7 1/2 hp VFD for cheap guy?

When I got my Clausing 1500 lathe with the 7.5 HP 3-phase motor I was advised that I needed a 15HP VFD to run it from single-phase power and still maintain most of the motor power. It's been long enough ago that I don't exactly remember why but the reason made sense at the time. And now, 3 years later, it's still running beautifully.

Stu
 
When I got my Clausing 1500 lathe with the 7.5 HP 3-phase motor I was advised that I needed a 15HP VFD to run it from single-phase power

That is correct, as a general rule if you look at VFD manuals, you should upsize by a factor of 2. You can fudge down to 1.5x a by adding a DC choke or input reactor to decrease the harmonic distortion. The issue is increased current spikes and harmonic distortion running a 3 phase input VFD on single phase can lead to premature failure of the VFD. Even if running a 7.5 Hp motor on a 15 Hp VFD, the input wiring and fusing need to be rated at 15Hp x 125%. Other than the HY and other generic VFDs listed on eBay, the largest single phase VFD is 5Hp. Speedstar does make a 7.5Hp but at $1k it is expensive. I would stick with an RPC to start out with.
 
The amount to upsize VFDs when using single phase "depends".

If you need absolute full power and 100 % duty cycle, double the size.

If you don't need that, then you can back off. Most lathes seldom run full power, 50% oversize is plenty conservative.
 
Funny about the 2 times the rated motor thing. I contacted Westinghouse to size my my TECO 510, and they got my motor specs and recommended a 2HP unit for my 2HP bridgeport pancake motor. They must have built in a 'safety factor' It has been running flawlessly for 2 years now.
 
They even have air conditioners in Minnesota?
I am originally from Minnesota. They only have two seasons there, called shovel and swat. The humidity gets so high that you need A/C even at 70F. We never had A/C, and melted at night in a pool of sweat. The high here today in Sacramento was 99F, but only 21% humidity. Feels fine, no A/C needed. We also get a nice sea breeze that comes in from the coast most evenings, cools things down a lot. IMO, Minnesota is a great place to be from, far from... ;)
 
To properly size a vfd never derate...always RTFM...

Look in the manual and select the size indicated for the unit in service.

Just derating by getting larger unit may get close but it is a guess at best.

Everyone brags about being precise but just guesses on these...RTFM...

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