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Unipolar and Bipolar motors.

Some drives were designed to use unipolar motors. Others use bipolar motors. Unipolar motors with 5-leads cannot be used on Bipolar drives. Bipolar motors with 8-leads can be used on Unipolar drives. Some Bipolar motors with 6-leads can be used on Unipolar drives.

To easiest way to wire an 8-lead motor for a Unipolar configuration is to first wire it in a Series configuration. This creates two centertaps: an A-centertap and a B-centertap. To complete the Unipolar configuration, those two centertaps are tied together to create V+. The four remaining wires are A+, A-, B+, and B-.

CAUTION: The current used with an 8-lead motor wired in a Unipolar configuration should be 0.707 times the rated current for that motor in a Series configuration.

The amount of current allowed through the windings is limited by thermal considerations. Only so much heat can go into the motor before it melts. A motor has two halves to a winding: A+ to A-centertap and A-centertap to A-. Let's say that each half has a resistance of 10 Ohms. The current in a Unipolar drive flows from the A-centertap to either of the A+ or the A-, but only one at a time. Let's say the drive is providing 1 Amp. That 1 Amp only flows through half the winding, which is 10 Ohms. The power lost that heats the motor is P=I^2*R or P = 10W. Using the same motor on a bipolar drive where the current flows from A+ to A-, the current needs to be derated by 0.707. By derating, P=(0.707^2) * (2 * 10 Ohm) = 10W. The motor sees the same amount of power running through it with both methods.