Electrical Equipment Handbook-7-Speed Control of Induction Motors [PDF]

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Source: ELECTRICAL EQUIPMENT HANDBOOK



CHAPTER 7



SPEED CONTROL OF INDUCTION MOTORS



Until the advent of solid-state drives, induction motors were not used in many applications requiring speed control. The normal operating range of an induction motor is within less than 5 percent slip. At larger slip, the efficiency of the motor will drop significantly because the rotor copper losses are directly proportional to the slip of the motor (PRCL ⫽ sPAG). The speed of an induction motor can be controlled by varying the synchronous speed or the slip for a given load. The synchronous speed can be varied by changing the electrical frequency or the number of poles. The slip can be changed by varying the rotor resistance or terminal voltage.



SPEED CONTROL BY CHANGING THE LINE FREQUENCY The rate of rotation of the stator magnetic field depends on the electrical frequency. The noload point on the torque-speed curve changes with the frequency (Fig. 7.1). The base speed is the synchronous speed at rated conditions. The speed of the motor can be adjusted by using variable frequency control. A variable frequency induction motor drive can control the speed from 5 percent of the base load to twice the base speed. There are limits on the voltage and torque as the frequency is varied to ensure safe operation. When the speed is being reduced below the base speed, the terminal voltage to the stator should be decreased linearly with decreasing stator frequency. This process is called derating. If the motor is not derated, the steel in the core will saturate and large magnetization current will flow in the machine. The flux in the core of an induction motor is given by Faraday’s law: d␾ υ(t) ⫽ N ᎏ dt Solving for the flux ␾ gives



冕 1 ⫽ ᎏ 冕V sin ␻t dt N



1 ␾ ⫽ ᎏ υ(t) dt N



M



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SPEED CONTROL OF INDUCTION MOTORS 7.2



CHAPTER SEVEN



If the electrical frequency decreases by 10 percent while the voltage remains constant, the flux in the core will increase by 10 percent. The magnetization current will also increase by 10 percent in the unsaturated region of the motor’s magnetization curve. The magnetization current will increase by much more than 10 percent in the saturated region. Since induction motors are designed to operate near saturation, the increase in flux due to the decrease in frequency will cause a large magnetization current to flow. The stator



FIGURE 7.1 Variable-frequency speed control in an induction motor: (a) The family of torque-speed characteristic curves for speeds below base speed, assuming that the line voltage is derated linearly with frequency. (b) The family of torque-speed characteristic curves for speeds above base speed, assuming that the line voltage is held constant.



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SPEED CONTROL OF INDUCTION MOTORS



SPEED CONTROL OF INDUCTION MOTORS



7.3



FIGURE 7.1 (Continued) (c) The torque-speed characteristic curves for all frequencies.



voltage is usually decreased in direct proportion to the decrease in frequency to avoid large magnetization currents. The flux in the motor remains approximately constant when the voltage is decreased with frequency. Since the power supplied to the motor is given by P ⫽ 兹3 苶VLIL cos ␪ the maximum power rating must decrease linearly with decreasing voltage to protect the stator from overheating. Figure 7.1a illustrates a family of torque-speed characteristic curves for speeds below the base speed. The stator voltage was assumed to vary linearly with frequency. When the frequency applied to the motor exceeds the rated frequency, the stator voltage is held constant at the rated value. Although the applied voltage can be raised above the rated value without reaching saturation, it is limited to the rated voltage. This is done to protect the winding insulation of the motor. As the frequency increases while the voltage remains constant, the resulting flux and the maximum torque will decrease with it. Figure 7.1b shows a family of torque-speed characteristic curves for speeds higher than the base speed, assuming that the stator voltage is held constant. Figure 7.1c shows a family of torque-speed characteristic curves for speeds higher and lower than the base speed, assuming that the stator voltage is varied linearly with frequency below base speed and is held constant at rated value above base speed (the rated speed for the motor shown in Fig. 7.1 is 1800 r/min). Changing the line frequency with solid-state motor drives has become the preferred method for induction motor speed control.



SPEED CONTROL BY CHANGING THE LINE VOLTAGE Since the torque developed by the induction motor is proportional to the square of the applied voltage, the speed of the motor can be controlled within a limited range by varying the line voltage as shown in Fig. 7.2. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.



SPEED CONTROL OF INDUCTION MOTORS 7.4



CHAPTER SEVEN



FIGURE 7.2 Variable-line-voltage speed control in an induction motor.



FIGURE 7.3 Speed control by varying the rotor resistance of a wound rotor induction motor.



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SPEED CONTROL OF INDUCTION MOTORS



SPEED CONTROL OF INDUCTION MOTORS



7.5



SPEED CONTROL BY CHANGING THE ROTOR RESISTANCE The shape of the torque-speed curve of wound rotor induction motors can be changed by inserting extra resistances into the rotor circuit, as shown in Fig. 7.3. However, inserting additional resistances into the rotor circuit will reduce the efficiency of the motor significantly. This method is usually used for short periods.



SOLID-STATE INDUCTION MOTOR DRIVES The solid-state variable frequency induction motor drive is the preferred method for speed control. A typical drive is shown in Fig. 7.4. The drive is very flexible. Its input can be single-phase or three-phase; 50 or 60 Hz; and any voltage in the range of 208 to 230 V. The output is a three-phase voltage whose frequency can vary in the range of 0 to 120 Hz and whose voltage can vary in the range of 0 to the rated voltage of the motor. The control of the output voltage and frequency is achieved by using the pulse-width modulation (PWM) technique. The output frequency and output voltage can be controlled independently. Figure 7.5 illustrates how the drive controls the output frequency while the root-mean-square (rms) voltage is maintained at a constant level. Figure 7.6 illustrates how the drive controls the rms voltage while maintaining the frequency at a constant value.



FIGURE 7.4 A typical solid-state variablefrequency induction motor drive. (Courtesy of MagneTek Drives and Systems.)



MOTOR PROTECTION The induction motor drive has a variety of features for protecting the motor. The drive can detect and trip the motor under any of the following conditions: 1. 2. 3. 4.



An overload (excessive steady-state currents) Excessive instantaneous currents Overvoltage Undervoltage



THE INDUCTION GENERATOR Figure 7.7 illustrates the torque-speed characteristic of an induction machine. It shows clearly that if an induction motor is driven at a speed higher than the synchronous speed by



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SPEED CONTROL OF INDUCTION MOTORS 7.6



CHAPTER SEVEN



FIGURE 7.5 Variable frequency control with a PWM waveform: (a) 60-Hz 120-V PWM waveform; (b) 30-Hz 120-V PWM waveform.



FIGURE 7.6 Variable voltage control with a PWM waveform: (a) 60-Hz 120-V PWM waveform; (b) 60-Hz 60-V PWM waveform.



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SPEED CONTROL OF INDUCTION MOTORS



SPEED CONTROL OF INDUCTION MOTORS



7.7



FIGURE 7.7 The torque-speed characteristic of an induction machine, showing the generator region of operation. Note the pushover torque.



a prime mover, the direction of the induced torque will reverse and it will act as a generator. As the torque applied to the shaft increases, the power generated increases. However, there is a maximum possible induced torque in the generator region of operation (pushover torque). If the actual torque is higher than the pushover torque, the machine will overspeed. An induction machine operating as a generator has severe limitations. An induction generator cannot produce reactive power because it does not have a separate field circuit. In reality, it requires reactive power. An external source of reactive power must be provided to it at all times to maintain its stator magnetic field. The induction generator cannot control its own output voltage because it does not have a field circuit. The terminal voltage of the generator must be maintained by the external power system which is connected to it. The main advantages of the induction generator are its simplicity and its ability to operate at different speeds (higher than synchronous speed). Since no sophisticated regulation is required, this generator is suitable for windmills and supplementary power sources connected to an existing power system. In these applications, the power factor correction can be provided by capacitors, and the terminal voltage can be controlled by an existing power system (the grid).



Induction Generator Operating Alone The induction generator can operate independently of any power system, if capacitors are available to supply the reactive power required by the generator and by the load. This arrangement is shown in Fig. 7.8. The magnetization current required by the induction machine as a function of the terminal voltage can be found by running the machine as a motor at no load and measuring its armature current. This magnetization curve is shown in Fig. 7.9a. Therefore, the induction generator can achieve a given voltage level if the external capacitors are supplying the magnetization current corresponding to that level. The reactive current produced by a capacitor is directly proportional to the voltage applied to it (straight-line relationship). Figure 7.9b illustrates the variation of voltage with current for a given frequency.



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SPEED CONTROL OF INDUCTION MOTORS 7.8



CHAPTER SEVEN



FIGURE 7.8 An induction generator operating alone with a capacitor bank to supply reactive power.



The induction generator must be flashed by momentarily running it as a motor. This is done to establish residual flux in the rotor, which is needed to start the induction generator. When the induction generator is starting, a small voltage is produced by the residual magnetism in its field circuit. A capacitive current flow is produced by the small voltage which increases the terminal voltage. The increase in terminal voltage increases the capacitive current, which increases the terminal voltage further until the voltage is fully built up. The main disadvantage of induction generators is that their voltage varies significantly with changes in load (especially reactive load). Figure 7.10 illustrates a typical terminal voltage-current characteristic of an induction generator operating alone with a constant parallel capacitance. The voltage collapses very rapidly when the generator is supplying inductive loads because the capacitors must supply all the reactive power needed by the load and the generator. Any reactive power diverted to the load moves the generator back along its magnetization curve. This results in a major drop in generator voltage. A set of series capacitors is included in the power line in addition to the parallel capacitors. The capacitive reactive power increases with increasing load. This compensates for the reactive power demanded by the load. Figure 7.11 illustrates the terminal characteristic of an induction generator with series capacitors. The frequency of the induction generator varies slightly with the load. However, this frequency variation is limited to less than 5 percent because the torque-speed characteristic is very steep in the normal operating range. This variation is acceptable in many applications such as isolated or emergency generators. The induction generator is ideal for windmills and energy recovery applications. Since most of these applications operate in parallel with the grid, the terminal voltage and frequency are controlled by the grid. Capacitors are used for power factor correction.



INDUCTION MOTOR RATINGS Figure 7.12 shows a nameplate for a typical high-efficiency induction motor. The most important ratings are 1. Output power 2. Voltage 3. Current



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SPEED CONTROL OF INDUCTION MOTORS



SPEED CONTROL OF INDUCTION MOTORS



4. 5. 6. 7. 8.



7.9



Power factor Speed Nominal efficiency NEMA design class Starting code



FIGURE 7.9 (a) The magnetization curve of an induction machine. It is a plot of the terminal voltage of the machine as a function of its magnetization current (which lags the phase voltage by approximately 90°). (b) Plot of the voltage-current characteristic of a capacitor bank. Note that the larger the capacitance, the greater its current for a given voltage. This current leads the phase voltage by approximately 90°. (c) The noload terminal voltage for an isolated induction generator can be found by plotting the generator terminal characteristic and the capacitor voltage-current characteristic on a single set of axes. The intersection of the two curves is the point at which the reactive power demanded by the generator is exactly supplied by the capacitors, and this point gives the no-load terminal voltage of the generator.



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SPEED CONTROL OF INDUCTION MOTORS 7.10



CHAPTER SEVEN



FIGURE 7.10 The terminal voltage-current characteristic of an induction generator for a load with a constant lagging power factor.



FIGURE 7.11 (a) A “compounded” induction generator, one with both “shunt” (parallel) and series capacitors. (b) The resulting voltagecurrent characteristic of the generator for a load with a constant lagging power factor.



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SPEED CONTROL OF INDUCTION MOTORS



SPEED CONTROL OF INDUCTION MOTORS



7.11



FIGURE 7.12 The nameplate of a typical high-efficiency induction motor. (Courtesy of MagneTek, Inc.)



The voltage limit is based on the maximum acceptable magnetization current flow because as the voltage increases, the iron becomes more saturated and the magnetization current increases. A 60-Hz induction motor can be used on a 50-Hz power system only if the voltage rating is decreased by the same proportion as the decrease in frequency. The current limit is based on the maximum acceptable heating in the motor’s windings. The power limit is determined by the combination of the voltage and current ratings with the power factor and efficiency.



REFERENCE 1. S. J. Chapman, Electric Machinery Fundamentals, 2d ed., McGraw-Hill, New York, 1991.



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SPEED CONTROL OF INDUCTION MOTORS



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