REALLY big motors

When you say 10hp motors and up, do we assume you mean 3-phase motors? Just about all machines of 5 HP and over have 3-phase motors. You invited comment on the feasibility of operating "such motors" on typical 100-200amp residential panels. General commentary would be:

Single-phase motors up to 20 HP could be operated (but normally aren't) from residential panels. Such a machine, fully loaded, would "draw" in the neighborhood of 75 amps or so; however a single phase machine of this size would require special starting circuitry and would probably incur a penalty from the power company. A 20 HP single-phase machine is a rare beast, indeed, and probably would be special order and very expensive. Five HP (single-phase) motors are the practical maximum for residential use and they are generally more expensive than their 3-phase counterparts.

A 10 HP, 3-phase machine could be easily adapted to run on "residential power" through the use of a phase converter. Either a static phase converter or a rotary phase converter made from a 10 or 15 HP 3-phase motor would be required.

Bob Swinney

Are they not three phase machines?Do you have three phase service at home?

i

"> Are they not three phase machines?Do you have three phase service at

He didn't say, but "100-200amp residential panels" implies single-phase service.

Bob Sw>> Some of the machines I am looking at have 10hp motors and up. Anyone

Everything I see in those sizes is 240v 3 phase. I have a phase converter with a 5hp idler. The starting current is going to determine the wire size, right ? What is the typical start current on a 10hp motor ? Would I need to get a bigger idler to help it start ?

Thanks.

You most likely don't want to start a 10 Hp motor right off the mains. The starting surge will be huge, and all the lights in the house will dim for a second. While a motor like this might start in 100 ms on a 480 V

3-phase mains, it will take a lot longer on the weak supply you have, and may cause other appliances to malfunction, especially if they just happen to try to start at the same moment.

What you want is a big VFD, in the 15 Hp range. I have a 15 Hp Tosvert VFD running a 7.5 Hp lathe, and it works great. I can set the start time so it doesn't require a huge surge (1 second seems fine) and I also get dynamic braking as part of the bargain, which a rotary converter doesn't provide. You can probably get by on a 10 Hp VFD, as you likely will not be running at 10 Hp very often.

Jon

Scott Moore wrote in news:DBwid.464771$mD.414860@attbi_s02:

Start up current would depend somewhat on the static load applied to the motor at start up. I don't have an example of a 10 hp, but a recent example I have of a 20 hp motor direct driving a blower pulls ~140 A @

460 V 3-phase for in excess of 10 seconds.

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.

Thanks, that sounds like good advice. Unless the VFD for 15hp costs more than the machine ! (> $2000 or so).

So edumacate me. The starting surge has to come from somewhere, right ? Does the big VFD store that energy in large capacitors ?

Peak current at start-up is known in the industry as "locked-rotor current" It is determined by the instantaneous source voltage and the motor design and is independent of the motor's static load condition. Static load will affect acceleration time.

Randy

A normal 10 Hp VFD is perfectly capable of running a standard 10 hp motor in a continuous overload condition. Just look at the rated output current of the VFD and compare with the motor's full load current. There is no need for oversizing.

Randy

I've got a 10HP 1745 rpm 3 phase motor I'd part with, but it is located in Maine. Shipping probably makes it pretty expensive, unless you're nearby.

You would probably need a bigger idler to run on a phase converter. You might be able to start it if completely unloaded (i.e. bare shaft). Be sure you don't over current your 5HP trying it or running it. You could also add additional 5HP motors in parallel instead of using one big idler. This way, you start your existing phase converter, then switch in the next idler, then the next as needed.

Steve

Scott Moore wrote:

Our 15Hp 230v 3 phase air compressor draws 42 amps running, and a heck of alot more on startup.

FWIW,

I'm running a 20 hp lathe off my residential service. I built a huge phase converter that has a 15 hp followed by a 20 hp idler motors. Runs great. Haven't done it yet, but I plan on being able to also run a 10 hp mill at the same time.

Karl

Karl,

What are you using for start-up of the 15 and 20 HP idlers?

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

Sounds like the whole neighborhood's lights dim when you go online :-)

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 ?

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.

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