Is it correct to believe that the NEMA 17 stepper motors in the store appear to be rated at 3A based on specs stating “Current Per Phase: 1.5A” and “ NO.of Phase: 2”?
According to the RAMBo board specs on the RepRap site, the controller board supports the following:
Motors, 5A 12-24V (fuse can be exchanged for up to 10A capacity) 10V-26V input voltage
If you have 5 stepper motors rated at 3A each, wouldn’t that draw too much current or am I somehow misinterpreting the board or stepper spec?
My thought was to buy slightly larger steppers for my build upfront and didn’t want to exceed what the board was capable of.
So a value of 138 means the steppers will draw about 0.8A each (maximum) or 4A total for all five motors. Based on threads on this forum, I know people have turned them up some, but then sometimes have overheating issues. It has been suggested on this forum by folks that know far more about steppers that I do, that rather than increase the current draw of the motors, you could increase the voltage. The Rambo board will handle up to 24V. This will increase the torque available when moving the router at faster speeds.
With that said, Ryan’s MPCNC design is well balanced and designed for this class of stepper at the specified current draw. Your stronger motors may try to drive the bit through the wood harder than the design allows resulting in deflection.
No. The drivers won’t send full power to each coil at the same time. So even if you had everything perfect, this motor is spec’ed for 1.5A. The driver usually sends max power to one coil, or the other, or splits it in between (for microstepping).
But in practice, you can’t get max power. They will cook the motor mounts (enough for them to deform, the rule of thumb is "keep it under 50C) and the drivers will overheat before you reach 1.5A.
In my past reading on steppers, I thought when the steppers were not moving, both coils are engaged. Or does the driver throttle back when energizing both coils at the same time for holding?
The coils point the motor in different directions. If you send all the power to one coil, the motor will hold in that direction. If you send it to the other coil, it will hold in that direction (That’s one whole step). If you then send the power reversed down the first coil, that is the next step, and then reversed down the second coil. Those are the 4 phases. The motor then has a bunch of magnets (100?) that make it take 200 steps to do a complete rotation.
If you send all the current to coil A, and you’re at step 0. If you then reduce that current to half, and send half to the second coil, you would be halfway between step 0 and step 1. This is microstepping. We configure them to have 1/16 or 1/32 microstepping, so each “step” from the microcontroller’s perspective is changing the amount of current from one coil to the next by a fraction of the total current. Microstepping makes for much smoother motion, and higher resolution.
Thanks for the description, and it clarifies some things for me, but I’m still unclear about the heart of my question. When I finish running a g-code file and go to change the endmill, the steppers are locked. I cannot push them in either direction. My reading indicated that 1) both coils are energized when the steppers are not moving, and 2) this is the sate they draw the most current. So assuming the drivers are set at 0.8A per coil, does my stepper draw 1.6A when the motors are not moving?
I could be wrong, but my understanding is that they are basically always using the current setting. When they are moving, where the current is going changes from coil to coil. When they are stopped, the same amount of current is flowing, but it isn’t changing where it is going. In my example full step above, when it is stationary, all the current is going through one coil. When you move, the current moves from the first coil to the second and then the reverse first, and then the reverse second. The current isn’t really changing.
It is easier to explain with full steps, but we are really doing microstepping, and when you are halfway between two steps, you are really using a bit more than the full step current, because each coil isn’t getting 0.5x, it is getting 0.7x.
I have seen some good visualizations for this. I can’t remember where now.