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Freewheeling diodes and stepper motor

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I'm currently building a small project, which is a stepper motor driver. Circuit is very simple: consists of 2 current sources based on LM317, low side switches made of TIP122 (+SK26 diode across CE junction). Whole thing is driven from PIC18f2550.

This is the rough schematic:

By poorchava at 2012-04-28

Motor parameters are unknown. It was made by ECM motor company, but I can't find any P/N. As the motor is NEMA 23 size I assumed 0.6A to be safe current level. It's an 8 wire one driven in unipolar fashion.

Now the problem:
I'm able to get motor to spin. But for some reason there is audible sound coming from PCB (can't say where from exactly). Also when I ass some diodes across motor windings for current recirculation the motors turque decreases ALOT and so does the maximum speed.

Why is that? I thought that adding a freewheeling diodes helps recirclate current better and faster so it should actually improve the performance, not diminish it  :o

The only thing that I can think of is that it has something to do with driving the steppers from a constant current source rather than a CV source.  Off of the top of my head I'm not sure what the freewheeling diodes will do in that case.

You might use an appropriate constant voltage source to see if things improve.

The freewheeling diodes short the back-EMF. You want to use high voltage transistors so the internal diodes will start to conduct in reverse at low currents. For very high speeds only energize one winding instead of two. BUT... the best way to drive a stepper at high speed is to use a 4 phase bipolar sine wave (from a class-D amplifier). More complicated but adds microstepping as a bonus.

Technically, in this configuration the freewheeling diodes don't short the stepper coils.  Instead it will connect the coils to the source and ground.  (The source is typically a voltage source.) 

A short across the coils would cause the decay to be slower.  The decay is sped up by providing the current a low impedance path (through the diodes) from ground to the (voltage) source.

In the case of this particular circuit the source is a a current source.  This current source likely looks like a very high impedance to the load.  I don't know what that does to the decay but I'm pretty sure it doesn't help.

power supplies cannot absorb energy. their output impedance while delivering current may be low , but when you feed them current the output impedance goes infinte. simply becasue the transistor in the supply can only conduct one way....

As for flyback diodes. when you send current through  a coil and interrupt it , it wants to keep flowing IN THE SAME DIRECTION.

so the didoes across your transistor do nothing.... they would block the current. the flyback diode needs to be across the coil. the energy stored in the coil now has a path through the diode. this current does not flow back into the supply ! this is a common mistake. the current runs in a circle through diode and coil. if the diode and coil were superconducting the current would run forever. unfortunately the coil has resistance and the diode has forward drop.... so you are actually burning off the stored energy as heat. (I2R in the inductor and Ix0.6 in the diode )
if you want to burn off the energy fast you need to add a resistor in series with the diode that has the same resistance as the winding resistance. ( you can do the math and draw power curves if you want. your maximum power dissipation happens when you match the resistance.)

now , since you are dealing with a motor... this decay has a side effect that it also produces a mechanical force that will actually slow your motor down. especially if two windings re energized. The motor becomes a transformer ! do not think of the currents through the motor as DC currents... they are actually AC currents when the motor is spinning ( current rises and falls continously... ) since the other winding is energized as well by a changing current you get this transformer effect....
you could even saturate the motor core and then the field would collapse ...

there is more than meet the eye.
motors like stepper motors and BLDC should be driven from H bridges and not from simple single ended drivers with flyback diodes.


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