Speed Control Of Synchronous Motor - Synchronous Reluctance Motor
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The document discusses the speed control of synchronous motor drives, explaining the principles of synchronous motors and their operational modes. It highlights the use of variable frequency sources for speed control, the types of synchronous motors, and various starting methods, as well as the advantages and disadvantages of different operational modes. The document also covers the specifics of permanent magnet synchronous motors, including their construction, types, and control mechanisms.
Speed Control Of Synchronous Motor - Synchronous Reluctance Motor
- 1. Speed control of synchronous motor drives drives 1
- 2. Synchronous motor - A synchronous motor is constructionally same an alternator - It runs at synchronous speed or it remains stand still - Speed can be varied by varying supply frequency because synchronous speed, Ns = (120f/p) - Due to unavailability of economical variable frequency sources, this method of speed control was not used in past & they were mainly used for constant speed applications - The development of semiconductor variable frequency sources such as inverter & cycloconverter allowed the use of synchronous motor in variable speed applications - It is not self starting. It has to be run upto near synchronous speed by some means & it can be synchronised to supply - Starting methods : a) using an auxiliary motor b) using damper windings 2
- 3. Types of synchronous motors - Commonly used synchronous motors are 1. Wound field synchronous motor (Cylindrical & salient pole) 2. Permanent magnet synchronous motor 3. Synchronous reluctance motor 4. Hysteresis motor - All these motors have a stator with 3 phase winding which is - All these motors have a stator with 3 phase winding which is connected to an AC source - Fractional horse power synchronous reluctance & hysteresis motors employ a 1 phase stator Operation of a wound field synchronous motor - Rotor is provided with a DC field winding & damper windings - Stator contain 3 phase winding & is connected to AC supply - Stator contain 3 phase winding & is connected to AC supply - Stator produce the same number of poles as DC field produces - When a 3 phase supply is given to stator, a rotating magnetic field revolving at synchronous speed is produced - The DC excitation in rotor produces a field - This field interacts with rotating magnetic field to produce a torque which is pulsating in nature & not unidirectional 3
- 4. - As a result synchronous motor is not self starting - Normally the motor is made self starting by providing damper windings on rotor - Due to the presence of damper windings, motor will start as an induction motor - When speed of motor reaches near synchronous speed, DC excitation - When speed of motor reaches near synchronous speed, DC excitation is given to rotor - Now the rotor poles gets locked with rotating magnetic field poles in stator & continue to rotate at synchronous speed Load angle/power angle/torque angle (δ) - The rotor poles are locked with stator poles & both run at synchronous speed in same direction - As load on motor increases, the rotor tends to fall back in phase by some angle - This angle is known as load angle (δ) - The value of δ depends upon the load 4
- 5. Pull out torque The power produced by synchronous motor, Where, V = stator supply voltage E = Field excitation voltage Torque, δ Sin X VE P s m 3 = δ Sin VE P T m 3 = = Torque, For a given value of supply voltage, frequency & field excitation, the torque will be maximum when δ = 90 i.e, - The maximum torque is known as pull out torque δ ω ω Sin X T s s s = = s s X VE T ω 3 max = - The maximum torque is known as pull out torque - Any increase in torque beyond this value will cause the motor to slow down & the synchronism is lost - This phenomenon is called pulling out of step 5
- 6. Variable frequency control of Synchronous motor - Synchronous speed α frequency - So by varying frequency, speed can be controlled - Like in induction motor, upto base speed, the V/f ratio is kept constant & for speed above base speed, the terminal voltage is maintained at rated value & frequency is varied maintained at rated value & frequency is varied - In variable frequency control, synchronous motor may operate in two modes a) True synchronous mode /open loop mode b) Self controlled mode a) True synchronous mode a) True synchronous mode - Here the stator supply frequency is controlled from an independent oscillator - Frequency from initial value to desired value is varied gradually so that the difference between synchronous speed & actual speed is always small 6
- 7. 7
- 8. - A drive operating in true synchronous mode is shown in previous slide - Frequency command f* is applied to a VSI through a delay circuit so that rotor speed is able to track the changes in frequency - A flux control block changes stator voltage with frequency to maintain a frequency to maintain a constant flux below base speed & constant terminal voltage above base speed 8
- 9. - Under steady operating conditions, a gradual increase in frequency causes the synchronous speed > actual speed & torque angle δ increases - To follow this change in frequency, motor accelerates & settles at new speed after hunting oscillations which are damped by damper windings - A gradual decrease in frequency causes the synchronous speed to - A gradual decrease in frequency causes the synchronous speed to become < actual speed & δ become negative - To follow this change in frequency, the motor decelerates under regenerative braking - Motor settles down at new speed after hunting oscillations - The frequency must be changed gradually to allow the rotor to track the changes in revolving field, otherwise the motor may pull out of the changes in revolving field, otherwise the motor may pull out of step - This method is employed only in multiple synchronous motor drives requiring accurate speed tracking between motors - E.g, fibre spinning mills, paper mills, textile mills 9
- 10. b) Self controlled mode - A machine is said to be in self controlled mode if it gets its variable frequency from an inverter whose thyristors are fired in a sequence, using the information of rotor position or stator voltages i) Rotor position sensor i) Rotor position sensor - here a rotor position sensor is used, which measures the rotor position w.r.to stator & sends pulses to thyristor - Hence the frequency of inverter output is decided by rotor speed - Here the supply frequency is changed so that the synchronous speed is same as rotor speed & hence rotor cannot pull out of speed is same as rotor speed & hence rotor cannot pull out of slip & hunting oscillations are eliminated - A self controlled motor has properties of a DC machine both under steady state & dynamic conditions - There fore it is called a commutator less motor 10
- 11. ii) Stator voltage sensor - Here the firing pulses for inverter switches are derived from stator induced voltages (stator induced voltages depends on rotor position) - The synchronous machine with the inverter can be considered to be - The synchronous machine with the inverter can be considered to be similar to a line commutated converter where the firing pulses are synchronised with the line voltage - Variable speed synchronous motor drives are generally operated in self controlled mode 11
- 12. VSI fed Synchronous Motor Drives - VSI fed synchronous motor drives can be classified as 1. Self control mode using a rotor position sensor or stator voltage sensor - Here the output frequency is controlled by the inverter & voltage is - Here the output frequency is controlled by the inverter & voltage is controlled by the controlled rectifier - If the inverter is PWM inverter, both frequency & voltage can be controlled within the inverter - Upto base frequency, V/f ratio is kept constant & above base speed f is varied by keeping V at rated value 12
- 13. 2. True synchronous mode where the speed of motor is determined by the external independent oscillator - Here the output frequency & voltage is controlled within the PWM inverter inverter - If the inverter is not PWM controlled, then the voltage is controlled by using a controlled rectifier & frequency is controlled by the inverter 13
- 14. Advantages & drawbacks of True synchronous mode operation - Multi motor drive is possible - Involve hunting & stability problems - Can be implemented by using VSI & CSI - Power factor can be controlled in a wound field synchronous motor by controlling the field excitation by controlling the field excitation Advantages & drawbacks of Self controlled mode operation - Eliminates hunting & stability problems - Good dynamic response - Can be implemented by using VSI & CSI - Can be implemented by using VSI & CSI - Load commutation of inverter is possible & no need of forced commutation - Power factor can be controlled in a wound field synchronous motor by controlling the field excitation 14
- 15. Self controlled synchronous motor drive employing a load commutated thyristor inverter - A CSI fed synchronous motor drive may employ a load commutated thyristor inverter - When a synchronous motor is fed from a CSI, it can be operated in self controlled mode or true synchronous mode self controlled mode or true synchronous mode - When fed from CSI, synchronous motor is operated at leading power factor so that the inverter will work as a load commutated inverter - A load commutated inverter fed synchronous motor under self controlled mode is shown in figure - The source side converter is a 6 pulse line commutated thyristor converter converter - For a firing angle range 0<αs<90, it works as a line commutated fully controlled rectifier delivering positive Vd & Id - For a firing angle range 90<αs<180, it works as a line commutated inverter delivering negative Vd & Id 15
- 16. - When synchronous motor is operated at leading power factor, thyristors of load side converter can be commutated by motor induced voltages in the same way, as thyristors of a line commutated converter voltages in the same way, as thyristors of a line commutated converter are commutated by line voltages - Commutation of thyristors by induced voltages of load is known as load commutation - The load side converter will work as an inverter for 90<αL<180 - For 0<αL<90, it work as a rectifier 16
- 17. Motoring operation – for 0<αS<90 & 90<αL<180, source side converter works as rectifier & load side converter as inverter causing power to flow from AC source to motor Generating operation - for 90<αS<180 & 0<αL<90, load side converter work as rectifier & source side converter as inverter causing power to flow from motor to AC source flow from motor to AC source - The DC link inductor Ld reduces ripples in the DC link current - Due to Ld, load side converter works as a CSI - For operating in self controlled mode, rotating magnetic field speed should be same as rotor speed - This condition is achieved by making the frequency of load side converter output voltage equal to frequency of voltage induced in the converter output voltage equal to frequency of voltage induced in the armature - Normally hall sensors are used to obtain rotor position information - The difference between CSI fed induction motor drive & synchronous motor drive is that induction motor drive uses forced commutation & synchronous motor drive uses load commutation 17
- 18. Advantages - High efficiency - Four quadrant operation is possible with regenerative braking - Higher power rating (upto 100MW) - Ability to run at higher speeds (6000 rpm) Permanent magnet synchronous motor drives - In a permanent magnet synchronous motor, the DC field winding in - In a permanent magnet synchronous motor, the DC field winding in rotor is replaced by a permanent magnet - Advantages of using a permanent magnet are * Elimination of field copper loss * Higher power density * lower rotor inertia * more robust construction of rotor * more robust construction of rotor * higher efficiency - Drawbacks of using permanent magnets are * Loss of flexibility in field flux control * Demagnetization effect * Higher cost 18
- 19. Permanent magnet materials - Commonly used materials for permanent magnets are * Alnico * Ferrite * Cobalt-Samarium * Neodymium-Iron-Boron * Barium and Strontium ferrites Construction - The main parts are stator & rotor - Stator contain a 3 phase winding placed in stator slots - Rotor contain permanent magnets - Rotor contain permanent magnets Based on construction of rotor, permanent magnet synchronous motors are classified into 2 1. Surface mounted permanent magnet motor - Here the permanent magnets are mounted on the surface of rotor19
- 20. - Permanent magnets are glued on the rotor surface using epoxy adhesive - Rotor has an iron core which is made up of laminations - Since the rotor is having a salient pole structure, this motor is not used for high speed applications 2. Interior permanent magnet motor 2. Interior permanent magnet motor - Here the permanent magnets are placed inside the slots in rotor 20
- 21. - Since the permanent magnets are placed inside the rotor, rotor is having a non salient pole structure - So this motor is used for high speed applications Types of permanent magnet synchronous motor drive - Based on the nature of voltage induced in stator, the motor is classified into classified into 1. Sinusoidal PMAC motor 2. Trapezoidal PMAC motor (Brushless DC motor) - The speed of PMAC motors are controlled by feeding them from variable frequency voltage/current source inverter - They are operated in self controlled mode - Rotor position sensors are employed for operation in self control - Rotor position sensors are employed for operation in self control mode - Alternatively stator induced voltages can be used to achieve self control - MOSFET inverters are used for low voltage & power applications and IGBT inverters are used for high voltage & power applications 21
- 22. 1. Sinusoidal Permanent Magnet AC motor - Here the stator carries a 3 phase winding which is sinusoidally distributed in stator slots - The stator windings are excited from a 3 phase supply to produce a rotating magnetic field - The rotor contain permanent magnets (interior or surface mounted) embedded in the steel rotor to create a constant magnetic field - The rotor contain permanent magnets (interior or surface mounted) embedded in the steel rotor to create a constant magnetic field - The rotor poles are so shaped that the voltage induced in a stator phase has a sinusoidal wave shape - A Permanent magnet AC motor is not self starting like a wound field synchronous motor - Here we can't use damper windings on rotor to make the motor self starting starting - These motors require a variable frequency AC source for starting Speed control of sinusoidal permanent magnet AC motor - The speed of the motor can be varied by changing the stator supply frequency - For speed control below base speed, v/f ratio is kept constant 22
- 23. - For speed control above base speed, voltage is kept at maximum rated value & frequency is varied (field weakening operation) - Permanent magnet AC motors are fed from variable voltage variable frequency inverters - The inverter switches are fired according to the rotor position information information - The inverter can operate in 120/180 degree conduction mode 23
- 24. 120 degree conduction mode - Here at any given point of time, only two switches conduct and six switching combinations are possible - The gating signals are shown below 24
- 25. 180 degree conduction mode - Here at any point of time, at least three switches are on. - The gating sequence for this mode of operation is shown below 25
- 26. 2. Trapezoidal PMAC motor - This motor is also known as BLDC motor - Here the stator carries a 3 phase concentrated winding - Rotor contain permanent magnets with wide pole arc so that the stator induced voltages are trapezoidal in shape - BLDC motor is supplied from an inverter - This motor operates only in self control mode, i.e, rotor position - This motor operates only in self control mode, i.e, rotor position information is required for operation - The rotor position information is obtained by using hall sensors or from stator terminal voltages Working - The motor is supplied from an inverter - The inverter switches are turned ON/OFF in a sequence to ensure proper commutation proper commutation - Here two phases are energized at any instant - Permanent magnets create rotor flux and energized stator windings create electromagnetic poles - The rotor is attracted by the energized stator poles 26
- 27. - By using an appropriate sequence to supply the stator phases, a rotational field on stator is created and maintained - Now the rotor poles follow the stator rotating magnetic field poles - Here stator is having concentric winding, so induced emf (back emf) is trapezoidal in nature 27
- 28. - Here two switches are ON at any instant, so that two phases are energized - Each switch conducts for 120 degree - The switching sequence and waveforms are shown below 28
- 29. - A trapezoidal PMAC motor is similar to a DC motor without commutator and brushes - Like a DC motor, here the voltage induced in stator is proportional to speed and torque is proportional to current - The stator and rotor magnetic fields remain stationary with respect to each other each other - In BLDC motor the commutation is electronic commutation, achieved by proper switching of inverter switches 29
- 30. Comparison of Sinusoidal & Trapezoidal PMAC motor Trapezoidal PMAC Sinusoidal PMAC Synchronous machine Synchronous machine Trapezoidal back emf Sinusoidal back emf Stator flux position changes at every 60 Continuous stator flux position variation Stator flux position changes at every 60 degree Continuous stator flux position variation Only two phases energised at any instant Three phases are energised at any instant Torque ripple at commutation No torque ripple at commutation Low order current harmonics at audible range Less harmonics due to sinusoidal excitation 30 range excitation Higher core losses due to harmonic content Lower core loss Less switching losses Higher switching losses Control algorithm is simpler Control algorithm is complicated
- 31. Microcontroller based Permanent magnet Synchronous motor drive - The schematic diagram of a microcontroller controlled permanent magnet synchronous motor drive is shown below 31
- 32. - dsPIC16F2010 is a 16 bit microcontroller - 28 pin configuration - Default 6 PWM output channels - RB3, RB4 & RB5 are I/O ports. Here they are used to give the hall sensor output signals to the controller - RC14 is digital input port, used to give ON/OFF command - RC14 is digital input port, used to give ON/OFF command - AN1 & AN2 are used to give analog inputs (here, reference speed and current data) to controller Working - According to the rotor position information from hall sensor, controller will generate PWM signal to control the inverter controller will generate PWM signal to control the inverter - Inverter output is given to motor & motor operates according to the voltage from inverter 32