I have been using a balanced rotary phase converter in the shop now for a coupla years with no problems. Now I have acquired a couple of VFD drives. One of the drives is rated for 220V single or 3-phase. Should I use these drives fed from the single phase, or should I put them on the 3-phase side of the RPC? I'm leaning toward using them downstream of the RPC so that the VFD's are not derated. Is there any electrical reason not feed them from the RPC? Thanks lg no neat sig line
The only reason that I can think of depends on how well balanced the RPC is -- at the load which the VFD and its load produces. If the balance is not petty good, you will likely get the same poor distribution of current through the rectifier diodes at the input to the VFD that running from single phase produces.
The VFD is not a resistive load. Nor is it an inductive load. Instead, the input view of the VFD is a three-phase bridge rectifier feeding into a *big* electrolytic capacitor.
The result is a current spike whenever one of the peaks goes above the voltage in the capacitor (plus the forward voltage drop of two arms of the three-phase bridge). The result should be a spike of current near the peaks (positive and negative) of each phase relative to the others. Normally, unless the output of the VFD is quite heavily loaded, the spikes are of very short duration.
The problem, in my mind, is that if the voltage of the three phases is not well balanced, you will get your current spikes only between the two arms which represent the highest voltage, and it will be little different than running it from single phase.
So -- I feel that you should assume the derating will be needed, since while things may be well enough balanced at the moment, if the value of the tuning capacitors shifts, it will be running in single-phase mode anyway. Better to pay for the extra capacity now than to discover that your VFD has been taken out by a drift in the balance of the RFC.
CNC equipment is probably a mix of inductive loading (any motors, including the spindle motor if you aren't running that from the proposed VFD), and the spikey capacitive loading from the power supplies running the computer circuitry, the stepper motors (if that is what you are using), or the servo motors.
So -- it is mostly seeing spikey capacitive loading, and probably some VFDs (perhaps invisibly) built into the CNC machines to run the spindle motors -- unless those are DC motors.
Quite a set of equipment -- and a more serious idler than in most HSM RPCs, so you might get good enough balance to benefit the VFDs, but I still think that it is safer to assume that you'll (sometimes) need the derating that the vendor suggest for single-phase operation.