Hi, got a sparkfun easy driver and stepper motor 12v. All work fine but I need more torque - so started looking at the Sparkfun Big Easy driver and a larger rated bipolar stepper motor.
Problem:
The steppers I am looking at are rated at strange voltages like 2.8v and 3.2v. Then I saw this blog post http://bildr.org/2012/11/big-easy-driver-arduino/ and that states not to worry about the motor voltage as the driver looks after that?????? Is that right?
e.g. If I connected a 2.8v 1.5amp rated stepper to a Big easy driver an then connected a 12v 2amp power supply to the big easy it would be OK?
Struggling to understand how I am supposed to power a 2.8v stepper.
Yes, don't worry about it, the controller controls the current, not the voltage, and uses a chopper
circuit to perform buck-conversion so that you take less current from the supply than the motor
uses.
And don't trust the description, if you want more than an amp out of that chip, some active cooling
will be strongly advised (small fan should do).
With bipolar motors the limit to rotation speed is
(a) the supply voltage to the chopper-drive - higher overcomes back EMF and overcomes inductance.
(b) the winding inductance - this limits the rate of change of current.
(c) the number of steps per rotation - more steps means slower (but most are 200 steps anyway,
though I've seen some 400 step motors).
So motors with low inductance windings will go faster - this automatically means low winding resistance too
(so higher currents and lower nominal voltages). Winding resistance around 1 ohm or lower means high performance.
The back EMF when the motor is spinning fast will be large, and unless the supply is high enough
to overcome it you won't get any torque at that speed. Some stepper drivers go as high as 140V
to get more RPM.
nemo1966:
e.g. If I connected a 2.8v 1.5amp rated stepper to a Big easy driver an then connected a 12v 2amp power supply to the big easy it would be OK?
The stepper motor spec is saying that if you feed it 2.8V (with no current limiting) it will consume its maximum 1.5A of current. But the BigEasyDriver performs current limiting, as MarkT said, so that's why you don't worry about it.