Stepper motor types
Permanent Magnet Stepper Motor (PM)
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 45o to 90o.
Variable Reluctance Stepper Motor (VR)
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 5o to 15o.
Hybrid Stepper Motor
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 0.9o to 5o, 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 (5o 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 60o, 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+5o from the first, and the third 60+5o 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) 5o back, that are pulled to the rotation direction, and another 3 South poles (blue) 5o front that are pushed to the rotation direction. The coil that is actuated is always between the North and South poles.
Awesome, thank you. I did find it just a bit slow for me but that is because I knew some of this before coming here.
That said it has given me a great deal of new knowledge and insight that will help me connect up my first stepper motor and have it actually work.
Thank you again.
Great tutorial! I suggest you slow the animations down, perhaps leaving a 3 second interval between steps. As it is now, it is visually difficult to keep up with each step of the sequence and study how the current flows through each winding.
Thanks for your efforts.
Fantastic explanation and didatics! I disassembled a disquette drive and found a very strange looking motor inside. Now I know it`s a stepper motor. Could you please inform with what software you designed the 2 demo videos?
I have a bunch of stepper motors that have failed. When installed in the machine they just vibrate back and forth. I know it is the motor because when I put in a new motor they machine works just fine.
Do you think there is anyway to repair these stepper motors?
It is a Hybrid .9 degree NEMA 23. Do you think a coil shorted or is it something to do with the magnetism? Or??
OK that animation mid-page on the right finally clicked in for how real steppers work and how you can get to smaller steps. All the beginner diagrams only show 4 poles and compass style rotors with giant 90 deg steps. that animation is fantastic, thanks so much!
@Vimee Bakori first disconnect completely the wires from the motor. You will need either 4 multimeters, or 4 LEDs or other source of light. Then , you must see between each of the 4 wires and the ground a pulse. The pulse must follow the pattern of a full or half step as explained in this article.
One question pls...
In a step motor with 4 wires (1=R, 2=S, 3=T AND 4=COMMON ),
how can measure by multimeter the voltage in order to understand if signal is correct?
i.e. I can measure sometimes 31V Only to one of three cables, or only to 2 of 3, or to all 3 cables...
How is the constraction of 4 wire motor with common ?
1. Depends on the stepper. For example, one stepper may have 5 ohms coils. Then, you will measure 5 ohms between the sides of the coil, and 2.5 ohms between one side and the middle wire.
2. You don't need to recognize them, they are bipolar motors. Only bipolars have 4 wires.
3. As i said in point 1, a 5 (or 6) wires motor has 2 coils. So, you must find one big and one small resistance between the coils. When you measure the resistance and you find the "big" resistance, then you are sure that you are holding the ends of the coils. Now be careful!!! If the motor has 5 wires, this means that the middle coils are internally connected! What this means is that you may find a big resistance between the ends of ONE coil, or between the ends of the 2 coils!!! That will trick you. No matter what, big resistance comes only from end-side of coils. Small resistance (which is half the big resistance) means that one wire is the middle.
Hi there,very good explaining about the stepp motors and how they work.I have 3 questions about them.
1.If I messure resistance on the coils,what I will get?
2.I have 2 stepp motors,they have 4 wires.And how can I recognize them?
3.The common pin how I can recognize it from the others,is there any resistance or something like that so I can recognize it?
I really appreciate your presentations. I happen to be a experimental biologist student, and wanted to understand dc and stepper motors so i could understand what is to be believed the mechanism of bacterial flagella.