Controlling Induction Motors Using the Vector Control Method - Assignment Example

Running Head: Institution: Student Name: Course: Date of submission: Table of Contents Introduction 3 Controlling Induction motors using the vector control method 4 The vector control concept 5 The method 5 Implementation of the vector control method 8 Voltage to frequency control in induction motors 10 FD system description 11 Pole changing 11 Stator Voltage control 12 Supply frequency control 12 Advantages of using frequency control in induction motors 13 Rotor resistance control 14 References 19 Introduction The basic principle behind the functioning of an induction motor is electromagnetic induction; which results in the production of a voltage in a conductor primarily due to the effect of the magnetic field (August and Hand 25). Different approaches can be implemented in order to control the induction motors, such as the use of voltage to frequency control and vector control. The simplest approach to controlling an induction motor can be based on the alteration of the orientation of the stator design and winding structure which ensures that the starting current is reduced to desired levels. Another basic way of controlling induction motors is through the use of pole changing technique, whereby the numbers of magnetic poles in the stator are changed to desired levels (August and Hand 36). In the design of modern induction motors, the stator voltages and respective currents are subject to be under control in order to ensure that the induction motor functions optimally. In the steady state conditions, these parameters are defined by magnitude and frequency. A control technique that basically involves the adjustment of magnitude and frequency are typically referred to as scalar control techniques. The use of scalar methods in the control of induction motors is generally known to produce transient effects that may turn out to be undesirable due to rapid changes in the magnitude and frequency can result to a disturbance in the torque of the induction motor. Vector control methods usually involve the changes in torque variables (Emadi 56). Vector control is effective in high performance drive systems. The vector control methods primarily rely on the concept of the space vectors for induction motors which employs the use of instantaneous values that are obtained in the respective three phase variables of the induction motors. Under the vector control technique, the vector variables are manipulated according to the desired control algorithm and the technique is primarily tailored to maintain a constant value of the induction motors torque during the rapid changes in the magnitude and frequency. Vector control methods are generally complex in implementation compared to scalar control methods; current and voltage sensors always play a vital part in the control of induction motors (Emadi 60). Controlling Induction motors using the vector control method The controlling of an induction motor is more difficult in cases where the LC filter is deployed. Such contexts do not warrant the use of a complex vector control. Vector control is used in cases whereby high control performance is required (Gottlieb 58). The inductor current and the capacitor voltage in most cases is controlled by deadbeat controller, the correct voltage reference is obtained by use of a high pass filter stator voltage and also employs the use of the multi-loop feedback controller. A key challenge in the use of the vector control method in inductor controlling is to maintain the required variables low in order to facilitate control reliability (Gottlieb 65). This control technique makes use of the vector concept. Conventionally, there are three alternating currents in an AC induction motors that are normally displaced by 1200 at the stator coils within the motor (Gottlieb 90). The resultant flux that is evident in the stator winding causes a current to be induced in the coils of the rotor; as a result the rotor generates a field in order to counteract the alternating current induced by the stator which results in a balancing of the torque. In a DC induction motor, the control of the currents in the can be initiated by an external source, this therefore implies that the currents are controlled by the interaction between the stator fields and the resulting currents that have been induced in the rotors of the induction motor. The limitations of using vector control is that optimal torque cannot be produced since the physical design implementation of the AC induction motors comprises of separate rotor and stator in the design (Hambley 102). The vector control concept Vector control methods in AC induction is somewhat similar to the control of a separately excited Direct Current motor whereby the field flux, denoted by Φ1 is produced by the field current Ia is perpendicular to the armature flux Φ2 which is respectively produced by the armature current (Shepherd and Hulley 125). The fields are decoupled and stationary in relation to one another. This therefore implies that the armature current is primarily controlled in order to control the torque; the flux of the field in such context is unaffected and therefore facilitates fast transient response (Trzynadlowski 250). This is of ultimate importance in the control of torque in the DC induction motors. It can be noted that the Torque is directly proportional to the armature current and the field current. Vector control techniques primarily seeks to recreate the orthogonal components that are found in an AC induction motor in order to control the resultant torque that produces current that is separated from the magnetic flux so as to enhance the transient response that is associated with DC motor (Shepherd and Hulley 152). The method Vector control is sometimes known as the Field Oriented Control is an inductor control method that is deployed in variable frequency drives primarily to control the torque and the speed of the 3 phase AC induction motors through the control of the input current that is being fed to the induction motor (Emadi 132). The vector control method is outlined below. The phase currents in the stator are determined and then transformed to the equivalent vector space which is in complex form. The transformed vector space is further transformed into a coordinate system that is in alignment with the rotating rotor of the induction motor; this implies that the current position of the rotor has to be known meaning that a speed measurement is a key requirement. The position of the rotor can be obtained by evaluation of the integral of the speed (Emadi 133). The next procedure in vector control method is the evaluation of the rotor flux linkage vector, which is found through the product of the stator vector current and the magnetizing inductance which is denoted by Lm which is then passed onto a low pass filter with the rotor having a zero load time constant; which is determined by the ratio of the inductance of the rotor to the resistance of the rotor. With the flux linkage vector available, the stator current vector undergoes further transformation that results to state current vector being expressed in a coordinate system with the real x-axis being in alignment with rotor flux linkage vector (Emadi 135). The x axis component of the stator current vector, which is real, can be helpful in controlling the flux linkage of the rotor. The imaginary component is further used in controlling of the torque of the induction motor (Emadi 139). The reference values of the above currents can be monitored and corrected through the use of proportional Integrators. Other approaches that can be used in controlling the rotor currents is through the use of bang-bang type current control, which is known to offer greater and better dynamics. When the PI controllers are used, the output of the PI forms the x-y components that are used as a voltage reference vector representation for the stator (Emadi 150). Decoupling technique is applied at the PI in order to enhance the control performance in situations whereby they are transient response such as rapid alterations in the motor speed, current changes and changes in the flux linkage. In most cases, low pass filters are deployed both at the input and output of the PI controller in order to curb the ripple effect of the current which is caused by transistor switching. The low pass filtering also serves to stabilize the induction motor control process. A significant limitation of using low pass filtering is that it limits the various dynamics that are used in the control system of the induction motors. This therefore implies that higher switching frequencies of more than 10 kHz are needed in order to facilitate minimum filtering that is a requirement for high performance motor drives like the servo motor (Emadi 153). The next step involves the transformation of the voltage references into stationary coordinate systems, which is done by using the rotor d-q coordinate system. The transformed voltages are then fed into a modulator that uses the Pulse Width Modulation strategy to determine the width of the pulse of the stator phase voltages and also aids in controlling of the transistors (Emadi 160). The vector control method generally uses the following attributes / variables in the control process (Gottlieb 156): a) Changes in the reference values imply a change in the speed and torque of the induction motor. b) The modulator determines the switching frequency and ensures that they remain constant as required c) In cases where the Proportional integrators are used, the step response is marked by an overshoot. d) Speed is primarily used to obtain the rotor position through evaluation of its integral. e) The control parameters of the induction motors play a significant role in determining the accuracy of the torque. Errors may be reported due to factors such as changes in the rotor temperature. Implementation of the vector control method It is conventional that Iq and Id of the rotating reference frame should be controlled in order to offer good control of the induction motor (Hambley 126). The vector control method uses closed loop ordered control variables of Id and Iq through comparing of the actual values that are measured from the induction motor. Establishing the motor values requires transformations on the measured values of the three phase stator currents to direct and quadrature variables of the rotating reference frame (Trzynadlowski 123). The resulting error values are then transformed back to the original three phase variables which are then applied to the induction motor. The figure below shows the process of vector control implementation process. The role of the flux position calculator is to majorly produce the correct orientation of the field through ensuring that ids are in alignment with the rotor flux (Shepherd and Hulley 125). The Angular orientation of the rotor flux in the induction motor can be either measured directly through the use of the sensors that are embedded in the architecture of the induction motor. The indirect method involves the calculation of the slip angle between the fields of the stator and rotor, through the use of the known characteristics of the rotor and a summation of the characteristics with the physical position of the rotor. The physical position can be determined by use of an incremental encoder which is fitted to the shaft of the motor (Shepherd and Hulley 130). Errors are represented by the difference that exists between the ordered and the actual Id and Iq variables that are input to the induction motor and the Proportional Integrators (PI). PI controllers are not part of the vector control technique but they are sometimes used to provide optimum closed loop control for the induction motors (Shepherd and Hulley). The output variables from the Proportional Integrators are transformed back to the static frame through the use of inverse transforms which are further transformed into three phase components by obtaining the inverse transform of the equation 2. The inverse Clarke and park transforms are indicated below (Shepherd and Hulley 133). Iu = ids Iv = [ 3/2( iqs) – ( ids)/2]0.5 Iw = iu – iv this represents the inverse Clarke Ids = idsr Cos θr – iqsr Sin θr Iqs = idsr Sin θr – iqs Cos θr this equation represents the inverse park The figure 1 above represents a vector control system that can be used for an asynchronous motor which can be used in the control of PMSM. The flux component in PMSM is produced by the use of permanent magnets, the rotor flux that is produced by the PM should rotate at the same speed with respect to the rotor field; this implies that there is no slip (Gottlieb 121). The ordered component Id is put to zero and the rotor angle is predetermined by integration of the rotor speed. The configuration for the system is as indicated in the figure 1 above. Voltage to frequency control in induction motors Voltage to frequency control is a scalar method of controlling the speed of an induction motor. Induction motors basically are divided into types: the Wound-rotor induction motor and the squirrel cage induction motor (Hambley 45). The various voltages to frequency control parameters that are used in induction motors include: a) Pole changing method b) Stator Voltage control method c) The Supply frequency Control approach d) Eddy current Coupling method e) Slip power recovery method and f) Rotor Resistance control method Different types of induction motors use different approaches in controlling the speed of the motor. FD system description A variable frequency drive system actually consists of an AC motor, controlled and an operator interface. Pole changing The frequency speed of an induction motor varies inversely with the number of poles present in an induction motor. This implies that synchronous speed and the motor speed can be adjusted by changing the number of poles in an induction motor. The induction motor manufacturers therefore have to provide for adjustments in the number of poles through the use of pole changing motor and through multi speed induction motor (Shepherd and Hulley 152). The number of poles has to be the same with the winding of the stator for squirrel cage induction motors. The orientation of the wound rotor poses the requirement for provision of the changing the number of poles. Therefore pole changing is only applicable to the squirrel cage induction motor (Shepherd and Hulley155). One of the simplest, though an expensive way of adjusting the number of stator poles in an induction motor is through the use of two separate windings that have been wound for different stator pole numbers. This means that any changes in the connections of the coil groups respectively results in the changes in the number of stator poles. This can be viewed theoretically through the dividing of winding into multiple coil groups, and subsequently taking out the terminals of the groups, different stator pole numbers can be obtained using different arrangements and as a result enhancing pole changing (Shepherd and Hulley160). Stator Voltage control Stator voltage control is a primarily lays emphasis on the slip of the motor. The method uses a constant frequency with varying voltage that is supplied to the stator of the induction motor. The supplied voltage should be below the value that the induction motor is rated (Gottlieb120). To ensure enhance control performance of the induction motor that is functional and operates at a full load slip, the supply voltage must be decreased by a factor of 1/ (2)0.5 if the slip of the induction motor is doubled. The corresponding stator current increases by 20.5 with respect to the full load value. The limiting factor of stator voltage control is the heating effect of the increasing stator current. This therefore implies that the method is not suitable for speed control. The method is preferable in the single phase induction motors that have a constant impedance to counteract the current that the stator draws (Shepherd and Hulley 160). Supply frequency control The synchronous speed of an induction motor is denoted by Ns =120 f/P The motor speed is denoted by Nr = (1-s) Ns It can be seen that changes in the synchronous speed, which can be brought about by variations in the supply frequency, can result to corresponding changes in the motor speed (Gottlieb 89). The voltage that is induced in the stator varies directly as the product of the air gap flux Φm and the supply frequency fs. A decline in the supply frequency, with the stator voltage constant, results to an increase in the air- gap flux (Hambley 56). Induction motors should therefore be designed to function at the knee point of the magnetization curve in order to maximize on the use of the magnetic material. Increasing the flux causes the induction motor to saturate, as a result increasing the magnetizing current which in turn affects the line current and the stator voltage, increase the power loss in the core and the stator of the induction motor which is represented by I2R. Decrease in the amount of flux in the induction motor should be eliminated in order to maintain the torque capability of the motor. This implies that the supply frequency with the voltage should be adjusted to a value defined by V/f (Gottlieb 95). Advantages of using frequency control in induction motors The use of the variable frequency control facilitates the overall enhanced control performance and transient response due to the following attributes of the process: a) Copper losses are usually low coupled with the fact that efficiency and the power factor of the induction motor are high. b) It is possible to control the speed and the breaking operation of the induction motor ranging from the zero speed to the base speed. c) The difference in terms of speed from the full load to no load is small. d) During the rapid changes in the stator voltages, operation can be done with the torque being at maximum and the current reducing, this result in good transient response. Another advantage of using variable frequency control is that a variable speed drive that possesses the above characteristics of good operation and transient response can be built from a squirrel cage induction motor (Emadi 203). The squirrel cage induction motors is more advantageous compared to the DC motor. Some of the advantages of the squirrel cage induction motors are that it is cheap, requires little or no maintenance since the design does not include the commutator and brushes, it is long lasting compared to the DC motor and can still be operated in a rugged and explosive environment with higher speeds, higher voltages and power ratings. The variable frequency control in induction motor has a vast use in industrial application such as: a) It can be deployed in underground water installation projects b) It can be used in applications that require contaminated and explosive environments c) It can be used in applications such as steel mills, blowers, motor drives, power generator and many more Rotor resistance control The torque of an induction motor basically depends on the rotor resistance. However the maximum value of the torque is not determined by the rotor resistance. The slip that is depicted at the maximum torque is also dependent on the maximum torque. It therefore implies that the if there are changes in the value of the rotor resistance, the maximum torque for the induction motor changes to higher values, while the constant torque is maintained (Shepherd and Hulley 195). The slip that is observed when the maximum torque occurs increases with a corresponding increase in the rotor resistance. The starting torque also increases with an increase in slip. The rotor resistance control can also be employed in induction motors that generally require high starting torque. Voltage and Frequency Control of the Grid Source (Shepherd and Hulley 152). A voltage is described as the measure of electrical push a circuit has.Voltages are usually used to control how much each module does and what it is designed to do. Impementation of Voltage to Frequency Control Voltage –to- Frequency Converter (VFC) Voltage-to-frequency converter (VFC) circuit is shown in the diagram below and normally the Circuit employs 555IC which is perceived as a core function of the circuit. Source (Shepherd and Hulley 152). The above circuit actually uses a +5V and -5V supply voltages though the input is usually limited to approximately 0.2V.It is possible for anyone to adjust the zero point through grounding the input and by adjusting the 4k7 pot primarily to obtain the lowest frequency oscillation as much as possible. This Frequency to Voltage Converter actually operates on the principle of charge balance, the oscilation process which is usually seen as a long term balance of charge or average current between the input signal current and the reference current. In order to maintain the speed of the motor more constant when loads are applied, a control application of the circuit is usually applied. In such cases a H-Bridge circuit is usually used to control the speed as well as the direction of the motor(Shepherd and Hulley 152). The diagram below actually shows the H-Bridge together with the 4 inputs in relation to the external power supply. In such cases the control application is usually used to allow the motor operation in a forward and reverse directions. From the diagram the motor will rotate in one direction. Example if Q4 and Q2 are on (Q1 and Q3 are off) and vice versa. If, transistors (Q3 and Q2) and/or (Q4.From the diagram the power supply will be shortened to the ground. The transistors in normal circumstances are usually protected from large reverse voltages by currents of the diodes (D1-D4), while Resistors R1-R4 limit the currents into the bases as per the circuit configurations (Shepherd and Hulley 152). (Shepherd and Hulley 152) Comparisons of the methods Both methods work on the basic principle of electromagnetic induction; which results in the production of a voltage in a conductor primarily due to the effect of the magnetic field. Secondly, both methods are normally used during specific control situations and there implementation is almost the same. In conclusion the best method of control is Voltage to Frequency is easy to be used and simple in nature. Moreover, it can be adjusted to suit particular voltage control situations. Voltage to Frequency is the best controlling method since it comprises of two simplest approaches to induction motor controlling (Shepherd and Hulley 152). The first simplest approach is based on the alteration of the orientation of the stator design and winding structure which ensures that the starting current is reduced to desired levels. Another basic way is through the use of pole changing technique, whereby the numbers of magnetic poles in the stator are changed to desired levels (August and Hand 36).Unlike the Vector control which is only effective in high performance drive systems. Additionally, the vector control methods primarily rely on the concept of the space vectors for induction motors which employs the use of instantaneous values that are obtained in the respective three phase variables of the induction motors(Emadi 56). References August, Hand and Augie Hand. Electric motor maintenance and troubleshooting. New York: McGraw-Hill, 2001. Emadi, Ali. Handbook of automotive power electronics and motor drives. Boca Raton : Taylor & Francis, 2005. Gottlieb, Irving. Electric motors & control techniques. New York : TAB Books, 1994. Hambley, Allan. Electrical engineering: principles and applications, Part 1. New York: Prentice Hall, 2005. Shepherd, William and Norman Hulley. Power electronics and motor control. Cambridge: Cambridge University Press, 1995. Trzynadlowski, Andrzej M. Control of induction motors. San Diego: Academic Press, 2001. Read More