Vfd's Rpc's And 3 Phase Power At Home

I see a TON of questions on RPC's and VFD's on here and I figured I would type out a knowledge base for what they are and how they apply to the home shop.

We as machinists work with industrial equipment at home. It's obvious if you are reading this that you know that industrial stuff is typically 3 phase and you want it to run in your home shop where there is no 3 phase power. For a long time there was little option for this except the rotary phase converter or RPC for short. There are static converters. And for a single machine, in SOME instances, they are a viable option. But for a home machinist, we know a few things going into our hobby. The first is that no matter what we have, we will want to upgrade it, and there are always more machines to buy space and money permitting. So an RPC is a very viable option for running multiple machines at once, providing power to a dedicated 3 phase panel that all machines are connected to. As of posting this, I have 3phase welders, and a 3 phase mill I run off a 15HP RPC and have no problems with doing so. I built my RPC and have built several others for people so I know a bit about them and what works, and frankly what don't. I will share that with you here.
Be aware that I may add to this from time to time as I get additional knowledge that is worth sharing.

ROTARY PHASE CONVERTERS
The base components of a RPC are as follows

1. 3 phase idler motor. This is sized a bit larger than the largest motor you will be starting. While a 10 HP motor will start a 10 HP equipment motor, it will only do so if that motor is started under no load. A 3 HP mill or gear head lathe that has a clutch that can be disengaged will start off a 3 HP rotary converter. But a 5 HP 3 phase air compressor with no head dump will struggle when trying to start off a 7.5 HP RPC. a 10 HP RPC is needed to reliably spin up a compressor with no head dump. You will want to choose a motor rated for continuous duty for an idler motor. Pump motors are a good choice. But it needs to be able to run for hours at a time.
   
2. A RUN capacitor bank. These capacitors are wired in from L1 to L3 and L2 to L3 to creates a resonant L-C circuit across the inductance of the windings of the idler motor and the capacitor bank. You can NOT effectively run a rotary phase converter without a proper run capacitor bank. The capacitors are connected through a contactor that engages AFTER the start capacitor bank has the motor spun up. Run capacitors are NOT start capacitors, and can NOT be used as such. Run capacitors are typically metal cased, oil filled, and are of a lower value than the start capacitors and are rated for continuous duty. Another note on run and start capacitors. They do occasionally fail, and their failure results in them exploding. They MUST be installed in an enclosure to contain them when they fail.
   
3. Start capacitors. Start capacitors are not oil filled, have a larger rating than a similar sized run capacitor and are black plastic cased. They too will fail explosively and must be contained in an enclosure for this reason. They run from L1-L3 typically and are always connected into the circuit via a contactor that is on a timer of some sort that will switch them in for the time it takes to spin up the 3 phase idler motor and then out of circuit. Trying to run them in circuit continuously will cause them to explode.
4. Contactors and control circuits. NO RPC should ever be built without a proper contactor and ac fail dropping circuit. By this I mean a push button start / stop circuit that if the AC power fails that the converter will remain off when power is restored. The reality is that no industrial machine should be run without this but as long as the RPC has is, and the output of the RPC is completely isolated from the machine after a power fail is sufficient for safety. One additional note on design. The start and run capacitors should never both be in the circuit at the same time. Using a timer relay to control both is a good design idea. When the start capacitor timer run it's cycle and drops the contactor out it can put the run capacitor bank in. I personally have my run capacitors connected to my output contactor. I run an input contactor, an output contactor and a contactor for the start caps. This gives full isolation of the input power from the RPC. And the AC fail loop is created through the input contactor so if I have a failure of pop the main, the RPC remains off when power is reapplied to the input line.

I am not going to get deep into the design of a RPC. There is much information on the internet about the design, and building of an RPC that will get you going. For those wanting to build their own, the web will provide, and questions can always be ask here for specific direction. Understand your limitations. I have a good bit of experience with electricity and motor controls. If you don't like working with electricity, find a buddy that does. Bribery of BBQ and beer is a strong motivator for many. Just remember to hold off on the beer until after the RPC is built and running. Beer and electricity don't tend to mix real well.

Sizing an RPC
Sizing an RPC is done with simple math. Motors are easy as the sizing matches. A horse power is just that. With that knowledge, 746 watts is also 1 horsepower. If you have a welder that draws 7460 watts, that's 10 HP. Something else to know for the guys building their own. Once the idler motor is running, as you add motors, you add capacity. This means that if you have a 10 HP idler, and start up a 5 HP dust collector, you now effectively have a 15 HP RPC running. If there are devices that will typically always be running, like a dust collector, and you are building your own converter. When you are fine tuning your run capacitor values to match the L1 to l3 and L2 to L3 voltages, have the second device running when you are making your voltage readings. Having the run capacitor bank a bit oversize for the idler motor will not hurt anything while your second device spins up and becomes part of the RPC system.

Also know that you will only be able to effectively capacitor start idler motors up to a certain size. 25 HP is the largest reasonable motor you should try to start via capacitors. And some smaller motors by design are too difficult to start as idler motors. Case in point is my idler. My idler is a 15 HP combustable gas environment motor. It has a 2.5 inch splined output shaft and is as big as a 75 HP standard frame motor. It's armature is VERY heavy and it will NOT start with capacitors. It gets manually started, which is an option for any RPC, but a requirement for mine. IF more than 25 HP is needed, build two RPC's one with capacitor start and the second start off the first then using contactors add in the second capacitor bank. You can build to over 100 HP of capacity this way. this is typically overkill but it's good information to have going forward.
Something else to know about the additive nature of motors and RPC's. ONLY motors that are directly wired via a contactor or switch to the system will add to it's capacity. Meaning that if you have a 20 HP CNC lathe or mill that the main motor is controlled via a VFD or is a DC motor, it does NOT add to the system, it's like connecting a purely resistive load like a welder. Also, when sizing, take all motors of a device into consideration. If you have a large CNC lathe with a 20 HP main motor, also remember the loads of the chip belt, tool changer, bar feeder, coolant pump and anything else the machine may have. It's almost easier to look at the wattage requirement than simply the spindle motor size when sizing an RPC for such a device. While most of us are not going to drag home a HAUS CNC mill or lathe with all the trimmings, it's not outside the realm of possibility.

VARIABLE FREQUENCY DRIVES
VFD's or variable frequency drives are new on the scene. They offer some things that the RPC has no ability to but come with a list of short comings as well. the single to 3 phase design was not meant for running machines in reality. They were a low cost solution to needs of a high torque variable speed system for certain applications to use off the shelf 3 phase motors in equipment that required high starting torque at lower RPM. We stumbled on these as home machinists and they were an answer to many prayers, but caused some damning of them as well. For single motor machines like a Bridgeport mill, or a simple lathe without additional coolant pumps or motorized power feeds, they are a God sent. For any equipment with multiple 3 phase motors, the damning begins. Machines with a single motor can have their directional controls removed (actually needs to be removed) and the motor runs directly off the VFD and it controls direction and braking. For multiple motor machines however, the output being wired to the power inlet on the back causes a host of issues. Coolant pumps and power feeds run at the same slow speed as the spindle or quill motor and create an operational issue. Control circuits in lathes and mills go nuts being fed from a VFD. So there are some applications that they work well on and others where they fail miserably. Now there are 3 phase in 3 phase out VFD's as well. These can be applied in place of older technologies where resistor banks and other methods were used for quill and spindle motors specifically and the rest of the machine is run off a RPC. Properly designed RPC's are a MUST for running a VFD off of them. Improperly balanced L1 L3 and L2 L3 voltages will make magic smoke pour out of a VFD rendering it useless. So make sure that your voltages are good prior to applying a VFD to a machine running from a RPC.

Short comings of the VFD
This is not an attack of VFD's but they do have some shortcomings that the uninitiated may end up letting the magic smoke out of their shiny new VFD if they are not aware.
First thing to remember, VFD's are fairly new to the scene. We being home machinists are not dragging home brand new mills and lathes with new high efficiency motors. We typically are using older (sometimes MUCH older) equipment with equally old motors. These older motors will work with a VFD but there are limits. Know that you can NOT simply take a Bridgeport mill and set the spindle speed to the mechanical maximum and then use the VFD to slow the spindle down to 10 RPM. It may physically do it, with no load but torque is going to be non-existent. To that end, VFD's tend to not like stalled motors being connected to them. Magic smoke will leak out of the VFD and possibly the motor as well.
You need to have a good understanding of what the settings are on a VFD. There are things on VFD's that can be set in such a way that the VFD will not self protect and burn up. If you are unsure, ask someone that knows what they are and what they should be set at. Tech support is your friend with this.

VERY IMPORTANT
VFD's are to run ONE MOTOR that is DIRECTLY WIRED to the VFD. There should be no contactors, switches or other means of interrupting the connection between the VFD and the motor it controls. Meaning you can't run a multiple motor lathe, mill or other device off a single VFD. And you absolutely should never buy one VFD and connect it to a 3 phase panel on the output side of the VFD with the hopes of running all your shop off the one VFD. The circuits in a VFD always expect to see a load on the output. Having the VFD running and dropping the load via a contactor opening or a switch being turned off can cause the VFD to fail due to voltage spikes in the VFD.

 

Hi Keith Well written article. Power and power supply is properly the thing we consistently have to look at in our workshops. As we add on and upgrade our workshops, the power problem is always present.

Luckily I have 3 phase power supply to my workshop (garage actually). I have a 63 amp circuit breaker which is more than enough to supply 3 mills, 2 lathes, shaper, drill and other bits and pieces. I just revamped the workshop and I am redoing the power supply which over the years I had just added on which no real plan and it became a mess. So new cables and circuit breakers. Neatened up the cable ducting.

What I really wanted to write about is that as amazing as vfd drives are, they need a lot more thought. I added a drive to my colchester lathe which was 3 phase but the previous owner had fitted a single phase motor. So because I have 3 phase and refitted a 3 phase motor the same size as was in the lathe originally. All was going well until I needed to drill a big hole (32mm) and the motor stopped as i entered the drill. What i figured out was that although the vfd could handle the speed, the torque reduces to such a degree at the lower speeds that you risk burning the motor and drive. So think carefully about just adding a vfd, you may need to change out the motor. Either a newer type motor which is better suited to working with a vfd or a larger motor which will be better able to handle torque at lower speed.

Another option if you cannot change the motor is to change the pulley/gear ratios to increase torque at lower speed, which could affect the top end speed. But there is a solution to this as with a vfd you as using the frequency of the power to control the speed of the motor and you can increase the frequency from the 50/60 Hz standard to 100 Hz with the vfd drive

So my last comment is have lots of fum in your workshop, but remember ELECTRICITY is not a toy and must be treated with care and respect. If you not sure of some thing get a expert to help, the costs may be worth it

 

What I really wanted to write about is that as amazing as vfd drives are, they need a lot more thought. I added a drive to my colchester lathe which was 3 phase but the previous owner had fitted a single phase motor. So because I have 3 phase and refitted a 3 phase motor the same size as was in the lathe originally. All was going well until I needed to drill a big hole (32mm) and the motor stopped as i entered the drill. What i figured out was that although the vfd could handle the speed, the torque reduces to such a degree at the lower speeds that you risk burning the motor and drive. So think carefully about just adding a vfd, you may need to change out the motor. Either a newer type motor which is better suited to working with a vfd or a larger motor which will be better able to handle torque at lower speed.

Another option if you cannot change the motor is to change the pulley/gear ratios to increase torque at lower speed, which could affect the top end speed. But there is a solution to this as with a vfd you as using the frequency of the power to control the speed of the motor and you can increase the frequency from the 50/60 Hz standard to 100 Hz with the vfd drive

So my last comment is have lots of fum in your workshop, but remember ELECTRICITY is not a toy and must be treated with care and respect. If you not sure of some thing get a expert to help, the costs may be worth it

Yes, I fully agree that the VFD is not a beat all end all answer to all your 3 phase needs. I have seen several questions about using them to do things like direct feed a 3 phase panel and run multiple machines from them which of course is a no no. I think I may add some instances that you can't use a VFD and why in the write up. My first exposure the VFD's were an application where a 220 3 phase motor was being run off 110. The machine was a pallet shrink wrapper and the motor was used to drive the spin table. It started out very slow and then would increase speed, turning a specific number of turns while a second standard reversable motor ran a screw assembly moving the wrap holder up and down. This was a number of years ago, and I was truly in awe of the technology. Then after researching them I started to see their weaknesses similar to the ones you mentioned. Things like allowing a motor to spin at unreasonably slow speeds. Sure you can get them to do it but the torque is non existent, and they don't react well to stalled armatures. The bigger problem is those that are not aware of the short comings may decide to adjust the over current and other settings that would otherwise protect the VFD and motor from things of this nature and cause harm to one or both. The application I saw them in used a gear reduction output motor, which as you mentioned addressed the extreme slow speed needed for the application instead of trying to get the motor to run at 50 RPM.

Thanks for bringing this stuff up. I will add some stuff to the write up about this.

 

VFDs are not a drop in replacement for 3 phase machines, and they have their plus and minuses. They are not a replacement for an RPC, but they often can work well with an RPC to provide additional functions/wider working envelope for a specific machine. The VFD technology has come a long way in features, device protection, power delivery characteristics and reduction in their size over the last decade. A newer VFD matched with a inverter/vector motor, using the the sensorless vector VFD with such a motor can offer full torque down to 0 RPM. A TENV or blower equipped inverter motors are not limited in their low speed cooling abilities compares to a TEFC motor. A VFD used in substitute for a RPC on a single machine, you should have the same or better motor performance as a RPC at the base frequency of the motor. If you start to change the VFD frequency, then you are dealing with apples and oranges when comparing the two. If a control system for a machine is designed to run off of 60Hz, you would not run it off of a VFD and vary the Hz, if multiple motors are run off of a VFD, you loose many of the benefits of the pairing of the VFD to a specific motors operation parameters. I often think of the marriage of a RPC and a VFD provides the optimum relationship if one has multiple machines, but only needs the variable speed adjustability and/or VFD control features on a single machine. To get the most out of a VFD, this usually requires some modification of the machines control system to interface with the VFD control inputs. There are also some misnomers about Hp and Torque output of a VFD compared to a standard 60Hz motor with some form of mechanical reduction system to change the output shaft speed. Torque on an optimized motor/VFD combination can be increased by up to 200% for short periods below a motors base speed, Hp will drop off in a linear fashion. Above the base speed Hp will remain constant to about 150% of the base speed for most newer 3 phase motors, and an inverter/vector motor well past 200% of their base speed. Torque falls off above the base speed, in a somewhat linear fashion to ~150% for newer motors and over 200% of base speed for inverter vector motors. If you reduce a motor speed by 50% below its base speed, the Hp is reduced by 50% and the torque remains the same; but a mechanical system, the apparent output shat Hp and toque would be 200% because of the mechanical ratio. This is one reason that I often try to change the work envelope of an invertor motor system toward the higher RPM range when feasible by using a smaller motor pulley.


If one is using a machine with a mechanical variable speed head, or a machine with geared speed steps, above the motor base speed if one where to double the output shaft speed the effective torque at the output shaft above the motors base speed will the similar to that of a VFD/motor system, the Hp will be greater for a VFD machine when one factors in the mechanical system ratio of the speed range. IF you double the speed of a system using a mechanical gearbox, you reduce the Hp and torque both by 50% at the output shaft, the same system without changing the mechanical ratio, but increasing the VFD to say 120Hz, will provide full Hp, and torque at about 60% vs 50% of the mechanical system. There is also benefits to increasing the motor size in a VFD system to optimize the usable speed range, so in a lathe with a VFD one can use a 2 speed gearbox instead of 16 speeds, they may use a 5Hp vs a 3Hp in their non-VFD model. A VFD has its place, but they are not necessarily drop in replacements and they have their limitations. Certainly for high Hp motors and multiple motor systems an RPC is more ideal to feed multiple machines when limited to single phase power source.

 

Great info in your write ups guys. Really appreciate it. As an electrician, I do lots of installs on these types of equipment, control wiring of somewhat sophisticated designs at times, and tons of other connections and installs, but the nuts and bolts of how the equipment works internally and the theory behind it is quite often beyond my reach as there just isn't time on the job to get to that understanding and it is quite often beyond the scope of our job. Quite often, the maintenance electricians will get a much better understanding of this end of the equipment after the install and we are gone from the job.

So, here is a situation I find myself in that may also help others on this forum. I have a 3 phase CNC mill I am rehabbing and upgrading the electronics on. I am adding upgraded control boards and a VFD for the main motor. This machine has variable speed I believe through a variable adjustable pulley and I will be adding a VFD to the motor feed so I can run the 1 1/2 hp 3 phase motor off my home shop single phase power. The 2 hp TECO FM50 VFD will only be feeding the 3 phase motor and the rest of the motors are single phase or 24 volt servo motors and will be fed from the incoming power disconnect separate from the VFD feed. Now, I need to determine how to set the variable speed on the machine.
Given the VFD will probably reduce torque at low settings, where do I set the variable speed on the machine for start up, given the speed range is 60 - 3000 RPM on the adjustment dial to get the best results from the combination of variable speed and VFD range/torque? In the middle of the range, top end of the range, or low end of the range? I understand I can adjust the machine variable speed once it is running for particular projects that may require higher or lower than "normal" speeds but for the average project, I would need a place to start and program the VFD to my machine.

Thanks again for the write up and help here.

Bob

 

I went the cheap route with VFD's. I have 2 Fosdick 24" 4BM drill presses, my Van Norman 22LU main motor, and feed motor. Each has its own VFD. All setup just for the 3 phase 60HZ, power on, and fwd & rev. The drill presses, and main VN motors are also set up using the VFD to brake the motors faster then just letting them coast to a stop.

I played around with using the VFD to control speeds, but decided it was not worth the hassle (all have gear boxes for speed changes).

I debated with changing the motors, and also looked at static, and rotary phase convertors. In the end, cheap VFD's were the way I went.

 

I like my homebuilt RPC. I might want to add some run caps but it does a good job.

 

Given the VFD will probably reduce torque at low settings, where do I set the variable speed on the machine for start up, given the speed range is 60 - 3000 RPM on the adjustment dial to get the best results from the combination of variable speed and VFD range/torque? In the middle of the range, top end of the range, or low end of the range?

Great question Bob. I do most of my speed changing with the variable belt drive and leave the VFD set at 60Hz. If I need a quick slow down for some light work like reaming or small tapping at less than 500 RPM, then I turn the VFD down rather than go into low gear. If I need to do some heavy work at less than 500 RPM, then I switch to low gear and leave the VFD at 60Hz. My VFD does not have Sensorless Vector Control, so I don't the torque range that one so equipped would have. The system is happiest running at 60Hz, but works fine between 30 and 90 Hz.

 

I like my homebuilt RPC. I might want to add some run caps but it does a good job.

Adding a run capacitor bank to it will decrease the idler motor current draw and increase start capacity. It will also balance the L1 to L3 and L2 to L3 voltages. if you run your RPC alot, you may get an added benefit of a lower electric bill.