Stepper Motor - Hybrid (cont.): Conventional hybrid stepper motors are rarely used in closed-loop systems because torque falls rapidly as current increases above the peak torque point -- putting them outside typical control limits. The stepper motor's torque also decreases as speed rises. If driven too fast, hybrids lose position accuracy by skipping steps.
A stepper motor's peak torque is limited by the flux level that saturates the rotor and stator teeth. But an enhanced stepper motor is now available that reduces saturation effects and produces 50 to 100% more torque than conventional stepper motors for the same input power.
Both conventional and enhanced stepper motors develop maximum torque when the rotor teeth are offset by one-quarter tooth pitch from opposing poles in the energized phase. The stepper motor pole pairs develop appreciable torque even at zero current. Torque increases as current approaches the rated value.
At or near rated current in conventional stepper motors, a larger part of the air-gap flux traverses the gap from stator slot to rotor slot rather than from tooth to tooth, thus producing less torque.
The enhanced stepper motor uses a relatively new stator design to get around this problem. Here, samarium-cobalt or neodymium-iron-boron magnets are embedded in slots between the teeth. More concentrated flux lines result between the stepper motor's rotor and stator teeth with fewer flux lines lost to the slotted air gap. These new slot magnets focus the air-gap flux, reduce leakage, and allow the stepper motor to produce more torque.
Torque is also produced by a second pair of poles of the same phase placed 180° away from each other, and 90° away from the first pair. The second pole pair of the conventional stepper motor produces a torque that opposes the positive-acting pair. This negative torque is large at low currents but diminishes near rated current.
Enhanced stepper motors also have large negative torques at low current. But positive-acting flux from the permanent magnets in the stator overcomes the small negative torque generated at rated current. The resulting torque then aids the pole pair producing the primary positive torque.
The slot magnets in enhanced stepper motors provide peak torques reaching twice that of conventional stepper motors. Moreover, these stepper motors can handle three times rated current compared to only two times for conventional stepper motors. Depending upon the inertial load, these new stepper motors reach speeds of 5,000 to 10,000 steps/sec. Corresponding torques are 200 oz-in. to 3,100 oz-in. in 2 to 4-in.-diameter packages. Hybrid stepper motors also generally have high inertia (30 to 40,000 gm-cm2), small step angles (0.5 to 15°) and high accuracy (± 3%).
Stepper Motor: Basics of Stepper Motor Design Engineering