stepper motor voltage/current - Page 1
Forum section: electronics / beginners

author: V_King
author_rank: Regular Contributor
timestamp: October 08, 2013, 05:06:23 pm
content: Hello,I am confused with stepper motor voltage/current ratings.I need NEMA17 and NEMA23 stepper motors with max available holding torque (reasonably priced), best at 12V power supply (will do tests with both sizes to see which one to use).Some of the stepper motor datasheets show very clearly, that the motor is rated for example at 4V and 1.2A. But other datasheets just mention voltage and current ratings up to some very high values. Some of the steppers sold on ebay do not even have ratings posted.I wonder if I can use Ohms law and feed 12V to 4V rated stepper with 0.4A (for 1.2A rating)?Also torque at very low revs is very important to me. In one of the datasheets I sav, that doubling the voltage, increases torque at low revs considerably, but may have the heat dissipation issues. Is it a good practice? The datasheet was from german motor manufacturer.Finally, I want to use the BigEasyDriver from Sparkfun, and it's datasheet says that the motor power input is 8-30V, so if I have 4V rated stepper motor, can I still use it with a driver? There is only current adjustment pot on it.Thanks for any input.Cheers

author: DavidDLC
author_rank: Frequent Contributor
timestamp: October 08, 2013, 05:14:30 pm
content: Read Microchip AN1307 ( application note ) and it will give some explanation about voltage/current relationship.David.

author: Kremmen
author_rank: Super Contributor
timestamp: October 08, 2013, 06:02:36 pm
content: The stepper losses are mainly I^2R or heat dissipation in the winding resistance. Voltage is of secondary importance, what matters is the current through the coil resistance.Ohm's law is still if effect so at DC you get nominal current at the low voltages the motor mfgs quote. This is quite relevant when the motor is at standstill or turns at very low revs.The situation changes however when the motor is turning fast, whatever that means in practice. Then the coil inductance enters the picture and starts to dominate the evolution of current through the coils. The well known formula di/dt = V/L defines the rate of current increase and if you put the numbers in place, you will note that usually the coils will not develop anywhere near nominal current at nominal voltages when the motor turns fast. So you increase the coil supply voltage in order to achieve faster di/dt and thereby more torque at high speed. One driver mfg recommends to supply the system with a voltage that is 32 times the square root of the coil inductance in mH. This is just a rule of thumb and it also assumes that the system has a current limit circuit that prevents gross overcurrents at low speeds.The smarter stepper drivers dynamically adjust the current limit such that at constant speed the limit is nominal, during acceleration and deceleration some overcurrent is allowed and at standstill the coil current is reduced to decrease dissipation.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: October 08, 2013, 06:17:24 pm
content: It's very common to overdrive a stepper's "Voltage" for torque and acceleration, and to overcome it's inductance at higher speeds. Like a resistor, a stepper motor doesn't really know voltage and current (within extreme limits), it really cares about heat. EDIT: answered better already.For holding torque without overheating the motor, typically you pulse it when stopped. You have all the switching circuits already so it's just a matter of software if you're using a computer to drive it. Alternatively if you don't need high speeds, consider a worm drive. They're really inefficient but low speed torque and braking won't be an issue.If you need a large holding torque, larger than your drive needs, consider adding or getting a stepper with a brake.I found this very helpful:http://www.geckodrive.com/step-motor-basics

author: cosminnci
author_rank: Regular Contributor
timestamp: October 08, 2013, 06:32:53 pm
content: I bought from ebay 3 nema23 motors 270oz in with a driver with tb6560 and a 24v power supply, but I am not very pleased with them. The holding torque and peak torque / rpm is very important to read there is a graph of rpm and torque . But so far nema 23 I seen on 24v, 1.5 -3A and depending on the current you adjust the torque. If you want holding torque you will need a serious driver with radiator to dissipate heat.

author: wobblycogs
author_rank: Contributor
timestamp: October 08, 2013, 08:41:09 pm
content: I was going to provide a link to the Gecko article on stepper motors but I see someone has beaten me too it. It's well worth a read.FWIW, the two rules of thumb I've used / read about for stepper voltage are: 20 times the rated voltage or 32 times the inductance. If you are running the steppers at reasonable speed you can often go a bit higher than that too (my steppers are rated at 4.2V and I'll be running them at about 70V). If you are running at very low speed ease back on the multipliers a bit.

author: V_King
author_rank: Regular Contributor
timestamp: October 11, 2013, 10:19:04 pm
content: Thanks for all the answers. Microchip and geko info answered all my questionsAlthough I could not fgure out the power calculations from the explanation in the geko website