4, 5, 6, and 8-wire Stepper Motors

'Help'. This is a common word during discussions involving peculiarities of step motors, which can have 4, 5, 6, and 8 wires. But today, I can say that this type of engine is no longer such a difficult challenge, as they are starting to get more and more standardized.

Here, we will talk about the most common motors: of 4, 5, 6, and 8 wires, which can be unipolar or bipolar. The best known of these is the bipolar, 4-wire, which is what we use, for example, with several drives, such as the TB6600, with the printer driver Router 4988, and with the DRV8825, among others. The 5-wire motors are unipolar. In 6-wire and 8-wire, these are rated as unipolar or bipolar depending on how the driver is connected.

Most of the up-to-date stepper motors that I have are with eight windings. It is not because only two coils appear on the schematic that is just that.

So, as we're talking about this, I’m going to answer a question from a follower, Natan Bittencourt, and explain this better: "Fernando, I did not quite understand the issue of stepper motor resolution. Does it have to do with the amount of coils inside the engine? Can I vary the amount of steps within a complete rotation just by varying the intensity of the electric current in the coils?”

The question essentially is this: if there were more coils in the engine, would it have more resolution? The answer is no. What causes resolution in the stepper motor is the number of phases with the number of teeth. It's almost always that. So when you search the web, you will find these diagrams with north pole with south pole.

This has the good and bad side. It serves synthetically, it has its utility, but it is bad to make a parallel with the stepper motor in the direction of the teeth, because when you play the sequence on the coil, you are millimetrically grinding your teeth. So beware of this kind of representation.

But another thing I want to address today is the waves.

In this image, we have the single step wave, the half step wave, and the micro step wave of 1/8. So the more you increase the micro pitch, 1/8, 1/16 (used frequently by 3D printers), 1/32, the drivers tend to throw a sine wave over the coils. However, I don’t see this in practice. This is because most of the drivers we buy are cheaper ones. Even the TB6600, 8825, and 4988 are of the more simplified type and cheaper as a result. I have to say, however, that they are intelligent, do a lot of things, control micro-steps, but they don’t go as far as the last consequence of generating a sine wave at the output.

So, what causes engine resolution? It is given by the number of teeth times the number of phases, influencing if this engine can, for example, have 400 or 200 steps per revolution. In the bipolar motor, the coils are connected together. There are usually eight coils, but only two circuits.

I personally prefer the bipolar motor. First, I prefer it because it is more standard. Second, because most of the new drivers coming out are also bipolar. Thus, I think 95% of the engines you'll find going forward are bipolar. But I can be wrong, of course. Engineering is something that can change any minute. However, TODAY, my personal preference is bipolar.

This drawing is from a Chinese factory that I found very interesting when dealing with this subject. The image shows 50 teeth of a Nema 14 precision pitch motor. Therefore, 50 teeth times four phases of the bipolar motor will give 200 steps. Then, if you get 360 divided by 200 we get 1.8, which is a Nema 17 engine, one of the most common. We can say the same for Nema 23.

In China, they also have a 100-pitch stepper motor. Simply stated, this means much more precision. There are 400 steps per revolution. This brings many benefits, the first being that it avoids resonance regions. I’ve already done several stepper motor tests, and I see entering that region of resonance is bad because it makes noise and loses torque.

The Unipolar motor driver has the hour coil energized, and time not connected. Thus, it only has one pole, as opposed to being bipolar. In the image, we have a simple representation of a driver made with field effect transistors. When we polarize the Gate of a certain transistor, it lets the current flow. For this, you need to do a binary sequence. I also show a ULN2003 connected in a unipolar 5 wire configuration.

In the case of the bipolar driver, the currents flow in both directions, and then invert the positive and the negative. That means we have two full bridges. The output power module of the TB6600, in the background, is with this logic. In contrast is the logic of ULN2003 or ULN2803, which is unipolar.

Table Nema 17

This is a datasheet of a Nema 17, 6-wire, which can be connected as bipolar or as unipolar. This shows that the stepper motor, in general, starts at a low RPM, and the torque decreases as the speed increases.

Speaking specifically of holding torque, the datasheet shows that if you turn the motor on a unipolar driver, it loses 30% of the torque.

Table Nema 23

In this Nema 23 datasheet, the torque curve almost reaches 750 RPM. The motor that was 9kgf.cm of torque goes at 1kgf.cm. So when purchasing a stepper motor, it is important to observe this datasheet to see in which RPM you are going to operate the motor. The holding of torque also fell when connected in unipolar. When you connect the coil in parallel, the torque doesn’t change.

In this space, we make a comparison of the bipolar torque with the unipolar torque. Of course, in both types, as we increase the RPM, the torque is falling. But, the bipolar already starts 30% higher in the unipolar. What I saw, however, is that the unipolar reaches a higher RPM, which the bipolar doesn’t reach. This is ideal for a given application. Thus, it is necessary to know both types of engines to choose which is the best for your project.

Let's talk a little about:

Torque - Power - Speed. In this image, the dash is the power, and the solid line is the torque. Horizontal marking deals with speed. Then, the graph shows the relationship between these three elements. The best position is when the power is in a region that doesn’t expend much power and the torque is high.

You can adjust the torque of the motor also by increasing the current or voltage you use with the driver.