What voltage should I use to run my steppers?
The 20 times rule is to answer the question "how high can I take the supply voltage without burning down the motor?". The limit is a motor rated case temperature of 100C. That is very hot. Grabbing a 100C motor is no different than plunging your hand in a pot of boiling water. I certainly don't like things that hot and you probably don't either.
The physics of the matter is simple. Motor output power increases linearly with supply voltage, heat increases as the square of that voltage. That means heat has a curve that gets progressively steeper with voltage. If it's a comfort, think of it as God's or nature's way of keeping you from getting 100 HP from a NEMA-23 by using a 10,000V power supply.
This subject is best looked at in terms of energy density. Fancy concept but really simple. A really good car engine gets 1HP per cu-in of cylinder displacement. You could hang a turbocharger, use nitromethane fuel and get more but where is the limit? At some point the whole mess would come apart. Same thing with steppers. A really good double-stack NEMA-23 will top-out at 100W mechanical with a really good drive. At 100W, the motor will be very, very hot. It's a small motor, 12.5 cu-in and it delivers 100W for an energy density of 8W per cu-in.
My swimming pool pump is rated at 1 1/2 HP and has a volume 300 cu-in. (5" by 5" by 12"). It is a 220VAC, high efficiency motor. 1 1/2 HP is 1117.5 Watts. It's not working at the limits of its design (a good thing), its power density works out to 1117W /300 cu-in or 3.725W per cu-in. That is a little less than 50% of what a little size 23 stepper can do, Watts / cu-in.
So, maybe interesting, but what does all this mean? How do you chose the right supply voltage to run things? First, figure how much power you need! Don't say "all I can get". Multiply IPM by in-oz, divide the result by 531. The answer will be in Watts.
Don't want to it that way? Run your motor/drive at 24VDC. Note the speed at things stall at. Say it's 40% of what you want. Your correct voltage to get where you want to be will be 24V/40% or 60VDC. It's easy because it's proportional. Mind the motor temperaure. Never operate a stepper where it sizzles when you touch with a wet finger.
Steppers today are not your "grandfather's Oldsmobile". The new, square steppers are as good pound for pound as any servomotor.
Posted by Mariss of Geckodrive.com on multiple forums.