How to determine stepper motor running current

[mayd85]

Hello,

I am working on a stepper motor project where I know the required torque and the speed at which I am supposed to run the motor. The motor runs with 1.8 degrees full step. Here is the torque curve of the motor



My torque requirement is 0.8 Nm @ 1/4 step with 1000 steps/s. From the graph, the motor delivers maximum 1.8 Nm torque when running at 1000 pps in full step. From this information, how to calculate running current @ 1/4 step with 1000 steps/s?

I understand that the torque deteriorates with increase in speed. But, is there any direct relation between torque and stepping angle too, at a fixed speed?

 

[barry]

This all depends on how you are driving the step motor. There are at least two methods: L/R and PWM. In the L/R method a series resistor limits the maximum current into the motor, the inductance of the motor limits the dynamic current. In the PWM method, a high voltage is applied to the motor, and this voltage is pulse-width modulated to maintain a constant average current.

I didn't answer your question, but as you can see, it's a complicated situation (You also need to consider back emf.) I think you need more information about your system.

You will note, however, that at the top of your graph it says: 6A and specifies a particular driver and voltage. Maybe that's an answer....

 

[mayd85]

Hello Barry,

I am driving the motor in PWM mode. As you rightly pointed out, I am trying to generate detailed requirements for stepper motor operation such as current, optimum stepping angle etc. with which I can configure my motor driver for efficient operation.

For determining micro stepping, I want to know how micro stepping and torque are related, if I want to keep the speed of rotation constant (My application involves lot of back and forth rotation in a short duration, which kind of determines the speed of rotation).

Also, the T/S graph is specified for full step and rated current. But for a particular speed of rotation, if I do not want to generate full rated torque (for preventing overstress on the mechanical part of the system), what should be the driving current? Can that be calculated from the T/S curve somehow?

 

[barry]

I've never encountered a situation where you want to control torque of a step motor; I'm not an expert, but I'm not sure it can be done. A step motor is not like an 'ordinary' motor. When you give it a step command, it's going to try its best to move from A to B. If there's not enough torque available, it will not move and you now have a situation where you have "lost steps", your step sequence is off and everything will be messed up.

If you are trying to limit torque to your load, maybe you need some kind of mechanical clutch.