An energized, holding stepper has a relatively high holding torque rating. The torque available is a function of motor speed, load inertia, load torque, and drive electronics as illustrated on the speed vs torque curve below. Most steppers are a few inches in diameter with a fraction of an n-m or a few in-oz torque. A small “dime” size stepper has a torque of a hundredth of a newton-meter or a few inch-ounces. They have torque ratings to a thousand in-oz (inch-ounces) or ten N-m (newton-meters) for a 4 kg size unit. They are truly small low power devices compared to other motors. If they do rotate continuously, they do not even approach a sub-fractional horsepower rated capability. Since stepper motors do not necessarily rotate continuously, there is no horsepower rating. For example, a 12-pole stepper has 6-pairs of poles, three pairs per phase. There may also be multiple pole pairs per phase. Thus, a 4-pole stepper motor may have two phases composed of in-line pairs of poles spaced 90 ° apart. These phases are frequently split into pairs. There may be as few as two winding phases or as many as five. Stepper motor coils are wound within a laminated stator, except for can stack construction. You can determine that the rotor is a permanent magnet by unpowered hand rotation showing detent torque, torque pulsations. More often than not the rotor is a permanent magnet. The rotor is a cylindrical solid, which may also have either salient poles or fine teeth. Stepper motors are rugged and inexpensive because the rotor contains no winding slip rings or commutator. Thus, we treat stepper motors as a class of AC synchronous motor.Ī unipolar drive of a center-tapped coil at (b), emulates AC current in single-coil at (a) As the windings are energized in sequence, the rotor synchronizes with the consequent stator magnetic field. In other words, DC can be switched to the motor so that it sees AC. The bipolar magnetic fields may also be generated from unipolar (one polarity) voltages applied to alternate ends of a center-tapped winding (figure below). ![]() With the direct line voltage, we have a 2-phase drive.ĭrive waveforms of bipolar (±) square waves of 2-24V are more common these days. The capacitor generates a 90 ° second phase. Slo-syn synchronous motors can run from AC line voltage like a single-phase permanent-capacitor induction motor. Thus, a stepper motor controller is less complex and costly than a servo motor controller. The stepper motor drivers are less complex solid-state switches, being either “on” or “off”. ![]() A considerable design effort is required to optimize the servo amplifier gain vs phase response to the mechanical components. The servo amplifier is a linear amplifier with some difficult to integrate discrete components. ![]() However, the high speed and accuracy required of modern hard drive head positioning requires the use of a linear servomotor (voice coil). They were once used for the same purpose in hard drives. Otherwise, the default is the stepper due to simple drive electronics, good accuracy, good torque, moderate speed, and low cost.Ī stepper motor positions the read-write heads in a floppy drive. High acceleration or unusually high accuracy still requires a servo motor. The expense and complexity of a servomotor are due to the additional system components: position sensor and error amplifier) It is still the way to position heavy loads beyond the grasp of lower power steppers. Today this is a higher cost solution to high-performance motion control applications. And, microprocessors readily interface to stepper motor driver circuits.Īpplication-wise, the predecessor of the stepper motor was the servo motor. Modern solid-state driver electronics was a key to its success. The widespread acceptance of the stepper motor within the last two decades was driven by the rise of digital electronics. When stopped but energized, a stepper (short for stepper motor) holds its load steady with a holding torque. The rotor moves in discrete steps as commanded, rather than rotating continuously like a conventional motor. A stepper motor is a “digital” version of the electric motor.
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