Lathe Motor Amperage Rating

[npalen]

Attached are the motor specs for my Grizzly G0602 metal lathe which is listed as 1.0 HP. I'm trying to understand why the amperage shown is so high. Testing the draw with an amprobe shows about 8.0 amps under no load and about 11.0 amps under as much load as I could put on it with a turning tool.

Looking at other 1.0 HP motors on the web, I see that 8.0 amps, give or take is typical of that size motor. I get the feeling that this motor should be rated at closer to 2.0 HP based on the higher amperage rating. Please help me understand the difference.

The reason I'm questioning this is looking at replacing the single phase motor with a three phase and VFD to get variable speed, reversing etc.

 

[Mendonsy]

My trusty Square D motor data calculator says that a 1 hp motor at 115 volts should draw 16 amps. That seems a bit high to me also.
Remember that going to a 3 phase motor will reduce that and most of the inverters convert the 115v input to 230 volts out. That is what I did with my drill press several years ago to get variable speed and reversing for tapping holes.

 

[npalen]

Yes, when you figure 1.0 HP as 0.75 KW and then do the math, the 15 or 16 amps does seem high but then again my understanding is that a 1.0 to 1.5 HP motor is about all that a 120VAC 20AMP circuit will normally handle.

Attached is the nameplate of the motor I'm looking at using:

 

[hayden]

If you are seeing 8A on a rating plate, it's for 240V, not 120V. The lower draw you are seeing (11A out of 16A max) is just because you aren't fully loading the motor. Power draw will track load.

 

[npalen]

Yes, I would have to agree. But looking at the three phase Leeson motor above, it shows only 3.2 amp on 230 VAC for 1.0 HP. Is there that much difference between single and three phase amp draw?

 

[Roadworthy]

There is a significant difference between current draw for single phase versus three phase as well as for 240 volt versus 120 volt. Just remember you are feeding the freq drive from a single phase source and calculate your breaker accordingly.

 

[npalen]

I understand the current draw for a 240 versus 120 being half. Didn't realize that much difference between single and three phase for the same HP, however.

Something else that surprises me is a fellow suggesting that a 120VAC 30 amp service is needed to run a VFD with 120 VAC input to powerthe Leeson 230 VAC 3-phase motor, shown above. I had never heard of more than 20 amps to feed a 120 VAC circuit. Learn something everyday, I guess.

 

[BravoXray]

The reason I'm questioning this is looking at replacing the single phase motor with a three phase and VFD to get variable speed, reversing etc.

I converted my Craftsman drill press to a 3 phase motor and VFD a couple of years ago, and love it. No more moving belts around, just dial in the speed I want and drill holes. I used a Teco 510 VFD and a Connex 1 HP 3 phase motor I picked up at a local motor repair shop, made a new control panel and added a tach display to it.
I'm looking at doing the same to my Jet B920N lath

e.

 

[strantor]

13.6A is about right for a 1HP motor on 120V single phase. Per this:

• 115 volts motor - single-phase : 14 amps/hp
• 230 volts motor - single-phase : 7 amps/hp
• 230 volts motor - 3-phase : 2.5 amps/hp
• 460 volts motor - 3-phase : 1.25 amps/hp

An equivalent 3ph motor on a VFD will require just a little more since the VFD introduces a bit of inefficiency.

Whoever told you 30A is required, is wrong. It should work on a 20A breaker, and probably even a 15A breaker.

 

[AWSubie]

Another option would be a DC motor and controller.