Step motors are current driven devices. In order to get any sort of performance out of them they need to be driven with a fairly high voltage, usually 5-20 times the voltage that would 'normally' result in the rated current thru the windings.
Ie. if the motor is rated 2.4V, 3A you can't use a 2.4V power supply. You should use something like 24V but applying 24V without any way of limiting the current would result in 30A thru the winding and a destroyed motor.