When the current through a solenoid coil is reversed, the polarity of an electromagnet is also reversed. Due to their configuration, current can only be directed through the coils of a Unipolar Motor in one direction – this means that, as their name suggests, Unipolar Motors can only power individual phases at fixed polarities. Bipolar Stepper Motors, on the other hand, can reverse their phase polarity with the use of an H-bridge.
Unipolar Stepper motors are very similar to Bipolar Stepper Motors, but are manufactured with a central tap that connects back to the power source, essentially splitting each coil into two smaller coils that can be powered independently. If required, the central tap can also be left disconnected, allowing the Unipolar Stepper Motor to be converted into a Bipolar configuration.
Bipolar Stepper Motors do not feature a central tap for dividing their solenoid coils - this makes their internal wiring slightly less complex than that of a Unipolar Motor. For this reason, smaller Motors such as NEMA 14 Stepper Motors are more commonly available in a Bipolar configuration due to their reduced size.
Unipolar Vs Bipolar Motors
Unipolar and Bipolar Stepper motors both have their own distinct advantages and disadvantages, dependent on their application.
Bipolar Stepper Motors use a single, larger coil per winding, this creates larger, more powerful magnetic fields than Unipolar alternatives, which allows for greater torque forces to be achieved. Because the coils of a Bipolar Stepper Motor can change polarity, this also means that all phases can be put to work at once, further increasing potential torque output. The main disadvantage of a Bipolar Stepper Motor is the requirement for H-bridge circuits to reverse the polarity of the circuit.
Unipolar Stepper Motors use tapped coils, of which each side can be independently magnetized. Because the current running through each coil will be running in a different direction dependent on which side of the coil is magnetised, the polarity of each phase can be reversed without reversing the current of the entire circuit. However, because only half of each coil is magnetised, Unipolar Stepper Motors suffer from reduced magnetic force, and thus reduced torque. Unipolar Motors are simpler to install and operate than Bipolar Stepper Motors, because their circuitry does not require the use of H-bridges.
Older and smaller electronic devices tend to use Unipolar Stepper Motors due to their simple, space-saving circuitry requirements, but with advancements in technology and reductions in the size of components, many modern electronic devices now favour Bipolar Stepper Motors.
Hands On Demonstration
For a detailed demonstration of the differences between and how to implement Unipolar & Bipolar Stepper Motors with Arduino controllers, this video from DroneBot Workshop is an excellent resource