Nema 23 motor question

[ed vitanovec]

I see several ratings on the motors, 2.8a, 3a, 3.5a, torque ratings of 281oz, 381oz, 425oz. The machine is 48" x 24" with dual drive on one axis and ball screw. My question. Is the bigger the motor rating the better? I will do wood routing and occasionally light aluminum. Can you traverse thru your cuts faster with lower or higher torque motors? I appreciate and comments and feedback.
Thanks! Ed

[Chuck Saunders]

Higher torque means that the motor can move through more resistance without loosing steps. This comes at a cost of higher power requirements and your power supply and drivers must have the capacity.

[ed vitanovec]

I am thinking 3.5a motors with 495 oz torque. The power supply is 48v, 7.5 a. The controller ratings are 3.5a max. If I use a 12a rated power supply would that be a better choice?

[Gary Campbell]

Ed...
Take the combined amperage of your motors. Your minimum power supply should have 70% of that number. (minimum)

[John McClanahan]

Also keep in mind that stepper motors produce maximum power at low speeds. As the RPMs increase, the torque starts dropping off. Revving the motors to get better speed will result in lower power. If you run them too fast, they will even stall. 2-3 turns of the motor for each inch of travel seems to work well.

John

[Gary Campbell]

John makes a good point. Most steppers are "viable" up to 500 rpm. Some will "run" up to 3000 rpm. His recommended 2-3 motor revs per inch (25mm) of travel really hits the sweet spot for most cutting applications. There is one caveat however, be careful of microstepping resolution settings when using mechanical reduction in this range. The torque drops off 8-15% for each level (1/2, 1/4, 1/8, etc) of increase. Motors with advantageous mechanical reduction are best configured with 1/2 and 1/4 step settings. In many cases drivers that are set (or factory set) to 1/8 or 1/10 so severely kill the torque that they render many otherwise good designs close to useless.

Make sure you couple your motors with drivers that you can experiment with multiple configurations. Having a step resolution higher than 1000, often places unneeded limits on speed and power. A higher resolution number is not always better. There is a sort of urban myth about them.

[Art Mann]

There is one caution about comparing steppers only on torque. The torque specified is usually stall torque. You don't carve with the motor stalled. Higher quality motors have lower inductance and maintain a larger percentage of their stall torque at higher speeds. It is possible that a cheaper stepper may be rated for a higher torque but not perform as well as a higher quality stepper rated for less torque.

[ed vitanovec]

Thanks for all the feed back. How can I identify a hi quality motor, what do I look for?

[Gary Campbell]

Ed...
Do some homework. Look and compare brands that are sold on reputable sights. Usually branded items will be higher quality than "generic" or unbranded products. Look for and look at the torque curves for the motor and see what torque is available at what rpm. Take your final drive ratio and multiply it by your intended feedrates to find the available torque in the rpm range that you intend to use.

Couple it with a good quality driver that is programmable, preferably by comm cable, or at least with dip switches. Go digital for a few more $, you wont be sorry. Don't fall into the low price trap of multi channel drives that lock you into 1/8 or 1/10 microstepping unless you are looking to use under extremely light loads. Try to find motors that have matching drives, I prefer them in the 6-8 amp range. Look over AC drives or get a good linear power supply with numerous caps that can deal with back EMF and stay away from the switching type unless you are going for a low powered budget build.