Stepper Motors

After doing some reading here and there on the internet, I discovered that stepper motors run these CNC machine. Hmmm... OK well it sounds to me like I need to figure out what a stepper motor is and how they work. Where do you get them? Do you have to buy them? Can they be salvaged from something else? There I go again with the questions. Let's see what we can make of the stepper motor mess.

Before I start on this topic I want to remind you that there is a lot of information on this topic on the Internet and I strongly recommend that you take some time and look at it. Please do not take what I have to say as law. I could have it all wrong. I will give a resource of information on this project as I discover it in the resource section of this project.

Stepper Motors

Located in the resource section of this project, you will find a link to the control of stepper motors by Douglas Jones. This document can explain more about stepper motors than I could ever hope to convey on my own on the types available and how they operate. I would suggest reading that document. It is very informative and well written.

Types of Stepper Motors - Unlike normal AC or DC motors, stepper motors have many wires attached to them. These wires are used to run (or more accurately, step) the motor. Stepper motors come in two basic types. These are permanent magnet and variable reluctance. You can generally tell the two apart by turning the shaft of the motor. If it has noticeable step or cogs the motor is likely to be of the permanent magnet type. Of these two types of motors, I have only played with the permanent magnet motors.

Permanent Magnet Stepper Motors - Permanent magnet stepper motors usually have two coils in them. These coils can be with or without center taps. If the coils have center taps then they are an unipolar motor and if they are two separate coils without center taps they are bipolar stepper motors. The bipolar motors typically have four wires. Unipolar motors usually have either five or six wires. The number is determined based on the center taps. Some center taps are tied together therefore giving five leads others, the center taps are separate and give you six leads. I have only used unipolar motors at this time.

How to step the motor. Unlike a regular motor, a stepper motor will not run by simply applying power to the leads and throwing a switch. Stepper motors run by sequentially energizing the coils that make up the motor. There are two basic methods to do this. Either half stepping or whole stepping. A great example of this can be found in the resource section by going to Jone's Stepper Motor page. I see no need to reinvent the wheel here unless the good info drops off the net.

Where to find stepper motors. For the experimenter, stepper motors can be found pretty easily. Some places to look for them are old 5-1/4" floppy drives, printers, copiers, some xray equipment, etc. New stepper motors can be bought from Hobby CNC and other places around the net. At this time I have found or salvaged about 15 stepper motors. Most very small and not suitable for a CNC, but I figure they can be used for learning purposes.

Stepper Motor Characteristics - Stepper motors come in a bunch of different sizes and a few different shapes. So I think I should address what makes a motor suitable for a project and what does not. The two main characteristic I think you need to look at are (DEG/Step) or (Steps/Revolution) and how much the holding power of the motor is. The holding power is usually expressed as oz-in.

Steps per Revolution - Stepper motors move a certain amount of distance for each step they make. If the stepper motor takes 30 steps to make one revolution, then that motor is 30 Steps per Revolution. The steps per revolution can be expressed as a degree per step by dividing 360 by the number of steps in one revolution. In our above example 360 / 30 = 12 degrees per step. Generally speaking, the more steps/rev the finer control you can develop from the stepper motor. For example, a 30 Step/Rev motor coupled directly to a 16 TPI lead screw will advance the table or work .002" per step. While a stepper motor of 200 steps/rev would advance it .00003" per step. You can see how the number of steps per revolution could impact your project. Now if your stepper motor is not of a high enough step resolution don't worry just yet. You can double the steps in a motor by half stepping and if I have read sources correctly, you can even get them finer by micro-stepping.

Holding Power - Stepper motors are rated by how much pressure it takes to force them to move from a location when the coils are energised. This measurement is in ounce inches. Obviously, the higher this number the more holding power the motor has. For cutting foam I would guess that you would want a motor of at least 50 - 75 ounce inches. This may be wrong and could require more. At this time I have not really looked deep into this motor characteristic to fully understand what it means.

Now that we know a little something about what a stepper motor is, it is time to figure out what leads are what. After all, of all the stepper motors I have found, only three of them were marked. I have a couple of 5-1/4" floppy drive motors that I am going to use as a learning tool. See Bench Experiment 1 to learn how to identify the leads on a stepper motor.