The first and most basic type of stepper motors is the Permanent Magnet (PM). The rotor of the PM motor carries a permanent magnet with 2 or more poles, in a shape of disk. The operation is exactly the one described above. The stator coils will attract or repulse the permanent magnet on the rotor and will generate the torque. Here is a sketch of a PM motor:

PM stepper motors have usually step angle from 45^o to 90^o.

The VR motor does not have a permanent magnet on the rotor. Instead, the rotor is made of soft iron, and performs a teethed disk like a gear. The stator has more than 4 coils. The coils are energized in opposite pairs, and will attract the rotor. The lack of a permanent magnet has a negative affect on the torque that is significantly decreased. But it has a great advantage. These motors have no detent torque. The detent torque, is the torque generated by the rotor permanent magnets that are magnetized to the stator's armature, when no current flows within the coils. You can easily understand what this torque is, if you try to rotate an unconnected stepper motor by hand (NOT a VR stepper). You will feel the distinctive "clicks" of each step of the motor. Actually, what you feel is the detent torque that pulls the magnets against the armature of the stator. Here is an animation of a VR stepper motor in operation:

VR stepper motors have usually step angle from 5^o to 15^o.

The hybrid stepper motors are named so, because they combine the characteristics from both VR and PM stepper motors. They have excellent hold and dynamic torque, and very small step angles, from 0.9^o to 5^o, giving them A+ in accuracy. Their mechanical parts can rotate at high speeds relatively to the other stepper motor types. This is the type of motor used for high end CNC and robots. The major disadvantage is the cost.

A typical 200 steps per revolution motor, will have 50 North and 50 South poles, with 8 coils (4 pairs). Because such a magnet cannot be manufactured, an elegant solution has been given. There are actually 2 separate disks, each one with 50 teeth. A permanent cylindrical magnet is also used. The disks are welded one on the North and one on the South pole of the permanent magnet. Thus, one disk has North pole on its teeth and the other South. The trick, is that the disks are placed in a way that if you look them from above, you will see one disk with 100 teeth! The hills of the first disk, are aligned with the valleys of the other disk.

A permanent magnet with 50 North and 50 South poles is not possible to be manufactured...Therefore two disks are placed on top and bottom of a cylindrical permanent magnet		The hills of one disk are aligned with the valleys of the other. If you look the disks from above, it will be like looking a 100-teethed disk with 50 north and 50 south poles! An elegant solution!

The following animation shows a hybrid stepper motor with 75 steps per cycle (5^o per step). Worth to notice that the 6 coils are in pairs of two, each one with its opposite coil. Although someone would expect to find these pairs with angle difference of 60^o, it is not so. If we suppose that the first pair is the most top and most bottom coil, then the second pair is with angle difference of 60+5^o from the first, and the third 60+5^o from the second. This angle difference is the reason why the motor moves! Full and half stepping can be applied, as well as single-coil excitation for power saving. In this animation i use full step drive. With half step drive, the steps are increased to 150!

Don't try to follow the coils to see how it works. Just focus on one coil and wait. You will notice that, whenever this coil is actuated, there are 3 North poles (red) 5^o back, that are pulled to the rotation direction, and another 3 South poles (blue) 5^o front that are pushed to the rotation direction. The coil that is actuated is always between the North and South poles.

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