blowers. Those are DC motors, and the armature ammeter reads 0-15 kiloamps, at 700VDC!
Jon Elson wrote:

I don't mean to nitpick, but not every VFD will run from single phase. In the 10-20HP range, they _probably_ will. (And smaller ones even more so.) But As Jon points out, things run hotter on single phase. And there's the rub. The detailed explanation is probably boring to anyone who's not a VFD designer, but here it is:
In order to get UL listing the VFDs need to pass a single phase test. UL does not require the drive to run successfully on single phase input, it just needs to be safe. The manufacturer has a several ways to pass the test:
1) They can beef up the rectifier and DC bus caps, so that things work cool and reliable even on single phase. In that case, they often add single phase ratings to the drive nameplate. This is common on very small drives, say 5HP and less, because the beefier rectifiers and caps are pretty cheap. These drives can be used up to their single phase ratings, with no problems.
2) They can use rectifiers and caps that are sized for three phase. Things run hotter on single phase, but as long as the temperatures stabilize at a safe level, the drive passes the UL test. These drives are rated for three phase only - single phase operation is considered an abnormal condition. The extra heat will cause reduced component life and poor reliability if you run them at their full rating on single phase power. But if you derate by aproximately 1/3, they should be OK. Running them on single phase power may void the warrantee though, since the manufacturer doesn't rate them for that.
3) They can design protection circuits that detect a single phase condition and shut down the drive immediately. This is very common on large drives (100HP and up). No sane person runs that size drives on single phase, and loss of a phase will cause significant overheating of rectifiers and especially capacitors, so shutting down is the right thing to do. Drives with such protection circuits will not work on single phase at all, even if you derate them.
The odds of getting a "type 3" drive aren't very high if it's less than 50HP, but that depends on the manufacturer. Scott wrote:

Nope. A start cap, no matter how big, doesn't reduce the starting current. (Start caps only apply to single phase motors anyway.) Starting current is typically 5-10 times the running current.
To address your specific issue (10HP):
I just looked at a 10HP motor on the shelf in the lab. Nameplate data is "10HP, 230/460V 3phase, 26/13A". That means 26 amps per phase on 230V three phase, 13 amps per phase on 460V. To get the single phase equivalent, multiply by the square root of 3, which gives you about 45A (at 230V).
So it would take approximately 45A at 230V single phase to run the 10HP motor fully loaded. It would be roughly the same whether you use a VFD or a rotary phase converter. (Probably a little higher for the rotary, especially if it isn't well balanced - maybe 50-55A.)
However, if you attempt to start that motor across the line, (with the rotary converter), the starting current will be at at least 5 times the running current, or about 225A. (The rotary helps a little with the starting surge, but not much.) Pulling 225A from any residential panel is almost certainly going to cause the lights to flicker, and you also have to worry about tripping the branch breaker if you are on a 50A or 60A branch circuit.
On the other hand, if you use a VFD, it can accelerate the motor from zero speed to full speed over several seconds, while limiting the current to under 50A the entire time. So with a VFD, it is reasonable to run that 10HP motor on a 60A 230V single phase circuit (you might even get away with a 50A circuit, but that is marginal - code says the branch should be rated at 125% of the load, and 45A * 125% is 56A).
It's the starting surge that ultimately limits how big of a motor you can use on a rotary converter.
By the way, remember that 700HP motor I was joking about earlier? We did start that thing across the line a couple times (480V three phase, not 230V single phase). It's kinda scary. You can hear the 500MCM cables vibrating inside the conduit as it comes up to speed. The facility energy guys told me later it drew about 4500A for nearly a second. This is with no load other than inertia. On the other hand I routinely use a VFD to accelerate the same motor from zero to full speed in 15 seconds, drawing only about 100A from the line.
Regards,
John Kasunich
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I would love to see pictures of such motors.
i
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wrote:

Remind me on Monday and I'll post some that show a 11,250 HP with a spun armature ( the armature weighs 38,000 pounds). All this to fill a little ( 50,000 cu ft) air tank that only lasts us .1 to 20 seconds. Pat
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I'll try. Meanwhile, I would this interesting page:
http://www.dresser-rand.com/newsroom/photo_gallery.asp
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wrote:

spun
(
Where can I drop the pictures? Pat
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how big is your picture file? I have a website of my own, the question is how to get them to me. Try emailing to ichudov AT algebra DOT com. Thanks!
i
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wrote:

SNIP!
It is on the way. Pat
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I have not received anything yet... try ichudov AT yahoo DOT com
i
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wrote:

I am impressed. I'd like to see some pictures of these wooly mammoths myself. I thought I'd seen something when a 40 KVA aircraft generator had a shorted feeder and blew a gaping hole all the way through the #1 engine pylon on a 727. I can't imagine what kind of damage a load short would do with this equipment.
Garrett Fulton
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wrote:

Don't have any pics of the rolling mill motors. It was about five or six years ago when I was there. The motors are horizontal, with the shafts about knee high from the main floor. The rotors extend down into the basement. Shafts are about 2 feet in diameter, with 4 or 5 foot diameter by 4 to 6 inch thick flanges on the ends for coupling. (Flanges are bolted to each other by 30 or so 3" diameter bolts.) Rotors are about 8-10 feet in diameter, and stators about 20 feet. Each motor is about 30 feet long, and three are coupled together to drive one shaft.
The overall setup is actually three stages. First there are four large synchronous AC motors, running on 33KV if I recall correctly. They are about 13,500HP each, and run at constant speed. Each AC motor drives a set of three 700 volt DC generators (4,500HP each). The 12 generators are connected to the 18 DC motors (three 3000HP motors on each of 6 shafts, driving the 6 roll stands). They are connected in such a way that even with 2 of the 4 AC motors (and 6 of the 12 generators) out of service, they can still run the mill (at reduced capacity).
The electronics controls the generator field current, which controls the generator armature voltage, which is connected to the motor armatures and determines the motor speed. The motors are precisely controlled to roll the steel, maintaining the proper tension between each stage. As the steel is rolled, it gets thinner and longer, so the final stages need to run faster than the earlier stages. The motors vary accordingly - the first few stages are shorter and fatter to develop more torque. The very first stage goes thru a gear reduction before driving the rolls. A back of the envelope calculation says the torque at the first stage rolls is about two million foot-pounds. (9,000HP at 20-something RPM.)
The whole arrangement was just mind boggling - not only are the individual motors huge, but the football field sized room is just full of them - between the AC motors, the generators, and the DC motors there are 36 huge machines on 10 shafts, totalling over 150,000HP.
I _really_ wish I had pictures of it!
Here are some less impressive photos that I found on the web. Some of these motors are even bigger, but the photos don't do them justice. http://www.teco-wmc.com/Company/News/News050104_AEDC_Synchronous_Motors.htm
http://www.geindustrial.com/cwc/products?id=largedc (look closely at the right hand pic - there's a guy on a ladder inside the stator)
http://www.geindustrial.com/cwc/products?id motors
Regards,
John Kasunich
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Another couple pics. The one on the back of the brochure (where it says DC testing) is similar to a rolling mill motor, very high torque at rather low speed.
http://www.geindustrial.com/products/brochures/GEA-4098A.pdf
Regards,
John Kasunich
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Thanks, John, for taking the time. Very interesting, indeed.
Garrett
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Scott Moore wrote:

So I found this chart online:
HP     VOLTS     100FT     150FT     200FT     300FT     500FT =====================================================1.5     230V     12     12     12     12     10 1.5     460V     12     12     12     12     12 2     230V     12     12     12     10     8 2     460V     12     12     12     12     12 3     230V     12     10     10     8     6 3     460V     12     12     12     12     10 5     230V     10     8     8     6     4 5     460V     12     12     12     10     8 7.5     230V     8     6     6     4     2 7.5     460V     12     12     12     10     8 10     230V     6     4     4     4     1 10     460V     12     12     12     10     8 15     230V     4     4     4     2     0 15     460V     12     10     10     8     6
Of course, that is 3 phase. I assume that the 2 phase input to that would have to be larger. However, it seems that the start current would be confined to the 2 phase section (?) because the motor would get most of its start current from the idler.
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Our 15Hp 230v 3 phase air compressor draws 42 amps running, and a heck of alot more on startup.
Tony

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