UNIT 1 Electric Drives 1.1 INTRODUCTION Motor control is required in large number of industrial and domestic applications such as transportation systems, rolling mills, paper machines, textile mills, machine tools, fans, pumps, robots, and washing machines. Systems employed for motion control are calleddrives and may employ any of the prime movers. Drives employing electric motors are known as electric drives. Nowadays, in electric power stations generating large amounts of electric energy for agriculture, industry, domestic needs, and electrified traction facilities and in driving all kinds of working machines, electric motor is essential, which is the predominant type of drive so the term electric drive being applied to it. Electric drive becomes more popular because of its simplicity, reliability, cleanliness, easiness, and smooth control. Both AC and DC motors are used as electric drives; however, the AC system is preferred because: o It is cheaper. o It can be easily transmitted with low-line losses. o It can be easy to maintain the voltage at consumer premises within prescribed limits. o It is possible to increase or decrease the voltage without appreciable loss of power. In spite of the advantages of AC motor, sometimes DC motor is used because: o In some processes, such as electrochemical and battery charging, DC is the only type of power that is suitable. o The speed control of DC motors is easy rather than AC; thus, for variable speed applications such as lift and Ward Leonard system, the DC motors are preferred. o DC series motor is suited for traction work because of high starting torque. 1.2 BLOCK DIAGRAM OF ELECTRIC DRIVE Source 1-φ and 3-φ, 50-Hz AC supplies are readily available in most locations. Very low power drives are generally fed from 1-φ source; however, the high power drives are powered from 3-φsource; some of the drives are powered from a battery
Fig. Block diagram of electric drive Ex: Fork lifts trucks and milk vans. Power modulator Power modulator performs the following functions: o It modulates flow of power from the source to the motor is impart speed−torque characteristics required by the load. o It regulates source and motor currents within permissible values, such as starting, braking, and speed reversal conditions. o Selects the mode of operation of motor, i.e., motoring or braking. o Converts source energy in the form suitable to the motor. Electrical motors Motors commonly used in electric drives are DC motors, induction motors, synchronous motors, blushless DC motors, stepper motors, and switched reluctance motors, etc. In olden days, induction and synchronous motors were employed mainly for constant speed drives but not for variable speed drives, because of poor efficiency and are too expensive. But in nowadays, AC motors employed in variable speed drives due to the development of semiconductors employing SCRs, power transistors, IGBTs, and GTOs.
Load It is usually a machinery, such as fans, pumps, robots, and washing machines, designed to perform a given task, usually load requirements, can be specified in terms of speed and torque demands. Control unit Control unit controls the function of power modulator. The nature of control unit for a particular drive depends on the type of power modulator used. When semiconductor converters are used, the control unit will consists of firing circuits. Microprocessors also used when sophisticated control is required. Sensing unit Sensing unit consists of speed sensor or current sensor. The sensing of speed is required for the implementation of closed loop speed control schemes. Speed is usually sensed using tachometers coupled to the motor shaft. Current sensing is required for the implementation of current limit control. Advantages of electric drives There are a number of inherent advantages that the electric drive possesses over the other forms of conventional drives are: o They have comparatively long life than the mechanical drive. o It is cleaner, as there are no flue gases, etc. o It is more economical. o They have flexible control characteristics. o There is no need to store fuel or transportation. o It requires less maintenance. o Do not pollute environment. o It is the reliable source of drive. o The electrical energy can be easily transmitted by using transmission lines over long distances. o Available in wide range of torque, speed, and power. o High efficiency. o Electric braking system is much superior and economical. o Smooth speed control is easy. o They can be started instantly and can immediately be fully loaded. o They can operate in all the quadrants of speed torque plane. o Being compactness, they require less space. o They can be controlled remotely.
Disadvantages of electric drives The two inherit disadvantages of the electric drive system are: o The non-availability of drive on the failure of electrical power supply. o It cannot be employed in distant places where electric power supply is not available. TYPES OF ELECTRIC DRIVES Depending on the type of equipment used to ran the electric motors in industrial purpose, they may be classified into three types. They are: 1. Group drives. 2. Individual drives. 3. Multi-motor drives. Group drives Electric drive that is used to drive one or more than two machines from line shaft through belts and pulleys is known as group drive. It is also sometimes called the line shaft drive.This drive is economical in the consideration of the cost of motor and control gear. A single motor of large capacity cost is less than the total cost of a number of small motors of the same total capacity. In switch over from non-electric drive to electric drive, the simplest way is to replace the engine by means of motor and retaining the rest of power transmission system. Advantages o The cost of installation is less. For example, if the power requirement of each machine is 10 HP and there are five machines in the group, then the cost of five motors will be more than one 50-HP motor. o If it is operated at rated load, the efficiency and power factor of large group drive motor will be high. o The maintenance cost of single large capacity motor is less than number of small capacity motors. o It is used for the processes where the stoppage of one operation necessitates the stoppages of sequence of operations as incase of textile mills. o It has overload capacity. Disadvantage Even though group drive has above advantages, it suffers from the following disadvantages. o If there is any fault in the main motor, all the machines connected to the motor will fail to operate; thereby, paralyzing a part of industry until the fault is removed. o It is not possible to install any machine at a distant place. o The possibility of the installation of additional machines in an existing industry is limited.