A unipolar stepper motor has two identical sets of windings. Each winding's center tap is tied to a power supply and the ends of the coils are alternately grounded. The current is only allowed to flow in one direction through the motor winding. Each section of windings is switched on for each direction of magnetic field. A unipolar stepper motor is also known as a four-phase motor. A bipolar motor has no center taps. The advantage to not having center taps is that current runs through an entire winding instead of just half of the winding. As a result, bipolar motors produce more torque than unipolar motors of the same size. The draw-back of bipolar motors, compared to unipolar motors, is that more complex control circuitry is required. Current flow in the winding of a bipolar motor is bidirectional. A control circuit, known as an H-bridge, is used to change the polarity on the ends of one winding. Every bipolar motor has two windings, therefore, two H-bridge control circuits are needed for each motor.