How do Stepper Motor Drivers Overcome the Speed Restrictions Due to a Charging Coil?

[mattnedgus]

Hey Guys,

My apologies, I think I've made some real errors of judgement above! I'm still very much an amateur but here's what I think is happening for anyone that reads this and gets as confused as I did, in the future:

A coil in one phase will have current go from all in the +ve direction, through 0, to all in the -ve direction, and then back through 0 to +ve again in one revolution of the motor. In the case of a 4.2A motor it has a total of 16.8A to move (+4.2A --> -4.2A --> +4.2A) - so massminds worse-case calculation I linked to in an earlier post, I think, is correct after all (although in contradiction to the calculation on All About Circuits website - how?).

For my motor (I=4.2A, L=3.2mH, V=68V) I thus get a max speed of 380RPM. The first time I calculated this it didn't feel right so I did more reading and convinced myself that my gut was right (that 759RPM should be my max). Reading the info on the All About Circuits website seemed to persuade me of this - but it didn't expain why the graphs on some spec sheets for motors were shown with usable torque at even higher speeds.

BUT, it just dawned on me that this (380RPM) is the CORNER SPEED, NOT the MAX SPEED!! This is the speed under-which the coil should be able to fully saturate (build the maximum field strength) in both directions and thus maintain full torque (or very near to) upto this speed. This is what Geckodrives graphs show with the flat portion: https://www.geckodrive.com/support/s...or-basics.html

At speeds faster than 380RPM there is fall-off in torque because there is no longer time to fully saturate the coil with the maximum current. Torque drops off after this speed because its proportional to current and reaches its max speed when insufficient current can be pushed into the motor to make a step.