Ok, two (probably stupid) questions...

At substantial risk of proving my ignorance, and suffering the inevitable public floggi... er, re-education, here are two 'conceptual' questions regarding home shop 3 phase issues:

Question 1. I understand the risks of using a VFD as a RFC substitute, and the virtual certainty of smoking it by using the connected machine's original electrical controls. (...connecting and disconnecting the load on the VFD while the VFD is powered up.) Could I minimize this risk by continuously running another 3-phase motor as a full-time load, just spinning away in a corner turning a fan blade, or nothing? (So, instead of the load on the VFD vanishing when I switch off the mill, it just decreases from, say, 2 hp to 1/3 hp.) (Note: I know I'm not utilizing the major benefits of a VFD in this plan... I will later, but not for about a year, schedule-wise.)

Question 2. Instead of constructing and load-balancing a RPC, can I just use my 3 hp 3-phase motor as a true generator, powered by a 3-or-so-hp single phase motor? (So the only connection between the motor and generator would be a belt and a ground wire.)

That's them. And yes, it's possible that I overlooked oft-repeated answers to these questions -- if so, I apologize in advance. And I realize that there is an efficiency penalty in each scenario. My hobby use of the machines is currently low enough that electrical efficiency (in dollars) is a non-issue. The loads I place on them are nowhere near their mechanical and electrical limits, either. And for discussion purposes, the current inventory of 3-phase machines is: 2 hp Cincy Toolmaster mill with (3-phase) power feeds, 1.5 hp Clausing lathe with no additional feed motors, and .5 hp Delta shaper.

Thanks, and let the thrashing begin.
Chip

My answer to 1) is basically I don't know. I've never
seen that done or heard about it. So at your own risk to
try....

The answer to 2), if I undertand the setup you propose
correctly, is an unqualified "no." You can't just drive
a regular three phase induction motor via its shaft, with
any sort of prime mover, and expect to get reliable
three phase power out of it.

(parenthetical: there are folks who use induction motors
along, powered by gas engines, as small gensets, but
it's a specialty situation and the loads are tiny compared
to the motor ratings)

What you *can* do is simply take your larger motor and
use it as an idler to make three phase power. The three
hp motor is a tad on the small side to run the 2 hp load
motor. I'd rather see a 5 if possible, but the three
hp one would certainly run the smaller machines.

You can pony start it and do a halfway decent job of
tuning it with capacitors, it would work.

Jim

A poly phase electric motor "motored" by a second source will output voltage and frequency but the output is like an egg shell. At some load, The output will just fail. No amount of coaxing will restore the system until the load is removed. All to repeat again with no improvement. So, 'just about worthless!

A poly phase generator may have a wound rotor with an excitation supply. (the Wynn 5.5kW 208V unit in the shop is such a device) Rotation IN, Electric power out! What's not to like?)

A "rotary snubber" might work to limit the damage that may come from inadvertant disconnect of a VFD and it's load. Since the actual electricl performance requirements are not well understood, operation parameters for this buffer will be hard to define. Some applications and examples seem to "allow" the output disturbance, some examples and many recomendations do not allow VFD output disconnects. You may be on your own there!

If your loads are small enough, Try one of the "self starting, three phase motors on single phase power" designs. A static converter, used properly can be a very elegant solution.

regards

CalG

the virtual certainty of smoking it by using the connected machine's original electrical controls.

Click to expand...

Chip,

It is really easy to use the machine's forward/stop/reverse switch to control the VFD, and keep the VFD hard wired to the motor. Many users do that, while others mount the drive where the touchpad is easily reached, and use the touchpad instead of the switch on the machine. Either way is not difficult.

-----------------
Barry Milton

As Barry said it is quite easy to hard wire control switches to a vfd. And can make the result much more usable than the direction your questions indicate.

Is there a reason not to use the VFD controls in this application?

The control wiring in a lathe is almost pre-configured for 3 wire control on a vfd. You can tap directly into it and control the vfd via the hand lever.

Using the drum switch on a mill is more cumbersome (IMO) than simply converting it to momentary switches. You also have more options if you don't. For instance, you could wire in a loop for use with power feeds - when the table travel is reached, it hits a NC switch open and the motor stops (like how the table feed works).

It isn't a virtual certainty of smoking it, it just isn't what the VFD was designed for.

I have , for instance, a 2500W inverter, single-phase, which runs off batteries charged by solar panels. That inverter is DESIGNED to have loads , motors, whatever, connected and disconnected, switched on and off, etc.. It is 2500W long term, 5000W short term.

BUT it is virtually identical to one phase of a VFD..........

The difference with VFDs is that they don't expect you to do that, and have to make it to a price in order to sell. So the overload capability is quite limited, and the devices used are as small and cheap as possible for the usage.

Hence teh issues.

I know of people who messed up and DID disconnect under load, regularly for months or years, and had no problems.. No guarantees though, the VFD makers generally recommend you do NOT do that.

Here's how I'd do it...

Chip- having been down a similar road, here's how I'd do it now:

Clausing lathe- VFD it. Use any VFD of your choice... but make sure it'll do single-phase in, three-phase out, 240v. I'd use an Allen-Bradley 1305 type AA08A, and a pair of off-the-shelf wirewound resistors, a muffin fan, and two 100-ohm 100w wirewound resistors, and the lathe's existing control wiring, plus one 10K 1/4w three-turn potentiometer for speed control. Figure the cost to do this will be about $120 and (at a learner's pace) three weekend days, plus some web-surfing and surplus hunting to find the drive and get the pot. I'll send you a drawing to simplify, and if you follow it, you can have it running in under an hour- the rest of the time will be just packaging it to look pretty and be safe. If your Clausing has the cone-sheave variable-speed setup, set the mechanical speed in the lower quarter of it's range, and use the VFD to control the actual spindle speed.

1/2hp Delta shaper: If it's a standard arrangement, swap to a single-phase motor. If not, use lathe (converted above) to make parts to adapt a garage-sale treasure. If absolutely not possible, VFD it with an AA06A, or a small OMRON (a 3G3JV), perhaps even a small TECO.

2hp Cincy Mill:
--Main drive motor, use an AA12A setup otherwise identical to the lathe.
--For the power-feeds, make a tiny RPC using a 1/3hp push-button start idler, tuck it into the base of the machine. If it were my machine, I'd tie in the idler with the start contactor (that energizes the spindle motor's VFD) also with lights, fans, etc., so it all comes on with one push. Frankly, those power feeds won't take much idler- just enough to make sure they go the right direction.