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driving bipolar stepper with only 3 leads
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newmaster:
I have this problem I can´t figure out myself:
I have a dead 4 axis CNC controller, that I can/t repair.  instead, I started to think about buliding a new controller. it only needs to move 3 axis table to the right spot and then all the work will be done with rotating axis from there.  the controls can be extremely simple. even if I have to manually enter coordinates for each axis, it will be fine. drive speed is not important, exept for the rotating axis.

anyways, the system is built so, that they have a regular 4 wire motors, but 2 poles are connected together in the housing covering the motors, making it 3 wire drive. wiring is burried inside the machine and raplacing it would be extremely difficult. also, only one motor is reasonably well accessible.

are there any off the counter stepper drivers available that can control the motors with 2 poles tied together, through 3 wires?
I tried to search, but my technical english vocabulary might not be good enough. I didńt find anything useful.

also, can anyone explain, what could be behind this kind of design decision? are there any benefits to it?
ajb:
Are you sure the steppers are 4-wire?  Three-phase steppers are a thing, and there are off-the-shelf drivers for them, though they're not as common as four-wire systems.  If the motors really are four-wire with two wires connected together, then you would need a bipolar power supply to drive them.  Normal four-wire drivers rely on full-bridge outputs to be able to drive current in both directions through the coil, but if the motor coils are tied together then the simplest solution is to connect the coil common to ground and use a pair of half bridges with positive and negative supplies to drive each coil in either direction.   If the CNC system is old, then maybe the cost in terms of silicon was enough of a reason to design it this way.
richard.cs:
Measuring the resistance between all three pairs will tell you if they are three phase (all similar resistance) or 2 phase with a common wire (two similar, one double).
newmaster:
they are definetly bipolar motors with 4 wires.   I opened up the rotating axis motor connection box to verify.  unfortunately, other motors are hard to access, so rewiring them is almost impossible without disassembling the linear table.
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