Whenever the term electric motor or electrical generator is used, we tend to think that the speed of rotation of these machines is totally controlled only by the applied voltage and frequency of the source current. But the speed of rotation of an electrical machine can be controlled precisely also by implementing the concept of drive. The main advantage of this 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 a electric motor (may be several) and a sophisticated control system that controls the rotation of the motor shaft. Now days, 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.
This drive system is widely used in large number of industrial and domestic applications like factories, transportation systems, textile mills, fans, pumps, motors, robots etc. Drives are employed as prime movers for diesel or petrol engines, gas or steam turbines, hydraulic motors and electric motors. The very basic block diagram an electric drives is shown below. The load in the figure represents various types of equipment’s which consists of electric motor, like fans, pumps, washing machines etc.
Types of Electrical Drives:
- Reversible types drives
- Non reversible types drives.
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.
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 modulator, 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 modulator, source and control unit. 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. 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. 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 mechanically stronger and require 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 a days used as an effcient alternative to DC servo motors because they don’t have the disadvantages like commentation problem. Beside of these, stepper motors are used for position control and switched reluctance motors are used for speed control.
Power Modulators – are the devices which alter the nature or frequency as well as changes the intensity of power to control electrical drives. Roughly, power modulators can be classified into three types,
- Variable impedance circuits,
- Switching circuits.
As the name suggests, converters are used to convert currents from one type to other type. Depending on the type of function, converters can be divided into 5 types –
- AC to DC converters
- AC regulators
- Choppers or DC – DC converters