I made a single phase to 3 phase with a 3 phase motor, but its showing on the meters from hot to ground.
1 shows 118.8; 1 shows 121; then the 3rd shows 180 volts. I need to get the 180 down around 120. what to do?

[email protected]

 

Those arrangements are inherently imbalanced, They can be made to work after a fashion. You can change things by changing the capacitor values, but since the caps are available in only certain large values it is difficult to really tune them in and a change in load will likely change things. If your horsepower needs are small, a single phase to 3 phase VFD will be nearly as cost effective, a far better solution, and much less screwing around. If you are above the range for 1 to 3 phase VFDs, there isn't much to do but swap things around until you get the best balance. I have seen such arrangements work for years where 3 phase power is simply not available, but they are a kludge and regarded as only better than nothing. If you're doing a version without capacitors, you may just need to learn to like 180 V. Cheer up though, some of the commercial converters aren't much better.

Regards
cww

 

Your question is a bit unclear. I assume you have a single phase motor driving a three phase motor. I also assume the three phase motor is an induction motor. By driving the three phase motor with a prime mover (the single phase motor), you're looking for three phase output. There are a few problems. First, how are the windings of the three phase motor connected? Again, I assume "star" or "wye" (not delta). In this case, the center point of the windings should be "grounded" or "earthed". Then the voltages on the three legs should be relatively balanced with respect to ground. However, for an induction generator to work properly, it needs excitation. This is usually provided for by a shunt capacitor bank. The problem is that the level of excitation (and capacitance) is a function of load. It is unlikely that your load will be constant, so this system probably won't work. Induction generators are usually only used when the induction generator is connected to an infinite grid that can supply the reactive current; and the grid does so in a variable fashion as needed. If you want reliable three phase power from your generator on an isolated grid (the generator being the only power source), you'll need a synchronous generator (or motor). A synchronous generator does not require a source of reactive power from the grid. However, the controls for the synchronous generator are quite complicated.

 

Curt, to replicate the single to three phase VFD thing in larger horsepowers all you'd need is a VFD that allows current to be introduced into the DC bus externally, which I believe is most VFDs because they'll do common bus applications. Then you get a large single phase input, DC supply, take the output of that into the DC bus terminals on the drive, and you should be good to go.

 

Yes, I've worked on the setups with a single DC supply and a rack of drives. They are big in the printing industry. But the cost begins to get where it would be more economic to simply replace the 3 phase motor with single phase. I right or wrong, assumed that the reason for a DIY "phase converter" was cost VS the more conventional solutions. Also most of the bulk DC setups require 480 volts as the currents quickly become excessive at the 240 V single phase most of us have available in residential settings. Since I have a lathe and mill in my basement, I think I know where he is coming from. But if the units could be obtained surplus and you can find a large transformer cheap, you could go that way. There really isn't a great way to run large 3 phase motors without 3 phase service.

Regards
cww