The main feature of a stepper motor that makes it unique when compared to other motors is that it rotates in “steps” (i.e. in fixed amounts of degrees with pauses in movement) rather than with continuous motion.
However, just like any other electric motor, a stepper motor still has one part (known as the rotor) that moves and another part (known as the stator) that is stationary. Stepper motors can have three types of internal structure designs for their rotors. Each design has an impact on the performance of the motor and determines how it is controlled. A stepper motor can have a permanent magnet rotor, a variable reluctance rotor, or a hybrid rotor that uses elements of both of the first two.
Permanent Magnet Rotors
A rotor with a permanent magnet has sides that are always north and sides that are always south. These fields align in turn with the magnetic fields created by the separate DC currents in the different pairs of windings. Depending on design decisions, the number of coils and the number of poles on the magnets can be increased in order to improve the accuracy by directly increasing the number of steps of the stepper motor. With this rotor, a good amount of torque is usually created as well as a detent torque, so the motor will resist a change of position whether or not a coil is energized (however, it is not necessarily a very strong resistance). The disadvantage of this construction is that it offers the lowest speed and lowest resolution in comparison to the other two types.
Variable Reluctance Rotors
A variable reluctance rotor is made with a soft iron core that is specifically shaped to have fewer rotor teeth (i.e. fewer poles) than the number of windings (i.e. stator poles). The “off” rotor will be perpetually pulled into alignment with the closest energized coil, thereby knocking another set of teeth out of alignment and continuing the “stepping” cycle. With this rotor, higher speeds and resolution are attainable in comparison to a permanent magnet rotor. However, the disadvantage is that it offers no detent torque and often has reduced torque in comparison to the permanent magnet.
As implied by the name, a hybrid rotor is a combination of a permanent magnet and variable reluctance rotor. Magnetized axially with a unique shape, this type of rotor is constructed with two caps that have alternating teeth, allowing it to capitalize on the strengths of both of the other rotors. With a large number of teeth, the precision of a hybrid rotor is increased with an inverse relationship to the decreasing in the angle of the step. This rotor has high resolution, speed, and torque, but the disadvantage of it is that the more complex structure directly results in a higher cost.
Unipolar or Bipolar?
The motion of the stepper motor requires the coils to be energized in order to create movement, additionally, the direction of the current must be controlled as well. There are two approaches to controlling this current that utilize either bipolar or unipolar design.
With a grounded center tap in between the two leads of the winding, a unipolar configuration only allows current flow in half of the winding at one time. Because only half of the copper is magnetized at a time, the magnetic field created is half of what it could be if the entire wire was magnetized. Although this design uses relatively simple circuits and components and only two semiconductors are needed, they do require more leads and are therefore more difficult to construct.
On the other hand, in a bipolar stepper motor, each coil only has two leads available and an H-bridge is necessary in order to control the direction of the current. A bipolar design does require a more complex driving circuit in comparison with a unipolar arrangement, but, because all of the copper in one coil is used at once, this configuration allows the motor to create the maximum magnetic field per the amount of copper that is used, this directly results in maximized torque. As technology has progressed, the favor has gone to bipolar steppers; today, they are used far more frequently than their counterpart.
What Can We Do For You?
Are you in need of a stepper motor but have questions about picking between a permanent magnet, variable reluctance or hybrid rotor and a unipolar or bi-polar design? With so many options available, the decision can be a little overwhelming. Don’t hesitate to contact an expert here at Telco to help you weigh the options and ensure that you come to understand what motor type will be the best for your unique circumstances.