Servo motors are fancy DC motors. You can push more current through them and get higher torque. Continuous torque is based on the long-term temperature rise in the coils. For short periods of time you can put a whole lot of current through the coils (peak torque) before they overheat. The current is always flowing the same way through the winding, and you just vary its magnitude slowly to accelerate and decelerate.
Stepper motors are operated at a constant current. At a standstill that also means constant voltage in the coils, and that is the most torque they will ever make. To step a stepper motor, you reverse the current in the coils in a specific sequence. As the RPM increases, the current reversals happen at a faster rate, and the inductance of the coils limits the rate of the voltage change (i.e. there is a time constant) so they are not developing as much torque as when at the standstill.
That's why, for the best high-speed performance of a stepper motor you want one with a low resistance (thus inductance since there are fewer turns of thicker wire) and then use a power supply of 20x or more of the motor's "rated voltage" since the driving circuit will keep the current at a constant level by pulsing the voltage going through the coils (at a very high frequency relative to the switching I mentioned above.)
Best regards,
Randy  |