REALLY big motors

It's got start caps that come in for three seconds on motor one. Then a time delay of five seconds and motor two comes in with another set of start caps for three seconds. I did it all with timers cause I had them. Tried to use just one set of start caps twice and let A LOT of smoke out. The size of the start banks was specified by a fellow from Iowa, I don't remember the amounts. Same fellow specified the run caps. I used motor starter contactors for each motor and a pushbutton start/stop circuit to bring in and stop the converter, this allows starting the unit from the shop with the converter in the back room.

Karl

I'm still waiting to hear about the really big motors! the air compressor here is 11,250 HP, 6.9Kv with a startup surge of over 1500Amps. Thats a big motor! Pat

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 > Scott Moore 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 protecti> I'm starting to wonder. This is for a motor that most certainly

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

I would love to see pictures of such motors.

i

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

I'll try. Meanwhile, I would this interesting page:

Gotta be someone here who works in a power plant... just motors in reverse :)

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

Well, it wasn't a motor, but I used to have to throw switchgear on an experimental electolytic cell. 200 VDC, 18,000A. We could also switch to 10 Hz AC to heat up (about a six week process) or do maintenance. The low frequency was to limit inductive losses. The cell was more of a battery, except consuming electricity to drive a reaction instead of the other way around, consisting of 51 cells in series.

Pete Keillor

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

Actually, if you ramp up the voltage to the motor, which is what the VFD does, there isn't much of a starting surge. The surge comes from applying across the line voltage to the motor with its rotor stopped. If you ramp up voltage, and to some extent frequency, the surge can be essentially non existent. Non existent surge doesn't have to come form anywhere.

Fitch

Yes -- you do have to ramp the frequency at the same time as the voltage, or the motor will find it difficult to start. But since that is automatic in a VFD anyway, that is no problem. Voltage is proportional to the frequency (relative to 60 Hz being full voltage in the US, or presumably 50 Hz being full voltage in the UK.

Glad to see you back into reading and posting again.

Enjoy, DoN.

Cool. Is there a good way to be sure the motor in question will tolerate this ?

PS. VFDs in the 15HP aren't cheap. I am starting to think in terms of just getting a really big rotary converter with big wires back to the main panel.

This actually costs less than a VFD.

Does anyone know if a 15HP rotary converter cares if it is underloaded ? I.e., most of my machines are 2hp or so, so the converter would be runnning underloaded most of the time.

Thanks again.

Have a 10 horse rotary. Sometimes run 1/2 hp off it. Maybe not efficient but seems to work fine.

bob g.

Scott Moore wrote:

I have an early 1940's copy of Popular Science that has an article about a motor on a wind tunnel at Wright-Patterson Air Base in Dayton, Ohio. Don't recall exactly what they said the hp was but W-P was required to give Dayton Power & Light 24 hours advance notice whenever they planned to switch it on. Mike

87,000 HP Yikes !!!

The wind tunnel here at Moffet field has been decommissioned, but when I took a tour, they said that they had to call the local power company and ask permission to turn it on. Otherwise the surge might well take out the city's power !

Where can I drop the pictures? Pat

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

SNIP!

It is on the way. Pat

I have not received anything yet... try ichudov AT yahoo DOT com

i