i know it's very similar , but it's not the same there is no energy stored in the inductor in this case... the inductor (stepper's windoing) is converting the energy to mechanical, not storing it and releasing it... so technically it's not a buck converter.
This is probably a far more subtle argument than you are aware; however, it would only interfere with buck type operation on one condition[1], which isn't met here.
The reason it is subtle is:
- An induction motor performs work based on applied flux, not by applied current. Yes, the current shows up as a necessary, inseparable part of operation, but the work performed is proportional to EMF. Such motors are mechanical-to-EMF converters. Examples: DC (brushed, or brushless with static (e.g. Hall effect) commutation), synchronous AC, AC induction (non-synchronous) less slippage.
- A reluctance machine (whether a motor, solenoid, relay coil, etc.) does operate by current; however, the work is again dependent upon flux. As the inductance goes from low to high (say, as the solenoid armature closes, reducing space for magnetic field and therefore removing work from the magnetic field and converting it to mechanical work), in order for the current to remain constant, more and more flux must be added to the coil, which means if you merely apply a fixed voltage, current must dip as the solenoid closes, or if you apply a constant current, the voltage must spike.
- What's even more subtle than this, is that a real stepper motor is a combination of these types. This is a necessity resulting from the small step size: to implement a stepper using an induction design would require N poles for 180/N degrees per step, and would be expensive to produce. A pure reluctance machine can be easily designed for fine steps (by notching the rotor and driving complementary notched stators in quadrature), but has weak holding force (the force is due to the difference in magnetic field strengths between the two sides of each notch, which isn't much distance for the field to drop off over, especially when made of iron parts). Instead, a hybrid combination is used, so that nearly the strength of an induction machine is produced, while doing it with only a pair of coils and a little machining. (I forget exactly how this is achieved, but this is the general result. Proof: if a stepper motor produces voltage -- as a generator -- for every step as it spins, it is of this type. If it does not generate, or the amount it generates is suspiciously small, it's a reluctance type.)
The consequence is, electrically, a stepper motor must be driven as a hybrid of these two methods. You must deliver enough flux (i.e., a spike of voltage for some duration of time) to build the magnetic field, in addition to the flux (voltage / frequency) required to advance the rotor due to its EMF.
If you look at only the V/I conditions at the terminals, averaged over a typical cycle (at constant frequency, say at a typical 'cruising' speed), you will measure real and imaginary components of current flow. The real part can only ever do work or dissipate power; the imaginary part can only ever recycle it. Physically, some of that real work will be done by induction as well as reluctance mechanisms, and some of the imaginary work will be done by stray fields (i.e., true leakage inductance) as well as mechanical effects (i.e., alternately stretching and squashing the magnetic field). But the point is, electrically, we don't care what the physical mechanisms are; we just care that, there's a real part doing work, and the imaginary part we can recycle with diodes, or play tricks with on energy (like using it for an implicit buck converter).
[1] And so, here's the catch, and where my footnote comes from, all the way at the top:
The condition is, as long as the input frequencies are orthogonal to the rotational frequencies, over the course of one cycle -- there cannot be any real power consumed of those frequencies. They must be recycled, reactively and therefore inductively*, back to the circuit.
(*Well, I guess inductively isn't a necessity. It could be capacitive under suitable conditions -- maybe not for a stepper motor, but a synchronous AC machine can be used to produce capacitive power factor. But whichever phase it is, it's not on the real axis!)
Indeed, often times this is forgotten -- bad motor controllers will succumb to such failure modes at the edges of operation. An old fashioned L297/298 pair will typically PWM at 10-20kHz; if you attempt to spin the motor above maybe 1kHz, operation shifts noticeably, as the number of "on" pulses per winding drops from more than 4 (reaching a reasonably steady-state current regulation condition before switching off), down to 2 and 1, and finally to zero as the winding isn't even able to develop rated current in the allotted time (or discharge fully to zero while off!).
Although the pulses of the L297 are not fixed frequency or phase, the orthogonality condition still applies as it does. When the applied flux gets terminated early (the winding gets turned off, due to peak current control, before the clock frequency would've turned it off ideally), the holding force or available torque both drop accordingly. When the current-control pulses are so coarse that they take up appreciable chunks of the operating time, the drop in torque becomes so severe that, eventually, it simply stops rotating at all, and stalls (torque < friction). This is ultimately why all steppers have a maximum operating frequency, but this is furthermore what limits the practical operating frequency of a PWM type stepper motor driver.
Now, I'm not saying this is what's happening in the OP's situation; this is just about steppers.
FYI,
Tim