Single phase, 3-phase, VFDs, RPCs, and DPCs... time for some schoolin'

[ArmyDoc]

@ArmyDoc it's obvious you did your homework, thanks. It makes whatever the follow-up questions easier. So what are the follow-up questions?

This thread is so well covered that I barely have anything left to add other than my opinion that the price of VFDs now is so low that they ought to be the shotgun answer for just about everything in the hobbyist's shop. The typical hobbyist's shop has no motors >5hp and most of the applications are simple re-powers of simple 3ph machines that never had a control system, just a motor with start/stop/reverse. There is no concern about using one VFD for multiple machines* so long as you stick to V/Hz mode** and you don't plug/unplug/reverse (via mechanical reversing switch) motors under power.

* machine implies a simple single motor machine like a lathe or mill. In the case of a more advanced machine with multiple 3ph motors (lube pump, coolant pump, etc), RPC is a better bet.

**V/Hz mode is the simplest mode and does not use feedback (amps, back-EMF, etc.) to control the output voltage. Theoretically you should be able to run a multiple-motor machine from this, and probably even disconnect under power. But I don't recall seeing any VFD manufacturer explicitly condone this, so I won't either.

All of my (what now seem to be simplistic) questions were answered by the above posts - I have a better understanding of when an RPC or DPCs need to be used vs VFDs. And I understand now why you can't use one VFD for multiple machines - in that you can't put a switch after the VFD, so it has to be wired directly to the motor. (although you imply there may be exceptions...) Unfortunately, the discussion also raises / suggests more complex questions, which I don't know enough to properly form into questions.

VFDs are far more complex than I initially realized. If, as you suggest above, you have a 3 phase motor with either just an on-off switch, wiring in a VFD seems pretty straight forward, and that was what I was envisioning. But now it is clear there are multiple modes of operation, programable features etc. I find myself in a situation where "I don't know what I don't know". That makes it harder to know what to ask. But a couple questions are obvious:

What are the various modes a VFD can run in, and how do they work / how are they different?

What exactly is V/Hz mode?

What are the alternatives to V/Hz mode, how do they work and how are they different?


Another question is related to Mark's post:

...

VFD's for the most part need to be direct wired to the motor, and usually used for single motors when run in sensorless vector mode. They are relatively inexpensive for units in the 1-3 Hp range, beyond 3 Hp running off of single phase they get expensive and there is derating for 3 phase input models as well as additional expenses for a DC choke and noise filters due to increase electrical noise generated. There are also a lot of install considerations when installing VFD's and additional expenses for enclosures and such. Since the VFD output is PWM there are issues with insulation breakdown, and electrical noise, but in general this is not an issue with 240VAC motors and shorter cables. There can also be some issues with bearing currents and damage to bearings, this is more of an issue with higher voltage and industrial settings. Pluses for VFD is small size/weight, inexpensive for smaller motors, have the ability to be programmed for multiple functions (soft start, variable speed, fast braking, etc.), and ability to drive a motor at very high overload for short periods with overload protection. As far as voiding warranty, well that varies and you need to check with the individual distributor. My experience is it does not void a mechanical warranty issue.

The bolded part is concerning to me, even though it states it is more of a consideration for higher voltage and higher Hp applications. Hate to spend $20k and more on equipment to have it damaged by using VFDs, even with the improved functionality.

It is tempting to put in an 10, 15 or even 20hp RPC for the whole shop, just for the simplicity of it. After all, three-phase machines were designed to run as the arrive, on native 3-phase. Why go to all the trouble of wiring in a VFD if you can just plug it in and it will work as designed?

 

[Dabbler]

VFDs are far more complex than I initially realized

A typical VFD has hundreds of configuration parameters. However, the majority aren't essential to know for simple situations, and most of them are nearly self-expanatory. When I bought my TECO VFD, I spent more than it would have cost for an offshore one - for exactly your reason. TECO has a 1-800 help desk number, who were very helpful on my first VFD install. They told me to set 4 parameters a certain way, and to forget about the rest until I felt the need to get fancy.

The bolded part is concerning to me

Don't be too concerned - the Clough42 video I referenced is very complete about how to install one to the highest order. My current installation is very simple: wire, toggle switch, VFD, motor. That's it. Use 600V rated wire (it is hard to get lower rating in Canada anyways) and keep your output wire to under a meter long to avoid most of the radiated EMF. What will get radiated in < 1M is primarily above150MHz with a steep falloff for every (octive) frequency doubling after that. At some point, purchase an EMF limiter on the output phase. For cheap input EMF protection, use an filtering power bar (if you are using a 110V VFD) or grab the chokes and ferrites from a power bar and install as per the Clough video.

There's nothing too complex here other than a few simple rules, which he goes over in the video.

 

[strantor]

I understand now why you can't use one VFD for multiple machines - in that you can't put a switch after the VFD, so it has to be wired directly to the motor. (although you imply there may be exceptions...).

Here's the thing... if you google "VFD multiple motors" you'll find app notes from every VFD vendor that all read the same. They're all talking about multiple motors connected concurrently, and they all use conveyors as an example. Say you have a 5HP VFD and (4 or 5) 1HP conveyor motors, all connected in parallel, starting and stopping at the same time, powered by the VFD. The VFD can't sense the current of individual motors, so individual motor protection is required. They all show this installed between the each motor and the VFD. in this application (V/Hz only, more on that later), these individual overload devices, if they open under load, will not harm the VFD. I've worked with these setups in real life and and you can turn those individual motors overloads on and off under load, and it doesn't cause a problem. But outside of this one common scenario the manufacturers are silent. I see no reason why you couldn't use a VFD in V/Hz mode to power a multiple-machine motor, other than the fact they tell you not to (and then give this conflicting scenario where all the sudden it's magically OK).

now it is clear there are multiple modes of operation, programable features etc. I find myself in a situation where "I don't know what I don't know". That makes it harder to know what to ask. But a couple questions are obvious:

What are the various modes a VFD can run in, and how do they work / how are they different?

What exactly is V/Hz mode?

What are the alternatives to V/Hz mode, how do they work and how are they different?

In V/Hz mode the VFD has a linear relationship between volts and hertz. You could plot it a graph and predict output voltage based on your frequency command. Because that's all the VFD is doing. You tell it "give me 60Hz" and it gives you 60Hz @ 240V. You tell it "give me 30Hz" and it gives you 30Hz @ 120v* and so on (* the graph does not start @ 0,0, so 30Hz wouldn't correspond exactly to 120v). The VFD does not do any monitoring of the motor, it does not know if the motor is spinning the proper speed for the given frequency and it does not care.

Sensorless vector mode is a more advanced mode. In sensorless vector mode, you must "tune" the drive to match the motor. During the tuning process, the drive will measure the electrical characteristics of the motor (resistance, inductance, inertia, et. al.) and use to create a profile of the motor to accurately estimate its speed based on current output ramps, voltage, amps, etc. If it determines that the motor is running too slow for a given frequency it will increase the voltage, maybe the frequency too, depending on whether you give it a setpoint in the form of RPM or Hz.

Sensored vector is like sensorless vector with the addition of an encoder to verify the motor speed.

Some VFDs have another, simpler encoder feedback mode which is like somewhere between V/Hz and vector, with encoder feedback.

So from that maybe it is more clear that the VFD is really intended to be implemented directly to a motor only, not a machine. It is not intended to be be used as a general purpose 3ph power supply. However, in V/Hz mode since it is not keeping track of its motor, it could conceivably be used as a general purpose supply in limited applications.

It is tempting to put in an 10, 15 or even 20hp RPC for the whole shop, just for the simplicity of it. After all, three-phase machines were designed to run as the arrive, on native 3-phase. Why go to all the trouble of wiring in a VFD if you can just plug it in and it will work as designed?

Because most RPCs are tuned with capacitors to roughly match a given load. The 3 phases will only have the correct amplitude and phase separation at that load. If load increases or decreases, it will stray from the optimum. Usually not a problem, if for example you build one for your 2HP mill, under full load, then at idle it will be not quite optimal, but not quite unacceptable either. But if you build one sized for some 20HP fully loaded motor and try to run a 1/4hp motor on it, you might have problems.

Some RPCs are better than others; most are shop-built. I have an American Rotary ADX30, 30HP RPC which has some load sensing and control gadgetry which swaps capacitors into & out of the circuit as needed, to keep the output correct regardless of load. It works pretty well. I have it reverse-connected to a 240-480 3ph transformer and it functions as the 3ph 480v supply that i use for testing motors and control panels. The load is always different for what i do, so that's why I paid out for such a widget. It wasn't cheap, but sounds still much cheaper than the SPC that you described. I would recommend that over a home made RPC if that's the way you want to go.

 

[mksj]

The install and parameter recommendations for the 1236/1340/1440 type lathes have been outlined in the basic install threads to help people get started and setup their lathes, other information has been posted on mills. They are based on the Hitachi WJ200 VFD because it is what QMT sells and in 99% of cases provides all the control features and has proven to be very reliable. I have dealt with 100's of installs with this model and have yet to have had a DOA or field failure. I use a few other brands such as Yaskawa, Teco, etc. mostly mid-priced range with a warranty and some US technical support.

The main reason for using a VFD is size, inexpensive for single machines and enhanced control features. The main drawback is they are not a plug and play install, and require some safety interlocks so you do not end up missing any parts. Parameter wise, some VFD's may have a very limited number where others may have several hundred and more when integrated into control systems. In the application you would be using it there are probably 20-30 parameters that need to be changed. These tell the VFD where to get their control and speed information from, speed set points, acceleration/braking information, motor characteristics, and such. These adjustments are one of the major reasons for using a VFD. Like learning a language or something like insert designation, at first it seems hard to grasp, after you start have some exposure and it becomes more understandable. My first VFD build took 3 months and programming was very daunting, I now can program and tweak a VFD run parameters in 20-30 minutes.

VFD's run either in a straight V/Hz which is a fixed ratio set on the motor base speed (it is not dynamically adjusted), or sensorless vector which is an encorderless feed back which can to some degree adjust the motor voltage/current to better control the motor RPM/power delivery. It uses the back emf/current to dynamically adjust the motor RPM under different loads. This provides significant more speed control as one decrease the HZ/higher load. Sensorless vector typically will maintain 0.1% speed control. The ratio of V/Hz is based on the motor's base speed, which typically in the US is 60Hz (but is motor/RPM specific), above the base speed there is no more voltage so torque falls off, below the base speed torque is more or less constant and Hp drops off in a linear fashion. One needs to understand these concepts in that turning down the VFD from say 60 Hz to 30 Hz your Hp is now 50% less and you loose the mechanical advantage of gearing down. One can compensate for this by using a vector motor and running it to higher RPM. No free lunch, but it depends on how much power you need and also the size of the motor. In VFD systems, motors are often oversized.

VFD's generate a lot of electrical noise due to the use of PWM nature and these can have other consequences on components such as wire, bearings and such. The bottom line is it is pretty much a non-issue at 240VAC and the context of the motor size and that they are not being run in a 24/7 environment. I would not worry about it in this application, but if you want to read up on it there is a wealth of information on-line.

 

[Dabbler]

I would recommend that over a home made RPC if that's the way you want to go.

+1 to that.

RPCs from commercial sources such as Rotophase and American Rotary are excellent value for the money. Mine is a Rotophase 15HP and it runs 2 HP and 7.5 HP motors equally well. If the 2HP is the only load you can hear a *tiny* increase in winding noise on my SG, but it never affects the finish. I'd never try to save money trying to homebrew a cobbled solution for a power source. (In my opinion it is a fire waiting to happen).

I use a VFD for speed control on the machines that need it. It is plugable to move from machine to machine. All of them are 2 HP, and they all have a very similar rated current draw.

 

[strantor]

+1 to that.

RPCs from commercial sources such as Rotophase and American Rotary are excellent value for the money. Mine is a Rotophase 15HP and it runs 2 HP and 7.5 HP motors equally well. If the 2HP is the only load you can hear a *tiny* increase in winding noise on my SG, but it never affects the finish. I'd never try to save money trying to homebrew a cobbled solution for a power source. (In my opinion it is a fire waiting to happen).

I use a VFD for speed control on the machines that need it. It is plugable to move from machine to machine. All of them are 2 HP, and they all have a very similar rated current draw.

Important note about the one I have: quality matters, but I can power sub-1Hp loads from my 30HP RPC, not because it's high quality, but because it's a "smart RPC" with a microcontroller circuit to control the output.
@ArmyDoc You could spend good money on a RPC from a reputable manufacturer, only to find that it isn't suitable for your needs, because it's "just a RPC." Best to read the manufacturers documentation. The American Rotary website has a nice tool to determine your needs.

I don't know anything about Rotophase but I suspect they probably have similar and I've heard good things about that brand as well

 

[ArmyDoc]

Because most RPCs are tuned with capacitors to roughly match a given load. The 3 phases will only have the correct amplitude and phase separation at that load. If load increases or decreases, it will stray from the optimum. Usually not a problem, if for example you build one for your 2HP mill, under full load, then at idle it will be not quite optimal, but not quite unacceptable either. But if you build one sized for some 20HP fully loaded motor and try to run a 1/4hp motor on it, you might have problems.
...
Important note about the one I have: quality matters, but I can power sub-1Hp loads from my 30HP RPC, not because it's high quality, but because it's a "smart RPC" with a microcontroller circuit to control the output.
@ArmyDoc You could spend good money on a RPC from a reputable manufacturer, only to find that it isn't suitable for your needs, because it's "just a RPC." Best to read the manufacturers documentation. The American Rotary website has a nice tool to determine your needs.

As far as RPC, I was looking at these:

North America Pro-Line Rotary Phase Converters for Sale

Pro-Line Rotary Phase Converters CNC Rotary Balanced, by design! PRO-LINE exceeds industry performance standards; powering sensitive modern CNC and traditional three-phase equipment. North America's Pro-Line rotary phase converters for sale go further with efficient removable back plates...

www.northamericaphaseconverters.com

and

Smart Phase™ Digital Rotary Phase Converters

Smart-Phase™ Digital Rotary Phase Converters Excellence in a box! Smart-Phase™ is a highly engineered digital rotary converter with tight voltage balance control. Best suited for ultra voltage sensitive machines, high performance machines and/or multiple loads running simultaneously...

www.northamericaphaseconverters.com

The site says they can are balanced all the way down to zero load. (about half way down the FAQs)

What is a Rotary Phase Converter & Other Rotary FAQs

Rotary Phase Converter General Questions What is a rotary phase converter? A rotary phase converter is an efficient way to produce 3 phase power from a single phase power source. It generates one line from a generator motor and combines that with your two single phase lines. What does CNC...

www.northamericaphaseconverters.com

I don't honestly know what way I'm going to go. The shop won't be built till April so I have plenty of time to think about it. I'm not even sure at this point what I want to get first - a mill or a lathe. I go back and forth. It sounds like if I get a mill, it might make more sense to get an RPC due to multiple motors (assuming the x and z drives are 3 phase in addition to the spindle? Looking at the Acra LCM50) If I get a lathe, I guess the VFD makes more sense, especially with all the material available on this site (looking at the PM 1340GT).

 

[mksj]

Mill is the easiest to install a VFD on, and a lot less expensive. The drives (typically Align) are all single phase, even on older 3 phase mills they are usually DC drives driven off of a transformer. So you are just driving a single motor in either case. The LCM50 comes in a digital drive version, it will operate off of single or 3 phase. Alternative is the belt drive and you add your own. On the mill I found it easier to go with the factory digital head, there was not much savings otherwise and a lot of build time. If you go RPC then most people go with the mechanical variable speed head mill, install your own VFD then pulley head. Lathe is a bit different situation, I find the variable speed on the fly and controlled braking to be more significant factors. Three phase motor will last a lot longer, do not have the start/stop restrictions of single phase, and are suitable to variable speed with very good performance envelope. If bigger Hp machines and multiple machines then an RPC becomes more cost effective. In a few larger shops run off of an RPC, I have still installed VFD's on some of the machines because it offered additional features individuals wanted. They are not mutually exclusive.

 

[strantor]

As far as RPC, I was looking at these:

North America Pro-Line Rotary Phase Converters for Sale

Pro-Line Rotary Phase Converters CNC Rotary Balanced, by design! PRO-LINE exceeds industry performance standards; powering sensitive modern CNC and traditional three-phase equipment. North America's Pro-Line rotary phase converters for sale go further with efficient removable back plates...

www.northamericaphaseconverters.com

and

Smart Phase™ Digital Rotary Phase Converters

Smart-Phase™ Digital Rotary Phase Converters Excellence in a box! Smart-Phase™ is a highly engineered digital rotary converter with tight voltage balance control. Best suited for ultra voltage sensitive machines, high performance machines and/or multiple loads running simultaneously...

www.northamericaphaseconverters.com

The site says they can are balanced all the way down to zero load. (about half way down the FAQs)

What is a Rotary Phase Converter & Other Rotary FAQs

Rotary Phase Converter General Questions What is a rotary phase converter? A rotary phase converter is an efficient way to produce 3 phase power from a single phase power source. It generates one line from a generator motor and combines that with your two single phase lines. What does CNC...

www.northamericaphaseconverters.com

I don't honestly know what way I'm going to go. The shop won't be built till April so I have plenty of time to think about it. I'm not even sure at this point what I want to get first - a mill or a lathe. I go back and forth. It sounds like if I get a mill, it might make more sense to get an RPC due to multiple motors (assuming the x and z drives are 3 phase in addition to the spindle? Looking at the Acra LCM50) If I get a lathe, I guess the VFD makes more sense, especially with all the material available on this site (looking at the PM 1340GT).

Your second link is like the one I have, "smart" RPC with AVR and dynamic phase shifting. That is the kind I would recommend for powering [anything you might ever have in the future]. The first link is, as far I can tell from skimming, "just a" RPC, with some protection features. I interpret that FAQ with a bit of suspicion. Maybe I'm just paranoid but it seems they left a lot of holes where very important words should be; the kind of words that could reverse the meaning of the whole sentence. Like a lawyer wrote it. Maybe that pro-line DOES do the same thing as that smart model. But if so, why make both? I would inquire with them.

I don't know anything about that brand but it seems decent quality and in fact I'd put money on that motor assembly being the exact same one, from the same factory as mine.

 

[Dabbler]

One of my friends VFD'd his 7.5 HP lathe, and modified the entire control package to make it seamless. A lot of costly work, but he likes it a lot.

A bit of food for thought: It it were just a mill and lathe, with the usual controls, well, you can get away with either approach. You could do 2 VFDs as most mills are 3HP or less, and that lathe in 3 phase 2 HP or less. this is in the inexpensive range for a pair of VFDs. That said, it is a smallish increment to go to a 5HP RPC which would do both machines, and if you order your lathe in 3PH, no additional costly wiring.

Another friend bought a GV1440 in single phase, and within the year wired it for 3PH. it took him a week of full days.

I think that @strantor and @mksj are spot on. Perhaps get the smart rotary phase converter. If you later need speed control, you can convert the lathe to VFD later...