What stepper motor size (torque)

[Lee Roberts]

As I understand it, it’s also based on motor Inductance isn’t it?

The lower Inductance the faster the motor driver can talk to the motor, so what you got to do is go for the highest torque you can get with the lowest Inductance at the same time. That also being a trade off ?

[irving2008]

As I understand it, it’s also based on motor Inductance isn’t it?

The lower Inductance the faster the motor driver can talk to the motor, so what you got to do is go for the highest torque you can get with the lowest Inductance at the same time. That also being a trade off ?

Steppers are constant power devices Power = revs * torque, i.e. as revs increase, torque drops off. Max torque is at or near stall. However as revs (= step rate) increase this linear relationship breaks down because of what you refer to Lee, which is the L/R constant of the motor. The rate of rise of current in the coil is proportional to the inductance: Since torque is directly proportional to current, for larger inductances you need to drive the coil harder (e.g. higher voltage) to reduce this torque drop off at higher step rates.

The formula is V = 32 * sqrt(L) where L is in mH. A 10mH coil ideally therefore needs a drive of 100v, but of course most steppers have a winding breakdown of 65 - 80v and 100v drivers are expensive anyway. What it means is that such a motor on a 48v driver (e.g. a Gecko) will not maintain torque above about 60% of max step rate. A better motor for a 48v drive would have an inductance no more than 2.25mH.

But too large a motor can be as much a problem as too small. Big motors have more rotor inertia which can reduce the ability to accelerate (e.g. change direction when cutting). You need to choose a motor thats in the 'sweet spot' else you can end up with nasty resonance issues (as well as pay more than you need to for both motors and drivers). The 'sweet spot' is an operating torque at cutting speed about 40% margin over what's needed. Thats NOT the same as stall torque which is how motors are rated. Its also important to ensure that the step rate at cutting speed is below that at the knee point (where the L/R effect starts). That may mean the steppers need to be geared to the leadscrews which also changes the inertial and torque dynamics. Hence the need to do the calculations.