Electric Motor Drives and Control1(Electrical)

ELECTRIC MOTOR DRIVES AND CONTROL-PAST, PRESENT AND FUTURE

Abstract-Many industrial applications require variable-speed motor drives. Traditionally, dc motors have been used in sucb applications. However, recent advancements in semiconductor power electronics and microelectronics have made it possible to use ac motors in many variable speed drive applications. Implementation of new control techniques such as field-oriented control, variable-structure control with sliding-mode features, and others have made ac motors viable alternatives to dc motors in high-performance drive applications. The advent ofMicroprocessors/microcontroUers/microcomputersh as made it possible to implement these complex control techniques.

Electric drive technology has undergone a dynamic evolution during the last three decades. This paper presents a comprehensive review of the state of the art in the & Id of electric motor drives and control Strategies. Trends in the technological evolution are also indicated.

INTRODUCTION

EL ECTRIC machines have been the workhorses of industry for many years. The three basic electric machines [l]-dc, induction, and synchronous-have served industrial needs for nearly a century. In recent years, intense research effort has made other variations of electric machines, such as brushless dc machines, permanent magnet machines, and
Switched reluctance machines, viable alternatives in many applications.

DC machines [l], [2] have traditionally dominated the domain of drive systems. Even now most industrial drives use dc machines. Although the machine is more expensive,
the control principles and the converter equipment required are somewhat simpler in dc drives. However, the main disadvantages are its commutators and brushes and the frequent maintenance required for its operation. Ac machines [l], [3], [4], on the other hand, are rugged and less expensive. Historically, they have been favored for constant-speed drive applications. The equipment required to use them in variable-speed drives has a history of being complex and expensive. However, during the last three decades, there has been intense research on the development of ac drive technology.

As a consequence, the cost and performance of ac drives have improved considerably. Their popularity in industry is definitely on the upswing. Traditionally, electric motors were controlled manually [1]-resistance control of dc motors and variac control of ac
motors being examples. Electronic control started with the advent of gas tubes such as thyratrons and ignitrons in the 1930’s. The modem era of control began with the advent of power semiconductors in the 1950’s. Subsequent progress in power electronics and microelectronics has profoundly influenced the operation and performance of drive systems, in particular, ac variable-speed drives. Ac machines exhibit highly coupled, nonlinear, multivariable structures, as opposed to dc machines (separately excited), with their much simpler decoupled control structure. Comparable control performance of ac drives generally requires more complicated control algorithms implemented by fast real time signal processing.

Recent advancements in power electronics, microelectronics, and microcomputers have made it possible to implement sophisticated control tasks at reasonable cost.

Technology breakthroughs have made ac drives viable alternatives to dc drives in many applications. In high-performance drive applications, for example, in the areas of robotics, machine tools, and rolling mills, drive systems are required that can provide fast dynamic response, parameter-insensitive control characteristics, and rapid recovery from speed drop caused by impact loads. Conventional linear controllers (PI, PID) cannot achieve these requirements simultaneously. In recent years, intense research efforts have been focused on the use of modem control technique in drive systems. Such techniques as model reference [5], adaptive control [6], and variable-structure control (sliding-mode control) [7] have shown promise in meeting the needs of high-performance drives.

II. DC MOTOR DRIVES

Control of dc machines is simple [2]. The field rnmf and the armature rnmf are decoupled, as can be seen from Fig. 1. The torque depends on armature current and field flux (T 01 I, af), and the field flux depends on field current (cpf 01 If). This decoupled feature provides enhanced speed of response for torque and speed. The control of torque is normally achieved by controlling the armature with constant field current. Field weakening is employed to increase the speed beyond a basespeed. The simplicity and flexibility of control of dc motors have made them suitable for variablespeed drive applications. Fast torque response has favored their use in high-performance servo drives. In fact, a majority of industrial drives today use dc machines.
Fig. 2 illustrates a typical dc motor speed drive system in which an outer speed loop and an inner current loop are implemented. The addition of the inner current loop-which indirectly provides the torque control - can limit the excursion of current, as can be seen from Fig. 3. However, because of time delay involved in the imperfect analog
Devices normally used for implementation (such as in a PI controller for speed loop), the initial excursion of current cannot be limited (Fig. 3(a)). A variation of the speed controller, known as the IP controller [8], 191, shown in Fig.4 can overcome this problem as shown in Fig. 3(b).

The converters in Figs. 2 and 4 can be phase-controlled rectifiers or choppers, depending on the supply available. Two quadrant converters can provide regeneration capability
[2]. A dual converter can provide fast speed reversal and are frequently used in mill drives [2].
A dc machine, although is ideal from the standpoint of control, is, in general, bulky and expensive compared with an ac machine. In addition, commutators and brushes require
Periodic maintenance and make the dc machine less reliable and unsuitable to operate at high speed or in an explosive environment. As a consequence, for more than a quarter
Century, attention has been diverted to develop ac drives as available alternative to dc drives in many applications.


111. INDUCTION MOTOR DRIVES

The induction machine is a rugged, reliable, and less expensive ac machine. It has been the economical workhorse for use in ac motor drive applications during the past quarter
Century. It has been used for both low-performance as well as high-performance drive applications. Basically, there are two types of induction machines: the squirrel cage induction machine (SCIM) and the wound rotor induction machine (WRIM). The SCIM is less expensive, more robust, and has been extensively used in a wide range

of power ratings. It will continue to play a prominent role in ac drive systems in the future.

Control of the SCIM
Different control methods of varying degrees of complexity have been proposed and used for control of induction machines the nature of application dictates the acceptance
of a particular method.
A simple and economic method of control is to vary the stator voltage [lo] at supply frequency using thyristors (or triacs is shown in Fig. 5. This method of control is
Characterized by poor dynamic and static performance. Although
it is inefficient because of high slip power loss, it is used in fans, pumps, and blower drives.
An efficient method of speed control for induction motors is to change the stator frequency [ 13, [3], [4]. Since the speed is close to synchronous speed, the operating slip is small, and slip power loss in the rotor circuit is small. However, this will require a frequency converter, which is expensive.
In drive systems, it is desired that the machine flux is regulated to provide better utilization of the machine. A requirement for maximum possible transient dynamics is to
operate the motor at its rated flux level. Indirect flux regulation schemes such as the “volt/Hertz” control [ll] and the “slip-current” control [ 121 use variable frequency control and have been extensively used in industry.
B. Flux Regulation by Stator Voltage and Frequency
Variable-frequency drives originally used open-loop, volt/Hertz control to regulate machine flux. They were found to be satisfactory for low-performance, cost-effective industrial drives. Closed-loop control with slip regulation was introduced later for improved drive performance. The air gap flux can be regulated if the air gap voltage and
Frequencies are varied simultaneously at a constant ratio Maximum torque per ampere of stator current can be obtained by coordination between torque and flux at a particular loading condition. The concept of variable-voltage variable-frequency control is illustrated in Fig. 6. Below the base speed, constant flux operation it used. Beyond the base speed, constant flux operation is used. Beyond the base speed, the motor terminal voltage is constant, and as the decreases (which is known as field weakening). The machine can be operated in constant power mode, as is shown in Fig.6. Fig. 7 shows a closed-loop volt/Hertz control scheme with slip frequency control, which provides a limit on the maximum torque.
C. Flux Regulation by Current and SlipFrequency
Another method that is often used for flux regulation is based on a coordination of stator current and slip frequency. The current-slip frequency relationship is shown in Fig. 8(a).
A closed-loop system based on this technique is shown in Fig. 8@). A current source inverter is suitable for this drive. Unlike the volt/Hertz control, the current-slip
frequency control technique is independent of stator parameters (resistance, leakage inductance). Hence, flux regulation can be achieved even at low speeds.
D. Field-Oriented Control
Both the volt/Hertz and current-slip frequency control provide satisfactory steady-state performance. The volt/Hertz control scheme is quite simple to implement. On the other hand, the current-slip frequency control scheme requires closed-loop current regulation as well as accurate speed measurement and, therefore, is somewhat complicated to
Implement. However, both these methods fail to provide satisfactory transient performance. Fig. 9 shows the typical transient characteristics of the current-slip frequency control scheme. As can be seen from the figure. The machine torque and the air gap flux experiences flux oscillation during transient. High performance drives like robotics, rolling mills and machine tools require fast and precise torque response. To achieve this, the dynamic structure of the machine has to be taken into account. The induction machine is a on linear multi variable highly coupled device. Several methods [14]-[18] have been proposed to obtain fast torque response with flux regulation. However, the emerging consensus is to use field-oriented control (FOC) [ 141, [ 151. Field orientation is a technique that provides a method of decoupling the two

components of stator current: one producing the air gap flux and the other producing the torque. Therefore, it provides independent control of torque and flux, which is similar to a separately excited dc machine. The magnitude and phase of the stator currents are controlled in such a way that flux and torque components of current remain decoupled during dynamic and static conditions.

Fig. 12 shows such an indirect method of control The scheme is simpler to implement than the direct method of FOC (Fig. 11); hence, there is an increasing popularity towards the indirect method of FOC.


Fig. 13 shows the experimental results of the torque and flux responses for step changes in command torque from zero to its rated value. As can be seen from this figure, the
machine torque response is almost instantaneous, and the average torque is controlled. The high-frequency torque pulsation is mainly caused by the motor current harmonics. The figure also shows that the machine flux is maintained constant during torque transition. This demonstrates the decoupling control of torque.


Both the direct and indirect methods of FOC are machine parameter dependent unless means are included for directly measuring the rotor flux component. Inductance parameters vary about f 20 %, whereas rotor resistance changes dramatically
(F 100%) with temperature. Without the exact knowledge of the machine parameters, optimum decoupling and torque linearization cannot be achieved [20]. Considerable
amount of research effort has been directed to developing parameter adaptation schemes for optimum decoupling of field-oriented control. The proposed schemes are based on modified reactive power compensation [2 11, estimation of magnetic flux [22], indirect measurement of instantaneous rotor resistance [23], and identification of rotor resistance by signal injection [24]. Thus far, no standard solution for parameter adaptation has emerged. In most parameter adaptation schemes, the identification is more effective at higher speeds and loaded conditions.
Decoupling control can also be achieved by orienting the air gap flux or stator flux [ 191. Stator flux orientation provides direct control on the saturation level of the machine. However, implementation of these schemes requires more real time
Computations than indirect rotor-flux orientation.


E. Control of the WRIM
In earlier days, the SCIM was used for essentially constant speed drive, and the WRIM was used for variable-speed drive systems. Although the WRIM is more expensive and
less rugged than the SCIM, it has been favored for use in high-power applications in which a large amount of slip power could be recovered. It may be noted that an attractive
feature of WRIM control is that only the slip power is handled by power electronics, which may be only a fraction of the rated machine power.
Classically, speed of the WRIM was changed by mechanically varying external rotor circuit resistance [11. The performance can be improved by using a chopper to control the equivalent rotor resistance [25], as is shown in Fig. 14. The static Kramer [26] or static Scherbius [4] systems, shown in Fig. 15 and 16, respectively, allow recovery of slip power and have been used for pump and blower drives. The Kramer drive provides sub synchronous speed control. The use of a cycloconverter in the Scherbius method allows bi-directional power flow, and hence, the drive can operate in both subsynchronous
and supersynchronous mode.

Field-oriented control can also be applied in WRIM’s [27] to provide decoupled control of real power and reactive power. These features are extremely beneficial in high-power
applications. Fig. 17 shows the field-oriented control of a WRIM. In this case, it is more convenient to use stator flux field orientation than rotor flux field orientation.
A novel use of the WRIM for high-power cycloconverterfed drive [28]-[30] is shown in Fig. 18(a). The scheme incorporates field-oriented control of high dynamic performance.
The WRIM is connected in a series fashion (Fig. 18(b)) such that the stator currents and rotor currents are equal in magnitude but reverse in phase sequence. Under balanced excitation, average torque production is possible only if the rotor rotates at twice the speed of the stator MMF wave [28]-[30]. Hence, speed doubling is achieved, and this allows speed enlargement with cycloconverter-fed operations. The field-oriented control of a series-connected WRIM has less parameters involved than the FOC of SCIM’s or
doubly fed WRIM’s. The machine parameter in question is the mutual inductance, the variation of which under flux regulation is expected to be small (much less than the variation of resistive parameters with temperature in the SCIM). Hence, the use of parameter adaptation is not required in this case.

VIII. MODERN CONTROL TECHNIQUES

The conventional linear controllers such as PI, PID have been used in many applications. The IP controller [8], [9] has also been applied with dc drives. However, these controllers
are sensitive to plant parameter variations and load disturbance. The performance varies with operating conditions, and it is also difficult to tune controller gain both on-line and
off-line. The increased productivity and improved product quality demands fast response and parameter-insensitive robust dfive systems.
.
The model referencing adaptive control (MRAC) (Fig. 36) is also being applied in electric drive systems [5]. In MRAC, the output response is forced to track the response of a reference model (idealized model with fixed plant parameters) irrespective of plant parameters variations. The controller parameters are adjusted to give a desired closed-loop performance. This adjustment is based on an adaptation algorithm that utilizes the difference between the reference model output and the plant output as its input.

Both the self-tuning or the MRAC techniques involve intricate control algorithms. The variable structure control using sliding mode was recently introduced into the field of controlled electric drive systems to compete with the former two adaptive control schemes. With sliding mode control (SLMC), the control system can be designed to provide parameter-insensitive features, prescribed error dynamics, and simplicity in implementation. Researchers have reported the application of SLMC in dc drives and ac servo drives [56]-[59] (using synchronous and induction machines). In SLMC, the drive system is forced to follow a predefined trajectory in the phase plane (Fig. 37) irrespective of plant parameter variation. This is achieved by using a set of switching control laws]. The structure and design of the SLMC are relatively simple. A typical SLMC drive system is shown in Fig. 38. In position control drives using
the SLMC, the actual position and speed are required as feedback signals. They are easy to obtain

Fig. 39 shows the first-order parameter invariant response for a step change in position command. In speed control drives, the speed and acceleration are required as feedback
signals. It is difficult to sense an accurate acceleration signal. Observers may be used to estimate the acceleration .
FUTURE TRENDS


The technology of electric motor drives spans many diverse disciplines such as electric machines, power semiconductor devices, converter circuits, control theory, signal processing, and microelectronics. This multidisciplinary technology has made impressive progress during the last two decades and continues to mature. Evolution in each discipline has contributed to the overall improvement in drive technology.

PWM techniques will be used for both converters and inverters to improve performance. Microcomputers will be used to obtain PWM patterns to optimize inverter switching losses and machine harmonic losses. SCR type current-source inverters will become obsolete in the future. The PWM GTO current-source inverter will challenge the popular PWM voltage- source inverter. In high-performance drives, the current-controlled PWM voltage-source inverter will be widely accepted.

In the future, researchers will attempt to use artificial intelligence or expert systems in conjunction with high-performance electric drives. Expert systems have the potential
for use in real-time control applications. In such systems, the controller could possibly interpret the dynamics of the system operation and self-adjust accordingly. Systems incorporating artificial intelligence could permit diagnosis and correction of faults in a complex system to supplant the need for human intervention.


X. CONCLUSIONS

This paper has presented a comprehensive review of the state of the art in the field of electric motor drives and control strategies. Drive technology has seen impressive growth during the last three decades. AC drive technology has been maturing rapidly and will likely overtake dc drive technology in many industrial applications. New high-speed, high-efficiency switching devices, new motor structures, new converter configuration, new control techniques, and new high speed micro controllers will contribute to the further development of high-performance motor drives. Interest and research
in this area will continue relentlessly.