Introduction Control Techniques in Electric Drives:

An electric drive is a well established industrial drive as it has several ad­vantages and special features. Its Control Techniques consists in starting, speed control, braking and speed reversal, and also maintaining the drive conditions required by the process or work being performed by the drive.

Modern electric drives employ thyristors and thyristor power converters for feeding the electric motor for the purpose of speed control, e.g., they provide a variable voltage to the armature of a dc motor; dc link converters or cycloconverters are used to provide variable voltage variable frequency supply to ac motors. These converters are static devices and their use makes the drive system compact, small in size, light weight, and less bulky. They have a high amplification factor. The overall efficiency of the drive improves because of insignificant losses in the static equipment. These drives employ automatic closed loop control.

The automatic Control Techniques of the drive has the following advantages:

  1. It permits increased productivity and improves the quality of production.

  2. It reduces running costs and hence production is economical.

  3. It reduces the expenditure on electrical energy.

  4. It improves the reliability of the system.

  5. It provides better working conditions.

  6. It simplifies the operation of the equipment.

  7. It makes remote control possible, particularly when the drive is inaccessible and the local control is difficult.

Thyristor controlled electrical drives having automatic control of current and speed are very popular. They can be controlled during starting, speed control, regenerative braking and speed reversal. It is well known that soft starting of an electric motor is possible with these drives. Using proper control the drive can be started and accelerated at constant torque and current. The motor does not see its blocked rotor behavior. In an ac motor this is possible by simultaneously controlling the frequency and voltage of the motor using what is called slip control, in which slip frequency is kept constant. To maintain constant current during acceleration closed loop control is necessary. Speed control of the drive motor requires the simultaneous control of voltage and frequency to maintain constant flux conditions in the motor. This also requires closed loop control. During braking up to zero speed and speed re­versal from thereon, the thyristor converters are so controlled that the kinetic energy of the motor is returned to the mains during braking, and soft starting is made available for acceleration in the opposite direction. Control Techniques during braking and speed reversal also require closed loop automatic control.

Therefore modern electric drive systems employ closed loop controls and the principles of feedback control theory. They are found to be versatile and are becoming very popular. They are also becoming price competitive, as the price of thyristors is coming down. Very sophisticated drive systems are being developed with excellent dynamic and steady state response.

The performance of the closed loop drive is of primary interest. A suitable drive system using closed loop control using speed feedback must be stable. It should provide acceptable transient and steady-state response to input commands. The system must be less sensitive to parameter variations. The steady-state error, which is a measure of steady-state response and ability of the control system to follow the input, should be minimum for the inputs. The system must be able to eliminate the effects of undesirable disturbances.

Even though a feedback control system is complex and costly, one of the foremost and fundamental reasons to employ the feedback in the drive systems is its improved performance with regard to reduction of steady-state error of the system. A closed loop system has a steady-state error which is several times less than that of open loop systems. However it is impossible to realize an optimum control system having all these requirements. Some adjustments may have to be made to improve the performance. Sometimes a compromise may have to be arrived at between conflicting and demanding specifications in choosing the system parameters to provide acceptable performance.

Therefore, in order to assess the behavior of these drives, the techniques of conventional feedback control theory have to be applied. The analysis and synthesis of drive systems form a special case of conventional feedback theory. Conventional transfer function methods can be applied to determine the time domain and frequency domain behavior of the system. The stability of the drive which is a necessary but not sufficient condition may be analysed using the conventional Routh-Hurwitz and Nyquist stability criteria. Based on these methods, the design of the controllers for stabilization of the system is possible both in the time domain using root locus techniques and frequency domain using Bode plots. The ac drive systems utilizing induction and synchronous motors may be considered to be multivariate systems. These can be analysed using the methods of modern control using state space techniques to determine the drive behavior. The controllers may be designed based on these methods.

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