I am trying to see whether Pololu has a NEMA23 stepper motor that would be a good fit for a project I’m planning.
I am planning on using the A4988 motor driver using 12 VDC battery. For this application, the max rotational speed that is needed is about 150 RPM. Within these constraints, I need as much torque as I can get without exceeding the current limits of either the driver or motor.
I know that the motors would offer better performance at higher voltages, but to keep the project costs down, I’m trying to stick to 12 VDC.
Thanks in advance,
How much torque would that be, exactly?
Motors have power, speed and torque. All three must be specified by the application requirements, if a design is to be successful.
I was targeting something greater than 100 Oz-in… the higher the better. The torque will define the max payload of the linear actuator I’m designing.
When looking at the motors available on the Pololu webstore, I was thinking this one might work #1477 (pololu.com/product/1477). I just don’t know how well it would operate at 12VDC. What attracted me to this one was the higher relative torque rating, and the current rating of 1A which the A4988 would like better in terms of heat generation than ones that ran at ~2A.
The voltage is not an issue for that motor, as long as it is greater than 8.6 V and does not exceed the driver maximum. Higher voltages = faster step response and higher torque when the shaft is rotating.
The 190 oz-in torque listed is the holding torque and torque goes down as the speed increases. See this general summary: pololu.com/file/0J628/SY57STH76-1006A.pdf
According to the data sheet, at 150 rpm or 500 steps/sec, you can expect about 120 N-cm or 170 oz-in torque with a 30 V motor power supply. It will be less at 12 V.
The current rating is 1 ampere/winding, so to avoid overheating the motor, it must be limited to that value. Either the A4988 or DRV8825 will work, but be sure to follow the directions to set the motor current properly.
Understood… I realize the A4988 motor driver modulates the applied voltage to keep the current below the setpoint and that needs to be set for the specific motor chosen. I also understand that the torque goes down with increasing RPM. My main concern was that I would not be able to achieve the target ~150 RPM with 12V applied to the motor driver. As you indicate, higher voltages = faster step response (e.g. RPM). The datasheet specs provided are listed at 30VDC, I am not sure what to expect at 12VDC. Thoughts on that?
There will be no problem achieving the target 150 rpm at 12 V. The available torque will be a lot less than at 30 V.
The larger question is: why do you want to use a stepper in a linear actuator? A suitably geared brushed DC motor will provide much higher torque than a stepper of the same physical size, at the same rpm.
The choice of using a stepper motor is to allow repeatable, step and repeat, type motion… much like a single axis on a CNC machine.
Encoders used with brushed DC motors allow more accurate step and repeat operations than do stepper motors, and with encoders, you can be certain that steps are not skipped.