Stepper Motors

Stepper motors use a magnetic field to move a rotor. Stepping can be done in full step, half step or other fractional step increments. Voltage is applied to poles around the rotor. The voltage changes the polarity of each pole, and the resulting magnetic interaction between the poles and the rotor causes the rotor to move. Stepper motors provide precise positioning and ease of use, especially in low acceleration or static load applications. 

Important performance specifications to consider when searching for stepper motors include shaft speed, terminal voltage, current per phase, continuous output power, and static or holding torque.  Shaft speed is the no-load rotational speed of output shaft at rated terminal voltage.  The terminal voltage is the design DC motor voltage.  The current per phase is the maximum rated current or winding for a stepper motor.  The continuous output power is the mechanical power provided by the motor output.  Static or holding torque is the maximum torque a motor can develop to hold its rotor in a stationary position.

Motor types for stepper motors can be permanent magnet, variable reluctance, or hybrid.  Permanent magnet (PM) motors use a permanent magnet on the rotor.  Step angles range from 1.5 to 30 degrees.  Permanent magnet motors are the most common and versatile stepper motor.  This includes both unipolar (bifilar) and bipolar types.  Variable reluctance (VR) motors have a free-moving rotor; no residual torque is produced due to the lack of a permanent magnet.  The rotor is instead composed of a soft iron metal.  Rotor is also composed of its own very prominent poles, tending to stick out more than a rotor found on the PM version. Step angles:  7.5 to 30 degrees single power source required (like a bifilar PM motor).  This is the least expensive stepper motor.  Hybrid motors consist of a heavily toothed PM rotor and toothed stators, plus prominent rotor poles like a VR rotor.  They are capable of very fine step angles:  0.5 to 15 degrees and have a high-speed capability (less chance of a stall). There is a higher available torque than PM or VR stepper motors.  Most effective but most expensive stepper motor type.  The step angle is the degrees per step of the motor.  Stepper motor configurations can have different numbers of leads depending on the specific winding wiring.  For example, bipolar PM motors can have 4, 5, or 6 leads, unipolar PM motors can commonly have 5 or 6 leads (two windings with two ends plus center taps, which may or may not be tied together), hybrid motors frequently contain 8 leads, and multiphase motors can have different lead configurations (for example, a motor wired for 5-phase power could have 5 or 10 leads).  Consult with manufacturer for specific winding wiring and lead information.

Gearmotor or gearhead options for stepper motors include motor configuration, gearing if applicable, gearbox ratio, and gearbox efficiency.  Feedback choices include integral encoder, integral resolver, and integral tachometer.  Other specifications to consider when searching for stepper motors include shaft orientation or type and number of shafts, design units, motor shape, dimensions of width and length, NEMA frame sizes, enclosure options and special or extreme environments.  Features common to stepper motors include integral driver electronics, integral brakes, integral clutches, and brake or clutch combinations.

• Advanced Energy
  With advantages such as high efficiency, compact size, and high torque density, permanent magnet (PM) motors are becoming increasingly competitive in many applications. PM motors use permanent magnets...
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• Effect of Exceeding Rated Current in Stepping Motors
  Both voltage and current electrically rate all stepping motors. It is common for system designers to run stepping motors beyond their rated voltages (see EADmotors Tech Note...
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• Motor Fundamentals
  Motors come in many different types, shapes, and sizes. Most of the motors used in motion control can be divided into two categories: stepper motors and servo motors. This document describes these two...
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Part Numbers for Stepper Motors