3 phase for a tool with 2 motors

[Ultradog MN]

Sometimes the old ways are not the best way.

Could I run 7 different machines with your unit? Not all at the same time of course.
But I have idled my 3 hp lathe and the two motors on my mill = 1 1/3 hp, at the same time just to test it.
And of the 2 I've built, neither cost me over $200.
Lastly, an rpc with a kit from the place I linked to was simple to build and I'm kind of a caveman about electrics.
To be fair, my 7 1/2 hp unit does burn most of 500 watts just to have it up and running.

 

[Lo-Fi]

No, but I've no idea why you'd want to. It's insanity burning angry pixies every minute you're not running a machine. Which for a hobbyist is quite a lot.... That's not to mention the potential safety issues with floating voltages and suchlike you can get if you're not careful on an RPC. No thank you! I'll take speed control, soft start, electrical braking and all the other trimmings you get with a VFD every time.

 

[verbotenwhisky]

Helping a friend get a mill set up, and he's going to need some sort of phase converter - he has 240v single phase, but the mill is 3 phase.

A RPC would be easy but a used one is hard to find, and the few I've seen are obscenely expensive.

I'm wondering if a VFD would work in this specific situation. The mill has 2 3 phase motors - one for the spindle and one for the power feed. It's an integrated power feed, not an add-on, like a Bridgeport.
I've always heard that you can only run a single motor on 3 phase. Is that true for this case as well? The VFD would no be used for a speed control - it would always run at 60hz.

I think you can run multiples off a single VFD if it is sized properly and phase conversion is all you are planning to use it for. if you are planning to regulate the operation/speed of either motor it will likely not work.

Be careful though because a VFD can setup a nasty feed back through your electrical system that will cause you issues.

 

[Ultradog MN]

All of us are hobbyists.
We follow our interests but have to make tradeoffs.
I'm a bottom feeder.
I buy very little stuff new. All used, old school, stuff. Lo tech but simple and reliable electrics.
Am a manual guy anway
Lathe has 16 speeds and a brake on the clutch handle. That works here.
You might like newer stuff and more tech.
Like soft start, infinately variable speed and braking - all with a knob. That's cool.
Can a caveman wire it?
Amortised out does it cost as much as my higher electricity bill?
The OP asked a question.
He's getting multiple opinions. Maybe just what he wanted?
He, like the rest of us, will have to make tradeoffs too.

 

[Shotgun]

Can a caveman wire it?
Amortised out does it cost as much as my higher electricity bill?

Yes. Two hot legs from your 220 power socket to the in terminal. Three legs to the motor. Set it to run at 60Hz and leave the knob alone. Bring the little control pad out to where you can push the start stop button. Maybe the forward and reverse. It will now act like a standard single speed motor. Any of the other fancy feature require some setup work.

For a 2 or 3 Hp motor, it's going to set you back $75 plus shipping. I get charged about ten cent per kWH for electricity. If the RPC is burning 500W idling, first of all, "Ouch! Something isn't right there. That's a lot of heat going into your workshop." But, that would cost me a dime every two hours. The VFD would pay for itself after about 1500 hrs (?). Can someone check my math?

 

[twhite]

My machines are all 50 to 75 years old with the original motors. My mill is a variable speed and the lathe is belt driven 12 speed. My static converter has served me for 30 years. Just the addition of the lathe has caused a slight inconvenience. As I tend to be old school for better or worse. I am more interested in a RPC.

Noise is not an issue for me. As I have spent my entire adult life in noisy machine shops. That means I am practically deaf.


Cutting oil is my blood.

 

[OTmachine]

I worked at a shop in CO where the whole shop, (2 CNC mills, 3 CNC lathes, 2 BP’s, a couple of O.D. and surface grinders, drill presses, and tumblers = 5 man shop), ran off one 25 HP RPC with no problems. I hadn’t stuck an amp clamp on my 3 HP RPC to see what it is doing, but the electric bill has never been OMG high. Works for me.
Put the RPC together before VFD’s were an affordable thing. Have VFD’s and still use the RPC.

 

[matthewsx]

I think you can run multiples off a single VFD if it is sized properly and phase conversion is all you are planning to use it for. if you are planning to regulate the operation/speed of either motor it will likely not work.

Be careful though because a VFD can setup a nasty feed back through your electrical system that will cause you issues.

Wouldn’t try that. VFD’s like to be ”tuned” to the motor they‘re running. Plus you loose the advantage of variable speed. They are cheap enough to get one for each motor anyway.

I have both static converters and VFD’s in my shop, horses for courses. I also have a 5hp three phase motor I’m hanging onto in case I want to build a rotary converter (actually started setting it up but decided I didn’t need it yet).

john

 

[Ulma Doctor]

I work on packaging equipment that uses a single inverter to operate 2 unmatched motors simultaneously.
There is no special tuning to the inverter to accomplish this task.
The drive frequency is adjustable, both motors run at the same frequency.
One motor is often 2.2Kw , the motor operating simultaneously is often .25 to .5Kw
I have not seen complications from this non-theoretical implementation

 

[Shotgun]

I've not experienced it myself, but all the documentation says that a switch AFTER the VFD can destroy it. Blows the MOSFETs, or something like that. I think that would be the impediment of using two motors at once.