Thread: Motor winding config clarification

I have this old 15hp motor that I’m planning to use to build a larger rotary phase converter.



Here is the label that was on it showing the winding connection options




Am I correct in taking it that the motor 380-440v rating is with it Delta connected - indicated by the small triangle stamped in the “conn” section on the nameplate?

Steve

Think I’ve been able to answer my own question.
The original motor starter was a star-delta type, which means it would have been Delta connected for running.
That also fits with my interpretation of the stamped triangle on the data plate.

Steve

Spot on.
Tell us more about about your project.E.g. why you are going that way, rather than VFD; how you are setting it up; what circuit diagram and how you plan to get the synthetic 3rd phase close to the other two.

Thanks Joe.

I think there’s always a place for both a RPC and VFDs if you’ve got a few machines. RPC is great for dumb machines like bandsaws that you just turn on and never need speed change on. Saves having to buy a VFD for each machine (cost soon adds up) They also work well for machines that have a few controls and limit switches - saves having to rewire the whole control side. Also for ones that have multiple motors such as traverse and coolant etc.
VFDs are also great - both for single motor machines that you want to just plonk next to 240v power point in multiple places, ones that you need speed control for, and also for larger motors where the controlled ramp up time can allow you to run motors that you simply couldn’t run off an RPC.

I’ve got a small RPC that I build a few years back that I’ve been using to power my Takisawa lathe, and the TOS mill (which also has a 3ph-3ph VFD for the spindle).
The RPC is functional but not particularly well balanced - not a design fault but a personal one (I just never got around to tuning it properly!!).

Design is pretty common/simple.
Transformer to get the input voltage up to around 415v, idler motor, start capacitors on contactor controlled by a timer to get the idler spinning and then run capacitors to keep it going and balance the phantom third leg.
Also an output contactor to make sure the output to the load isn’t connected during startup.
The balancing on such a simple design is never perfect, as you really need different capacitance for different loads. Going down that track means you’re into control systems and circuit boards etc.
Practically you just balance it so it’s reasonable in the load range you mostly run it in, and live with it.

Steve

Hi Steve,

Originally Posted by OxxAndBert

RPC is great for dumb machines like bandsaws that you just turn on and never need speed change on.

VFDs are great for that too. No reason why you can't just have an ON/OFF switch on the VFD.

Another plus for VFDs I reckon, is when you sell the machine it can be sold as a single phase unit.

For sure, but at a minimum you still need to mount the VFD and mess around with switches etc.

With an RPC it’s just plug in and go (usually after sorting out the phase rotation so the motor goes the correct way. Weird how a 50/50 odds situation seems to give the wrong rotation 9 times out of 10!! )

Not sure what the “when you sell the machine” situation is. Isn’t one meant to just keep accumulating them until there’s no room to move

Steve

A little OT but what is option 3?

Originally Posted by OxxAndBert

Isn’t one meant to just keep accumulating them until there’s no room to move

Definitely. I'm there already!

Steve I have been thinking i would like to build a wood gasifier and I have been keeping an eye out for a 3 phase petrol generator to run on the gas... would a rotary phase converter work ok being run with petrol/gas engine, i have some
3 phase motors and a 20 hp b&s...and a 25hp kohler????

Hi Shed,
I think what you are asking is can a rotary phase converter be used as a generator - in which case I think the answer is no.
A RPC basically uses single phase input and some capacitors to spin a 3ph motor, and once its spinning the magic magnetism stuff generates the third phase. So the electricity creates the rotation and magnetism to create the extra phase - no external power required.

I do recall back in the 80's when I first looked at building a RPC there were designs around that used a single phase motor to drive a 3 phase motor as a generator. Obviously there was some additional magic required as just spinning an induction motor won't generate anything, but I've long since forgotten the detail.

Steve