Vector Control for Induction Motor - Case Study Example

Executive summary The purpose of this report is to explore the V/F control and vector control methods for the control of induction motors. The report introduces these two methods in details by focusing each at a time. The report then examines the uses of each control method, its advantages as well as the disadvantages. Comparisons are then made to indicate both the similarities and the differences that exist between them. The report concludes by reviewing which each of the method is and how it is used in controlling the speed of induction motor. However, it is realized that the vector method is the best between these two. Table of Contents 1.0 Introduction 3 2.0 Induction control methods 3 2.1 V/F control for induction motor 3 2.1.1 Description of V/F control for induction motor 3 2.1.2 Uses of V/F control for induction motor 4 2.1.3 Advantages of V/F control for induction motor 5 2.1.4 Disadvantages of V/F control for induction motor 5 2.2 Vector control method 6 2.2.1 Description of Vector control for induction motor 6 2.2.2 Uses vector control for induction motor 7 2.2.3 Advantages of V/F control for induction motor 7 2.2.4 Disadvantages of V/F control for induction motor 8 3.0 Comparisons between the vector control and the V/F control for induction motor 8 3.1 Similarities 8 3.2 Differences 8 4.0 conclusion 9 5.0 References 10 Introduction 1.1. Mukhtar (2010) argues that advances in solid state power devices have resulted into the development of variable speed AC induction motors that are powered by switching power converters. 1.2. The switching power converters control both the magnitude and frequency of the current and voltage applied to a motor. For much higher performance and efficiency, the motor drives should generate less noise. 1.3. The speed of an induction motor which is given by N=120f/p (1-S) where f= frequency of supply, p=no of poles, s=slip; can be changed by either controlling the number of poles, slips or the frequency. 1.4. The common principle applied is the use of constant V/Hz in which the magnitude and frequency of the voltage applied to the stator of a motor maintains a constant ratio. In this manner, the magnitude of the magnetic field is kept at an approximately constant level throughout the operating range (Atmel 1994). 1.5. Atmel (1994) says that another principle which can be applied is the use of vector control. This technique regulates both the amplitude and phase angle of the AC excitation voltage. 2. Induction control methods 2.1. V/F control for induction motor 2.1.1. Description of V/F control for induction motor 2.1.1.1. The induction motors can be controlled using the Volts/Hz control in which the frequency of supply f is varied (Mukhtar 2010). It is possible to obtain a variable frequency supply with good quality output wave shape as shown in the diagram below. Fig 1. Voltage verses frequency curve under the constant V/F control derived from Trzynadlowski 1994. 2.1.1.2. However, the frequency control also requires proportional control in applied voltage, because the stators flux, λs =νs/ωs (neglecting the resistance drop) remain constant. Otherwise, if frequency alone is controlled, then the flux will change. 2.1.1.3. Increasing the frequency results into a reduction in the flux and thus decreasing the torque by the motor. 2.1.1.4. Decreasing frequency results into an increase in the flux and this leads to the saturation of the magnetic circuit. 2.1.1.5. The drives in the PWM inverters are fed from a PWM nerve since their voltage and frequency can be controlled independently. 2.1.1.6. In such a control scheme, the motor is supplied by three phase supply through the filter, diode rectifier and PWM inverters. 2.1.1.7. Depending on the desired speed, the method uses frequency command which is applied to the inverter and thus directly generating the phase voltage command from the frequency command by a gain factor, and input dc voltage of the inverter is controlled. 2.1.1.8. The voltage command signal for the inverter is also generated using frequency command and V/F function generator. 2.1.1.9. When the ratio V to f is made to be constant with a change in f, then the magnitude remains constant with the torque being independent of the frequency supply. Thus, the ratio between the frequency and magnitude of the stator voltage should be based on the rated values of these variables. According to Tzou & Hsu (1997), both low frequency and low voltage leads to a drop across the stator resistance which cannot be neglected and thus must be compensated. 2.1.2. Uses of V/F control for induction motor 2.1.2.1. The method is used in applications such as blower and fan drives where the speed response at a lower end is not serious (Rashid 2006). 2.1.2.2. The open loop Volts/ Hz control is a popular speed control method for induction motor drives that do not require high accuracy. 2.1.2.3. Besides, this method is used in low performance application where precise speed control is not required. 2.1.3. Advantages of V/F control for induction motor 2.1.3.1. In this method, the stator flux remains constant despite of the change in frequency supply and thus the torque developed depends on the slip speed only. Regulation of the slip speed makes it easy to control the speed and torque of an AC induction motor by the use of the V/Hz principle (Mukhtar 2010). 2.1.3.2. The use of the closed loop speed control through the control of the slip speed of the motor results into accuracy in speed response and thus keeping the motor speed at its set value. 2.1.3.3. It is only the rated, maximum or minimum frequency information is required to implement the method since the maximum voltage (rated voltage) is applied to the motor at a rated frequency. 2.1.4. Disadvantages of V/F control for induction motor 2.1.4.1. This method does not precisely control the speed of the motor since the frequency control only controls the synchronous speed. 2.1.4.2. According to Mukhtar (2010), there is also a small variation in speed of the motor under load conditions. Such a variation is not much when the speed is high. When working at low speeds the frequency is low, and if the voltage is also reduced then the performance of the motor deteriorates due to large value of static resistance drop. For low speed operation, the relationship between the voltage and frequency is given by v=v0+kf where v0 =voltage drop in the stator resistance. 2.1.4.3. For applications that do not have a concern for the accuracy in speed response such as heating air conditioning, fan and/ or blower applications, an open loop speed control is used whereby the supply frequency is determined by the assumption that the motor follows a synchronous speed and is based on the recommended speed. In this case, the speed error as a result of the slip of the motor is regarded as acceptable and yet it should not be the case. 2.1.4.4. Another disadvantage is that for frequencies that are greater than the rated value, the principle applied in this method has to be violated in order to avoid the breakdown of insulation by ensuring that the stator voltage should not exceed its rated value (Rashid 2006). 2.2. Vector control method 2.2.1. Description of V/F control for induction motor 2.2.1.1. The induction motors can also be controlled using the vector control method where the phase angle and the magnitude of the excitation current are controlled (Nagrath & Kothari 2004). 2.2.1.2. In this method, the stator current is changed into a reference frame where the two parameters can be controlled independently, one for the electromagnetic torque and the other for the rotor flux. 2.2.1.3. The basic principle of vector control is to separate the components of the stator current which is responsible for the production of flux and the torque. The diagram below shows the vector control algorithm for induction motor Fig.2. Vector control algorithm for induction motor derived from Rashid, 2006. 2.2.2. Uses of V/F control for induction motor 2.2.2.1. The vector control method is used in AC machines to overcome the effects of inductance, stator resistance and induced voltage just as in DC machines. 2.2.2.2. It is also used to maintain a 900 spatial orientation of the rotor flux with respect to the stator torque producing current component using sensor feedback signals and model based computations. 2.2.2.3. The vector control methods that are based on dq modeling are used for permanent magnet synchronous machines. 2.2.3. Advantages of Vector control for induction motor 2.2.3.1. Nagrath & Kothari (2004) argues that vector control allows the decoupling of the torque and flux thus making the control of the induction to be similar to the control of a separate excited DC motor which is much easier to control. 2.2.3.2. Vector control achieves a superior performance under speed and torque change than the widely used separately excited dc motor in the industry. 2.2.3.3. In this method, there is no magnetic coupling between the armature circuit and the field circuit and thus, the armature current can be changed independently, and the torque can be controlled faster, keeping field flux constant (Ustun 2008). 2.2.3.4. It controls the spatial orientation of the EMF in the machine leading to the coining of the term FOC which is used to controllers that maintain a 900 spatial orientation between the critical field components. 2.2.4. Disadvantages of Vector control for induction motor 2.2.4.1. Vector controls are only applied to machines where it is possible to transform the machine electrical variables into a suitable reference frame. 2.2.4.2. In this method, the control of the flux and the torque (thereby speed) in the induction motor requires the control of the phase and the magnitude of three stator currents using a fast inverter (Nagrath & Kothari 2004). In performing such a purpose, there is need to highly involve a current controlled voltage source inverter (CC-VSI). 3. Comparisons between the vector control and the V/F control for induction motor 3.1. Similarities 3.1.1. Both the v/f control and the vector control methods are used to control the motors since they include minimum harmonics in accordance to other PWM techniques. 3.1.2. In both methods, to control the induction motor there must be a three phase supply of power through the filter, diode rectifier and PWM inverters (Zhou &Wang 2002). 3.2. Differences 3.2.1. In vector control the variables (the torque and flux current vectors) are controlled independently unlike in V/F control where a change in the frequency supply leads to a change in the flux. 3.2.2. According to Nagrath & Kothari (2004), the vector/ field oriented control has some properties that make it more favorable to offer a high level of dynamic performance as compared to the V/F control. Besides, the closed loop control associated with such a drive provides a long term stability of the system in the vector control than in V/F control. 3.2.3. The V/F method is a scalar control method which does not have fast response characteristics desirable for traction applications while the vector control method has desirable characteristics (magnitude and phase angle) which are both controlled. 3.2.4. The V/F control is a simple technique that does not require much knowledge in controlling the speed of the induction motor while the vector control is a complex technique which requires the knowledge of rotor flux whose computation requires the use of advanced algorithms of voltages and terminal current using the AC induction motor. 4. Conclusion The induction motors are controlled using various methods where the V/F control (a scalar control method), and vector control are among them. The V/F method is used to control the induction motors by changing the supply frequency while the vector control can be used by controlling the magnitude and phase angle of the current. However, the vector method is regarded as the best when considering the above discussed methods. 5. References Atmel A. (1994). AC Induction Motor Control using the constant V/f Principle and a Natural PWM Algorithm, New York: Tata McGraw-Hill Husain, I. Electric and Hybrid Vehicles: Design Fundamentals. California: Springer, Leonhard, W. (1996). Control of Electrical Drives, (2nd Ed), New York: Springer Mukhtar, A. (2010). High Performance AC Drives: Modeling Analysis and Control, New Jersey Springer Nagrath, I.J & Kothari, D.P. (2004). Electric Machines, New York: Tata McGraw-Hill Rashid, M. H. (2006). Power Electronics Handbook: Devices, Circuits, and Applications, London: Academic Press Toliyat, F. A & Campbell, S.G. (2004). DSP-Based Electromechanical Motion Control. London: CRC Press  Trzynadlowski, A. (1994). The field orientation principle in control of induction motors. USA: Springer Tzou, Y. Y & Hsu H.J. (1997), “FPGA Realization of Space-Vector PWM Control IC for Three-Phase PWM Inverters.” IEEE Transactions on Power Electronics, 12(6), pp. 953-963 Ustun S. V. (2008). “Optimal tuning of PI coefficients by using fuzzy-genetic for V/f controlled induction motor Expert Systems with Applications.” An International Journal, 34 (4), pp. 228-326. Zhou, K. &Wang, D. (2002). “Relation between space-vector modulation and three-phase carrier-based PWM.” IEEE Transactions on Industrial Electronics, 49(1), pp. 186-196 Read More