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 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. 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. 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. 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.