How Strict is the Max Vsupply Calculation for Steppers?

[mattnedgus]

Thankyou, I think you may have mentioned that Gecko Guide before - that's where both of the methods for calculating the max voltage comes from.

My issue is that the voltage I'm using is 20% higher than the max recommended by those methods. In terms of designing Im wondering if I can go over those maximums for better performance and if so how much?

For example, in the 3 motors I was looking at (below), one has a really low inductance of 1.18mH and a recommended maximum voltage of between 32 and 35V. A supply of 68V seems way too high?

(I can't find speed-torque curves for Motor A and B (D57CM31 and D57CM31-4A) so I've emailed Leadshine for them to see how they might fair compared to each other.)


3.1NM NEMA 23 A
I = 4A
L = 2.8mH
V = 2.5V
Max Vsupply = 54V
Upper Vsafe = 50V
Corner Speed = 321RPM (based on 48Vsupply)

3.1NM NEMA 23 B
I = 6A
L = 1.18mH
V = 1.6V
Max Vsupply = 35V
Upper Vsafe = 32V
Corner Speed = 381RPM (based on 36Vsupply)

4.5NM NEMA 34
I = 5.5A
L = 4mH
V = 2.53V
Max Vsupply = 64V
Upper safe V = 51V
Corner Speed = 232RPM (based on 68Vsupply)

[routercnc]

I can only comment on the stepper in the first post as that is the same specification I have. No issues at 68V on that motor.

I can see what you are trying to do now you have laid them out but can’t really advise on the others. Best guess would be use the same percentage increase in voltage on the first motor as a guide to the percentage over voltage values to try for the other motors and compare corner speeds in the calculations.

But if you do it for real then get a second opinion before you burn up a motor ! You need help from the electrically minded on here . . .

[m_c]

I've not got much time, but in a nutshell regards motors, current = torque, and voltage = speed.

However in the case of stepper motors, current <> torque, as they run at full current (unless you use a driver with automatic current reduction), to hold them on position, but voltage limits how much current the driver can 'push' through the motor coils.
What this means in basic terms, is at standstill, the motors rated voltage is need to push the rated current through the motor coils (I.e. a 5V motor, needs 5V to push the rated current through the active coil). As speed increases, back-emf increases (think of it as the motor itself acting as a generator to 'push back/restrict' the current the driver is trying to push through), which means you need an increased voltage to maintain the current flow through the motor coils.

This then means that too high a supply voltage only comes into consideration at high speeds. At standstill, with say a 5V 1A motor, the motor is having to dissipate 5W, however if you then spin the motor fast enough to utilise a 68V supply while still forcing the full 1A through the motor, the motor is now dissipating 68W (not all will be converted to heat, as the motor is doing some useful work). And a large amount of that heat created is in the motors rotor, which has relatively poor cooling. Now get the rotor too hot, and it'll demagnetize.