Stepper Motor Not working at High Speed

I m Using PLC for stepper motor in which i m using PWM for generation of pulses .. My Ton is 80 ms and my TOFF is 40 ms , and Overlap is 40ms .. at this setting my motor is not working .. and if TOFF is increased to 45ms , and overlap is increased to 45 ms it Works ..

I have checked my program pulses on SCOPE and it is found that Ton is 80ms , Toff is 40ms, Overlap is 40 ms .. they are ok

It is a customized three phase motor with retainer .. motor current is about 0.5 A, and voltage is 24 V DC ...

To reduce high frequency switching I have used diode 1N4001 as freewheeling diode , but still my motor does not work..

What is probable reason that my motor is not working ...

pl guide

I m Using PLC for stepper motor

Are you lost? This is the Arduino forum, not a PLC forum.