![Industrial drives
and control
[idc]
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Basics of electric drives notes ty elect
- 2. Teaching Scheme Examination Scheme TH TU PR PAPER HRS TH PR OR TW TOTAL 03 -- 02 03 70 -- -- 25@ 125
- 3. TOPIC Marks 1 Basics of Electrical Drives 10 2 DC Drives using Converters 16 3 DC Drives using Choppers 16 4 AC Drives 16 5 Advances techniques of Motor Control 12 TOTAL 70
- 6. Objectives •At the end of this topic we will be able to Understand ➢ Electric Drives, type and Choice ➢ Parts of Drives ➢ Motor duty Class ➢ Motor power Rating ➢ Speed torque characteristic's of DC & 3 Phase IM
- 7. Electric Drives • Systems employed for motion control are called as Drives. • Drives employing electric motors are called as Electrical Drives. • An electrical drive can be defined as an electromechanical device for converting electrical energy into mechanical energy to impart motion to different machines and mechanisms for various kinds of process control. • An electrical drive is an industrial system which performs the conversion of electrical energy into mechanical energy or vice versa for running and controlling various processes.
- 8. BLOCK DIAGRAM OF AN ELECTRICAL DRIVES
- 9. • The energy transmitting shaft and the control equipment by which the motor characteristics are adjusted and their operating conditions with respect to mechanical load varied to suit practical requirements is called as electric drive. Drive system=Drive + load • Load: usually a machinery to accomplish a given task. Eg-fans, pumps, washing machine etc. • Power modulator: modulators (adjust or converter) power flow from the source to the motion • Motor: actual energy converting machine (electrical to mechanical) • Source: energy requirement for the operation the system. • Control: adjust motor and load characteristics for the optimal mode.
- 10. Power Modulator : • The power modulator regulates the output power of the source. • It controls the power from the source to the motor in such a manner that motor transmits the speed torque characteristic required by the load • They contain and control the source and motor currents with in permissible limits during the transient operations such as starting, braking, speed reversal etc. • They converts the input electrical energy into the form as required by the motors. • Adjusts the mode of operation of the motor that is motoring, braking are regenerative.
- 11. • Power modulators may be classified as, • Converters uses power devices to convert uncontrolled valued to controllable output. • Switching circuits switch mode of operation • Variable impedance
- 12. • Converters • They provide adjustable voltage/current/frequency to control speed, torque output power of the motor. • The various type of converters are, 1. AC to DC rectifiers 2. DC to DC choppers 3. AC to AC choppers 4. AC to AC –AC voltage controllers (voltage level is controlled) 5. Cyclo converter (Frequency is controlled) 6. DC to AC inverters
- 13. • Switching circuits • Switching circuits are needed to achieve any one of the following. 1. Changing motor connection to change its quadrant of operation. 2. Changing motor circuits parameters in discrete steps for automatic starting and braking control. 3. For operating motors and drives according to a predetermine sequence 4. To provide inter locking their by preventing maloperation 5. Disconnect under abnormal condition Eg: electromagnetic contactors, PLC in sequencing and inter locking operation, solid state relays etc.
- 14. • Variable impedance • Variable resisters are commonly used for AC and DC drives and also needed for dynamic braking of drives • Semiconductors switch in parallel with a fixed resistance is used where stepless variation is needed. inductors employed to limit starting current of ac motors.
- 15. Control unit • Controls for a power modulator arc provided in the control unit. • Nature of the control unit for a particular drive depends on the power modulator that is used. • When semiconductor converters are used, the control unit will consists of firing circuits, which employ linear and digital integrated circuits and transistors, and a microprocessor when sophisticated control is required. • It also generates the commands for the protection of power modulator and motor. An input command signal which adjusts the operating point of the drive, from an input to the control unit.
- 16. ADVANTAGES OF ELECTRICAL DRIVES • These drives are available in wide range torque, speed and power. • The control characteristics of these drives are flexible. According to load requirements these can be shaped to steady state and dynamic characteristics. As well as speed control, electric braking, gearing, starting many things can be accomplished. • The are adaptable to any type of operating conditions, no matter how much vigorous or rough it is. • They can operate in all the four quadrants of speed torque plane, which is not applicable for other prime movers. • They do not pollute the environment.
- 17. • They do not need refueling or preheating, they can be started instantly and can be loaded immediately. • They are powered by electrical energy which is atmosphere friendly and cheap source of power. • Cost is too low as compared to another system of the drive. • The system is more simple and clean. • The control is very easy and smooth. • Flexible in the layout. • Facility for remote control. • Transmission of power from one place to other can be done with the help of cables instead of long shafts, etc. • Its maintenance cost is quite low. • It can be started at any time without delay.
- 18. Disadvantages of electrical drives 1. The power failure ,completely disabled the whole of the system. 2. The application of the drive is limited because it cannot use in a place where the power supply is not available. 3. It can cause noise pollution. 4. The initial cost of the system is high.
- 19. Application • Electric drives are used in 1. traction systems, boats, 2. lifts, cranes, electric car, etc. 3. They have flexible control characteristics. The steady state and dynamic characteristics of electric drives can be shaped to satisfy the load requirements. 4. They are available in wide range of torque, speed, and power. 5. They can be started instantly and can immediately be fully loaded. 6. They can operate in all the four quadrants of the speed-torque plane. 7. They are adaptable to almost any operating conditions such as explosive and radioactive environments.
- 20. CHOICE OF ELECTRICAL DRIVES (i) Nature of electric supply Whether AC or DC supply is to be used for supply (ii) Nature of the drive Whether the particular motor is going to drive individual machine or a group of machines (iii)Capital and running cost (iv) Maintenance requirement (v) Space ad weight restrictions (vi) Environment and location (vii) Nature of load Whether the load requires light or heavy starting torque Whether load torque increases with speed remain constant Whether the load has heavy inertia which may require longer straight time
- 21. (viii) Electrical characteristics of motor • Starting characteristics, • running characteristics, • speed control and Braking characteristics (ix) Size, rating and duty cycle of motors • Whether the motor is going to the operator for a short time or whether it has to run continuously intermittently or on a variable load cycle (x) Mechanical considerations • Type of enclosures, type of bearings, transmission of drive and Noise level. • Due to practical difficulties, it may not possible to satisfy all the above considerations.
- 22. • There are three classification namely 1. Grope drive 2. Individual drive 3. Multi motor drive • Grope drive ➢One motor is used as a drive for two or more than machines. The motor is connected to a long shaft. All the other machines are connected to this shaft through belt and pulleys.
- 23. • Individual drive • In this drive, there will be a separate driving motor for each process equipment. • One motor is used for transmitting motion to various parts or mechanisms belonging to signal equipment. • Ex: Lathe • Multi motor drive • In this type of drive, separate motors are provided for actuating different parts of the driven mechanism. • Ex: cranes, drives used in paper mills, rolling mills etc., • In cranes, separate motors are used for hoisting, long travel motion and cross travel motion.
- 24. Classes of motor duty. • Continuous duty • Short time duty • Intermittent periodic duty • Intermittent periodic duty with starting • Intermittent periodic duty with starting and braking • Continuous duty with intermittent periodic loading • Continuous duty with starting and braking • Continuous duty with periodic speed changes
- 25. Continuous duty • This type drive is operated continuously for a duration which is long enough to reach its steady state value of temperature. • This duty is characterized by constant motor torque and constant motor loss operation. • This type of duty can be accomplished by single phase/ three phase induction motors and DC shunt motors. • Examples: 1. Paper mill drives , Compressors Conveyors, Centrifugal pumps and Fans ,
- 26. TL – Load torque in N-M, Ө- Temperature in Deg.centigrade, t- Time in seconds.
- 27. Short time duty • In this type drive operation, Time of operation is less than heating time constant and motor is allowed to cool off to room temperature before it is operated again. • Here the motor can be overloaded until the motor temperature reaches its permissible limit. • This type of duty can be accomplished by single phase/ three phase induction motors and DC shunt motors, DC series motors, universal motors. • Examples: 1. Crane drives , 2. Drives for house hold appliances 3. Turning bridges 4. Sluice gate drives 5. Valve drives and 6. Machine tool drives.
- 29. Intermittent periodic duty • In this type drive operation, It consists of a different periods of duty cycles • I.e. a period of rest and a period of running, a period of starting, a period of braking. • Both a running period is not enough to reach its steady state temperature and a rest period is not enough to cool off the machine to ambient temperature. • In this type drive operation, heating due to starting and braking is negligible. • This type of duty can be accomplished by single phase/ three phase induction motors and DC shunt motors, universal motors. • Pressing • Cutting • Drilling machine drives.
- 31. Intermittent periodic duty with starting • This is intermittent periodic duty where heat losses due to starting can't be ignored. • It consists of a starting period; a running period, a braking period & a rest period • Operating and rest period being too short to reach their steady state temperature value. • In this type of drive operation, heating due to braking is negligible. • This type of duty can be accomplished by three phase induction motors and DC series motors, DC compound motors, universal motors. • Examples: • Metal cutting, • Drilling tool drives, • Drives for forklift trucks, • Mine hoist etc.
- 33. Intermittent periodic duty with starting & braking • This is an intermittent periodic duty where heating during starting & braking can't be ignored. • It consists of a starting period, a running period; a braking period & a rest period are being too short to reach their steady state temperature value. • This type of duty can be accomplished by single phase/ three phase induction motors and DC shunt motors, DC series motors, DC compound motors, universal motors. • Examples: 1. Billet mill drive 2. Manipulator drive 3. Ingot buggy drive 4. Screw down mechanism of blooming mill 5. Several machine tool drives 6. Drives for electric suburban trains and 7. Mine hoist
- 35. SELECTION OF POWER RATING OF MOTORS • From the point of view of motor rating for various duty cycles it can be broadly classified as: • Continuous duty • Intermittent duty • Short time duty
- 36. Continuous duty • A motor with next higher power rating from commercially available rating is selected. • Motor speed should also match load’s speed requirements . • It is also necessary to check whether the motor can fulfil starting torque requirements and continue to drive load in the face of normal disturbances in power supply system • the latter is generally assured by the transient and steady state reserve torque capacity of the motor.
- 37. • If the motor has load torque of T N-m and it is running at 𝜔 radians/seconds, if efficiency in 𝜂 , then power rating of the motor is 𝑃 = 𝑇 ∗ 𝜔 1000 𝑁 − 𝑀 • Power rating is calculated and then a motor with next higher power rating from commercially available rating is selected.
- 38. Fluctuating and intermittent duty • Equivalent current, torque and power methods • This method is based on approximation that the actual variable motor current can be replaced by an equivalent Ieq, which produces same losses in the motor as actual current. • The equivalent current is determined as follows;
- 46. • Example • A constant speed drive has the following duty cycle. i) Load rising from 0 to 400 KW ; 5 min ii) Uniform load of 500 KW; 5 min iii) Regenerative power of 400 KW returned to the supply; 4 min iv) Remains idle for ; 2 min Estimate power rating of the motor. Assume losses to be proportional to (power)2