Homebrewed Electricity > Controls
Vfd motor matching
bigrockcandymountain:
Thanks Bryan,
I have since done a whole lot of thinking on this and i have decided to keep it simpe for some experimenting. I have a 4 pole 3hp single phase motor sitting on the shelf that i will mount and make some chips. I will be limited to 800 rpm max. That should get me started, so i can get a feel for what i really need.
I'm thinking ultimately the best might be a 3 phase 5hp 4 pole motor with a vfd so i can drive it 120hz. I would like to know for sure though before i spend any real money.
I think you are right that single phase vfds only go up to 3hp. The china ones go to 5hp which i assume is just 3hp with Chinese over rating.
I understand you can use a 3 phase input vfd with singe phase input if you derate it by half. So i would need a 10hp.
machinemaker:
If you are looking for VFDs or other components I have had good luck with: automation direct and factorymation.
Kent
SparWeb:
Hi BRCM
I currently have a very heavy plate chucked into my little lathe and it takes a while for it to come up to speed. Presumably the current is higher than the nameplate until it settles. I could measure with a clamp-on ammeter to get you an idea what a heavy lathe start-up load looks like. Scale up from my small 12" machine to this one of yours.
bigrockcandymountain:
Ya that would be great. The running amps etc is pretty easy to calculate, but the starting inrush current is quite a bit harder to predict.
Thanks machinemaker for the tips. I'll look those guys up if i decide to go the vfd route.
SparWeb:
This was fun.
My lathe has a 2HP motor running on single-phase 240VAC, so make adjustments accordingly. Because of the mucking about I'm doing to mount a 4-jaw chuck, you caught me right in the process of going from facing the new backplate to removing the 3-jaw chuck entirely. So I was going from heavy load on the spindle to no load and back. Perfect time for a comparison test.
I measured a bunch of things together to get an idea what's going on. No Load:
Speed 287 RPM
Line Voltage: 246.0 VAC
Startup Voltage drop: 242.2 VAC
Startup Current Peak: 32.4 A
Running Voltage: 244.4 VAC
Running Current: 6.2 A
The result is that the input power to the lathe is 1.5 kW (2HP exactly) while it's running, and the inrush goes up to 7.8 kW (10.5 HP) at the moment of startup.
Then with the chuck on and a circular plate almost as heavy as the chuck:
Speed 282 RPM
Line Voltage: 247.0 VAC
Startup Voltage drop: 242.4 VAC
Startup Current Peak: 36.1 A
Running Voltage: 244.7 VAC
Running Current: 6.5 A
Now it needs 1.6 kW (2.1HP exactly) to run, and the inrush went up to 8.8 kW (11.7 HP) at startup.
Just for fun I switched up the speed to "575" RPM though it actually ran at 562 RPM. Now it needs 2.4 HP to run and 13.3 HP to start.
The startup inrush current was 41.3 Amps. My breaker is 40! I haven't tripped it.
Conclusion: The ratio of inrush/nominal current is about 5x in light loads, and about 6x for heavier loads.
I bet I just proved a bunch of stuff I could have looked up in a book.
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