I am comparing the performance of a Copley Motion Control Stepnet STP-075-07 driver to a TI DRV8811 stepper controller while driving a NanoTech LP2515 stepper motor.
The both drivers use voltage PWM to drive the motor at constant current however the Stepnet controller uses a PI loop at 24V while the DRV8811 uses a current chopping scheme with fixed t_off (as you are well aware) at only 12V. Both controllers limit the current to 100mA. I have confirmed these values (the voltages and currents) on an oscilloscope.
I am finding that the Stepnet controller is capable of driving the stepper motor at significantly higher speeds than the DRV8811. When connected to the DRV8811, the stepper motor stalls around 700 steps per second but when connected to the Stepnet controller the stall speed is pushed out to about 1000 steps per second.
I believe this may be because the higher voltage of the Stepnet driver is capable of overcoming higher counter EMF voltages. My understanding is that counter EMF is generated by the magnetic flux generated by the spinning rotor and can be summarized thus:
V_supply = I * R + V_emf + V_cc
Where V_driver is the supply voltage to the driver, I is the current in the motor coil, R is the resistance of the windings, V_emf is the voltage created by the spinning rotor, and V_cc is the voltage drop of the internal circuitry of the stepper controller. V_emf is proportion to rotor speed.
With a differential probe, I measured the voltage across a single motor coil. I also measured the current in the same coil with a current probe. What I expected to see is that either as the motor speed increases, the voltage across the coil decreases until it drops to a level such that the motor stalls or that the current would decrease in the coil to a point that the motor stalls. I did not see either of those scenarios.
So, would somebody please describe what happens to current, voltage, and motor torque in a stepper motor as the rotation speed increases and stall occurs? Is it possible (or perhaps likely) that the reason the Stepnet controller is capable of driving the same motor at higher speed because it is using a higher voltage and able to overcome a greater counter EMF voltage? And finally, are there any techniques (such as impedance matching) that will push will increase stall speed or is a higher driver voltage the only option?
Thank you for your help, sorry for the lengthy post.
