Unipolar and Bipolar Stepper Motor Selection

There are two basic types of stepper motors for circuit design: unipolar and bipolar. A stepper motor will move one step when the current flow changes direction in the field coil(s), thus reversing the magnetic field of the poles, and this reversal is what separates unipolar and bipolar stepper motors.

Torque

Bipolar stepper motor circuits have the advantage of having just one winding and a low winding resistance. The two changeover switches are a disadvantage to using a bipolar stepper motor in that they need more semiconductors than their unipolar counterparts.

Unipolar circuits need just one changeover switch; however, they have the crippling disadvantage of requiring a double bifilar winding, meaning that at a specific bulk factor the wire is thinner and the resistance is considerably higher. Because unipolar motors seem simpler with regard to use with discrete devices, they remain popular today, although bipolar motors can be driven with the same number of components as a result of modern integrated circuits.

A bipolar stepper motor produces the most torque, which is proportional to the intensity of the magnetic field of the stator windings and can be increased with additional windings or by boosting the current; but increasing current can saturate the motor’s iron core and, more importantly, will increase the maximum temperature of the stepper motor. Because of the double cross section of the wire, unipolar motors have the advantage of having half the copper resistance of bipolar motors. At its power loss limit a bipolar stepper motor will still provide roughly forty percent more torque than a unipolar motor built on the same frame.

Current Driving

To keep the power loss of a stepper motor at a reasonable limit you must control the current in the windings. The simplest, most popular solution is to supply only the voltage needed, using the winding’s resistance to limit current. There’s a more efficient and precise way to address this issue, but it is also more complicated than the previous example: include a current generator to achieve independence from the resistance of the winding.

George

George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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