First of all let me preface all of this by saying that I pretty much bought a motor at random (very cheap on eBay) so I’m not suggesting that this is a suitable motor for your project.
So here is what I purchased (at around $11 for two)
This page gives links to the following two datasheets:-
which are less than informative. My motors arrived with no wiring/plugs etc so the colour charts were less than useful – at which I point I wondered if I would ever make them work!!
However:- this is what I could learn from the datasheets:-
So long as you have two of the last three parameters then you can calculate the other using the equation Volts = Amps x Ohms
There is one potentially important statistic missing from these datasheets that can be calculated from the data given. If we can issue a maximum of 300 steps per second, (300Hz), and – due to the gearbox – we need to issue 2400 steps for a full revolution then the fastest we can turn the output shaft is (2400/300) ie once every 8 seconds. This may be rather slow for an output shaft if it is connected to wheels but could be fine if we are rotating the turret of a tank. So this is an important fact to ascertain before purchasing your motor.
So what have we learned? The gearing, steps per revolution and the maximum frequency produce trade-offs between torque and speed. Whether the motor is uni-polar or bi-polar can also influence the required current draw, and torque, as discussed earlier.