View Full Version : going oversize on 3-phase converter

10-27-2003, 10:38 AM
well i'm piecing together my parts for the converter. i initially have a machine with a 3HP motor i want to run. from reading various articles it appears i want to go with a 5HP minimum idler motor for my rotary converter. i have found a few 5HP motors, and a few 10HP motors, but no 7.5HP ones (i'd really like to use a 7.5 if i could find one). to start with i would only be running one machine at a time on the converter, mainly because i am only one person.

my questions are, what negatives are there to going with the 10HP motor in the converter vs. a 5HP? if i go with the 5HP and at some point do decide to run both a 3HP and a 2HP motor off the converter at the same time, will this work, or will i be wishing i had the 10HP? how much extra power will i be using to run a 3HP machine off of a 5HP converter vs. a 10HP converter? I was under the assumption the current draws are pretty low for a rotary converter when it is just idling with no load on it. i don't think i will ever have three machines running at once, and i can't ever foresee me buying something with a 7.5 or 10HP motor in the actual machine i will be running (that would be a rather large piece of equipment).

thanks for any insight.

andy b.

10-27-2003, 11:55 AM
If I understand it, and I don't, use an idler motor larger than the largest motor you plan to run. Running additional motors will actually help the situation after that.
A 5HP motor should be plenty for what you have in mind.

10-27-2003, 03:10 PM
there are some 7.5 HP motors out there...




10-27-2003, 03:32 PM
For $100.00 plus $65-$75 shipping minimum, you have almost bought a VFD before you have even started.
If you are trying to do it on the cheap, used 3 phase motors are usually inexpensive, check surplus dealers. You may be lucky enough to find one with a beat up shaft that someone is willing to dump.

Jerry B
10-27-2003, 04:31 PM
I was in the same boat you are in a few years ago. The general rules of thumb are:

1. Use an idler that is as large if not larger than the motor on your machine.

2. You can safely run up to twice the rated horse power of the idler motor on any combination of you machines.

Michael Az
10-27-2003, 04:43 PM
I am in the same boat. The mill HP that I need to run is 5 HP and I was thinking about looking for a 10 hp for the idler. Wonder what a VFD would cost for my application?

10-27-2003, 08:19 PM
well, i guess i'm leaning towards the 5HP motor. i didn't make it clear in my original post that i found some motors locally. the 5HP motor is $50, and there is no shipping (since i can just stop over and get it). if i get something on ebay it pretty much shoots my budget.

i'll let you guys know what happens. it might be 5 months though, since i have to finish building my lathe table and building the drive set-up for that.

andy b.