Which compressor?

Starting a 10hp motor with a 15hp idler is kinda dicey according to motor folks I've spoken to. If the 10hp motor can be started with a small load it would probably work fine. Starting the 15hp idler will draw lots of current unless it is already spinning. This is a common way of starting 3 phase converters. In fact, you can even wrap a rope around the shaft of the motor and pull it to spin the motor up. Might be hard to spin up a 15hp motor that way though. Some folks use a single phase motor mounted on a hinge to start the larger motor spinning. Once it's up to speed the idler motor is energized and the belt from the single phase motor to the idler is flipped off by raising the smaller motor to relieve the belt tension. I have a converter that is well balanced and is based on a 15hp idler motor. It runs the whole shop and starts 2 10hp lathes with DC drives. But the starting is adjusted a little soft, about 1.5 seconds. Before I got the big converter I used one built around a 7.5hp motor and it could only start the lathes with a 6 second soft start. These are DC drives though and the motors are shunt wound DC units and so may draw less current when starting than an AC induction motor. I don't know. You could use the 10hp motor with a smaller motor pulley in order to lessen the starting current. This would mean the compressor would not pump as fast but maybe that isn't a problem for you. ERS

You also might consider a smaller motor pulley for a greater reduction, lower top speed to reduce the power requirement (also the output).

Pete Keillor

Also, the high power draw of the compressor at full speed may limit what other tools you can run when it kicks ni (e.g a plasma cutter).

i

Ignoramus8571 wrote: (snip)

I'm not doing that well with my search strings. Any suggestions for what to look for? There are some used Allen-Bradley stuff I'm seeing that looks like it might work that is only a little over my budget. (And way oversize on capacity)

Stuff like:

I'd go for a VFD for a compressor, because that tends to be automatic equipment that will start and stop unattended. Much less to go wrong with a VFD than with a homebrew rotary converter.

And if you need to derate the 15 HP compressor to use a smaller motor, you can slow down the compressor with a smaller motor pulley and reduce the HP needed to drive it. Check the paperwork on the compressor head first, some of them have a minimum speed and HP draw.

Yes, but Iggy gets enough "Interesting Stuff" through Mil-Surp bids and other "finds" that having a 277V/480V 3-Ph 4-wire service installed in his shop area would be very useful. If for nothing else than testing items before resale so he can say "It Works."

The bugaboo is if the utility will /allow/ him to have a 3-Ph service installed at a residence, and that varies wildly by the utility rules and state/local laws & codes.

And once you have a 480V service, dropping it down to 240V or 208V is as simple as a dry transformer or three. Taking voltage down is easy, up is hard - you run out of amps on the service way fast.

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Definitely do NOT use search strings. Look in two categories: "AC drives" and "inverters".

It is a soft starter. Here's a brochure for it:

Based on a quick look, it is not a VFD, it is more like a gradual solid state switch. It does not make three phase, it simply increases amount of three phase available to the motor gradually. Someone else may be able to comment more accurately.

i

The OP Can check this out

If he can find a 10 HP motor and a smaller pulley, this drive would make his compressor into a useful 10 HP compressor. I am not saying that it is the way to go -- I would look for a 30 HP drive to run the

15 HP motor, but just showing what is out there.

Ignoramus8571 wrote: (snip)

(snorp)

Well, I called up the local Quincy service rep, and found out that it has already been sized down as far as the motor goes. The compressor is a 20 HP model. So the 15 HP motor is about as low as it will go. The service rep said that a 10 HP motor won't run that cylinder. So, 30 HP VFD it is. Just need to find a good one.

Btw, thanks for all the help and suggestions.

Todd

Todd, I routinely monitor the drives categories for good deals, and I will keep an eye open for 30 HP drives.

i

(snip)

Well, I just noticed this:

Probably a 480V model, but I've got a question into the seller anyway. All the bad image links in the description make me a bit nervous though...

Call him at his phone listed at

****), and find out. I think that he messed up his website and URLs (they contain bad characters etc).

i