When applying step motors, designers need to consider several factors, including output torque, rotor inertia, step angle, and speed. Having already discussed the effects of torque and inertia (August 2002), we turn our attention to step angle and speed.
The step angle of a hybrid step motor is a function of the device’s mechanical construction. A fixed value, this fundamental increment of movement is often called the “cardinal” step. For twophase hybrid step motors, the most common cardinal step angles are 1.8 degrees and 0.9 degrees; for five-phase hybrids, the cardinal angles are 0.72 degrees and 0.36 degrees. Phase refers to the number of wire bundles snaking through the stator. Each wire bundle, or phase, can wrap around any number of stator poles along its course through the stator.
The actual determination of the step angle requires an understanding of motor construction. In Figure 1 we see a cross-section of a typical two-phase hybrid step motor. A toothed stator surrounds a toothed rotor. If you’re familiar with gears, you will find some similarities in operation. The teeth in a step motor “mesh,” not by contact as in gear meshing, but through the forces of a magnetic field.
In simplified terms, the stepping action of the motor is a reflection of the teeth on the rotor “lining up” with the teeth on the stator — over and over — during rotation. Each step, each incremental movement, results from magnetized rotor teeth being repelled on one side and attracted on the other by the magnetic forces produced in the windings of the stator.
The cardinal step angle is obtained from a simple calculation based on the number of poles (the radial posts) in the stator, and the pitch of the rotor teeth. A typical two-phase hybrid step motor has four poles in each phase, and a tooth pitch of 7.2 degrees (50 teeth). If we divide the tooth pitch by the number of poles, then we come up with the step angle (i.e. 7.2 degrees/4 = 1.8 degrees). A “high-resolution” two-phase step motor simply has twice the number of rotor teeth (100) on a 3.6-degree pitch. Here, the step angle is 3.6 degrees /4 = 0.9 degrees.
One advantage to a smaller step angle is that the motor produces less vibration and noise. Obviously, it takes a certain amount of energy to turn the motor shaft one revolution. If we divide this revolution into much smaller increments or steps, the energy required to move the rotor is likewise broken into smaller amounts.
When the rotor moves and its teeth line up with the stator, they do not do so in a precise movement — they don’t “stop on a dime” so to speak. Rather, there is a bit of overshoot due to the energy being applied and the magnetic forces at work. This is known as “ringing” and can be characterized as being oscillatory in nature. Eventually, the teeth will line up with each other and come to rest. The time it takes for this is known as settling time, and for a given amount of current flowing through the motor windings, this time decreases with the step angle, all else being equal.
Look at it this way: If we pull a stretched rubber band two inches and let it go, it will snap back and oscillate back and forth for a brief moment. If we now pull that same stretched rubber band four inches, will it not snap back and oscillate just a bit longer? Of course it will. We are applying more energy to move the rubber band farther, and as a result, the rubber band will oscillate more. The same is true for step motors. The larger the step angle, the more energy we have to apply to get it to move that step. Larger steps mean more overshoot, and that translates into more vibration.
Although this vibration can be compensated for electronically by microstepping (electrically proportioning current sinusoidally in the phases), the cardinal step angle remains the same and in some cases can have an adverse effect on the application. This can be especially true in high-speed applications such as printing, scanning, and cutting.
“How fast will it go?” is a common question asked of motors. To get an accurate answer, we need to look at the torque component. As with any motor, whether electric, gasoline, steam, or hydraulic, there is a characteristic speedtorque curve. Stepping motors typically have excellent low-speed torque, but high-speed torque leaves something to be desired. Because a stepper by nature is a high pole-count motor, it cannot deliver high useable speed. Small lowinductance step motors could possibly reach speeds of 8,000 rpm when coupled with the right driver, but such motors will have almost no torque at that speed and will stall under just a whisper of a load.
Fortunately, if a system design dictates a stepper due to cost constraints, simplicity, or accuracy, there is still hope if higher speed remains an objective. There are ways to get more torque at speed out of a stepper — just don’t expect multi-thousand rpm speeds.
The electronic “brains,” or driver, required to move a step motor has everything to do with how the motor operates, and the output speed is no exception. Pulse-width modulated (PWM) or “chopper” type drivers are commonly used in industry today. This type of driver has the advantage of being able to drive a step motor with a higher voltage than the motor’s rating without the use of big, power-robbing resistors. This is accomplished by “chopping” the voltage driving the motor, in effect, modulating the pulse width. Driving a motor with a higher chop voltage pushes current into the motor windings quicker, letting the coils become more saturated at higher speeds. This in turn allows the motor to produce more torque (which is proportional to magnetic flux) at those speeds.
The motor’s windings themselves also play a major role in how fast you can drive a stepper and still get usable torque. The lower the resistance and inductance, the higher the speed. This then begs the question “why even use a high-resistance or high-inductance motor?” The answer is current. Lower inductance motors typically draw a considerable amount more rated current. This means more cost in terms of drivers. Higher current capability requires the use of more stout components, sometimes ones that are physically larger. As is the case with power supplies, higher current translates to higher cost.
Finally, the way the motor is wired also plays a part in how fast it can operate. If the motor phases are tied together in series, high low-speed torque is obtained due to the fact that the full coil is now being used. The downside is that the inductance increases, thus causing the motor’s torque curve to drop very quickly. If the motor phases are wired in parallel (this can be done with an eight-lead motor) we gain not only high torque since the full coil is used, but also high speed because inductance drops due to the parallel layout.