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AC Motors

DC Motors

Special Motors

DC Motors

BRUSH-TYPE DC MOTOR ACTION

When a current-carrying conductor is placed in, and at a right angle to, a magnetic field, it will experience a force perpendicular
to the field and to itself. The direction of the force in relation to the field and current is shown in the figure below a and b. The force on this conductor is proportional to the flux density, current and the length of the conductor.

Using the above principle, we can explain the motor action of a simple single loop armature as shown in the figure below (c), where DC current enters the right side of the loop and exits the left. The resultant forces acting on the single loop armature generate a clockwise torque. However, the torque diminishes to zero as the plane of the armature coil becomes perpendicular to the field as shown in the figure below (d).

Upward and down ward forces created by interaction of field and armature flux.

Commutation:

In order to continue the clockwise motion of our simple single loop armature, we need a commutator arrangement as shown below (a). As the coil becomes perpendicular to the magnetic field, the direction of current in the coil reverses, causing the forces acting on the coil to switch their direction. The coil then continues to rotate in a clockwise direction.

The torque produced on the armature is proportional to the sine of the angle between the magnetic field and the plane of the rotating coil. The torque will produce a ripple type waveform as shown below (b). This figure shows that the resulting torque reaches zero at the two vertical positions during the armature (loop) rotation. This simple motor relies on the inertia of the armature to carry it through the zero torque points to continue its rotation.

To eliminate this effect and keep a level of torque always at some point above zero. a four-segment commutator and two armature coils may be used (see figure below - c). This arrangement staggers forces to keep the torque at an acceptable level. The torque/position curve will then look like the figure below (d). The more segments added to the coils and corresponding commutator armature, the closer the torque curve will approximate a straight line characteristic. See the figure below (e and f).

Relationship of commutator segments and torque:

a) two-segment commutator,
b) two-segment commutator torque curve,
c) four-segment commutator,
d) four-segment commutator torque curve,
e) 32-segment commutator, and f) 32-segment commutator
torque curve.

The figure below shows the position of a commutator in relation to the armature coils of a typical DC motor.

Commutator and brush position
in a typical DC motor design.

Counter emf and Armature Current:

When a DC armature is rotating in a magnetic field, there is an induced voltage produced in the armature which takes the form of an opposing or counter electromotive force (cemf). When the flux field is held constant, this voltage is proportional to the armature speed. Motor action will continue as long as the voltage supplied to the commutator is greater than the cemf. The cemf limits the current flowing in the armature according to the formula:

where V is the source voltage, I is the armature current and R is the armature resistance. It is inherent that the current in the armature is proportional to the load or torque produced. The current increases with an increasing load until the motor stalls, at which point the cemf is equal to zero.

Speed Control:

The speed of a DC motor is easily controlled by adjusting the voltage either in the field or armature or a combination of both. This can be accomplished by means of controls, variable resistors and other devices.

Having briefly reviewed the fundamental operation of commutator motors, we will now consider each electrical type individually.

BRUSH-TYPE DC MOTORS

Series Wound

Features:

Continuous or short time duty
AC or DC power supply
Usually unidirectional reversibility
Speed varying with load
Starting torque 175% and up of rated torque
High starting current

Design and Operation:

Series wound motors are among the most popular of fractional and sub fractional hp motor types. Capable of operation on either AC
or DC power supplies, series motors deliver high motor speed, high starting torque and wide speed capability, making them ideal drives for a variety of applications. See figure below.

Series wound motor.

The armature and field of a series motor are connected in series with respect to the line. This feature allows series motors to be
operated from either AC or DC supplies between 0 and 60 Hz. Because of their “dual” capability, series motors are often called “universal.” The performance difference of a universal motor between 50 and 60 Hz is generally negligible. It should not be assumed, however, that all series motors are universal. Some may be optimized for a particular power supply, and perform poorly or fail prematurely if operated on a power supply substantially different from that specified on their nameplates.

Actually, no universal motor has the same performance on both AC and DC. Usually, the motor will run slower on AC than on DC because the windings exhibit a higher impedance when operated on an AC supply. The speed difference is most apparent with higher loads. Sometimes the AC vs. DC speeds can be more closely matched if a properly specified resistor is placed in series with the motor when operated on DC.

At lighter loads, an opposite speed relationship may occur. Since the effective field strength is lower on AC, the motor may run faster.

Advantages:

In addition to their versatility, series wound motors have the highest horsepower per pound and per dollar of any motor that operates directly from standard single-phase AC power. This factor accounts, in part, for the popularity of series motors in household appliances
and power tools. The economics are closely related to the inherent high speeds of series motors. For example, a typical AC induction motor rated at 1/10 hp (75 watts) at 1725 RPM weighs approximately 15 lbs. (67 newton's). A series universal motor rated at 1/10 hp (75 watts) and 10,000 RPM can weigh under 4 lbs. (18 newton's).

Although there is a dramatic savings in weight and cost per hp delivered, there are other aspects to the comparison:

At the stated rating point in our foregoing example, the torque of the induction motor will be 58 oz-in. (410 mN-m),
compared with 10 oz-in. (71 mN-m) for the series motor.
The induction motor will have much better speed regulation (less change in speed with variations in load).
The induction motor will be significantly quieter because of its lower speed and absence of commutating brushes.
The induction motor will not have the maintenance and service life considerations associated with brush commutation.

In spite of these differences, series motors are uniquely suited to a variety of applications. In particular, series motors are the only small motors capable of more than 3600 RPM operating directly from a single- phase (60 Hz) AC power supply. Also, the series motor will provide higher starting torque than any other motor of equivalent physical size operated from similar power supplies. Used as a DC motor, the series design is practical up to about the 5" diameter size range. Above that, PM and shunt-wound motors become practical in a cost/performance trade-off.

Although series motors are usually supplied as unidirectional (to obtain greater efficiency and brush life) bidirectional series motors can also be produced. One method accomplishing this is a three-wire design which can be reversed with a simple single pole/double throw (SPDT) switch. However, for this arrangement, a split or double field winding is required, reducing the available
hp in a given frame.

An alternative to the three-wire method is the four-wire series motor which is made reversible by transposing the armature leads, usually with a double pole/double throw (DPDT) switch. With reversible series wound motors, the application must be able to tolerate some variations in speed between one direction and the other, due mainly to inherent differences in commutation until the brushes seat adequately in each direction.

In addition to the advantages discussed above, series motor speed can be adjusted over a broad range by using a rheostat, an
adjustable autotransformer or an electronic control. With the application of a mechanical governor attached to the motor shaft, a
series motor can also provide a constant speed over a wide torque range.

The no-load and operating speeds of series motors are usually quite high. No load speeds in excess of 15,000 RPM are common and are limited only by the motor’s own friction and winding characteristics. Normal operating speeds are from 4000 to 10,000 RPM. The excellent forced ventilation made possible at these speeds helps to yield much higher horsepower ratings than “common” induction motors operating at 1725 to 3450 RPM.

Disadvantages:

A series motor inherently provides poor speed regulation and is classified as having a varying speed characteristic. This means that the speed will decrease with an increase in load and increase with a decrease in load. The amount of change will depend upon the
particular motor design. Speed changes are more pronounced because the armature and field are connected in series.

As the load is increased, the motor must slow down to let more current flow to support the load. This increase in current, however, increases the strength of the field, and thus the counter emf, which has a limiting effect on current build-up. The result is a further decrease in speed to compensate for this change. However, the simultaneous change in field and armature strength cause the two to always be matched or balanced resulting in the excellent starting torque characteristic of the series motor.

Although high speed is often a significant advantage, it does not come without a “price.” Specifically, bearing and brush life are affected by high speed (household appliance series motors typically have a brush life of 200 to 1200 hours, depending on the type of appliance). Centrifugal forces must also be analyzed to prevent the destructive effects of imbalance at high
speeds. These factors generally limit series motors to intermittent duty applications. However, series motors have been successfully
applied in many continuous duty applications where operating conditions are favorable, or where the nature of the application provides for a moderate amount of servicing.

Cautions:

Because of the steepness of the speed/torque curve near the no-load point, operation at or near no-load is usually discouraged. See figure below. If consistent performance between motors or even in the same motor is desired, series motors should be operated at some load value beyond this point. The slope of the speed/ torque curve, along with the point of peak efficiency, can be altered slightly by the motor manufacturer to suit specific applications.

An additional caution-series motors designed and built for one direction of rotation should never be reversed (extremely poor brush life and performance can be expected).

Typical characteristic curve for
a series type (universal) motor.

Shunt-Wound

Features:

Continuous duty
DC power supply
Reversibility at rest or during rotation
Relatively constant and adjustable speed
Starting torque 125% to 200% of rated torque
Normal starting current

Design and Operation:

One of the earliest and most versatile types of DC motors, the shunt-wound design has always enjoyed considerable popularity as
an excellent electrically adjustable, relatively constant speed drive. With solid state control circuitry and its inherent relatively constant speed characteristics, the shunt wound DC motor is a valuable companion to advanced SCR (Silicon Controlled Rectifier) controls. See figure below.

Shunt-wound motor.

The shunt-wound DC motor has both a wound field and armature with spring-loaded brushes applying power directly to the armature by means of a segmented commutator. The term “shunt” is derived from the connection of the field and armature in parallel (shunt) across the power supply. See figure below. The field and armature may also be separately excited from two independent sources. This allows changes in armature voltage to vary the speed while still maintaining a constant field voltage.

typical shunt wound motor wiring diagram

Advantages:

The shunt motor inherently provides good speed regulation (changes in load only slightly affect speed within its rated torque range).

For example, a 1/4 hp shunt motor operating at a rated speed of 1725 RPM will generally not vary in speed from no-load to full load by more than 15%. With modern feedback-type controls, the speed regulation can be even further improved to ±1% or less over a defined speed range, without an add-on tachometer. Tight control over a wider speed range may require sacrifices in regulation to compensate for the wide speed range feature. A tachometer, feedback or closed-loop control may also be needed.

The most common means of controlling shunt motors is the adjustment of armature voltage while maintaining constant field voltage. Armature voltage control is normally used to decrease the motor speed below its base speed. Regulation and starting torque are generally not affected, except at the very lowest speeds. A totally enclosed shunt motor can be designed to operate at rated torque down to zero RPM without developing excessive temperatures.

Another method, field weakening, may also be used to vary motor speed. It is, however, usually used only to increase the motor speed above its base speed and is not often recommended unless the load is decreased to maintain a constant horsepower output. In addition, the percent of regulation is increased and the starting torque decreased with the field weakening method.

Normal NEMA* speed ratings (base speed) for shunt motors operated from electronic controls are 1140, 1725, 2500 and 3450 RPM, but a shunt motor can be wound to operate at any intermediate speed for special purpose applications. This same flexibility, within limits, also applies to shunt motor voltage ratings.

Shunt designs are reversible at rest or during rotation by simply reversing the armature or the field voltage. Because of the high inductance of the field circuit, reversing the armature is the preferred method.

*NEMA is the national Electrical Manufacturers Association.

Disadvantages:

If the shunt-wound motor is operated from a fixed voltage supply, a decrease in speed will occur as the motor is loaded. The decreasing speed with increased load tends to be linear over a range in which the magnetic characteristics are linear. As load is increased, further saturation begins to occur, resulting in what is commonly known as armature reaction and the resultant abrupt drop in speed, as shown in the figure below. The speed also increases linearly with increasing armature voltage, making the shunt-wound design valuable as an adjustable speed motor. The fact that speed varies proportionally with armature voltage makes it possible to vary speed over a wide range with electronic controls.

Typical shunt-wound motor performance curve

Cautions:

Reversing the armature while it is rotating is called “plugging” or “plug reversal.” Because of the counter electromotive force (cemf) or generated voltage in the armature, plugging will subject the armature to approximately twice the rated voltage and therefore should be
used with discretion.

Dynamic braking, while not as severe as plugging, should also be used with caution. A shunt motor can be dynamically braked
by “shorting” the armature after it has been disconnected from the line. Current-limiting resistors are generally used to reduce the
severity of this operation.

Brush life on a shunt-wound motor is usually good. However, severe duty cycles, like plugging and dynamic braking, can adversely affect brush life. Such applications should be carefully studied to prevent excessive stress to brushes and other motor parts. With direct current, an electrolytic action takes place which causes one brush to wear faster than the other. This is a normal condition. The quality of the DC wave shape coming from the control will also have an important effect on brush life. Recognizing these precautions
and using a careful and intelligent approach to shunt-wound motor application will usually guarantee long and successful brush
and motor life.

Permanent Magnet (PM)

Features:

Continuous duty
DC power supply
Reversibility at rest or during rotation with current limiting
Relatively constant and adjustable speed
Starting torque 175% and up of rated torque
High starting current, relative to full load running current

Design and Operation:

Historically, permanent magnet field motors provide a comparatively simple and reliable DC drive in applications requiring high efficiency, high starting torque and a linear speed/torque curve. With the great strides made in ceramic and rare earth magnet materials, combined with electronic control technology, the PM motor has taken on a new importance as an adjustable speed
drive delivering significant performance in a relatively compact size. See figure below.

The single design feature which distinguishes the PM field motor from other DC motors is the replacement of the wound field with permanent magnets. It eliminates the need for separate field excitation and attendant electrical losses in the field windings. The armature and commutator assembly in conventional PM motors is similar to those found in other DC types.

Permanent magnet motor.

Advantages:

Perhaps the most important advantage of PM field motors is their smaller overall size made possible by replacing the wound field with ceramic permanent magnets. For a given field strength, the PM ring and magnet assembly is considerably smaller in diameter than its wound field counterpart, providing substantial savings in both size and weight. See figure below. And since the PM motor is not susceptible to armature reaction, the field strength remains constant.

Stators for 1/4 hp (186.5 watt) ventilated shunt-wound field DC motor (right) and 1/4 hp PM DC motor (left). Note that the inner diameters of the two stators are the same, while the outer diameter of the PM motor is considerably smaller.

Armature reaction can act to weaken the magnetic field of a conventional shunt wound DC motor at loads beyond approximately 200% of rated value. This characteristic is generally responsible for the “drop off” in torque associated with shunt wound designs. See figure below.

Comparison of shunt and PM
motor curve shapes.

If we examine the field construction of the wound field and PM field motors, we can explain the differences in armature reaction and corresponding differences in speed / torque characteristics of the two motor types. The armature magnetizing force in a wound field construction “sees” a very high permeability (low reluctance) iron path to follow. In the PM field design, this armature magnetizing force is resisted by the low permeability (high reluctance) path of the ceramic magnet, which tends to act as a very large air gap. The net result is that the armature cannot react with the field in a PM motor, thereby producing a linear speed / torque characteristic throughout its entire torque range.

PM motors can be useful in a number of specific ways:

They produce relatively high torques at low speeds, enabling them to be used as substitutes for gearmotors in many
instances. PM motors operated at low speeds are especially useful where “backlash” and inherent mechanical “windup” of gearing in gearmotors can not be tolerated.
The linear speed / torque curve of PM motors, coupled with their ability to be easily controlled electronically, make them ideal for adjustable speed and servo motor applications.
The linear output performance characteristics of PM motors also make it easier to mathematically predict their dynamic performance. See figure below.

A typical family of speed / torque curves for a PM motor at different voltage inputs, with V₅ > V₄ > V₃ > V₂ > V₁.

Since the PM field motor is not affected by armature reaction, it can produce very high starting torque. This high starting torque capability can be a valuable asset in many “straight motor” (innermost) applications as well as inertial load applications requiring high starting torque with less running torque. PM motors function well as torque motors for actuator drives and in other intermittent duty applications.

The size reduction in PM motors is generally accomplished without any significant change in the temperature rise rating for a given horsepower. In fact, the electrical efficiency of the PM motor is very often 10% to 15% higher due to the elimination of field copper losses which occur in wound field motors. PM motors can be produced in TENV (totally enclosed non ventilated) construction, eliminating the need for fans and providing much greater application flexibility.

With their higher inherent efficiency, PM motors offer lower current drain for more efficient battery operation in portable applications.
The permanent magnets also provide some self-braking (less shaft coast) when the power supply is removed. PM motors require only two leads (shunt wound motors require four). The leads can be reversed by simply changing the polarity of the line connection. Dynamic braking is achieved by merely shunting the two leads after disconnecting them from the power source. PM designs also provide similar performance characteristics to shunt wound DC motors when used with all common control methods (except field
weakening).

Disadvantages:

While ceramic magnets now have properties which make them very reliable, certain characteristics of these materials must be thoroughly understood if proper operation of ceramic magnet PM motors is to be obtained. At lower temperatures (0°C and below), ceramic magnets become increasingly susceptible to demagnetizing forces.

Armature reaction (which is capable of producing the threshold limit for demagnetization) takes on greater importance at lower temperatures. Therefore, special attention must be given to overload current conditions including “starting,” “locked rotor” and “plug reversing” when applying PM motors to low temperature use. Plug reversing requires current limiting, even at normal temperatures.

The design of the motor’s power supply is also important. SCR circuits can be designed to provide current regulating and / or limiting features to protect the motor at low temperatures. The actual application parameters involved vary with each particular PM motor design, since the protection against demagnetization is part of the motor’s design and must be considered accordingly. It is best to consult the manufacturer if low temperature use or plug reversing is contemplated.

As operating temperature increases, the residual or working flux of PM motors decreases at a moderate rate. This flux decrease is much like the decrease of field flux strength in wound field motors as copper resistance increases with temperature.

On a motor-to-motor and lot-to-lot basis, PM motors are sometimes criticized for having somewhat greater variability in performance characteristics than wound field designs. Such criticism may be the result of greater variations encountered in both the quality of the raw materials and the processes employed in the manufacture of the magnet segments themselves. However, undue variation can be greatly minimized by the motor manufacturer. Proper magnetic circuit design and calibration of the magnetic assembly to a predetermined common field strength value (somewhat less than full saturation) can do much toward achieving consistent motor performance. Too often, calibration is ignored by some motor manufacturers because of cost, and in many cases, the variation in the
level of flux achieved by saturation alone is considered acceptable.

Another concern is whether a PM motor can be disassembled without loss of field strength and without having to provide any additional magnetic circuit keeper. The answer can be yes and no, depending primarily upon the characteristics of the
magnetic materials selected for a given design. Although newer ceramic materials permit disassembly without loss of magnetic
field strength, the user should consult the manufacturer before attempting to disassemble the motor.

Cautions:

Because of their high starting torque characteristic, care must be exercised in applying PM gearmotors. A PM gearmotor application should be carefully reviewed for any high inertial loads or high starting torque loads. These types of loads could cause the motor to transmit excessive torque to the gear head and produce output torque which exceeds its design limits. SCR controls having current
limiting circuits or overload slip clutches are often employed to protect gearing used with PM motors.

BRUSHLESS DC MOTOR ACTION

A segmented commutator rotating within a stationary magnetic field causes mechanical switching of the armature current. In a brushless DC motor, the magnetic field rotates. Commutation occurs electronically by switching the stator current direction at precise intervals in relation to the position of the rotating magnetic field. Solid state controls and internal feedback devices are required to operate brushless DC motors.

Brushless DC motors combine characteristics of both DC and AC motors. They are similar to AC motors in that a moving magnetic field causes rotor movement or rotation. They are similar to DC motors since they have linear characteristics. The figure below shows cross-sections of AC, DC and brushless DC motors. The AC motor has windings in the stator assembly and a squirrel cage rotor. The PM DC motor has windings on the rotor assembly and permanent magnets for the stator assembly. The brushless DC motor is a hybrid of the AC and DC motors. The rotor has permanent magnets and the stator has windings.

Cross-sections of: a) an AC motor (top), b) a PM DC motor (left), and c) a
brushless DC motor (right).

Brushless DC

Features:

Continuous duty
DC power supply
Reversibility at rest or during rotation with current limiting
Adjustable speed
Starting torque 175% and up of rated torque
High starting current

Design and Operation:

Brushless DC motors consist of two parts: the motor and a separate electronic commutator control assembly (see figure below).

Brushless DC motor.

The two must be electrically connected with a cable or wiring harness before motor action can take place. See figure below.

Schematic diagram of a brushless DC motor and control.

By energizing specific windings in the stator, based on the position of the rotor, a revolving magnetic field is generated. See figure below. Sensors mounted inside the motor detect the position of the permanent magnets on the rotor. For example, as the rotor moves through a specific angle or distance, one of the sensors will detect a change from a north magnetic pole to a south magnetic pole.

Phase current flow.

At this precise instant, current is switched to the next winding phase. By switching current to the phase windings in a predetermined sequence, the permanent magnets on the rotor attempt to chase the current. The current is always switched before the permanent magnets catch up. Therefore, the speed of the motor is directly proportional to the current switching rate. At any instant, two windings are energized at a time with the third one off. This combines the torques of two phases at one time, thus increasing the overall torque output of the motor.

The rotor consists of a four-pole permanent magnet and a smaller four-pole sensor magnet. As the sensor magnet rotates it will activate a series of sensors located 60° apart. The sensors can be photo sensors, Hall effect devices, magneto resistors or other devices which monitor the position of the shaft and provide that information to the controller.

The controller logic circuits contain a binary decoder which interprets the signals from the sensors regarding the position of the permanent magnet rotor. The logic circuit outputs a specific address which tells a drive circuit which windings should be energized.

The rotation of the motor is changed within the control logic which in turn reverses the phase energizing sequence. A toggle switch is usually provided to convert the logic from clockwise to counterclockwise. The figure below shows the truth tables for both clockwise and counterclockwise commutation.

Commutation sequence:
a) clockwise (top), and
b) counterclockwise (bottom).

Trapezoidal vs. Sinusoidal Torque Properties:

Timing of the "on' and "off" switching of different phase pairs is determined by the signals emanating from the sensors in the motor's commutation circuitry.

Trapezoidal torque characteristics of the phase pairs mean that fewer commutation signals are required than for a motor whose
phases exhibit sinusoidal torque properties. This simplifies the control design and minimizes its cost, while providing a motor torque output with low ripple properties.

Commutation circuitry is designed to switch as the torque output and the back emf in the individual phase pairs reach their maximum (and most constant) level. This produces the least ripple effect on the output torque and the lowest phase current
swing. The resulting smooth output torque makes it easier to implement digital and pulse width modulation control techniques.

Advantages:

Brushless motors are more reliable. They do not have commutator or brushes to wear out. The commutation is achieved through reliable solid-state circuit components, making them ideal for applications where downtime is critical or where drive system access is difficult. Brush sparking and associated RFI are eliminated.

Brushless motors are not sensitive to harmonics like AC motors. The brush noise associated with brush type DC motors and commutators is also eliminated.

Brushless DC motors provide predictable performance because of their linear characteristics. See figure below. They can exhibit high starting torques, precise positioning capability and controlled acceleration and deceleration. And more power can be achieved from a smaller size motor.

Brushless motors can be designed with low rotor inertia. This means they accelerate more quickly in less time and offer less power dissipation during the stop / start cycle. They are also capable of operating at high speeds since there are no mechanical commutator limitations.

Typical speed / torque curve
for a brushless DC motor.

Disadvantages:

Unlike conventional DC motors, electronically commutated designs cannot be reversed by simply reversing the polarity of the power source. Instead, the order in which the current is fed to the field coil must be reversed. Also, due to low friction inherent in brushless
motors, excessive coasting may be a problem after the current has been removed. Special damping circuits or other devices
may be added to remedy this factor, but cost will be adversely affected.

From a cost standpoint, the electronics needed to operate brushless DC motors are relatively more complex and therefore more expensive than those used with conventional DC motors. While electronically commutated DC motors are now closer to being competitive with conventional DC / tachometer feedback units, they are still costly when compared with conventional DC / SCR control drives.

Stepper Motors

Features:

Continuous duty
DC power supply
Reversibility at rest or during rotation
Adjustable speed
Normal starting current

Design and Operation:

The widespread acceptance of digital control for machine and process functions has generated a growing demand for devices that
translate digital commands into discrete incremental motions of known accuracy. The ability to interface stepping motors with microprocessors and / or mini-computer controls has enhanced their application potential (see figure below).

Stepper motor.

While conventional AC and DC motors operate from continuously applied input voltages and usually produce a continuous (steady state) rotary motion, stepper motors move in discrete steps (increments). Stepping occurs in strict accordance with the digital input commands provided. At very low stepping rates, the stepping action at the motor shaft may be visible. At high stepping rates, the shaft appears to rotate smoothly, like a conventional motor. Step error is non cumulative. The absolute position error is independent of the number of steps taken. Final shaft position is predictable within a maximum error determined by mechanical tolerances, and
from the motor’s static torque vs. angular displacement curve.

Although we refer to the angular position of the stepper shaft as the motor’s “output,” there are many applications where this rotation is converted to precise linear motion, for example, by means of the lead screw or rack and pinion.

DC steppers are divided into three principle types, each having its own unique construction and performance characteristics:

variable reluctance (VR),
permanent magnet (PM), and
PM hybrid.

Variable Reluctance:

Generally a lower priced drive, the variable reluctance stepper has a wound stator and a multi-poled soft iron rotor. The step angle
(determined by the number of stator and rotor teeth) varies typically from 5 to 15 degrees. Unlike the hybrid design, variable reluctance steppers have relatively low torque and inertia load capacity. They are, however, reasonably inexpensive and adequate for light load computer peripheral applications. Operating pulse rates vary over a wide range, depending upon the specific design and construction of a particular motor.

PM Steppers:

With step angles ranging from 5 to 90 degrees, PM steppers are low to medium-priced units with typically slower step rates (100 steps / second for larger units and 350 steps / second for smaller ones). They usually employ a wound stator with a PM rotor delivering low torque. Step accuracy is ≥ ±10%.

PM Hybrid:

The PM hybrid stepper combines the construction and performance aspects of both PM and variable reluctance type stepper motors. Both the rotor and wound stator are toothed. The toothed rotor is composed of one or more elements known as stacks. See figure below. Each stack has both hollow and solid laminations bonded together to form two cup shaped structures. A permanent magnet is
inserted in the space between the two cups. Rotor stacks are then fastened to a nonmagnetic (usually stainless steel) shaft.

1) Hollow laminations,
2) Alnico permanent magnet, and
3) solid laminations.

nonmagnetic (usually stainless steel) shaft. The perimeter of each lamination has multiple teeth with a specific tooth pitch (angle between tooth centers) depending on the degree of step required. Step angles vary from 0.5 to 15 degrees. See figure below.

PM hybrid stepper tooth pitch.

When the cups are pressed on the shaft to form a stack, they are positioned in such a way that the teeth of one cup line up with the slots of the other cup. The two cups of each stack are said to be offset from each other by half of one tooth pitch.

The stack configurations can vary. When more than one stack is used, nonmagnetic spacers are inserted between stacks to prevent coupling. See figure below.

Variable stack lengths for PM
hybrid stepper motors.

Without the spacer, the separate magnetic structures would combine, eliminating the advantage of multiple stacks. With adequate
space between them, magnetic flux will follow the path of least resistance through the stator core, multiplying the available torque by the number of stacks. This construction gives the PM hybrid higher torque capacity (50 to 2000 + oz in.) with step accuracies to ±3%. See figure below.

Flux path through rotor and stator.

The figure below shows a cross-section of a typical DC PM hybrid stepper with toothed rotor and stator. When the rotor is inserted into the stator bore, only one pair of stator poles will line up exactly, tooth for- tooth, with the teeth on a single rotor cup. The remaining poles will be out of alignment by some fraction of a tooth pitch. This misalignment is what makes it possible for a stepper to develop torque. Most PM hybrid steppers have four phases which are bifilar wound, but other phase arrangements and multiples are available.

When phases are energized in a specific sequence, PM hybrid steppers deliver specific angular output motions (steps) of known accuracy, provided that system inertia and friction do not exceed acceptable limits.

Cross-section of a typical DC
PM hybrid stepper with toothed rotor and stator.

Each angular displacement ends in a well-defined position of magnetic attraction called a detent position. These stable equilibrium
positions are created by the magnetic interaction between the permanent magnet rotor and the magnetic field produced by the energized phase windings. As the motor is stepped, the detent positions shift around the entire 360° rotation. The direction of rotation is determined by the phase energization sequence.

PM hybrid designs offer excellent speed capability1000 steps / second and higher can be achieved. Because the step angle
is fixed by the tooth geometry and step error is non cumulative, the shaft position of a motor loaded within its capacity is always
known within a fraction of one step. This open-loop operation eliminates the need for encoders, tachometers and other feedback
devices which add to system cost.

Advantages:

Steppers are popular because they can be used in an open-loop mode while still offering many of the desirable features of an expensive feedback system. Hunting and instabilities caused by feedback loop sensitivity and phase shifts are avoided.

Due to the non cumulative nature of stepper error, step motors also offer improved accuracy. The replacement of less dependable mechanical devices, such as clutches and brakes, with step motors provides considerably greater reliability and consistency. Predictable and consistent performance coupled with reasonable cost make the DC stepper an excellent positioning drive.

Disadvantages:

Stepper motors can be made to produce reasonably high torques (2000 oz-in. or more). However, they do have a limited ability to handle extremely large inertial loads. See Fig. 3-24. Since steppers tend to oscillate (ring) upon stopping, some sort of damping means is usually required. Stepper motors unfortunately are also not very energy efficient, but this is the price that must be paid to
achieve the truly unique performance characteristics available from the stepper motor. Resonance is sometimes a problem that can be remedied by a specialized electronic control design or by avoiding operation within the step rate ranges prone to resonance.

Typical torque vs. speed (steps / second) for a PM hybrid stepper

Constant Horsepower:

This type of load absorbs the same amount of power regardless of the speed.

Variable Torque:

Some loads require different torque at different speeds.

Load Inertia:

The load inertia is expressed as:

where M is the mass of the rotating parts and k is the radius of gyration.

Acceleration Time:

The difference between the friction torque required by the load and the torque delivered by the drive will affect acceleration time. Greater accelerating torque decreases the time required to get the load to full speed. It can be expressed as:

where:
t = accelerating time (seconds)
n₂ = final speed (RPM)
n₁ = initial speed (RPM)
T_a = accelerating torque (lb-ft.) available from the drive (Tdeveloped - Tfriction)
W = weight of rotating system (lbs.)
k = radius of gyration (ft.)