Voltage to Frequency Control vs Vector Control - Essay Example

Name: Instructor: Course: Date: Comparison between Voltage to Frequency Control and Vector Control INTRODUCTION Various control techniques for induction motors have been effectively implemented, facilitating the motor to be applied in a wide range of applications. Two commonly used techniques include scalar and vector, with the most popular scalar method being the voltage to frequency, also commonly referred to as volts to hertz control. Voltage to frequency (V/F) control assumes that the controlled voltage is sinusoidal, and therefore, adjusts only the frequency without regard for the spatial phase of the corresponding vector parameters. For this reason, this type of control finds wide applications in low-performance industrial drive systems. By contrast, vector control bases its principle of operation on the “Field Orientation Principle” to adjust the magnitude and spatial position of the vector parameters of the induction motors (Trzynadlowski 43). Vector controlled motors are applied in high-performance drive systems in which the instantaneous magnetic field and torque of the motor are controlled, both under momentary and constant operating conditions. This report provides a comparison between V/F control and vector control regarding their descriptions, applications, advantages, and limitations. V/F CONTROL Description A V/F control involves a relationship that remains essential to the operation of induction motors utilizing the concept of flexible frequency control. It bases its principle of operation on the theory that an AC induction motor generates torque due to its rotating field’s flux. As such, the control method aims to control the value of stator voltage in respect to the frequency, in an attempt to keep the flux of the motor’s stator steady and in turn, allow the stator to generate a full load torque. Based on this, Trzynadlowski argues that V/F control involves adjusting the supply frequency in order to regulate the speed of the rotating flux of the stator (43). The moment of rotation thus developed in the induction motor depends entirely on the slip speed – difference in stator and rotor speeds. Under steady conditions, the motor’s air-gap magnetic flux approximates to the relationship,, where represents the phase voltage’s amplitude of the motor, and represents the applied synchronous electrical frequency (Goldberg 33). The following diagram shows operation of a typical V/F control technique. The motor base point describes the characteristics of the V/F controller. Below this point, the induction motor ‘operates overexcited’ due to the steady state, while above the point, it ‘operates underexicited’ due to voltage limit of the DC-bus (Brejl, Princ, & Uhlir) In addition to the constant V/F control, other versions of V/F control include the closed-loop, current source, and variable voltage source systems. Among the three, the closed-loop system is the second, after the constant method, commonly applied V/F control. It applies an analog sensor in its control loop to regulate the actual speed of the motor. Advantages One of the chief benefits of V/F control regards its strength under steady-state conditions. In constant V/F control, the value of an increase in torque is usually 100 ms. As a result, Holtz argues that lack of a closed-loop mechanism, and limitation to stable performance make V/F based drive systems extremely strong (1365). V/F-regulated systems also show robust function even in the very low speed ranges, where vector-controlled systems fail to show robustness. In addition, V/F control is simple to implement, and in turn, allows it to be integrated in extremely complex electronic devices. Its simplicity makes it a cost-effective control technique to use in induction motors for light and low-performance industrial and home applications. Disadvantages V/F control remains inadequate for high demand drive systems, mainly because its principle of control is imprecise. It does not respond well to extreme speed fluctuations, which limits its application to industrial systems that need little servo performance. Applications V/F control is commonly applied in home and industrial appliances not only for its simplicity, but because it also provides a cost-effective way to ensure stability in light and low servo performance appliances. According to Kataoka, Sato, and Bendiadellah, industries find it economically- and technically-feasible to apply V/F-regulated induction motors due to their “novel control system orientations” (456). Common appliances using V/F-controlled motors include pumps, fan drives, grinders, centrifuges, etc. VECTOR CONTROL Description Also called field-oriented control, vector control involves controlling both the phase and magnitude of a vector variable. It decouples the moment of force generating currents supplied to an induction motor and the magnetizing flux and regulates them separately. Rashid argues that the control technique transforms instantaneous currents of the stator to a rotary reference plane parallel to the stator, motor’s rotating element, or air-gap flux vectors, to generate two current components (one for generating flux and the other for torque) (1046). By maintaining the flux component at a fixed known value, the torque of the motor linearly corresponds to the torque element of the current, making vector-controlled induction motors resemble, in operation, an independently energized DC motor drive system. The following block diagram shows how a vector control method operates. From the diagram, motor line currents, ia, ib, and ic are transformed to and in the fixed reference plane, and then to the simultaneously revolving plane d-q currents, ids and iqs. On the other hand, two inverse conversions occur in the controller: from simultaneous d-q plane to the fixed d-q reference plane, and from d+-Q+ to a+, b+, and c+ (Holtz 1369) Based on the above explanation, input command to a vector control comprises of torque or magnetizing flux, with the flux command being chosen depending on the operation conditions in the steady-state horsepower or steady-state moment of force. On the other hand, the output command consists of the 3-phase orientation voltages. Vector control method is implemented by regulating the motor phase currents in accordance with the orientation voltage commands. It is further classified into two: direct and indirect control methods. Direct field-oriented control applies an air-gap flux transducer or Hall Effect sensor to indicate the value of the air gap magnetic flux. In this way, it is able to regulate the flux in order to create an adjustable torque of the motor. Indirect vector control (open loop or closed loop) measures the flux from motor characteristics such as the current and velocity (Toliyat, Niazi, & Godbole). A closed loop field-oriented control applies speed feedback transducer to regulate motor speed and achieve optimum torque over a speed range extending from zero to base speed. On the other hand, an open loop vector control regulates current to the motor instead of the speed and as such, it has a narrower speed range compared to the closed loop method, which limits its ability to create holding torque when the motor is at rest. In addition to these two vector methods, sensorless field-oriented control is another vector control that finds wide application to control induction motors. Rather than relying on sensors to measure the position of the rotor, it applies other motor characteristics to regulate the motor coil position in order to generate the needed moment of force. Advantages Vector control offers an effective method to maintain the continuity of the torque control during the transient states of the motor drive system. This is because in vector control, space vectors of 3-line motor system are adjusted depending on the control algorithm. Moreover, the control method exhibits constant and dynamic performance compared to V/F control method. Another advantage includes enhanced efficiency, making it possible to replace larger drive systems with smaller ones without compromising speed and torque (Toliyat, Niazi, & Godbole) Disadvantages Vector control configurations are extremely complex to implement compared to V/F control configurations. They always require use of current and voltage transducers, and for enhanced performance level, the configurations may call for application of position and speed transducers. All these components make the overall control technique expensive and thus, cost-ineffective for industrial applications (Goldberg 34). Moreover, indirect vector-controlled drive systems make use of precision shaft encoders in order to ensure accuracy of motor operation. This calls for extra electronic components and other requirements, some of which may interfere with the stability required in induction motors. Applications Due to its ability to offer steady state and dynamic performance, vector control is widely used in industrial appliances operating at high-speeds and requiring high servo performance. Vector-controlled motors are commonly applied in generator and turbine drive systems, servos, actuators for robots, etc (Rashid 744). CONCLUSION Voltage to frequency control aims to regulate the value of stator voltage in respect to the frequency, in an attempt to keep the flux of the motor’s stator steady and in turn, allow the stator to generate a full load torque. V/F-regulated systems show operational stability even in the very low speed ranges, where vector-controlled systems fail to show robustness. However, it remains inadequate for high demand drive systems, mainly because its principle of control is imprecise. Common appliances using V/F-controlled motors include pumps, fan drives, grinders, and centrifuges. On the other hand, vector control decouples the moment of force generating currents supplied to an induction motor and the magnetizing flux and regulates them separately. Field-oriented control exhibits constant and dynamic performance compared to V/F control. Nevertheless, it requires use of current and voltage transducers, which makes its implementation and expensive for small or light industrial applications. As such, it is widely used for high servo performance drive systems such as turbines and generators. Works Cited Brejl, Milan, Michael Princ, and Petr Uhlir. “AC Induction Motor Volts per Hertz Control with Speed Closed Loop, Driven by eTPU on MPC5500.” Freescale Semiconductor AN3205 (2006). Web. 24 Dec. 2010. Goldberg, Lee. Green Electronics, Green Bottom Line: Environmentally Responsible Engineering. London, UK: Newness, 2000. Print. Holtz, Joachim. “Sensorless Control of Induction Motor Drives.” Proceedings of the IEEE 90.8 (2002): 1359-1365. Print. Kataoka, T., Y. Sato, A. Bendiabdellah. “A Novel Volts/Hertz Control Method for an Induction Motor to Improve the Torque Characteristics in the Low Speed Range.” Power Electronics and Applications 5 (1993): 455-458. Print. Rashid, M.H. Power Electronics Handbook: Devices, Circuits, and Applications. 2nd ed. San Diego, CA: Academic Press, 2006. Print. Toliyat, Hamid, Peyman Niazi, and Kedar Godbole. Overcoming Vector Control Challenges. Mechatronic-design, 5 Dec. 2007. Web. 25 Dec. 2010. Trzynadlowski, Andrzej. The Field Orientation Principle in Control of Induction Motors. Berlin: Springer, 1994. Print. Read More