Stepping Motors

Stepping motors offer many advantages. Although feedback is not usually required, stepping motors are compatible with feedback signals, either analog or digital. Error is noncumulative as long as pulse-to-step integrity is maintained. A stream of pulses can be counted into a stepping motor, and its final position will be known within a small percentage of one step.

Since maximum dynamic torque occurs at low pulse rates, stepping motors can easily accelerate a load. When the desired position is reached and command pulses cease, the shaft stops and there is no need for clutches or brakes. The motor is generally left energized at a stop position. Once stopped, the motor resists dynamic movement up to the value of the holding torque. An additional feature of the PM stepping motor is that when all power is removed, it is magnetically detented in the last position. A wide range of step angles are available -- 1.8 to 80°, for example -- without logic manipulation. Stepping motors have inherent low velocity without gear reduction. A typical unit driven at 500 pps turns at 150 rpm. The rotor inertia is usually low. Multiple stepping motors driven from the same source maintain perfect synchronization.

But efficiency is low; much of the input energy must be dissipated as heat. Load must be analyzed carefully for optimum stepping motor performance. And inputs must be matched to the motor and load. Damping may be required when load inertia is exceptionally high to prevent oscillation.

Excitation modes:Stepping motors can be excited in different modes, depending on stator winding and desired performance.

Two phase:One entire phase (stator winding) of the motor, end-tap to end-tap is energized at a given moment in time. Input current and wattage are halved (compared to four-phase excitation), and heat dissipation is decreased. Output can be improved by as much as 10%. In the two-phase modified mode, both windings (end-tap to end-tap) are energized simultaneously. Energy input in this mode is the same as four phase, but output performance is increased by about 40%. The control is complex and costly for this mode.

Three phase: Many variable-reluctance stepping motors use three-phase windings. In modified mode, two adjoining phases are excited simultaneously and the rotor indexes to a minimum reluctance position corresponding to the resultant of the two magnetic fields. Since two windings are excited, twice as much power is required as the standard mode (one phase at a time). Output is not increased but damping is improved.

Four phase: Each half winding is regarded as a separate phase, and phases are energized two at a time. Although this mode isn't very efficient, the controller is simple. Compared to single-phase excitation, twice the input energy is required. Torque output is increased by about 40%, and maximum response rate is increased.

Five phase: Five-phase stepping motors have 10 poles rather than the 8 poles typically used in other stepping motors. Rotor-to-stator offset becomes one-fourth to one-tenth the rotor tooth pitch. A 50-tooth rotor provides a full-step of 0.72°, and a 100-tooth version produces a 0.36° full-step (0.18° half-step). The motors run at 500, 1,000, or 2,000 steps/rev with improved loaded-position accuracy and stiffer response. In addition to higher resolution, five-phase motors produce less vibration than two to four-phase motors with virtually no resonance effects.

Variable reluctance: These stepping motors have soft iron multipole rotors and a wound stator. The number of teeth on the rotor and stator, as well as the number of winding phases, determines the step angle. Variable-reluctance motors are generally medium step-angle devices (5 to 15°) which operate at high step speeds. Torque is generally low. Rotor inertia and, thus, inertial load capacity are extremely low. Motors of this type operate at maximum pulse rates from 300 to 1,000 steps/sec and have a maximum load inertia capacity of about two-thirds of rotor inertia. When excited in an overlap mode, these motors can move at half step angles and double pulse rates. The net output velocity remains the same.

Permanent magnet: PM stepping motors generally are thought of as low-torque, large step-angle devices. Torque developed by the motors is far below that for equivalent-size hybrid step motors, and step angle generally is 90 or 45° . Position resolution, moreover, is on the order of +10% of step angle, a value that generally relegates the motors to unsophisticated motion-control applications. Maximum pulse rates are for 100 steps/sec for large units to 350 steps/sec for small units. Rotor inertia is moderate between 5 and 75 gm-cm².

Rare-earth magnets make possible PM stepping motors having a large number of poles. With a suitable number of poles, PM stepping motors develop more torque than either hybrid stepping motors or dc servomotors. Speed range for the motors is less than that for dc types but much higher than that for hybrids.

Position resolution of the PM stepping motors is less than that for hybrids. But unlike hybrids, some PM stepping motors perform well in closed-loop systems.

Both cemf and iron losses are proportional to the number of poles in the motor. Thus, available torque from a PM stepping motor falls off more slowly with speed than in hybrids and more rapidly than in dc motors. The result is that PM stepping motors operate effectively at higher speeds -- up to about 3,500 rpm -- than hybrids but not as high as dc types. The speed range for PM stepping motors, however, suits a wide range of servo applications.

Hybrid:Hybrid stepping motors are frequently chosen for a wide variety of motion-control systems because they are easy to use. Stepping motors can maintain accuracy and reliability in open-loop mode, requiring less complex drive electronics than closed-loop servocontrollers. And absolute positioning accuracy for stepping motors is comparable to closed-loop servocontrollers for many applications.

Conventional hybrid 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. Torque also decreases as speed rises. If driven too fast, hybrids lose position accuracy by skipping steps.

Peak torque is limited by the flux level that saturates the rotor and stator teeth. But an enhanced stepping motor is now available that reduces saturation effects and produces 50 to 100% more torque than conventional stepping motors for the same input power.

Both conventional and enhanced stepping motors develop maximum torque when the rotor teeth are offset by one-quarter tooth pitch from opposing poles in the energized phase. The pole pairs develop appreciable torque even at zero current. Torque increases as current approaches the rated value.

At or near rated current in conventional stepping 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 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 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 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 stepping motor produces a torque that opposes the positive-acting pair. This negative torque is large at low currents but diminishes near rated current.

Enhanced 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 motors provide peak torques reaching twice that of conventional stepping motors. Moreover, they can handle three times rated current compared to only two times for conventional stepping motors. Depending upon the inertial load, these new 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 stepping motors also generally have high inertia (30 to 40,000 gm-cm²), small step angles (0.5 to 15°) and high accuracy (± 3%).