Having given a general introduction to linear electric actuators in the preceding chapter, we now consider various types of LEAs in some detail. In this chapter, we begin with linear induction actuators (LIAs). These are essentially derived from rotary induction motors and resemble linear induction motors with restricted and controlled linear motion. Before we present an analysis of LIAs, it is worthwhile discussing some of the constraints under which these devices must operate.

PERFORMANCE SPECIFICATIONS

Linear induction actuators are used for short travel (between 2 and 3 m) and thus are characterized by rather small mechanical airgaps of about 1 mm. Also, the operating speed is generally less than 2 m/s. LIAs, as low-speed direct-drive systems, are characterized by the rated thrust Fxm for a given duty cycle and a peak (short-duration) thrust Fxp developed at standstill. LIAs are used only with variable-voltage variable-frequency static power converters or voltage-source inverters to obtain good energy conversion efficiencies and controlled thrust or positioning. The ac source—three-phase, in general—which supplies the static power converter is specified in terms of voltage and frequency: rated values and usual deviations. Few LIAs work continuously; short duty cycles are typical. In either case, the cooling system of the primary should also be specified as it determines the rated (design) current density in the LIA windings.

The performance of the control system of the LIA must also be specified.

In Chapter 3 we observed that the thrust/kVA and thrust/loss ratios for linear induction actuators (LIAs) are rather low. On the other hand, in Chapter 4 we concluded that these ratios are high for linear permanent magnet synchronous actuators (LPMSAs). These are relatively expensive, however, owing to the costs of magnets. Recent research in rotary reluctance machines has led to the development of linear reluctance synchronous actuators (LRSAs), which are relatively inexpensive and have good performance indices. Most LRSAs correspond to their rotary counterparts, including reluctance motors with traveling fields, switched reluctance motors, stepper motors, etc. LRSAs are especially suited for low-speed (less than 2 m/s) applications.

In analyzing an LRSA, it is to be noted that the winding concepts and details of LIAs (Chapter 3) are valid for LRSAs, and its speed being low, end effects in an LRSA may be neglected. However, the presence of high saliency on its secondary must be taken into account. In essence, theory pertaining to rotary reluctance motors is also applicable to LRSAs, except for the existence of normal forces in the latter, which merits special treatment.

PRACTICAL CONFIGURATIONS AND SALIENCY COEFFICIENTS

Like most linear electric actuators, LRSAs may be tubular or flat, single-sided or double-sided, with the secondary as the mover. The primary windings are three-phase ac windings as in LIAs. The secondary should have a high saliency.

Linear electric actuators are electromagnetic devices capable of producing directly (without any linkages, etc.) progressive unidirectional or oscillatory short-stroke motion. The motion occurs because of the electromagnetic force developed in the actuator. Linear electric generators are also linear motion electromagnetic devices which transform short-stroke oscillatory motion mechanical energy into single-phase ac electrical energy. Just as a rotary electric machine may operate either as a motor or as a generator, a linear motion electromagnetic device may be designed to work either as an actuator or as a generator. From this standpoint, linear electric actuators and generators are the counterparts of a corresponding rotary electric machine. In general, however, linear electric machines have been associated with long linear progressive motion, such as in transportation and similar applications.

Whereas primitive linear electric machines have been in existence for a long time, since the 1960s there has been a great deal of interest in linear machines for various applications—especially transportation. Several books and numerous papers have been published on the subject in the recent past. However, the literature on linear electric actuators and generators is relatively sparse. Clearly, the potential applications of these devices are too numerous to mention here. Judged from the present trend, it would suffice to say that the field of linear actuators and generators may lead to an industry of “linear motion control” with a large worldwide market.

In this book we deal with magnetic and electromagnetic devices. Specifically we discuss the theory, design, and analysis of linear motion electric actuators and generators. Whereas actuators are not precisely defined, we may consider them to be as devices which convert a form of energy into controlled mechanical motion. The form of energy may be electric, hydraulic, or pneumatic. In this book we are concerned with electric actuators, which convert electric energy into controlled mechanical motion of limited travel. Electric actuators compare favorably with hydraulic and pneumatic actuators, and are much superior to both in terms of efficiency, controllability, cost, and environmental safety. Of course, electric actuators may be either rotary generating rotary motion or linear leading to linear motion. Furthermore, a linear electric actuator may be direct operating, utilizing electromagnetic or piezoelectric effects to produce force and motion. On the other hand, a converting type of linear actuator uses an electric motor with gears and linkages to produce linear motion. The direct operating type of linear actuators are the subject matter of this book. Most of these devices invariably consist of coupled magnetic and electric circuits in relative motion, although piezoelectric actuators operate on a different principle.

Because the electromechanical energy conversion process is reversible, actuators may be operated as linear electric generators, in which case mechanical energy is transformed into electric energy.

Linear permanent magnet synchronous actuators (LPMSAs) have been proposed for applications in factory automation upgrading. Such actuators utilize high-energy product magnets and are characterized by high thrust density, low losses, small electrical time constant, and rapid response. The main drawbacks of LPMSAs are their high cost owing to the costs of the magnets and the presence of stray magnet fields, especially in single-sided configurations.

Thrust– or position-controlled flat LPMSAs may be used for travels up to about 3 m, with a constant airgap provided either mechanically or via a controlled suspension system. For travel lengths of less than 0.5 m tubular configurations may be preferred, since tubular structures make better use of materials, resulting in a compact actuator.

CONSTRUCTION DETAILS

LPMSAs may be flat single-sided or double-sided or may have a tubular structure. In either of these configurations, the primary may constitute the mover. On the other hand, the actuator may have moving magnets. The core of the primary of a flat LPMSA is made of longitudinal laminations with uniformly distributed slots, which house the windings. Often, the windings are three-phase distributed windings having one to four slots/pole per phase (q = 1, 2, 3, or 4). These windings are similar to those of induction-type actuators presented in Chapter 3. Because the windings are located in open slots, the effective airgap becomes considerably greater than the actual airgap.

In tubular structures the laminations of the primary core may be either longitudinal or disk-shaped as in linear induction actuators.

In this chapter we present a general introduction to linear electric actuators (LEAs) and linear electric generators (LEGs). We will use the acronyms LEAs and LEGs consistently throughout the text. We begin with the definitions of LEAs and LEGs, although these definitions are not standardized in the profession. Subsequently, we will present a general discussion pertaining to the principles of operation and certain applications (existing and potential) of LEAs and LEGs.

TERMINOLOGY

Linear electric actuators (LEAs) are electromechanical devices which produce unidirectional or bidirectional short-stroke (less than a few meters) motion. In this regard, an LEA may be considered an electric motor in that both LEAs and electric motors internally develop electrical forces resulting in mechanical motion. However, the LEAs considered in this book have linear motion (or motion in a straight line), whereas electric motors have rotary motion. Moreover, unlike most electric motors, LEAs are commonly used for bidirectional, or reciprocating, motion.

Linear electric generators are electromechanical energy converters driven by prime movers undergoing reciprocating motion and converting mechanical power into single-phase ac power. It is to be noted that LEGs are not used for three-phase power generation, since the phase sequence of the generated voltage will reverse as the prime mover reverses its direction of motion.

Just as rotary electromagnetic machines do, LEAs and LEGs share the blessing of reversibility; that is, the same machine can act as an actuator or as a generator.

Linear stepper actuators (LSAs) are devices which transform digital (or pulse) inputs into linear incremental motion outputs. They are counterparts of rotary stepper motors. When they are properly operated, the number of linear steps of the actuator equals the number of input pulses, and the mover of the actuator advances by one linear step increment, τss. LSAs have applications for short-travel linear step motion. An example of application is the Sawyer motor, which has been successfully applied in drafting equipment, head positioning, laser beam positioning for fabric cutting, etc.

Just like their rotary counterparts, LSAs may be either reluctance or hybrid (with permanent magnets). A unique feature of LSAs is that they can produce directly linear motion insteps smaller than 0.1 mm [1]. Other advantages of LSAs are as follows:

• They can operate as an open-loop system and yet yield a precise position control.

• They are robust and mechanically simple.

• They can be repeatedly stalled without any damage to the actuator.

• Required electronic controllers of LSAs are simple.

Among the disadvantages of LSAs are as follows;

• They have relatively high losses perthrust (unless the airgap is drastically reduced (to less than 0.2 mm).

• With open-loop operation, the step size is constant.

• Step response may have a large overshoot and subsequent oscillations.

• The mover weight tends to be high for the thrust produced.

• Frictional loads and linear ball bearings lead to position errors in open-loop operations.

• […]

BASIC ELEMENTS OF A DRIVE

A rotational mechanical load in which any one of a wide range of operating speeds may be required is often called an ‘infinitely variable speed drive’ or, more modestly, a ‘variable speed drive’ or ‘adjustable speed drive’. Suitable operating characteristics to provide a given range of load torques TL and speeds N might be provided by a pneumatic or hydraulic drive as well as by several forms of electrical variable speed drive. The basic components of such a drive, Fig. 4.1, are the motor or actuator that delivers torque to the load and controls its speed and the power controller that delivers power to the motor in a suitable form. In this book only those drives in which the drive motor is an electric motor are considered.

The output power developed by an electric motor is proportional to the product of the shaft torque and the shaft rotational speed. The value of the developed torque usually varies automatically to satisfy the demand of the load torque plus any torque associated with friction and windage. Increase of the shaft power due to increase of load torque is usually supplied by automatic increase of the supply current demanded by the motor. Any significant change in motor speed, however, must be obtained in a controlled manner by making some adjustment to the motor or to its electrical supply.

The advances in power electronics since this book was first published in 1987 have chiefly been in the development of more effective semiconductor switching devices. In particular, the future of high power switching applications will involve reduced use of the silicon controlled rectifier (SCR) and gate turn-off thyristor (GTO) and increased use of the metal–oxide–semiconductor (MOS) controlled thyristor (MCT). The most influential development, however, is likely to be due to increased ratings of metal–oxide–semiconductor fieldeffect transistor (MOSFET) devices and, in particular, widespread use of the insulated gate bipolar transistor (IGBT). Design data of these switching devices is widely available from manufacturers. Increases in the range of semiconductor switches and their nonlinear nature has influenced practitioners to move towards computer based design rather than analytical studies. Simulation techniques are widespread and expert systems are under development.

Chapters 1–3 have been extensively revised, compared with the original text, to incorporate much new material, especially concerned with modern semiconductor power switches. With regard to the switching properties of semiconductor devices the authors have adopted an analytically fundamental approach rather than the current industrial standard. This is educationally easier to understand and more conservative in solution than accepted industrial practice.

Some re-organisation of the original text has permitted expansion of the section on Adjustable Speed Drives, now in Chapter 4, to include a brief treatment of various types of synchronous motor.

The previous work on step-wave inverters has been concentrated into the new Chapter 11 and an additional chapter has been included on pulse-width modulation (PWM) control.