Q9Industrial Electronics

(a) Constructive Difference: Permanent Magnet vs Variable Reluctance Stepper Motor

Permanent magnet (PM) stepper motor: the rotor is constructed from permanently-magnetized material (a hard ferromagnetic or ceramic/rare-earth magnet), typically magnetized with alternating north and south poles around its circumference, so the rotor itself possesses its own persistent magnetic field, independent of any current flowing in the stator windings. The stator consists of salient poles wound with excitation coils, and torque is produced by the attraction/repulsion interaction between the rotor's permanent magnetic poles and the magnetic field established by the energized stator coils, with the rotor aligning itself so that its permanent poles align with (or are attracted toward) the appropriately-energized stator poles.

Variable reluctance (VR) stepper motor: the rotor is constructed from plain, unmagnetized soft magnetic material (soft iron or silicon steel), machined with a number of salient (projecting) teeth around its circumference, but possessing no permanent magnetization or inherent magnetic polarity of its own. Torque production in a VR motor relies entirely on the principle of reluctance minimization: when a particular stator winding is energized, the resulting magnetic field induces the nearest rotor teeth to align with the energized stator poles, since this alignment minimizes the magnetic circuit's reluctance (and correspondingly maximizes the circuit's inductance) — sequential energization of successive stator windings causes the rotor teeth to progressively align with each newly-energized winding in turn, producing the stepwise rotor rotation characteristic of stepper motor operation.

Key consequences of this constructive difference: because the PM rotor has its own permanent magnetic field, a PM stepper motor exhibits a 'detent torque' — a small residual holding torque present even with all stator windings de-energized, since the rotor's permanent magnet poles still interact (weakly) with the stator's ferromagnetic pole structure — a VR motor, lacking any permanent rotor magnetization, exhibits essentially zero detent torque when unpowered, and can be freely rotated by hand with negligible resistance when de-energized. Additionally, because a PM motor's torque direction depends on the interaction between a specific stator winding's current polarity and the rotor's fixed permanent-magnet polarity, reversing a given stator winding's current direction produces a different torque effect (attraction becomes repulsion, or vice versa) — whereas a VR motor's reluctance-based torque production is fundamentally polarity-independent (reluctance minimization occurs regardless of which direction current flows through the energized winding, since the resulting alignment force depends only on the magnitude of the field, not its polarity), meaning VR motor drive circuits need not provide bidirectional (reversible) winding current capability, generally simplifying their drive electronics compared to a PM stepper motor drive.

Hybrid Stepper Motor - A Third Type

In addition to the pure permanent-magnet and pure variable-reluctance designs, industrial stepper motor applications very commonly employ a third construction, the hybrid stepper motor, which deliberately combines features of both preceding types to obtain their combined advantages. The hybrid rotor consists of an axially-magnetized permanent-magnet core (providing the PM motor's persistent field and detent torque) sandwiched between two soft-iron end-caps, each end-cap machined with a set of fine salient teeth (offset from each other by half a tooth pitch), so the rotor simultaneously possesses a strong permanent magnetic field (as in a PM motor) and a fine, multi-toothed geometric structure that shapes and concentrates this field into a large number of discrete high-reluctance-gradient positions per revolution (as in a VR motor). This combination yields a stepper motor with a much finer step angle (commonly 1.8° or 0.9°, i.e., 200 or 400 steps per revolution) than a typical pure PM or pure VR design can economically achieve, together with useful detent torque and good dynamic torque/response characteristics — the hybrid design is, for these reasons, by far the most widely used stepper motor construction in modern industrial motion-control, CNC, and instrumentation applications, with pure PM and pure VR designs now comparatively less common outside of certain low-cost or specialized niche applications.

Torque-Speed Characteristics of Stepper Motors

A stepper motor's available torque is not constant with stepping speed but instead falls off as the pulse (stepping) rate increases, a behavior generally presented as two characteristic curves: the pull-in (start-stop) torque curve, which bounds the maximum load torque against which the motor can start, stop, or reverse without losing synchronism at a given stepping rate; and the (higher) pull-out (slew) torque curve, which bounds the maximum load torque the motor can sustain once already running synchronously at that stepping rate, without pulling out of step. Both curves decline with increasing stepping frequency, primarily because the winding current (and hence the developed torque) cannot rise to its full steady-state value within the progressively shorter time available per step at high stepping rates, owing to the finite electrical (L/R) time constant of the stator windings — this torque roll-off at high speed is why a stepper motor drive is typically operated with a gradual acceleration/deceleration ramp (rather than an instantaneous jump to the target stepping rate) when high-speed operation is required, allowing the motor to first build up speed within its pull-in torque capability before continuing to accelerate within its (higher-speed but still adequate) pull-out torque capability.

Practical Resolution Significance of the Result

The computed pulse rate of 1365 Hz for this motor (130 steps/rev at 10.5 rev/s) illustrates the direct, load-independent relationship between electrical pulse frequency and mechanical shaft speed that is the defining practical advantage of open-loop stepper motor positioning: since each of the 130 discrete steps corresponds to a fixed mechanical angle of 360°/130 ≈ 2.77° per step, a control system needing to index the shaft to any intermediate angular position simply needs to issue the corresponding whole number of pulses, while a continuous-speed application (such as this one) simply maintains the required pulse frequency for as long as that speed is to be sustained — no tachometer or speed-feedback loop is required to achieve this precise, repeatable correspondence between commanded pulse rate and actual shaft speed, provided (as discussed in part (b) of the following answer) the motor is not driven beyond its pull-out torque capability at the given stepping rate, underscoring why open-loop stepper drives remain a standard, cost-effective solution for moderate-precision industrial indexing and continuous-speed positioning tasks.

(b) Numerical: Input Digital Pulse Rate for Continuous Rotation

Given: steps per revolution = 130 (the paper's phrase 'steps per resolution' is understood to mean steps per revolution, the standard stepper motor specification); required speed = 10.5 revolutions/second.

In a stepper motor drive, each input digital pulse applied to the drive/translator circuit causes the motor shaft to advance by exactly one step. The pulse rate (the number of pulses per second that must be supplied to the drive) required to produce a given continuous rotational speed is therefore simply the product of the number of steps per revolution and the desired number of revolutions per second:

Result: the drive circuit must therefore be supplied with an input digital pulse train at a rate of 1365 pulses per second (1365 Hz) in order to produce continuous rotation of this stepper motor at the specified speed of 10.5 revolutions per second. This direct, linear proportionality between input pulse frequency and resulting shaft rotational speed is one of the defining practical advantages of stepper motor drive systems, since it allows precise, straightforward open-loop speed (and, by simply counting the total number of pulses delivered, position) control using only a digital pulse generator and a stepper motor drive/translator circuit, without requiring a closed-loop speed or position feedback sensor for many moderate-precision industrial positioning applications.