Induction motor speed control using solid state drives.pptx

This PPT explains the speed control of induction motor using solid state devices and circuits.

1. Induction Motor Drives Electric Drives and Control D.Poornima , Assistant Professor ( Sr.Gr ), Department of EEE, Sri Ramakrishna Institute of Technology, Coimbatore

2. INTRODUCTION 01

3. Induction Motor Drives 01 Used for variable speed applications in a wide power range, from fractional horsepower to multi-megawatts. Widely used because of its reliability, robustness, and low cost. The elimination of the commutator lowers cost and is broader in its application. The main problem is its difficult speed control. Advancements in power electronic converters rectified this problem - now inverters are being used. Applications pumps and fans, paper and textile mills, subway and locomotive propulsions, electric and hybrid vehicles, machine tools and robotics, wind power generation systems, etc.

4. Speed Control Of Induction Motor 1. Control from stator side a. By changing the number of stator poles b. By changing the applied frequency c. By changing the applied voltage 2. Control from rotor side a. Eddy Current Coupling b. Rotor rheostat control c. Slip Power Recovery E 2  ∝   V

5. Pole Changing For a given frequency, the synchronous speed is inversely proportional to the number of poles. Pole Changing provision has to be incorporated at the manufacturing stage and such machines are called, ‘ pole changing motors ‘ or ‘ multi-speed motors ‘. Squirrel-cage rotor - not wound for any specific number of poles - produces the same number of poles as stator winding has -pole changing arrangement is required only in the stator. In the wound-rotor motor, an arrangement for changing the number of poles in the rotor is also required, which complicates the machine. Pole Changing of Induction Motor method of speed control is only used with squirrel-cage motors.  

6. Pole Changing Fig (a) shows a phase winding consisting of 6 coils divided into two groups a-b consisting of odd-numbered coils (1, 3, 5) connected in series and c-d of even-numbered coils (2, 4, 6) connected in series. The coils can be made to carry current in the given directions by connecting coil groups either in series or parallel, as shown in Figs. (b) and ©, respectively. With this connection, the machine has 6-poles.

7. Pole Changing If current through the coils of group a-b is reversed Fig. (a), then all coils will produce north poles. Fluxes coming out of these north poles will now find a path through the inter-pole spaces for going out, producing south poles in inter-pole spaces. Thus, the machine will now have 12-poles. Required direction of current through coils can be obtained by connecting a-b and c-d either in series or in parallel as shown in Fig. (b) and (c). Each phase of machine winding has two coil groups, a-b and c-d , which can be connected either in series or in parallel for both pole numbers 6 and 12 .

8. Stator Voltage Control From the torque equation of induction motor, Rotor resistance R 2 is constant and if slip s is small then (sX 2 ) 2 is so small that it can be neglected. Therefore, T ∝ sE 2 2 where E 2 is rotor induced emf and E 2 ∝ V T ∝ sV 2 , if the supplied voltage is decreased, the developed torque decreases. For providing the same load torque, the slip increases with a decrease in voltage, and consequently, the speed decreases. Easiest and cheapest method, still rarely used, because A large change in supply voltage is required for a relatively small change in speed. A large change in supply voltage will result in a large change in flux density, this will disturb the magnetic conditions of the motor.   S uitable for applications where torque demand reduces with speed, like fan and pump drives.

9. Static Stator Voltage Control – Single Phase Variable voltage for speed control is obtained using AC voltage controllers. Domestic fan motors, single-phase, are controlled by a single-phase triac voltage controller . Speed control is obtained by varying the firing angle of the triac . These are preferred because of higher efficiency.

10. Stator Voltage Control – Three Phase Thyristor voltage controller for speed control of 3-phase motors are used for Three Phase Induction Motor. Motor may be connected in star or delta. Speed control is obtained by varying the conduction period of thyristors. For low power ratings, anti-paralleled thyristor pair in each phase can be replaced by a triac . Both single- and three-phase, allow a stepless control of voltage from its zero value, they are also used for soft start of motors.

11. Variable Frequency Control Synchronous speed of the rotating magnetic field of an induction motor is given by, where, f = frequency of the supply and P = number of stator poles. Synchronous speed changes with change in supply frequency. Actual speed of an induction motor is given as N = Ns (1 - s). Not widely used because At lower frequencies, the motor current may become too high due to decreased reactance. If the frequency is increased beyond the rated value, the maximum torque developed falls while the speed rises. It may be used where, the induction motor is supplied by a dedicated generator (so that frequency can be easily varied by changing the speed of prime mover).

12. An y reduction in the supply frequency, without a change in the terminal voltage, causes an increase in the air-gap flux. An increase in flux will saturate the motor magnetically. This will increase the magnetizing current, distort the line current and voltage, increase the core loss and the stator copper loss, and produce a high-pitch acoustic noise. A decrease in flux is also avoided to retain the torque capability of the motor. Variable Frequency Control below the rated frequency is carried out at the rated air-gap flux by varying terminal voltage with frequency to maintain the (V/f) ratio constant at the rated value . Constant V/F Control

13. Constant V/f Control For an Induction motor, where K is a constant, and L s and L′ r are, respectively, the stator and stator referred rotor inductances. Positive sign is for motoring operation and negative sign is for braking operation. When frequency is not low, (R s /f) ≪ 2π(L s + L′ r ) and therefore, This suggests that with a constant (V/f) ratio, motor develops a constant maximum torque, except at low speeds (or frequencies). Motor therefore operates in constant torque mode.

14. Constant V/f Control For an Induction motor, where K is a constant, and Ls and L′r are, respectively, the stator and stator referred rotor inductances. For low frequencies (or low speeds) due to stator resistance drop, the maximum torque will have a lower value in the motoring operation (-Eve sign) and a larger value in the braking operation (- ve sign). This is due to a reduction in flux during motoring operation and an increase in flux during braking operation. If same maximum torque has to be retained at low speeds and also in motoring operation, the (V/f) ratio is increased at low frequencies. This causes a further increase in maximum braking torque and considerable saturation of the machine in braking operation.

15. When either V saturates or reaches rated value at base speed, it cannot be increased with frequency. Therefore, above base speed, frequency is changed with V maintained constant. According to with V maintained constant, maximum torque decreases with increase in frequency (or speed). Variation in terminal voltage with frequency is shown V is kept constant above the base speed. Below the base speed (V/f) ratio is maintained constant, except at low frequencies where (V/f) ratio is increased to keep maximum torque constant. S peed torque curves suggest that speed control and braking operation are available from nearly zero speed to above synchronous speed.

16. Advantages: Speed control and braking operation are available from zero speed to above base speed. During transients (starting, braking and speed reversal) the operation can be carried out at the maximum torque with reduced current giving a good dynamic response. Copper losses are low, and efficiency and power factor are high Drop in speed from no load to full load is small. Can be used in underground and underwater installations, and also in applications involving explosive and contaminated environments, such as in mines and the chemical industry They have several other applications such as traction, mill run-out tables, steel mills, pumps, fans, blowers, compressors, spindle drives, conveyors, machine tools, and so on.

17. 01 02 03 04 05 Rotor Resistance Control Similar to that of armature rheostat control of DC shunt motor. Only applicable to slip ring motors, as the addition of external resistance in the rotor of squirrel cage motors is not possible. Maximum torque is independent of rotor resistance, but speed at which the maximum torque is produced changes with rotor resistance. For the same torque, speed falls with an increase in Rotor Resistance . Advantages Motor torque capability remains unaltered even at low speeds. Cost of Rotor Resistance Control is very low Employed in cranes, Ward Leonard Ilgener Drives, and other intermittent load applications. Major disadvantage - low efficiency due to additional losses in the resistor connected in the rotor circuit. Losses mainly take place in the external resistor they do not heat the motor.

18. Static Rotor Resistance Control The ac output voltage of the rotor is rectified by a diode bridge and fed to a parallel combination of a fixed resistance R and a semiconductor switch realized by a transistor Tr Effective value of resistance across terminals A and B, R AB , is varied by varying duty ratio of transistor Tr, which in turn varies rotor circuit resistance. Inductance Ld is added to reduce ripple and discontinuity in the dc link current Id. Rotor current waveform is shown Thus rms rotor current will be

19. Resistance between terminals A and B will be zero when the transistor is on and it will be R when it is off. Average value of resistance between the terminals is given by where δ is the duty ratio of the transistor Power consumed by R AB is From these Eqs . , power consumed by R AB per phase is This equation suggests that rotor circuit resistance per phase is increased by 0.5R(1 – δ). Thus, total rotor circuit resistance per phase will now be R rT can be varied from R r to (R r + 0.5R) as δ is changed from 1 to 0.

20. Slip Power Recovery Slip-power: a part of the air-gap power that is not converted into mechanical power, represented by sPg One of the methods of controlling the speed of an Induction motor. In rotor resistance control method, the slip power in the rotor circuit is wasted as I 2 R losses during the low-speed operation. The efficiency is reduced. The slip power from the rotor circuit can be recovered and fed back to the AC source to utilize it outside the motor. Overall efficiency of the drive system can be increased.

21. Slip Power Recovery 01 Meth od for recovering the slip energy and power recovery of an Induction Motor.

22. The basic principle - connect an external source of the EMF of the slip frequency of the rotor circuit. A portion of rotor AC power (slip power) is converted into DC by a diode bridge. The smoothing reactor is provided to smoothen the rectified current. The output of the rectifier is connected to the inverter. The inverter inverts the DC power to the AC power and feeds it back to the AC source. The inverter is a controlled rectifier operated in the inversion mode. Used in large power applications where the variation of speed over a wide range involves a large amount of slip power.

23. Slip Power Recovery Schemes 2 1

24. STATIC SCHERBIUS DRIVE Provides the speed control of a wound rotor motor below synchronous speed. The portion of rotor AC power is converted into DC by a diode bridge. The controlled rectifier works as an inverter and converts the DC power back into AC and feeds it back to the AC source. This drive can flow the power both in the positive as well as in the negative direction of the injected voltage. This increases the operating condition of the drive.

25. Drive is started by resistance control with S 1  closed and S 2  open. When the speed reaches within the control range of the drive, S 2  is closed to connect the diode bridge and the inverter is activated. Now S 1  is opened to remove the resistances.

26. The feedback power is controlled by controlling the inverter counter emf V d2 , which is controlled by controlling the inverter firing angle. The DC link inverter reduced the ripple in DC link current I d . The drive input power is the difference of the DC input power and the power fed back. Reactive input power is the sum of the motor and input reactive power. Thus, the drive has poor power factor throughout the range of its operation.

27. Where α is the inverter firing angle and n, and m are respectively the stator to the rotor turn ratio of the motor and source side The neglecting drop across the inductor. Substituting the equations (1) and (2) in the above equation we get where a = n/m

28. The maximum value of alpha is restricted to 165º for safe commutation of inverter thyristor. The slip can be controlled from 0 to 0.966α when α is changed from 90º to 165º. The appropriate speed range can be obtained by choosing the appropriate value of α. The transformer is used to match the voltage from Vd1 and Vd2. Equivalent circuit of motor referred to the rotor, neglecting magnetizing branch is shown

29. The torque equation is given by, The nature of speed torque curves is shown The drive is widely used in medium and high-power fan and pump drives, because of its high efficiency, and low cost, and requires speed control in the narrow range only.

30. Operating Modes of Static Scherbius Drives 01 Sub-synchronous Motoring – Slip and torque both are positive and so injected voltage is in phase with the rotor current. The power flows into the stator and feedback into the rotor circuit. Super-synchronous Motoring – T he speed of the motor is above the synchronous speed, slip is negative. The voltage and current are out of phase with each other. The power feeds into the rotor from the drive circuit along with input power flowing into the stator. Sub-synchronous Generating – For sub-synchronous speed, the torque is required to be positive, although the slip is positive. The power is fed into the rotor through the slip ring. Super-synchronous Generating – The speed of the motor is above the synchronous speed, slip and torque become negative. Thus, the injecting voltage is in phase with the rotor. The mechanical power is injected by the shaft and the output power is obtained from the stator and rotor circuit.

31. STATIC KRAMER DRIVE Controls the speed of an induction motor by injecting the opposite-phase voltage in the rotor circuit. Injected voltage increases the resistance of the rotor, controlling the speed of the motor. Also converts the slip power of an induction motor into AC power and supplies back to the line. This method is only applicable when the speed of the drive is less than the synchronous speed.

32. Working 01 The rotor slip power is converted into DC by a diode bridge. This power is fed into a DC motor which is mechanically coupled to an induction motor. The torque supplied to the load is the total sum of the torque produced by the induction and DC motor drive. The figure shows the variation of V d1 and V d2 with a speed of two values of DC motor field current. When the value of V d1 is equal to the value of V d2 then the steady state operation of the drive is obtained, i.e., at A and B for field current of I f1 and I f2 .

33. Working The speed control is possible only when speed is less or half of the synchronous speed. When the large range speed is required, the diode bridge is replaced by the thyristor bridge. The relationship between the V d1 and the speed can be altered by controlling the firing angle of thyristor amplifier. Speed can be controlled up to stand still with thyristor control.

34. Closed Loop Control of Drives

35. Open Loop and Closed Loop Control T here are two types of systems - open loop and closed loop In open loop control system - output does not affect the input Controlling phenomenon is independent of the output In closed-loop control - output is fed back to the input terminal which determines the amount of input to the system. In electrical drives feedback loops or closed loop control satisfy the following requirements. Protection Enhancement of speed of response To improve steady-state accuracy

36. Closed Loop Control D ifferent closed loop configurations which are used in electrical drives irrespective of the type of supply they are fed are Current Limit Control Closed Loop Torque Control Closed Loop Speed Control

37. Current Limit Control employed to limit the converter and motor current below a safe limit during transient operations. It has a current feedback loop with a threshold logic circuit. As long as the current is within a set maximum value, feedback loop does not affect operation of the drive.

38. Current Limit Control During a transient operation, if the current exceeds the set maximum value - the feedback loop becomes active and the current is forced below the set maximum value - the feedback loop becomes inactive again. If the current exceeds the set maximum value again, it is again brought below it by the action of the feedback loop. The current fluctuates around a set maximum limit during the transient operation until the drive condition is such that the current does not tend to cross the set maximum value, e.g. during starting When close to the steady-state operation point, the current will not tend to cross the maximum value - the feedback loop will have no effect on the drive operation.

39. Closed Loop Torque Control Applied in battery operated vehicles, rail cars and electric trains. Driver presses the accelerator to set torque reference T*. Through Closed Loop Torque Control, the actual motor torque T follows torque reference T*. Speed feedback loop is present through the driver. By putting appropriate pressure on the accelerator, driver adjusts the speed depending on traffic, road condition, his liking, car condition and speed limit.

40. Closed-Loop Speed Control Most widely used feedback loops for drives. Employs an inner current control loop within an outer speed-loop. Inner current control loop limits the converter and motor current or motor torque below a safe limit. Current may be controlled directly or indirectly. For example, in a variable frequency induction motor drives the current is controlled by controlling the slip.

41. Closed-Loop Speed Control Working An increase in reference speed ω* m produce a positive error Δω m . Speed error is processed through a speed controller and applied to a current limiter which saturates even for a small speed error. L imiter sets current reference for inner current control loop at a value corresponding to the maximum allowable current.

42. Closed-Loop Speed Control Working Drive accelerates at the maximum allowable current (and in some cases at the maximum torque). When close to the desired speed, limiter desaturates. Steady-state is reached at the desired speed (with some steady-state error) and at current for which motor torque is equal to the load torque.

43. Closed-Loop Speed Control Working A decrease in reference speed ω* m produces a negative speed error. Current limiter saturates and sets current reference for inner current loop at a value corresponding to the maximum allowable current. D rive decelerates in braking mode at the maximum allowable current. When close to the required speed, current limiter desaturates. O peration is transferred from braking to motoring. Drive then settles at a desired speed and at current for which motor torque equals the load torque. In those drive applications where the load torque is able to provide enough decelerating torque, electric braking need not be used. Current and speed controllers may consists of proportional and integral (PI), proportional and derivative (PD) or proportional, integral and derivative (PID) controller, depending on steady-state accuracy and transient response requirements.

44. VECTOR CONTROL OF INDUCTION MOTOR – Introduction Vector Control is used to have superior performance for induction motors than widely used separately excited DC motors in the industry. Separately excited DC motor is a doubly fed motor, the field flux produced by the field current is orthogonal to the armature flux produced by the armature current. The orthogonality makes them decoupled, i.e. field current controls only the field flux and armature current controls only the armature flux So it has faster dynamic performance than the induction motor. Blaschke , in 1972 introduced the principle of field orientation to realize DC motor characteristics in an induction motor drive.

45. VECTOR CONTROL OF INDUCTION MOTOR DC Motor like performance can be achieved in IM if the motor control is considered in the synchronously rotating reference frame (d e - q e ) where sinusoidal quantities appear as DC quantities in steady state. For the same, he used decoupled control of torque and flux in the motor and named it as transvector control . The cage IM with vector or field-oriented control offers a high level of dynamic performance and the closed-loop control provides the long-term stability of the system. The vector control is also called as an independent or decoupled control wherein the torque and flux current vectors are controlled. With vector control, i ds (IM) = I f (DC Motor) i qs (IM) = I a (DC Motor)

46. From the motor speed signal ( ω r ) and desired speed (ω* r ) the error ω e is determined. Speed controller calculates the motor torque (T o ) needed to correct the speed which is passed through a limiter to determine torque signal T*. In a parallel Field Weakening block the motor speed ω r generates another signal. These two signals are employed to calculate i * ds and i * qs (ideal quadrature currents) and a speed correction ω* 2 .

47. ω = ω r + ω* 2 is integrated which is then used to find the transformation e jψ . This transformation carried out on i * ds , i * qs which gives the final ideal set i * dss , i * qss 2/3 phase transformation on i * dss , i * qss yields the ideal stator current i * as , i * bs , i * cs The measured stator currents i as , i bs , i cs are compared with i * as , i * bs , i * cs by the current controller and the six signals are generated to control the currents fed to the induction motor.

48. Thank you