[RotarySMP]

Another way to look at it is to think of the magnetic force developed by the coil.

In a motor designed for unipolar use ( a six wire motor) you can saturate the iron core magnetically at rated current with a single coil. This will give you max torque. Any more current gains you nothing, as te iron core can not generate a stronger magnetic field (and thus torque). Could even demagnetise things if taken to extremes.

If you were to wire that six wire motor in bipolar series (isolating the center taps), you have twice the coil windings acting on the core, so you only need 0.7x (1/root 2) the current to saturate the cores.

The doubled number of coil windings increases the inductance, so the coil and iron core will resist changes in current. (Every step is a change in current to the coils). This make for a steep torque curve, with torque reducing rapidly with RPM.

An eight wire bipolar motor is designed to saturate the core with two coils on. That is why you will get a higher max torque rating for Bipolar as opposed to unipolar with an eight wire motor.

In bipolar series it acts just like the six wire with the center taps isolated above.

In bipolar parallel, you have two parallel paths for the current and the inductance is reduced. The coil and core resists changes in current less, meaning the motor can step faster.

At the end of the day the rated torque is holding torque when the machine is stationary. Not very useful. Far more useful is torque at higher speeds to give good rapid traverses.

This is achieved by running the lowest inductance motors for your drives current, at the highest voltage your driver can survive (for a choppering driver).