1. Definition
1.1. 1. Drives • Systems employed for motion control. • may employ any prime movers such as diesel or petrol engines, gas or steam turbines, hydraulic motors for supplying mechanical energy for motion control
1.2. 2. Electrical Drives • Drives employing electric motors • Motion control is required in a large number of industrial and domestic applications like the transportation system, rolling mills, paper mills, machine tools, etc.
1.2.1. • used to draw electrical energy from the mains and supply the electrical energy to the motor at whatever voltage, current and frequency necessary to achieved the desired mechanical output
2. Intro
2.1. 1. Study of electric drive system involves controlling electric motor in steady state and dynamic operation, taking into account the characteristic of mechanical loads and behaviours of power electronic converter
2.2. 2. Steady-State: A condition of a physical system or device that does not change over time, or in which anyone change is continually balanced by another, such as the stable condition of a system in equilibrium.
2.3. 3. Dynamic Condition: Occur in electric drive systems when the operating point changes from one steady state condition to another, following a change introduced in system variables
3. Factor affecting Drive Selection
3.1. • Steady-state operation requirements: Nature of speed-torque characteristics, speed regulation, speed range, efficiency, duty cycle, quadrants of operation, speed fluctuations if any, ratings. • Transient operation requirements: Values of acceleration and deceleration, starting, braking, and reversing performance. • Requirements are related to the source: Type of source, and its capacity, magnitude of voltage, voltage fluctuations, power factor, harmonics and their effect on other loads, ability to accept regenerated power. • Capital and running cost, maintenance needs, life. • Space and weight restrictions if any. • Environment and location. • Reliability.
4. Factor to Adjust Speed Drives
4.1. 1. Stable operation : The torque speed characteristic of the motor should be such that stable operation of the drive is assured in all the four quadrants over a wide range of speeds. The drive must be controllable with regenerative braking to maintain constant speed if the load overhauls the motor
4.2. 2. Have a good transient response : Drive should return to its original operating condition very quickly in case of disturbance. If there is a step change in torque or speed the drive must attain its new operating point quickly without any overshoots.
5. Basic Components
5.1. 1. Power source • Provides the energy that the drives system needs. • 2 major types are AC and DC • Sources can be either single-phase or three-phase systems • 3- phase 50 Hz supply is used for normal industrial and commercial electrical drive applications
5.2. 2. Converters • Main function is to transform the waveform of a power source to that required by an electric motor in order to achieve the desired performance. • Provide adjustable voltage, current, and/or frequency to control the speed, torque, or power of the motor. • 4 basic types of converters: 1. dc to ac 2. dc to dc 3. ac to dc 4. ac to ac
5.3. 3. Controllers • Basic function is to monitor system variables, compare them with some desired values and then readjust the converter output until the system achieves the desired performance.
5.4. 4. Motors • Electric motors should be selected to match the intended performance of the load. • Types of motors: 1) DC – separately excited, shunt, series, compound. 2) AC – Induction and synchronous • Fed from DC and AC supply.
5.5. 5. Mechanical Load • Exhibit wide variations of speed-torque characteristics. Load torques are generally speed dependent.
5.5.1. • Load characteristics can be grouped into one or more of the following types: 1)Torque independent of speed • The power that the load consumes is linearly dependent on speed. • Hoist or the pumping of water or gas against constant pressure 2)Torque linearly dependent on the speed • Mechanical power is proportional to the square of speed. • Motor driving a dc generator connected to a fixed-resistance load, and the field of the generator is constant 3)Torque proportional to the square of the speed • Load power requirement is proportional to the cube of the speed and may be excessive at high speed. • Fans, centrifugal pumps, and propellers. 4)Torque inversely proportional to the speed • Usually requires a large torque at starting and at low speed. • Milling and boring machine. • This is why the motor of an electric saw does not always get damaged when the saw disk is blocked