Rotary Phase Converter vs VFD for 3-Phase @ Home

[ToolsRCool]

I have a 2hp Bridgeport mill in addition to a metal lathe of slightly less power, both are 240v 3-phase machines. Buddy has a pile of brand new Mitsubishi VFD's and gave me one rated for 2hp. It is petty small, light, and supposedly has a ton of functionality. Programmable limits, alarms, reversible, slow or soft start, and a bunch of other things. Said to do 3-phase, and supposedly all I have to do is wire it up to the single phase 240v in my garage and done, it will be spitting out 240v 3-phase. Both equipment pieces have reversing switches on them, so I don't believe I'd even need the VFD to do that.

I won't be running both of these devices at the same time, so my initial idea was to just put an appropriately rated 3-pole rotary drum type selector switch to chose which device to send the 3-phase power to.

Is there any reason I should not use a VFD? This is just hobby level equipment usage of the mill and lathe, at my own home residence. No mass production, no automation, power off when done, etc.... Considering this point, is there any reason I should instead be considering to purchase a (used) rotary phase converter instead of using this new and free VFD in its most simplistic way possible?

 

[Packard V8]

Can't beat free and they work wonderfully well in your application. We all need a friend like him.

jack vines

 

[mm08822]

Vfd is fine to use. Verify it will take single phase input and what the output rating is.

Do not break the vfd output circuit with a drum switch. Use it on the input side of the vfd for digital inputs.

Since your buddy has a pile of vfds, get a second one and wire each permanently.

 

[cannuck]

What it sounds like is you are aware you can not run both machines from VFD at once - but based on capacity? The other consideration is if your Bridgeport or lathe has other motors than quill on mill and spindle of lathe. You can only run one motor at a time on the VFD, so if you have power feed it could be what eliminates just plugging in the VFD. On my mill the input 3phase is sent to a transformer and split up into a 120 variable speed motor circuit and another 120V coolant system while quill power bypasses that section. If you have something like this, you can run a completely separate single phase 120V supply to those systems.

I chose to go RPC to keep the mill wired up as is to move into 3ph shop later and nobody gave me a free 2HP VFD.

 

[dave*99]

Vfd is fine to use. Verify it will take single phase input and what the output rating is.

Do not break the vfd output circuit with a drum switch. Use it on the input side of the vfd for digital inputs.

Since your buddy has a pile of vfds, get a second one and wire each permanently.

This is the correct advice for using a VFD on your Bridgeport. I wired one up on same machine. Worked great.

OP - Be sure to understand the post. After disconnecting motor leads from the reversing switch on the Bridgeport, the VFD gets wired directly to the motor leads.

You will reverse the Bridgeport using controls on the VFD. Or wire the reversing switch on the Bridgeport to the digital control terminals on the VFD. This is a low voltage signal connection.

 

[micromind]

A VFD can supply motors only, nothing else. If there is a control transformer connected to the load side of a VFD, it'll burn up.

As noted, don't switch anything on the load side of a VFD, most will do it a few times but eventually they will burn up. It's ok to have a reversing switch between the VFD and motor but always stop the VFD before changing direction.

The vast majority of small VFDs will accept single phase power, you'll need to find out which of the 3 input terminals to connect it to and some VFDs will need to be programmed to accept single phase input.

 

[tool_scrounge]

VFDs are usually programmed for the motor’s max motor current. For what you are proposing, you would need to program it for the lesser max motor current of both motors. VFDs are great for things like mills with uncomplicated wiring . But if the lathe has contactors (relays) for direction reversal, multi-speed motors, 3 phase coolant pumps, etc. , VFDs will not be reliable in this situation. A rotary phase converter will work much better.

 

[rsanter]

I prefer a good VFD for most applications.
You can also use a twist lock plug to switch between machines

 

[LopezBart]

VFDs are usually programmed for the motor’s max motor current. For what you are proposing, you would need to program it for the lesser max motor current of both motors. VFDs are great for things like mills with uncomplicated wiring . But if the lathe has contactors (relays) for direction reversal, multi-speed motors, 3 phase coolant pumps, etc. , VFDs will not be reliable in this situation. A rotary phase converter will work much better.

On my 5 hp 15x40 YMZ, the motor was two speed, had contactors for everything, and the magic smoke had come out for the previous owner, leaving some a smoked drum switch, contactors, etc. I bought the lathe, removed all the contactors, reversing drum switch, etc. and "programmed" the VFD so that the logic would work with the original microswitches in the start/forward/reverse lever and foot brake/stop. I added a box w/ a pot to control the speed, and a meter that reads %full rpm or %load, all connected to the VFD. This works like a charm. There's no coolant pump, though, even though there's a place for one; if there had been one I would have replaced it with a single phase unit.

 

[RoninB4]

-From what I've read a VFD affects the finish on precision grinders (surface, OD, or ID). I went RPC for all but the lathe so I have no experience to speak from.

 

[ToolsRCool]

Good info thanks. Do please tell me more on the reversing and / or disconnection of the 3p output after the VFD. I'm guessing the VFD output is monitored within and such abrupt signal changes can make it mad?

My initial intentions were to use the stop and reversing controls on the lathe and mill as they are. But several replies are indicating this may not be healthy for the VFD? This was something I did not consider, but exactly why I posted this question here. Can somebody go just a pinch deeper on it? I think it is only a single push button on the VFD to reverse direction, but I would still prefer to use the equipment mounted control switches rather than changing direction at the VFD. I've done a few small operations at work and such using a production power tap where I am plunging it to a certain depth and then backing it off. That would be kind of troublesome to do or control if I had to actually step away from the machine to reverse the mill, for example.

I can post the VFD model number or a picture of it here if that helps. Thanks for advice.

 

[mike93lx]

I'm certainly no expert but if you want to run the machines with all of their built in controls, you should do a phase converter so that it acts as if its just plugged into a regular old 3 phase supply. Vfd's are really for controlling and powering individual motors.

You can easily have a single RPH to feed both machines

 

[txvwnut]

I have my mill wired to my VFD as if it were a phase converter, all switching is done at the mill as if it was wired to a three phase circuit and have had no issues for the 13 years it has been that way. I have the VFD configured for soft start and I think there was even a program code in the VFD to use the way I have it.

 

[tool_scrounge]

On my 5 hp 15x40 YMZ, the motor was two speed, had contactors for everything, and the magic smoke had come out for the previous owner, leaving some a smoked drum switch, contactors, etc. I bought the lathe, removed all the contactors, reversing drum switch, etc. and "programmed" the VFD so that the logic would work with the original microswitches in the start/forward/reverse lever and foot brake/stop. I added a box w/ a pot to control the speed, and a meter that reads %full rpm or %load, all connected to the VFD. This works like a charm. There's no coolant pump, though, even though there's a place for one; if there had been one I would have replaced it with a single phase unit.

i was going to mention that one could rewire the machine for VFD use and transfer the controls to the VFD inputs. But it seemed beyond the scope of what the OP was up for. In your case, you had no choice but fix the smoked your lathe’s smoked wiring.

Did you retain the two motor speed control or just use one of the motor speed windings with the VFD?

I am always a little wary of buying used machine tools where the previous owner substantially modified the electronics unless it is pretty simple. A lot of times safely wiring it and programming the VFD is beyond a lot of folks skill set. Obviously not in your case.

In my case, I specifically got a RPC because I found in Southern California, smaller 3 phase machinery sold at a discount as most machine shops did not want smaller machines and home shop types wanted single phase. i bought a barely used RPC to easily power everything.

 

[dave*99]

Good info thanks. Do please tell me more on the reversing and / or disconnection of the 3p output after the VFD. I'm guessing the VFD output is monitored within and such abrupt signal changes can make it mad?

My initial intentions were to use the stop and reversing controls on the lathe and mill as they are. But several replies are indicating this may not be healthy for the VFD? This was something I did not consider, but exactly why I posted this question here. Can somebody go just a pinch deeper on it? I think it is only a single push button on the VFD to reverse direction, but I would still prefer to use the equipment mounted control switches rather than changing direction at the VFD. I've done a few small operations at work and such using a production power tap where I am plunging it to a certain depth and then backing it off. That would be kind of troublesome to do or control if I had to actually step away from the machine to reverse the mill, for example.

I can post the VFD model number or a picture of it here if that helps. Thanks for advice.

The question is, can the free VFD your buddy offered accommodate disconnection from the motor while running.
The answer is in the owners manual for that exact model.

I have seen threads on GJ showing folks mounting the VFD on the head of the mill etc. There are options.
The search function on GJ is your friend.

The unit I wired to a Bridgeport had soft start, braking and speed control. More than just a reverse button. It modernized the machine a bit. Was it necessary? Maybe not, but the user now likes those features.

If you find a buddy with a free RPC, this changes everything.

 

[walta]

My initial intentions were to use the stop and reversing controls on the lathe and mill as they are.

I can post the VFD model number or a picture of it here if that helps.

The control switches on the machine could be wired as inputs to the VFD note the programing in the VFD will need to be adjusted for the new inputs. Any chance you buddy is an expert on VFD and could help you set it up?

I understand that having infinite speed control of the cutter /spindle is a huge advantage sometimes.

Yes post the VFD info with a link for its manual

Walta

 

[dougf]

Be careful with motor cooling when using a VFD, you'll potentially burn up your motor when running slower speeds for extended periods due to decreased fan cooling.

 

[LopezBart]

i was going to mention that one could rewire the machine for VFD use and transfer the controls to the VFD inputs. But it seemed beyond the scope of what the OP was up for. In your case, you had no choice but fix the smoked your lathe’s smoked wiring.

Did you retain the two motor speed control or just use one of the motor speed windings with the VFD?

I just used the high speed winding; I programmed the VFD to not do overspeed on the motor, although that would have been possible. The top speed on my lathe is 2000 rpm; I didn't feel like I needed to increase that.

I am always a little wary of buying used machine tools where the previous owner substantially modified the electronics unless it is pretty simple. A lot of times safely wiring it and programming the VFD is beyond a lot of folks skill set. Obviously not in your case.

It's actually far simpler for me)to program the VFD according to the manual for proper operation with the four microswitches (fwd/rev/stop/jog), than to attempt to diagnose a problem with the maze of burned wiring, a 2"x6" drum type rotary switch smelling of smoke and 3 contactors. I think it took a weekend or two to get it running with a small box to hold the 0-10v meter & pot.

In my case, I specifically got a RPC because I found in Southern California, smaller 3 phase machinery sold at a discount as most machine shops did not want smaller machines and home shop types wanted single phase. i bought a barely used RPC to easily power everything.

That works as well. I definitely find the variable speed feature w/o stopping with the VFD to be useful, though.

 

[LopezBart]

Be careful with motor cooling when using a VFD, you'll potentially burn up your motor when running slower speeds for extended periods due to decreased fan cooling.

Most machine tools don't run at a significant load in home use (I've never gotten the load meter on my 5 hp lathe to read much more than 10% or so). If I find myself running at a small fraction of the motors rated speed, I change gears or on the Bridgeport adjust the variable speed head.

 

[micromind]

With an inductive device (such as a motor) whenever a switch is opened under load, there will be an electrical arc. This arc will cause high voltage low current spikes, a lot like an ignition coil. With non-VFD stuff, these arcs are dissipated in the motor and power system, no harm done.

A VFD however is different. The motor will absorb the spikes on the load side of the switch, no harm done. The spikes on the line side of the switch, depending on the basic design of the VFD, will blow tiny holes in the semiconductor film inside the output transistors. Eventually, they will fail because not enough semiconductor is left. Some VFDs can handle this better than others.

There are basically 3 currents to a motor. The first is magnetizing current then locked-rotor then running current. Running and locked-rotor are obvious but magnetizing current is based on the voltage applied and the DC resistance of the motor winding.....pretty close to a dead short. It lasts roughly 1/60th of a second.

With a VFD, the amount of current that will flow depends on the impedance (resistance) from the filter capacitors in the VFD to the motor winding. Since the output transistors have resistance, they will heat up rapidly but the current doesn't last long enough to cause instant destruction. Instead the rapid heating will cause mechanical stress that will eventually cause separation of the semiconductor film inside and they will fail. Again, some VFDs handle this better than others.

 

[wyliesdiesels]

Both equipment pieces have reversing switches on them, so I don't believe I'd even need the VFD to do that.

Not a good idea to use a reversing switch in between the VFD and the machine. You can screw up the VFD

 

[dave*99]

With an inductive device (such as a motor) whenever a switch is opened under load, there will be an electrical arc. This arc will cause high voltage low current spikes, a lot like an ignition coil. With non-VFD stuff, these arcs are dissipated in the motor and power system, no harm done.

A VFD however is different. The motor will absorb the spikes on the load side of the switch, no harm done. The spikes on the line side of the switch, depending on the basic design of the VFD, will blow tiny holes in the semiconductor film inside the output transistors. Eventually, they will fail because not enough semiconductor is left. Some VFDs can handle this better than others.

There are basically 3 currents to a motor. The first is magnetizing current then locked-rotor then running current. Running and locked-rotor are obvious but magnetizing current is based on the voltage applied and the DC resistance of the motor winding.....pretty close to a dead short. It lasts roughly 1/60th of a second.

With a VFD, the amount of current that will flow depends on the impedance (resistance) from the filter capacitors in the VFD to the motor winding. Since the output transistors have resistance, they will heat up rapidly but the current doesn't last long enough to cause instant destruction. Instead the rapid heating will cause mechanical stress that will eventually cause separation of the semiconductor film inside and they will fail. Again, some VFDs handle this better than others.

Agreed. And it's important to note how many VFDs stop a motor. They soft stop by ramping down speed or apply braking to the motor electrically. This avoids generating the spikes mentioned above.

 

[NWOhioChevyGuy]

Not only less cooling but motors will run hotter at slower speeds.

I'm not a EE, but where I work some of our pump motors have a Min & Max Hz the can run on a VFD due to heat and insulation.

 

[ToolsRCool]

The control switches on the machine could be wired as inputs to the VFD note the programing in the VFD will need to be adjusted for the new inputs. Any chance you buddy is an expert on VFD and could help you set it up?

I understand that having infinite speed control of the cutter /spindle is a huge advantage sometimes.

Yes post the VFD info with a link for its manual

Walta

He is, actually.

 

[dave*99]

He is, actually.

If you have a VFD expert buddy that will donate VFD's you should be investigating what beer he prefers. I suspect you already know.

 

[jar944]

I have my mill wired to my VFD as if it were a phase converter, all switching is done at the mill as if it was wired to a three phase circuit and have had no issues for the 13 years it has been that way. I have the VFD configured for soft start and I think there was even a program code in the VFD to use the way I have it.

I wouldn't suggest anyone do this with a vfd.

 

[ToolsRCool]

I have my mill wired to my VFD as if it were a phase converter, all switching is done at the mill as if it was wired to a three phase circuit and have had no issues for the 13 years it has been that way. I have the VFD configured for soft start and I think there was even a program code in the VFD to use the way I have it.

Fav answer so far. ))

 

[LopezBart]

I have my mill wired to my VFD as if it were a phase converter, all switching is done at the mill as if it was wired to a three phase circuit and have had no issues for the 13 years it has been that way. I have the VFD configured for soft start and I think there was even a program code in the VFD to use the way I have it.

There are many VFDs that warn explicitly against doing this in their manuals:
From the 2 HP Teco manual:

• Do not connect or disconnect the motor during operation. This will cause the inverter to trip and may cause damage to the inverter.

 

[dave*99]

I have my mill wired to my VFD as if it were a phase converter, all switching is done at the mill as if it was wired to a three phase circuit and have had no issues for the 13 years it has been that way. I have the VFD configured for soft start and I think there was even a program code in the VFD to use the way I have it.

What brand and model do you have? I'm curious how the soft start works in this configuration?

 

[American Locomotive]

I wouldn't suggest anyone do this with a vfd.

Fav answer so far. ))

There are many VFDs that warn explicitly against doing this in their manuals:
From the 2 HP Teco manual:

• Do not connect or disconnect the motor during operation. This will cause the inverter to trip and may cause damage to the inverter.

Yup. Almost every VFD I've ever played with gets angry if you try turning the output on without anything connected. Additionally, most VFDs can't handle motor in-rush currents. A 10a 3-phase motor probably has an in-rush of near 80-100A, a little 3HP VFD isn't going to have transistors that can reliably handle that. Trying to line-start a motor with a drum switch on a running VFD will probably result in the drive instantly tripping out on overload.

Maybe txvwnut has some strange VFD that's capable of sensing when the outputs go closed-circuit and then starting up? I've never seen any option like that in any VFD I've programmed. Granted I've only played with like 10 different VFDs from 4 brands, and there's a lot more different ones out there.

 

[MichaelP]

I strongly prefer RPC. I have a 15HP one in my shop and run a lot of different 3-phase machinery without any need to rewire each of them: all controls remain original, and I have zero headaches and zero concerns when I buy new equipment or replace the old one. My 1700rpm old monster low noise motor is located out of the shop space, I wired the shop with multiple 3-Ph outlets and a single control panel on the wall.

If I knew that all I'd ever have would be a couple of 3-PH machines, or if I had a real need for soft stop or infinite speed control for a particular machine, I would start thinking about individual VFDs.

 

[txvwnut]

There are many VFDs that warn explicitly against doing this in their manuals:
From the 2 HP Teco manual:

• Do not connect or disconnect the motor during operation. This will cause the inverter to trip and may cause damage to the inverter.

What brand and model do you have? I'm curious how the soft start works in this configuration?

Teco(in the white case) and I wanna say 5hp but it's probably a 3hp as that is what my mill motor is and was probably the largest that Surplus Center had in stock. I do not instant reverse with it, if I am doing an operation where I know I'm going to need to reverse I'll shut it off early let the quill come to a stop then reverse. There have been times where I have done an instant reverse and you'd never known it was hooked up to the VFD. The reason I went with a VFD is my static phase converter would give fits when really loading the machine up with a heavy op. The VFD doesn't give any of the issues I had with a static phase converter and the only reason I went with a VFD was the price was incredibly low enough to sacrifice a few bucks for the sake of an experiment. I have everything to build a RPC just haven't done it yet. Now here's that part the will probably drive a few of you nuts. it's not mounted in an enclosure none of the wiring to it's pretty but not a hazard either. Just screwed to the wall kinda behind the machine, it's high enough no chips will get in it but it subject to all the dust and bs floating around in the shop. The one thing I did in my programming which was by accident was I have to cycle the VFD itself on or off after turning on the power to it. Maybe this has something to do with why I can do what you're not supposed to with it, I dunno. I just need the mill to make chips cause chips sometimes mean money in the pocket.

 

[fitter30]

Vfd for 3 phase has to twice the hp or more.

 

[mike93lx]

Vfd for 3 phase has to twice the hp or more.

For the vfd's that I have looked at, the HP rating is output, so I can't see why it would need to be double the amount needed. I just looked at a data sheet for a single phase input Teco to confirm. What am I missing?

 

[LopezBart]

For the vfd's that I have looked at, the HP rating is output, so I can't see why it would need to be double the amount needed. I just looked at a data sheet for a single phase input Teco to confirm. What am I missing?

You're not missing anything. The 2x the hp SWAG has been used to provision VFDs not explicitly rated for single phase input in that service; for VFDs explicitly rated for single phase input, the usual nameplate ratings are fine.

 

[Dagny]

Mount the drive where it is handy and use it for everything.

 

[manwithtools]

Vfd for 3 phase has to twice the hp or more.

Outdated rule of thumb that applied to 3 phase input only FVD's. Modern VFD's that are designed for single phase input do not require any derating. The output HP is what they will support. Note, not all modern VFD's will natively support single phase input. If they don't, the rule is to derate by 57.8%

 

[jar944]

Vfd for 3 phase has to twice the hp or more.

No, not if it's a single phase rated vfd.

 

[dave*99]

There are many examples of wiring reversing and drum switches on lathes and mills to VFDs on practicalmachinist.com.
I've only seen references to connecting those switches to the digital inputs of the VFDs. I'll admit I have not seen everything. No need to quote me if I'm wrong. I have a wife for that.

 

[fitter30]

No, not if it's a single phase rated vfd.

Show me never seen one. But 99% my work is 480 vac 3 phase except for controls.