A science of Electric Drive.docx

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  Chapter Four A science of Electric Drive 1.   Electric Drive  in the frame of other sciences Electric drive : the speeds of rotation of these machines can be controlled by: -   The applied voltage -   Frequency of the source current. -   Implementing the concept of drive . The main advantage of third concept is, the motion control is easily optimized with the help of drive. In very simple words, the systems which control the motion of the electrical machines are known as electrical drives . A typical drive system is assembled with electric motor (may be several) and a sophisticated control system that controls the rotation of the motor shaft. Now a day, this control can be done easily with the help of software. So, the controlling becomes more and more accurate and this concept of drive also provides the ease of use.  Functional diagram of Electric drives    2.   Parts of Electrical Drives The diagram which shows the basic circuit design and components of a drive also shows that, drives have some fixed  parts such as, load, motor, Power Converter, control unit and source. These equipments are termed as parts of drive system.  Now, loads can be of various types i.e. they can have specific requirements and multiple conditions, which are discussed later, first of all we will discuss about the other four parts of electrical drives  i.e. motor, power Converters, sources and  control units . Electric motors are of various types. The dc motors can be divided in four types  –   shunt wound dc motor, series wound dc motor, compound wound dc motor and permanent magnet dc motor. And AC motors are of two types; induction motors and synchronous motors. Now synchronous motors are of two types; round field and permanent magnet. Induction motors are also of two types; squirrel cage and wound motor. Besides all of these, stepper motors and switched reluctance motors are also considered as the parts of drive system.   So, there are various types of electric motors, and they are used according to their specifications and uses. When the electrical drives were not so popular, induction and synchronous motors were usually implemented only where fixed or constant speed was the only requirement. And for variable speed drive applications, dc motors were used. But as we know that, induction motors of same rating as a dc motors have various advantages like they have lighter weight, lower cost, lower volume and there is less restriction on maximum voltage, speed and power ratings. For these reasons, the induction motors are rapidly replaced the dc motors. Moreover induction motors are mechanical stronger and requires less maintenance. When synchronous motors are considered, wound field and permanent magnet synchronous motors have higher full load efficiency and power factor than induction motors, but the size and cost of synchronous motors are higher than induction motors for the same rating. Brush-less dc motors are similar to permanent magnet synchronous motors. They are used for servo applications and now days used as an efficient alternative to dc servo motors because they don’t have the disadvantages like commutation problem. Beside of these, stepper motors are used for position control and switched reluctance motors are used for speed control. Power Converters -  are the devices which alter the nature or frequency as well as changes the intensity of power to control electrical drives. Roughly, power converters can be classified into three types, a) Variable Impedances  are used to controlling speed by varying the resistance or impedance of the circuit. But these controlling methods are used in low cost dc and ac drives. There can be two or more steps which can be controlled manually or automatically with the help of contactors. To limit the starting current inductors are used in ac motors.  b) Switching circuits  in motors and electrical drives are used for running the motor smoothly and they also protect the machine during faults. These circuits are used for changing the quadrant of operations during the running condition of a motor. And these circuits are implemented to operate the motor and drives according to predetermined sequence, to  provide interlocking, to disconnect the motor from the main circuit during any abnormal condition or faults. c) Power Electronic Converters, As the name suggests, PE converters are used to convert currents from one type to other type. Depending on the type of function, PE converters can be divided into 4 types. I.   AC to DC converters II.   Choppers or DC-DC converters III.   Inverters IV.   AC-AC converters a.   Cyclo-converters  b.   AC regulators    AC to DC converters  are used to obtain fixed dc supply from the ac supply of fixed voltage. Choppers or dc-dc converters   are used to get a variable DC voltage. Power transistors, IGBT’s, GPO’s, p ower MOSFET’s are mainly used for this purpose. Inverters  are used to get ac from dc; the operation is just opposite to that of ac to dc converters. PWM semiconductors are used to invert the current. Ac-Ac converters   a. Cycloconverter s are used to convert the fixed frequency and fixed voltage ac into variable frequency and variable voltage ac. Thyristors are used in these converters to control the firing signals. b. Ac Regulators  are used to obtain the regulated ac voltage, mainly auto transformers or tap changer transformers are used in these regulators. Sources may be of 1 phase and 3 phases. 50 Hz ac supply is the most common type of electricity supplied in India,  both for domestic and commercial purpose. Synchronous motors which are fed 50 Hz supply have maximum speed up to 3000 rpm, and for getting higher speeds higher frequency supply is needed. Motors of low and medium powers are fed from 400V supply, and higher ratings like 3.3 kv, 6.6 kv, 11 kv etc are provided also. Control unit  –    choice of control unit depends upon the type of power Converter that is used. These are of many types, like when semiconductor converters are used, and then the control unit consists of firing circuits, which employ linear devices and microprocessors. So, the above discussion provides us a simple concept about the several parts of electrical drive. 3.   Classification of Electrical Drives The classification of electrical drives  can be done depending upon the various components of the drive system. Now according to the design, the drives can be classified into three types such as single-motor drive, group motor drive and multi motor drive. The single motor types are the very basic type of drive which are mainly used in simple metal working, house hold appliances etc. Group electric drives are used in modern industries because of various complexities. Multi motor drives are used in heavy industries or where multiple motoring units are required such as railway transport. If we divide from another point of view, these drives are of two types: Reversible types  and Non-reversible   types . This depends mainly on the capability of the drive system to alter the direction of the flux generated. So, several classification of drive is discussed above. 4.   Dynamics of electrical drives When a electric motor rotates, it is usually connected to load which has a rotational or translational motion. The speed of the motor may be different from that of the load. To analyze the relation among the drives and loads, the concept of dynamics of electrical drives  is introduced. We can describe the dynamics of electrical drive easily by the following instant.
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