Unipolar chopper design

[marklewry]

Hi all,
I previously posted a message here with regards to this problem but have been experimenting since and come up with some new results...
I have designed and built a Unipolar chopper drive for an a lead Bifilar wound stepper (4Amp 0.85Ohm 1mH approx). I have had good success running smaller motors but this one is giving me grief!!!
The current build up within the windings happens so quick that it seemed that I must of had duff sense resistors that seemed really inductive. This is not the case, it seems that because the motor is Bifilar wound it acts like a transformer. When the current builds up in the first coil the second coil releases its energy from the inductance through the freewheeling diode. This unfortuneatly seems to screw up the amount of useful continuos current in the windings - because it all happens too fast, the current builds up to about 4 amps in 2 micro seconds.

Can anyone give me some advice on what to do about this.
Mark Lewry

PS Thanks to all who previously helped

 

[tiki]

Hi marklewry,
you have had the right idea for the reason of fast current rising. The two bifilar winding act as a transformer and
the current, which wants to flow continuously, is "jumping" from one winding to it's complementary. The average current depends on the voltage and the sum of resistors in the motor loop. In one winding the current will go up, in the other winding it will drop.
If I=V/R with I: resulting current, V: voltage over motor circuit, R: the sum of resistors in the motor circuit, will be greater than the current, you want to control (you told about 4A), you will only see the fast current rising up to the average current.

Help: add appropriate inductors in the upper uH-range in series to the motor windings.
They are independent, cannot act as a transformer and you will get the wanted dI/dt. Check the current capability of the inductors without saturation! Check the possibly radiated EMI noise by chopped current through these inductors.
In my pspice-simulation (V=12V, MOSFETs MTP3055, winding-R=0.85Ohms, winding-L=1mH, addititional L=100uH, without current regulation) the resulting dI/dt is about 50mA/us and the average current through the windings is about 5A, sigificantly lower, than without inductors.
Please keep in mind, the freewheling diodes, if you youse the standard circuit, conduct the full current.
tiki

 

[cbarn24050]

hi, it sounds to me like you have a shorted winding, or you have them connected up wrong.An easy way to tell is to remove the power, then connect an osilloscope probe to each end of the winding.Connect the scope earth to the winding mid-point, rotate the motor by hand and you should get equal sine waves on each phase phase shifted 180 deg apart, repeat for the other winding.