1.1 Block Diagram of Drive Components of Electric Drive & their functions. Power Processor / Modulator. Control. Unit

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1 Introduction Motion control is required in large number of industrial and domestic applications like transportations, rolling mills, textile machines, fans, paper machines, pumps, washing machines, robots etc. A system which has ability to provide motion control is known as drives. It may have prime mover as diesel engine or petrol engine, steam engines, gas or steam turbine, electric motors or hydraulic motors for providing mechanical energy to have the motion control. Many industries require electric motors whose speed can be varied as per requirement. Robotics required high precision for controlling the position. So, high performance position controlled drives are employed in such systems. In many applications a reversal in direction of rotation is also required in addition to speed variation. An electric drive is an electromechanical system that employs an electric motor as the prime mover instead of a diesel engine, steam or gas turbines, hydraulics, etc. to control the motion and processes of different machines and mechanisms. 1.1 Block Diagram of Drive Components of Electric Drive & their functions Power Source Power Processor / Modulator Motor Load Control Control Unit Feedback Figure Figure Modern electric drive drive system employing power electronic converters The main components of Electric Drive are (a) Source (b) Power processor or power modulator (c) Motors (d) Control unit (e) Sensing unit (f) Load (a) Source Power source can be A.C. or D.C.in nature and normally they are uncontrollable. Hardik Lakhani, EE Department Control of Electric Drives ( ) 1

2 (b) (c) The parameters of A.C. like amplitude, frequency and phase may be fixed or variable. The D.C. power source like batteries provides fixed magnitude and zero frequency supply. A.C. sources can be o 3 Ф or 1 Ф o 50 Hz or 60 Hz o 240V/415V o 220V/380V o 120V/90V o 11kV/415V, etc. 3 Ф sources are normally for high power applications. Power Processor or Power Modulator Power modulator performs many tasks and they are as follow; It controls the flow of power from source to load in such ways that speed- torque characteristic is achieved required by load. It also converts nature of supply i.e. if the supply is D.C. and induction motor is connected to mechanical load then modulator converts D.C. source into variable frequency A.C. supply. It is referred as converter. During transients like starting, braking and speed reversal, it keeps the motor current within the acceptable limits. It also helps to select the mode of operation such as motoring or braking. Motors To drive the mechanical load electrical motors are required. Basically there are two types of motors: A.C. motors and D.C. motors. A.C. motors : o Induction motors: wound rotor, squirrel-cage & linear o Synchronous motors: wound field & permanent magnet D.C. motors: shunt, series, compound & permanent magnet Apart from these, BLDC, stepper motors and SRM (Switched Reluctance Motor) are used. In past, for constant speed operation either induction motors or synchronous motors were used. To achieve variable speed operation using these machines was very hard, too expensive and with less efficiency. Therefore, to perform variable speed operation D.C. machines were dominated. But, now days due to advancement in the semiconductor technology (converters with semiconductor devices like IGBT, GTOs, Thyristors & Power transistor), A.C. machines are used for variable speed operation. Hardik Lakhani, EE Department Control of Electric Drives ( ) 2

3 (d) Sensing Unit In order to provide command to the power modulator to control the flow of power from source to load, sensing unit provides input to the control unit. Depending upon the control algorithm either motor current, voltage, speed or torque is sensed and feedback. There are many sensors available to sense the specific parameters of motor. (e) Control Unit Control unit takes two inputs (i) reference signal & (ii) feedback signal. It does certain operation on these signals i.e. comparison. Based on error generated, the control unit initiates the command to the power modulator. Basically, power processor is a converter consisting controlled semiconductor devices. Control unit produces the firing pulses as a command to achieve required speed torque characteristic by load. Control unit consisting firing circuits may be implemented using linear & digital integrated circuits, transistors and microprocessor or DSP when sophisticated control is required. The complexity of the control unit depends on the desired drive performance and the type of motors used A controller can be as simple as few op-amps and/or a few digital ICs, or it can be as complex as the combinations of several ASICs and digital signal processors (DSPs) Analog - This is noisy, inflexible. However analog circuit ideally has infinite bandwidth Digital immune to noise, the bandwidth is obviously smaller than the analog controller s depends on sampling frequency DSP/microprocessor flexible, lower bandwidth compared to above. DSPs perform faster operation than microprocessors (f) Load Load is generally mechanically designed equipment to do specific operation i.e. pumps, trains, fans, machines tools etc. 1.2 Classification of Drives Electrical drives can be classified in many ways depending on various parameters Based on supply of type of motor used, drives can be D.C. drives & A.C. drives. Based on direction of flux generated, reversible type drives & non-reversible type drives. According to design, individual drive, group drive or shaft drive & multi-motor drive Individual Drive A single motor is dedicated to a single load. Applications include electric saw, drill, disk drive, fan, washer, dryer, blender etc. Hardik Lakhani, EE Department Control of Electric Drives ( ) 3

4 1.2.2 Group Drive or Shaft Drive If several groups of mechanisms or machines are organized on one shaft and driven or actuated by one motor, the system is called a group drive or shaft drive Multi-Motor Drive Each operation of the mechanism is taken care of by a separate drive motor. For example, robot (a) (b) (c) Figure 1. 2(a) Individual drive (b) Group drive or Shaft drive (c) Multi-motor drive 1.3 Overview of A.C. & D.C. Drives Before power semiconductor devices were introduced: A.C. drives were used for fixed speed operation. D.C. drives were used for variable speed operation. a.c. d.c. 20% 40% 60% 30% 70% 25% 75% 80% Figure 1. 3 A.C. Vs. D.C. electric drives market dynamics After power semiconductor devices were introduced in 1950s: Speed control with A.C. motors can be performed because variable frequency A.C. supply can be generated using inverters. Applications limited to medium performance application fans, blowers, compressor etc. High performance application dominated by D.C. motors traction, elevator, servos etc. Hardik Lakhani, EE Department Control of Electric Drives ( ) 4

5 1.3.3 After semiconductor devices were introduced in 1980s: Vector control drives were introduces. Because of that A.C. motors were begun to use in high performance application. Though control is complex and expensive A.C. motors became more favorable than D.C. motors. Day by day inventions in microprocessor and microcontroller technology cost of these components and semiconductor decrease. 1.4 Multi-machine System for Speed Control A.C. Motor Fixed speed D.C. Generator I r Variable D.C. I a D.C. Motor Variable speed Load Figure 1. 4 conventional variable speed electrical drive system The system consists of 3 electromechanical machines o An AC motor o A DC generator o A DC motor The ac motor, which drives a dc generator, is powered by a single or multiphase ac source. The speed of the induction motor is fairly constant. The output of the dc generator is fed to the dc motor The output voltage of the dc generator is adjusted by controlling its excitation current. Adjusting the field current of the generator controls the terminal voltage of the dc motor. Hence, the speed of the dc motor is controlled accordingly. 1.5 Comparison A.C. Vs. D.C Motors: D.C. requires frequent maintenance, is very heavy, expensive, speed restricted due to mechanical construction. A.C. requires less maintenance, is cheaper, robust, high speed operation can be easily achieved (esp. squirrel-cage type) Control Unit: D.C. drives: simple control can be implemented using simple analog circuit. Therefore, it is very cheap. Hardik Lakhani, EE Department Control of Electric Drives ( ) 5

6 A.C. drives: there are two types of control (i) scalar simple & cheap (ii) vector control complex & requires fast processor/dsp. Table 1. 1 Comparison between A.C. drives and D.C. drives D.C. Drives Power circuit and control circuit is simple Frequent Maintenance. Commutator makes bulky, costly and heavy Speed and design rating are limited due to commutation. This is used in certain location Fast response and wide speed range smooth achieved by conventional and solid state control Poor PF, harmonic distortion of the current Power / weight ratio is small A.C. Drives Power and Control circuit is complex Less Maintenance. Problems are not there, particularly squirrel cage motor. Ratings have no upper limits. Used in all location In solid state control speed range is wide and conventional method it is stepped and limited. For Regenerative drives the line pf is poor, for non-regenerative drives the line PF is better. Power / weight ratio is large. 1.6 Principle Factors Affecting the Choice of Drive Rating and capital cost Speed range Efficiency Speed regulation Controllability Braking requirements Reliability Power-to-weight ratio Power factor Load factor and duty cycle Availability of supply Effect of supply variation Loading of the supply Environment Running costs Hardik Lakhani, EE Department Control of Electric Drives ( ) 6

7 1.7 Applications of Electrical Drives Pumps, fans & Compressor Plant automation Flexible manufacturing systems Steel mills Paper & pulp mills Textile mills Automotive Application Conveyors, lift, escalator, elevators Appliances & power tools Mining equipment Cement kilns Electric propulsion Robotic actuators Spindles & servos Aerospace actuator ********* Hardik Lakhani, EE Department Control of Electric Drives ( ) 7