Selecting the right driver

[Gary]

A motor and driver is rated as per phase, so if a motor is 4A, then get a driver that is rated at 4A or over.
To get any power that is useful from the motor you also need volts, so dont just look at the current.
As an example if you see a 10A driver that is only rated at say 30V, this will be completely useless for larger motors, unless you are happy watching paint dry.
In other words you may get the torque, but you wont get the speed.

[irving2008]

Gary's point is well made. The bigger the motor the higher the inductance of the windings, therefore the longer it takes for the magnetic field to build up. even small cheap steppers can have big inductances. A higher voltage 'forces' a higher current through the motor at the start of the step when the inductive impedance is highest. A simple rule of thumb is:

volts = sqrt(32 * H^2) where H is inductance in milliHenries.

so a 5mH motor would require ideally sqrt(32 * 5 * 5) = 30v, while a 10mH motor needs 56v (hence big motors on cheap drivers is a waste of time)

This is partly why the bipolar parallel arrangement give the best torque at high speeds. For typical motor windings that are bifilar wound (as most are) this halves the inductance as compared to bipolar series connection which quadruples it (all relative to unipolar).