Download to read offline
Unit 5_INDUCTION MOTORS-STEPPER MOTORS.pptx
- 1. 1 UNIT 5- INDUCTIONMOTORS AND SPECIAL MACHINES Three Phase and Single-Phase Induction Motor-Construction and Working Principle- Special Machines: Stepper motor- Permanent Stepper Motor – Variable Reluctance Stepper Motor – Servomotor-AC Servomotor-DC Servomotor–Stepper Motor Selection and Control: An Industrial Case Study.
- 2. 2 Three Phase InductionMotor • A 3-phase induction motor is an electromechanical energy conversion device which converts 3-phase input electrical power into output mechanical power. • The three phase induction motor is one of the A.C. motors, which is widely used for various purposes in industry. These motors never run at a Synchronous speed but a little less than the synchronous speed. The speed of these motors depends upon the supply frequency. • Therefore, these motors are not generally used for speed control. However, we prefer D.C. motors where large variations of speed are required. These motors are preferred in industry because they have low price, simple & rugged construction, can be manufactured with characteristics to suit the industrial requirement. • These motors differ from other types of motor, in that there is no electrical connection between the rotor & supply. The required voltage & current are induced by induction from the stator winding that is why, the name given is induction motor.
- 3. 3 Construction of ThreePhase Induction Motor • Stationary part, known as Stator • Rotating part, known as Rotor.
- 4. 4 • 1. Stator •It is the stationary part of the motor. It has three main parts : • Frame or Yoke • It is the outer part of the three phase induction motor. Its main function of the frame is to support the stator core & stator winding. It acts as a covering, and it provides protection & mechanical strength to all the inner parts of the three phase induction motor. • Stator core • The main function of stator core is to carry the alternating flux. In order to reduce the eddy current loss, the stator core is laminated. The core is made up of thin silicon steel laminations. These are insulated from each other by varnish, the slots are cut on inner periphery of core stampings. The stator windings are placed in these slots. • Stator windings • Stator winding is made up of super enamelled copper wire. Three phase windings are placed in the stator core slots & six terminals are brought out. They may be star connected or may be delta connected. The windings are connected in star at starting.
- 5. 5 • 2. Rotor •It is a rotating part of the motor. It is mounted on the shaft. It consists of hollow laminated core having slots on its outer periphery. The windings placed in these slots (rotor winding) may be one of the following two types : • Squirrel cage rotor • Slip ring rotor or wound rotor or phase wound rotor.
- 6. 6 • 1. Squirrelcage rotor • The rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors. • The squirrel cage rotor consists of a aluminium, brass or copper bars. These aluminium, brass or copper bars are called rotor conductors & are placed in the slots on the periphery of the rotor • The rotor conductors are permanently shorted by the copper, or aluminum rings called the end rings. • To provide mechanical strength, these rotor conductors are braced to the end ring & hence form a complete closed circuit resembling like a cage & hence got its name as squirrel cage induction motor.
- 7. 7 2. Slip ringrotor or wound rotor or phase wound rotor • The wound rotor consists a slotted armature. Insulated conductors are put in the slots & connected to form a three phase double layer distributed winding similar to the stator winding. The rotor windings are connected in star. • The open end of the start circuit are brought outside the rotor and connected to the insulated slip rings. The slip rings are mounted on the shaft with brushes testing on them. The brushes are connected to three phase variable resistors connected in star. The purpose of slip rings & brushes is to provide a means for connecting external resistors in the circuit.
- 8. 8 Slip ring rotoror wound rotor or phase wound rotor
- 9. 9 Slip ring rotorSquirrel cage rotor Rotor consists of three phase winding Rotor consists of copper bar Construction is complicated Construction is simple Resistance can be added externally Resistance cannot be added Slip ring and brushes are present No slip ring and brushes Rotors are costly Rotors are cheap High starting torque Moderate starting torque Speed control possible by adding external rheostat Speed control is not possible by adding external rheostat since the rotors are short circuited Rotor copper loss is high, therefore less efficiency Rotor copper loss is less, so high efficiency Used in lifts, cranes, elevators, compressors Used in lathes, drilling machine, fans, blowers, water pumps, grinders and printing machines
- 10. 10 Principle of Operationof 3-Phase Induction Motor • When the 3-phase stator winding is fed from a balanced 3-phase supply, a rotating magnetic field (RMF) is produced in the motor. This RMF rotates around the stator at synchronous speed which is given by,
- 11. 11 • The RMFpasses through the air gap and cuts the rotor conductors, which as yet are stationary. Due to the relative motion between the RMF and the stationary rotor conductors, EMFs are induced in the rotor conductors. As the rotor circuit is closed with short-circuit so currents start flowing in the rotor conductors. • Since the current carrying rotor conductors are placed in the magnetic field produced by the stator winding. As a result, the rotor conductors experience mechanical force. The sum of the mechanical forces on all the rotor conductors produce a torque which moves the rotor in the same direction as the rotating magnetic field. Hence, in such a way the three phase input electric power is converted into output mechanical power in a 3-phase induction motor.
- 12. 12 • Also, accordingto Lenz’s law, the rotor should move in the direction of the stator field, i.e., the direction of rotor currents would be such that they tend to oppose the cause producing them. Here, the cause producing the rotor currents is the relative speed between the RMF and the rotor conductors. Thus to reduce this relative speed, the rotor starts running in the same direction as that of the RMF.
- 13. 13 Advantages of ThreePhase Induction Motor • It has simple and rugged construction. • It requires less maintenance. • It has high efficiency and good power factor. • It is less expensive. • It has self-starting torque.
- 14. 14 Disadvantages of ThreePhase Induction Motor • The 3-phase induction motors are constant speed motors; hence their speed control is very difficult. • 3-phase induction motors have poor starting torque and high inrush currents (about 4 to 8 times of the rated current). • They always operate under lagging power factor and during light loads, they operate at very worst power factor (about 0.3 to 0.5 lagging).
- 15. Single Phase InductionMotor • Stator carries single phase winding and is separated from the rotor by small air gap. • Rotor is squirrel cage type. • Stator winding carries alternating current and produces alternating flux. But this alternating flux cannot produce rotation in the rotor. Hence single phase IM is not self starting
- 16. Making single phaseIM self starting • It is temporarily converted into two phase motor by having an extra winding on the stator • By 2 phase motor action, revolving magnetic field is produced in the stator. • This revolving magnetic field can cause the rotation of the rotor. Hence motor is made self starting.
- 17. 17 Types of singlephase induction motor • Split Phase Induction Motor • Shaded Pole Induction Motor • Capacitor Start Induction Motor • Capacitor Start Capacitor Run Induction Motor • Permanent Capacitor Induction Motor
- 18. 18 Split Phase InductionMotor • In this type of motor, an extra winding is wounded on the same core of the stator. So, there are two windings in the stator. • One winding is known as the main winding or running winding and second winding is known as starting winding or auxiliary winding. A centrifugal switch is connected in series with the auxiliary winding. • The auxiliary winding is highly resistive winding and the main winding is highly inductive winding. The auxiliary winding has few turns with a small diameter. • The aim of auxiliary winding is to create a phase difference between both fluxes produced by the main winding and rotor winding.
- 19. 19 • The currentflowing through the main winding is IM and current flowing through the auxiliary winding is IA. Both windings are parallel and supplied by voltage V. • The auxiliary winding is highly resistive in nature. So, the current IA is almost in phase with supply voltage V. • The main winding is highly inductive in nature. So, the current IM lags behind the supply voltage with a large angle. • The total stator flux is induced by the resultant current of these two winding. As shown in the phasor diagram, the resultant current is represented as (I). It will create a phase difference between fluxes and resultant flux produces a rotating magnetic field. And the motor starts rotating. • Auxiliary winding only uses to start the motor. This winding is not useful in running condition. When the motor reaches 75 to 80 % of synchronous speed, the centrifugal switch opens. So, the auxiliary winding is out from the circuit. And motor runs on only main winding. • The phase difference creates by this method is very small. Hence, the starting torque of this motor is poor. So, this motor is used in low starting torque applications like a fan, blower, grinder, pumps, etc.
- 20. 20 Shaded Pole InductionMotor As compared to other types of single-phase induction motor, this motor has a different construction and working principle. This type of motor does not require auxiliary winding. This motor has stator salient pole or projecting pole and the rotor is the same as squirrel cage induction motor. The stator poles are constructed specially to create a rotating magnetic field. A pole of this motor is divided into two parts; shaded part and un-shaded part. It can be created by cutting pole into unequal distances. A copper ring is placed in the small part of the pole. This ring is a highly inductive ring and it is known as a shaded ring or shaded band. The part at which shaded ring is paced is known as shaded part of the pole and the remaining part is an unshaded part.
- 21. 21 • When analternating supply passing through the stator winding, an alternating flux induced in the stator coil. Due to this flux, some amount of flux will link with shaded ring and current will flow through a shaded ring. • According to Len’z law, the current passing through coil is opposite in nature, and flux produced due to this coil will oppose the main flux. • The shaded ring is a highly inductive coil. So, it will oppose the main flux when both fluxes are in the same direction and it will increase the main flux when both fluxes are in the opposite direction. • So, it will create a phase difference between the main flux (stator flux) and rotor flux. By this method, a phase difference is very less. Hence, the starting torque is very less. It is used in applications like toy motor, fan, blower, record player, etc.
- 22. 22 Capacitor Start InductionMotor • This type of motor is an advanced version of the Split phase induction motor. The disadvantage of split-phase induction is low torque production. Because in this motor, the phase difference created is very less. • This disadvantage compensates in this motor with the help of a capacitor connected in series with auxiliary winding. The circuit diagram of this motor is as shown in the below figure.
- 23. 23 • The capacitorused in this motor is a dry-type capacitor. This is designed to use with alternating current. But this capacitor is not used for continuous operation. • In this method also, a centrifugal switch is used which disconnects the capacitor and auxiliary winding when the motor runs 75-80% of synchronous speed. • The current through auxiliary will lead the supply voltage by some angle. This angle is more than the angle increased in a split-phase induction motor. • So, the starting torque of this motor is very high compared to the split-phase induction motor. The starting torque of this motor is 300% more than the full load torque. • Due to high starting torque, this motor is used in the applications where high starting torque is required like, a Lath machine, compressor, drilling machines, etc.
- 24. 24 Capacitor Start CapacitorRun Induction Motor • In this type of motor, two capacitors are connected in parallel with series in auxiliary winding. Out of these two capacitors, one capacitor is used only for starting (starting capacitor) and another capacitor is connected permanently with the motor (running capacitor).
- 25. 25 • The startingcapacitor has high capacitance value and a running capacitor has low capacitance value. The starting capacitor is connected in series with a centrifugal switch that will open when the speed of the motor is 70% of synchronous speed. • During running conditions, both running winding and auxiliary winding connected with motor. The starting torque and efficiency of this motor are very high. • Therefore, this can be used in the application where high starting torque is required like a refrigerator, air conditioner, ceiling fan, compressor, etc.
- 26. 26 Permanent Capacitor InductionMotor • The low-value capacitor is connected constantly with the auxiliary winding. Here, the capacitor has low capacitance. • The capacitor is used to increase the starting torque but it is low compared to the capacitor start induction motor. • The power factor and efficiency of this motor are very high and also it has a high starting torque that is 80% of full load torque. • This type of motor is used in the application like an exhaust fan, blower, heater, etc.
- 27. 27 Applications of SinglePhase Induction Motors • Single phase motors are not self starting and less efficient than three phase induction motor and available in 0.5HP to 15HP and still they are widely used for multiple purposes such as: • Clocks • Refrigerators, freezers and heaters • Fans, table fans, ceiling fan, exhaust fans, air coolers and water coolers. • Blowers • Washing machines • machine tools • Dryers • Type writers, photostats and printers • Water pumps and submersible • Computers • Grinders • Drilling machines • Other Home instrument, equipment and devices etc.
- 28.
- 29. Application of Capstart Cap run IM • These motors have high starting torque hence they are used in • conveyors, grinder, air conditioners, compressor, etc. • They are available up to 6 KW.
- 30.
- 31. Applications of ShadedPole Motor • Due to their low starting torques and reasonable cost these motors are mostly employed in small instruments, hair dryers, toys, record players, small fans, electric clocks etc.
- 32.
- 33. 33 • A steppermotor is an electromechanical device which converts electrical pulses into discrete mechanical movements. • Stepper motor rotates in discrete step increments when electrical command pulses are applied to it. • The sequence of the applied pulses is directly related to the direction of motor shafts rotation. • The angle through which the stepper motor shaft rotates for application of each command pulse is called step angle (β). • Where Ns= No.of stator teeth or pole; Nr= No. of rotor teeth or pole; m=No. of phases. The opposite poles of stator windings are connected in series considered to be one phase. • The No. of steps required to complete one revolution is resolution (Z) Z= No. of steps/ one revolution= 360 degree/ β STEPPER MOTOR
- 34.
- 35. 35 Variable Reluctance steppermotor • It is constructed from ferromagnetic material with salient pole. • Stator is made from stack of steel lamination and equally spaced six inward projecting pole each wounded with exciting coil. • Rotor also made from stack of steel lamination and equally spaced four outward projecting pole. • In stator opposite poles are connected in series considered as one phase. Here it is three phase (A,B,C) or 6/4 VR stepper motor. • Each phase is excited separately through a switch by the DC source. • The direction of rotation of the stepper Motor depends upon the sequence we excite the windings. • Generally, number of stator poles will not be equal to number of rotor poles for proper operation.
- 36. 36 Variable Reluctance StepperMotor • The principle of Variable Reluctance Stepper Motor is based on the property of the flux lines which capture the low reluctance path. • The stator and the rotor of the motor are aligned in such a way that the magnetic reluctance is minimum.
- 37.
- 38. 38 • The operationis based on various reluctance positions of rotor with respect to stator. • When any one phase of the stator is excited, it produces its magnetic field whose axis lies along the poles of the phase which is excited. • Then the rotor moves to minimum reluctance position. • Let us see the operation of the VR stepper Motor ,when the phases A,B,C are excited in sequence i.e., one after the other with the help of switches S1,S2 and S3. • When the phase AA1 is excited with the switch S1 closed, then stator Magnet Axis exist along the poles formed due to AA1 (i.e) vertically. • Then the rotor adjust itself in a minimum reluctance position and occupies along AA1 1, 3, rotor poles. This is the initial position (ie) 0°. • When the BB1 phase is excited with the switch S2 closed, and de- energise the AA1 phase with the switch S1 opened, turns through 30° step angle in clockwise direction.
- 39. 39 • =360/MxNrdegree • Where • M = the number of stator phase • Nr= the number of rotor phase • In the present case M=3, Nr=4 • =360/3x4 degree = 30 degree
- 40. 40 Variable reluctance steppermotor(full step operation)
- 41. 41 Half step operation(A, AB,B,BC,C,CA)
- 42. 42 Summarisation of variablereluctance stepper motor
- 43.
- 44. 44 Step angle calculationfor permanent magnet stepper motor • =360/MxNr degree • Where • M = the number of stator phase • Nr= the number of rotor phase • In the present case M=2, Nr=2 • =360/2x2 degree = 90 degree
- 45.
- 46.
- 47.
- 48. 48 Hybrid Stepper Motor •Step angle = (Nr-Ns)*360 / (Nr*Ns) • Nr- no.of rotor teeth • Ns- no.of stator teeth • Nr=5 • Ns=4 • Step angle =(5-4) * 360 / (5*4) = 18o
- 49. 49 Applications of StepperMotor • As the stepper motor is digitally controlled using an input pulse, they are suitable for use with computer-controlled systems. • They are used in numeric control of machine tools.(CNC machine) • Used in tape drives, floppy disc drives, printers, and electric watches. • The stepper motor also uses in X-Y plotter and robotics. • It has wide application in textile industries and integrated circuit fabrications. • The other applications of the Stepper Motor are in spacecraft launched for scientific explorations of the planets etc. • These motors also find a variety of commercial, medical, and military applications and are also used in the production of science fiction movies. • Stepper motors of microwatts are used in wristwatches.
- 50.
- 51. 51 Servo Motor • Aservomotor is a linear actuator or rotary actuator that allows for precise control of linear or angular position, acceleration, and velocity. • A servomotor (or servo motor) is a simple electric motor, controlled with the help of servomechanism. • It consists of a motor coupled to a sensor for position feedback. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servomotors. • If the motor as a controlled device, associated with servomechanism is DC motor, then it is commonly known as a DC Servo Motor. If AC operates the controlled motor, it is known as a AC Servo Motor.
- 52.
- 53.
- 54. 54 • There aresome special types of applications of an electric motor where the rotation of the motor is required for just a certain angle. • For these applications, we require some special types of motor with some special arrangement which makes the motor rotate a certain angle for a given electrical input (signal). • The servo motor is usually a simple DC motor controlled for specific angular rotation with the help of additional servomechanism. • The main reason behind using a servo is that it provides angular precision, i.e. it will only rotate as much we want and then stop and wait for the next signal to take further action.
- 55.
- 56. 56 DC Servo Motor •The motors which are utilized as DC servo motors, generally have separate DC source for field winding and armature winding. • The control can be achieved either by controlling the field current or armature current.
- 57. 57 Field Controlled DCServo Motor • In this arrangement the field of DC motor is excited be the amplified error signal and armature winding is energized by a constant current source.
- 58. 58 Armature Controlled DCServo Motor • Here the armature is energized by amplified error signal and field is excited by a constant current source.
- 59.
- 60. 60 AC Servomotor • Thecontrol phase is usually supplied from a servo amplifier. • The speed and torque of the rotor are controlled by the phase difference between the control voltage and the reference phase voltage. • By reversing the phase difference from leading to lagging or vice versa, the direction of the rotation of the rotor can be reversed.
- 61. 61 Application of servomotor • Remote-controlled toy cars for controlling the direction of motion • Widely used as the motor which moves the tray of a CD or DVD player. • Solar Tracking System • Servo motors control industrial spinning and weaving machines • Printing Presses/Printers • Automatic Door Openers • Robotic Vehicle
- 62.