There are two common configurations for stepper motors, the two phase bipolar motor and the four phase unipolar motor. In their simplest form, as shown in the figure, there is a permanent magnet on the rotor and two coils oriented at 90° to each other on the stator. When driven, the current in each of the coils produce magnetic vectors that sum together with the permanent magnet on the rotor aligning with this combined magnetic vector. Changing the polarity of the current will change the angle of the magnetic vector and thus generate the rotating magnetic field that will make the motor turn. For the bipolar motor, this is done by sequentially changing the polarity of the current in the two windings. For the unipolar motor, the coil is wound in a center tapped configuration with the center typically connected to a positive supply voltage. Alternately connecting the two end connections to ground will change the direction of the resulting magnetic vector.