Variable Frequency Induction Motor Drives
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The document discusses variable frequency induction motor drives, detailing the methods of controlling motor speed through voltage source inverters (VSI) and current source inverters (CSI), highlighting their advantages and drawbacks. It explains closed-loop speed control mechanisms, regenerative braking, and dynamic braking methods for induction motors, as well as concepts of vector control for independent torque and flux management. Additionally, comparisons between VSI and CSI fed drives are provided, emphasizing their differences in reliability, cost, and operational flexibility.
![Transformations in reference frame theory
Need of transformations
- The analysis of 3 phase electrical circuits are complicated since it
involve 3 time varying quantities ( 3 phase ABC reference frame)
- In 2 phase, there is only 2 time varying quantities, so the analysis is
less complicated (2 phase αβ reference frame)
less complicated (2 phase αβ reference frame)
- If the quantities are not time varying, then analysis become simpler
(2 phase dq reference frame)
- So we go for transformations in complex poly phase circuit analysis
- The process of replacing one set of variables to another related set of
variables is called transformations
- The general form of transformation equations is
[New variables] = [Transformation matrix][Old variables]
[Old variables] = [Inverse transformation matrix][New variables]
- Transformation matrix is a matrix containing the coefficients that
relates new & old variables 27](https://image.slidesharecdn.com/editedvsicsifedinductionmotordrives-240728205917-0367265b/85/Variable-Frequency-Induction-Motor-Drives-27-320.jpg)
Variable Frequency Induction Motor Drives
- 1. VSI & CSI fed Induction motor Drives 1
- 2. Variable frequency Induction motor drives - The speed of an Induction motor can be controlled by varying the supply frequency - Variable frequency control allows good running & transient performance - Variable frequency Induction motor drives are utilized in fan, pump, - Variable frequency Induction motor drives are utilized in fan, pump, blower, conveyor and machine tools applications - Variable frequency AC supply can be obtained by using 1. Voltage source inverter 2. Current source inverter 3. Cycloconverter 3. Cycloconverter Voltage source inverter (VSI) fed induction motor drives - VSI allows a variable voltage variable frequency AC supply obtained from a DC supply - Normally self commutating devices like MOSFET, IGBT, power transistors are used in VSI 2
- 3. - VSI can operate as a stepped wave inverter or a PWM inverter - In a stepped wave inverter, output frequency can be controlled by controlling the turn ON time of switches and output voltage can be controlled by controlling the input DC voltage - In a PWM inverter, the output voltage & frequency can be controlled within the inverter by PWM technique within the inverter by PWM technique - Figure below shows the configuration of a VSI fed Induction motor 3
- 4. Stepped wave inverter - When supply is DC, variable DC input is obtained by connecting a chopper between DC supply & inverter - When supply is AC, variable DC input is obtained by connecting a controlled rectifier between AC supply & inverter 4
- 5. * Stepped wave VSI fed induction motor drive has following drawbacks - The output voltage of inverter has large low frequency harmonic content - Low frequency harmonics increases motor losses & causes de-rating of motor - Motor develops pulsating torque due to harmonics (5,7,11,13) - Motor develops pulsating torque due to harmonics (5,7,11,13) - Harmonic content in current increases at low speeds. Machine saturates at low speeds due to high V/f ratio. These two effects overheats the machine at low speeds Features - This is advantageous for multi motor drive - Dynamic behavior is fairly good at high speeds - Dynamic braking is possible - Regeneration requires an additional converter connected antiparallel to line side one - Speed reversal is achieved by changing the phase sequence 5
- 6. PWM inverter - The drawbacks of stepped wave inverter can overcome by using a PWM inverter - Here output voltage can be controlled by PWM, no arrangement is required for varying DC input voltage - So inverter can be directly connected when supply is DC and through a - So inverter can be directly connected when supply is DC and through a diode rectifier when supply is AC 6
- 7. - PWM inverter has constant DC link voltage & uses PWM technique for both voltage control & harmonic elimination - Output voltage waveform is improved (more sinusoidal), with low harmonic content - The amplitude of torque pulsation is minimal even at low speeds - Power factor of the system is good as a diode rectifier is used on line - Power factor of the system is good as a diode rectifier is used on line side - Four quadrant operation is possible - Dynamic braking can be employed - Single & multi motor drive is possible - VSI fed induction motor drive are normally powered from AC supply - VSI fed induction motor drive are normally powered from AC supply & regeneration is possible if the rectifier used is a full converter or dual converter - Output voltage wave form of a stepped wave & PWM inverter are shown in next slide 7
- 8. Closed loop speed control of VSI induction motor drives - A closed loop speed control of VSI fed IM drive is shown in figure - It employs an inner slip speed loop & outer speed loop - For a given current, slip speed has a fixed value. So slip speed loop also - For a given current, slip speed has a fixed value. So slip speed loop also functions as an inner current loop - Drive uses a PWM inverter fed from a DC source which has the capability for regenerative braking & 4 quadrant operation 8
- 9. 9
- 10. - The actual speed(ωm) is compared with reference speed(ωm*) to get speed error - The reference signal (V*) for voltage control is generated from frequency(f) using a function generator - The speed error is processed through a speed controller (PI) & a slip regulator to produce a slip speed command (ωsl*) regulator to produce a slip speed command (ωsl*) - The synchronous speed obtained by adding actual speed & slip speed determines inverter frequency - V* is compared with actual stator voltage to get voltage error - The voltage error is processed by a voltage controller to produce the necessary modulation index variation 10
- 11. Current Source Inverter (CSI) fed Induction motor drive (stator current control) - Here the developed torque & hence the speed of motor is controlled by stator current control - The behavior of motor with stator current control is different from that obtained with stator voltage control obtained with stator voltage control - When an induction motor is fed from a current source, the analysis shows that the maximum torque produced by motor α (stator current)2 & is independent of frequency & rotor resistance - The speed- torque characteristics for different stator currents are for different stator currents are shown in figure 11
- 12. - When operating at constant flux, the operating points are located mostly on unstable region of torque – speed characteristics - Hence closed loop control is mandatory for stable operation - A constant current for 3 phase induction motor can be obtained from a 3 phase current source inverter (CSI) - Figure below shows the configuration of a CSI fed Induction motor - Figure below shows the configuration of a CSI fed Induction motor 12
- 13. - The two commonly used configurations for CSI fed induction motor drives based on the source available are shown below 1. When source available is DC source - Here a chopper is used in between DC source & Inverter to vary the DC link voltage 2. When source available is AC source 2. When source available is AC source - Here, 3 phase controlled rectifier produces a controlled DC voltage - Inductor converts this voltage to a constant current - CSI regulates output frequency & therefore the torque & speed of motor 13
- 14. - Here one more configuration is possible, i.e if we are using a diode rectifier, - Here one more configuration is possible, i.e if we are using a diode rectifier, the circuit configuration become as shown below - 3 phase diode rectifier produces an uncontrolled DC voltage. - It is regulated by using a chopper, which is then converted to current source by inductor - CSI regulates output frequency & therefore the torque & speed of motor14
- 15. Advantages & disadvantages of CSI fed IM drive - More reliable than VSI. In VSI, commutation failure will cause short circuit across source & current rises to dangerous values. So expensive semiconductor fuses are required for safety. In CSI due to the presence of large inductance, current will not increase to dangerous values & less expensive HRC fuses are sufficient. dangerous values & less expensive HRC fuses are sufficient. - Current input is unaffected by motor parameter variations - It produce harmonics in the system - Open loop operation is not possible - Only single motor operation is possible - Converter grade thyristors are sufficient - Converter grade thyristors are sufficient - There is stability problem at light load. A minimum current should be there for commutation - So it finds application in medium & high power drives 15
- 16. Comparison of VSI & CSI fed drives - CSI is more reliable than VSI - Because of large inductance in DC link & large inverter capacitors (for commutation) CSI drive has higher cost, weight & volume, lower speed range, slower dynamic response - The CSI drive is not suitable for multi motor drives. But a single VSI - The CSI drive is not suitable for multi motor drives. But a single VSI drive can feed a number of motors Braking of VSI fed Induction motor drives - During motoring operation, power flows from inverter to motor - The motor will run at a speed which is less than synchronous speed - During braking operation, the motor works as a generator & produces - During braking operation, the motor works as a generator & produces electrical energy - The induction motor will work as generator, when the actual speed of motor become greater than synchronous speed - For braking operation, the inverter output frequency is reduced, so that synchronous speed become less than actual speed 16
- 17. - Now motor will work as generator, produces electrical energy - This energy is converted to DC by the inverter, which will work as a controlled rectifier during braking operation - As a result, the direction of DC link current reverses - The Electrical power reaching the DC link can be utilized effectively by Regenerative braking or it can be wasted in a resistor by Dynamic Regenerative braking or it can be wasted in a resistor by Dynamic braking Dynamic Braking - In dynamic braking, the electrical power generated during braking operation is wasted in a resistor to get the desired braking effect - The circuit configuration for dynamic braking of PWM inverter fed induction motor drive is shown below induction motor drive is shown below 17
- 18. - For dynamic braking, the switch SW & a self commutated switch S in series with braking resistance R connected across the DC link - When operation of motor shifts from motoring to braking, switch SW is opened - Generated power flowing into DC link charge the capacitor & its voltage increases voltage increases - When voltage crosses a set value, switch S is closed, connecting the resistance across the DC link - The generated power & a part of power stored in capacitor flow into the resistance & DC link voltage reduces - When it falls below nominal value, switch S is opened - Thus the generated power is dissipated in the resistance giving - Thus the generated power is dissipated in the resistance giving dynamic braking - Dynamic braking is applicable to all Induction motor drives fed from an inverter 18
- 19. Regenerative Braking - In regenerative braking, the electrical power generated during braking operation is effectively fed back to the supply - Regenerative braking is not possible in all VSI fed induction motor drives - When supplied from a DC source, regeneration is easy - When supplied from a DC source, regeneration is easy - Here, during braking motor works as generator producing electrical power. - This power reaches the DC link through PWM inverter & direction of DC link current reverses - Now, power flow from DC link to source 19
- 20. - When supplied from an AC source, for regeneration the source side converter (rectifier) should be a full converter or dual converter - During regeneration, electrical power generated reaches DC link - The direction of DC link current reverses - Now the controlled rectifier/dual converter will fed back this DC link power to AC source to get regenerative braking power to AC source to get regenerative braking Regenerative Braking of CSI fed Induction motor drives - When inverter frequency is reduced to make synchronous speed less than motor speed, machine works as a generator - Power flows from machine to DC link & DC link current flow reverses 20
- 21. - If a fully controlled converter is made to work as an inverter, the - If a fully controlled converter is made to work as an inverter, the power supplied to DC link will be transferred to AC supply & regenerative braking will take place - Thus no additional equipment is required for regenerative braking Basic principle of Vector Control (Field oriented control) - In a separately excited DC motor, both field flux & electromagnetic torque can be controlled independently by varying field current & torque can be controlled independently by varying field current & armature current respectively in the machine - In an AC motor (eg. Induction motor) there is only one current, ie the stator current which produce both flux & torque in the machine - So in an AC motor, by using normal control techniques, independent control of torque & flux is not possible 21
- 22. - Independent control of torque and flux possible in AC motor drives by using Vector Control or Field Oriented Control (FOC) - In vector control, an induction motor is controlled under all operating conditions to get performance similar to a separately excited DC motor - The stator current (Is) of an induction motor can be resolved into 2 - The stator current (Is) of an induction motor can be resolved into 2 components - Flux producing component Im & torque producing component Iw - If we are able to control Im & Iw independently, then flux & torque can be controlled independently - In vector control we are controlling Im & Iw independently independently - There are 3 stator currents in a 3 phase induction motor & they together produce the required flux & torque inside the machine 22
- 23. - To control torque & flux independently, we transform the 3 phase stator currents (Ia, Ib & Ic) into 2 phase current by using ABC to dq transformation (Clarke & Park transformations) - Now the 2 phase currents are 1. Flux producing component, id – which produces the net flux in the motor motor 2. Torque producing component, iq – which produces the torque in the motor - There are 2 type of vector control 1. Direct vector control - here the actual speed of the motor is sensed by using a tachometer 2. Indirect vector control - here the actual speed is calculated from machine terminal voltages & no tachometers are used 23
- 24. 1. Direct vector control 24
- 25. - Here the actual 3 phase motor terminal voltages & currents are sensed & they are converted to 2 phase frame to get the actual values of torque producing & flux producing components of stator current - The actual motor speed is sensed by a tachometer & is compared with reference speed to get speed error - The error is processed by using a PI controller & its output is the - The error is processed by using a PI controller & its output is the reference torque producing component of stator current (Iqref) - The Iqref is compared with Iqact & the error is processed by using a PI controller - The output of PI controller is given to DQ to ABC transforming block - The reference flux current (Idref) is compared with actual (Idact) to get error & is processed by using PI controller error & is processed by using PI controller - The output of controller is given to DQ to ABC transforming block - The DQ to ABC transforming block will generate the three phase currents which will produce the desired flux & torque inside the machine. The firing pulse generator will produce the firing pulses for inverter switches 25
- 26. - When inverter switches operates based on the generated firing pulses, the inverter output will be such that to get desired torque & flux in the machine 2. Indirect vector control 26
- 27. Transformations in reference frame theory Need of transformations - The analysis of 3 phase electrical circuits are complicated since it involve 3 time varying quantities ( 3 phase ABC reference frame) - In 2 phase, there is only 2 time varying quantities, so the analysis is less complicated (2 phase αβ reference frame) less complicated (2 phase αβ reference frame) - If the quantities are not time varying, then analysis become simpler (2 phase dq reference frame) - So we go for transformations in complex poly phase circuit analysis - The process of replacing one set of variables to another related set of variables is called transformations - The general form of transformation equations is [New variables] = [Transformation matrix][Old variables] [Old variables] = [Inverse transformation matrix][New variables] - Transformation matrix is a matrix containing the coefficients that relates new & old variables 27
- 28. ABC to αβ transformation (Clarke transformation) (3 phase rotational reference frame to 2 phase rotational reference frame) - Let ia,ib & ic be the 3 phase currents & iα and iβ represents 2 phase currents - Now the transformation equations are given by − − i i 2 1 2 1 1 - This transformation is applicable for voltages also − − − = c b a i i i i i i 2 1 2 1 2 1 2 3 2 3 0 2 1 2 1 1 3 2 0 β α 28
- 29. αβ to ABC transformation (Inverse Clarke transformation) (2 phase stationary reference frame to 3 phase rotational reference frame) - Let iα and iβ represents 2 phase currents & ia,ib & ic be the 3 phase currents - Now the transformation equations are given by − − − = 0 2 1 2 3 2 1 2 1 2 3 2 1 2 1 0 1 3 2 i i i i i i c b a β α - This transformation is applicable for voltages also 2 2 2 29
- 30. αβ to dq transformation (2 phase rotational reference frame to 2 phase stationary reference frame) - The transformation equations are given by − = 0 0 1 0 0 0 cos sin 0 sin cos i i i i i i q d β α θ θ θ θ 30
- 31. dq to αβ transformation (2 phase stationary reference frame to 2 phase rotational reference frame) - The transformation equations are given by − = 0 cos sin 0 sin cos i i i i d θ θ θ θ α Concept of space vector - Space vector is a transformation for analyzing three-phase electric = 0 0 1 0 0 0 cos sin i i i i q θ θ β - Space vector is a transformation for analyzing three-phase electric systems. - The term “space” originally stands for the two-dimensional complex plane, in which the three-phase quantities are transformed - The transformation from 3 phase to 2 phase is called space vector transformation 31
- 32. - The 3 phase voltages, currents and fluxes of AC motors can be analyzed in terms of complex space vectors - With regard to current, space vector can be defined as follows - Let ia, ib, ic be the instantaneous currents in the stator phases, then the stator current vector is is given by - is = ia+ αib+ α2ic Where α=1<120 (ej(2π/3)), α2=1<240 (ej(4π/3)) s a b c Where α=1<120 (ej(2π/3)), α2=1<240 (ej(4π/3)) Space vector modulation - It is an algorithm for the control of pulse width modulation (PWM) - It is used for the creation of alternating current (AC) waveforms, most commonly to drive 3 phase AC powered motors at varying speeds from a DC source from a DC source - Consider a 3 phase inverter as shown in figure - The output may be given to a 3 phase induction motor - The switches must be controlled so that at at no time are both switches in the same leg turned & cause a short circuit of the DC supply 32
- 33. - This requirement may be met by the complementary operation of the switches within a leg. i.e. if A+ is on then A− is off and vice versa. - This leads to eight possible switching vectors for the inverter, V0 through V7 with six active switching vectors and two zero vectors 33
- 34. - To implement space vector modulation, a reference signal (Vref) may be generated - The reference vector is then synthesized using a combination of the two adjacent active switching vectors and one or both of the zero vectors 34
- 35. Advantages of Space vector PWM - Harmonics in output voltage decreases - Output voltage increases - Switching losses decreases - Smooth control of output voltage and frequency 35