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Welcome to the seventh chapter in the TI Precision Lab series on stepper motors. My name is Audrey Kuehler and today I will discuss the basics of unipolar stepper motors. There are two types of stepper motors, unipolar and bipolar, which differ based on the presence of a center tap and the current flow direction. We will cover their construction and operation later on in the video.

At TI, we commonly see unipolar stepper motors being used in textile machines, industrial printers, pachinko machines, and slot machines. While we primarily use stepper drivers to drive bipolar steppers, we have several drivers which can drive unipolar steppers, as well as bipolar stepper drivers, which can easily be adapted to suit the need for unipolar stepper motors.

Unipolar stepper motors have two windings per phase. This common wiring configuration for unipolar motors is a six-wire configuration with two VM taps and four wires connected to the motor windings at A, B, C, and D. There is a center tap on each phase which is generally connected to the motor voltage supply, VM. The center taps can be electrically isolated or they can be shorted electrically. Another common wiring configuration for unipolar steppers is a five-wire configuration, as shown here.

Bipolar stepper motors have only a single winding per phase. Four wires are connected to the motor windings and there is no center tap at VM. Besides wiring, the key difference between unipolar and bipolar stepper motors comes down to the direction of the current, which is also where unipolar and bipolar motors get their name.

In unipolar motor drivers, the current only goes one direction, so the current only has one polarity. Hence, unipolar stepper. A bipolar motor has current that operates in one direction and then another to help the motor spin. We will now discuss these differences in more detail.

Let's talk about bipolar steppers first. Bipolar motors use H-bridges to allow current to flow in both directions through the winding. Bipolar drivers also use the full amount of copper windings so the motor delivers more torque. The direction of the current determines the polarity of the magnetic field created by that winding. So the motor will rotate in response to the polarity change.

Now let's talk about unipolar wiring. To drive a unipolar stepper motor, all that is needed is a single supply connection. A unipolar motor is not the same thing as a unidirectional motor. A unipolar stepper motor can drive a motor in more than one direction without the need to reverse the current.

Furthermore, the unipolar stepper only needs low-side FETs to drive the motor. As you can see here, all you need to do is send positive current toward each of the FETs individually, first to A, then B, and so on and so forth.

Here is another way of looking at this. The waveform shows where we send current through A followed by B, C, and D to drive the motor. Again, unipolar stepper motors do not need any reverse current to reverse the direction. Instead, just energize the low-side FETs in reverse order.

As we just learned, you only need four low-side drivers to drive unipolar stepper motors. TI has integrated low-side drivers which can do just that. With some minor reconfiguration of the winding connections, you can operate a unipolar stepper motor as a bipolar stepper motor. Method number one is to ignore the center tap on the winding. To do so with a six-wire unipolar motor, leave the center VM taps in each phase disconnected and taped off with electrical tape.

This new configuration now allows bipolar current flow in the motor windings. But be aware, you cannot use a five-wire motor with this method. From here you can connect the motor to one of our bipolar drivers. This configuration energizes the full winding, thus achieving higher torque at lower speeds compared to the corresponding unipolar stepper motor. However, the resulting inductance causes the torque to drop at a faster rate than that of a unipolar stepper motor, thereby limiting the maximum speed of operation.

Method number two is to ignore one half of the winding. This can be done only if the center taps of a unipolar stepper motor are electrically isolated. This new configuration now allows bipolar current to flow in the motor windings and the unipolar motor can be used with a bipolar driver.

Compared to using the full winding, the half winding configuration reduces the coil inductance. Therefore, using only half winding results in lower torque at lower speed. But at the same time, it results in higher available torque at high speeds. Additionally, using only half of a winding results in a noisier voltage and current waveforms.

All of these points should be carefully considered at the time of selecting a topology to drive a unipolar stepper motor with a bipolar driver. If the application requires the motor to run at low to moderate speeds, then ignoring the center taps and using the full coil for driving is the recommended approach.

Another way to drive a unipolar motor is to use four-channel half-bridge drivers. These drivers all feature PWM input interface and therefore can be operated such that an external controller turns on the low-side MOSFETs in succession to drive a unipolar stepper motor. The PWM inputs can be controlled to drive a unipolar motor in full or in half-step mode.

Driving in full-step mode can be done with one phase on or two phases on together. Here we have a full-step mode with one phase on. This is the simplest method, turning on one low side MOSFET at a time. Check out full step with two phases on in the app note linked below.

Here you can see the truth table in the input and output waveforms when driving a unipolar stepper motor in half-step mode. Thank you so much for watching. I hope you enjoyed this video. To find more information on unipolar stepper drivers and other technical resources, please visit ti.com.

This video is part of a series