Asenkron motorlarda darbe genişlik modülasyonlu frekans çevirici ile hız denetimi

Abstract

Bu tezde, asenkron motorların sincap kafesli türleri için sözkonusu olan darbe genişlik modülasyonlu frekans çevirici ile hız denetimi yöntemi ve bu yöntemin temel prensipleri genel hatları ile incelenmiştir. Ayrıca, 1,5 kW'lik bir sincap kafesli asenkron motorun hızını de netleyecek güç elektroniği devreleri gerçeklenerek yöntemin üstünlükleri görülmüştür.

The induction motor is the electrical motor type most widely used in industry. This leading position re sults mainly from its excellent features, such as: - Simple and rugged construction. For the user this means a low purchase price and high relia bility. - High efficiency and low maintenance costs, which result in low overall operating costs. - Suitability for use in difficult environments. - Several speed options. - Standardized design. The induction motor, particularly the cage type, is most commonly used in adjustabe-speed ac drive systems. To control the speed of a drive system, many methods have been developed, ranging from mechanical and hydraulic sys tems to electrical and electronic systems using, for ex ample, d.c. shunt-wound motors whose speed can be control led directly. However, it has been the desire of drive system manufactures to vary the speed of the standard three-phase induction motor by electrical means. This aim has been technically possible for same years, and with resent advances in power electronic components and integ rated circuits it is now achievable with both cost and quality comparable to that of alternative systems. The speed control methods of induction motors are given in Fig.l. Among these variable frequency method has been used in this thesis. To provide an efficient means of speed adjustment the ratio of voltage to frequen cy must be constant for this aim. Typical torque-speed curves are shown in Fig. 2, where it can be observed that the shapes are similar, with the maximum torque value in dependent of frequency. In frequency controlled squirral cage motor drives, usually frequency converters fitted with intermediate cir cuits are used. A frequency converter of this type con sists of four parts, as shown in Fig. 3. Applied frequency rated frequency -*=. 0.4 y fc=0.7 y Rated frequency Arl.O V- - A=l.3 Figure 2. Induction motor torque-speed curves at different frequencies. Mains -iff- Rectifier Intermediate DC Circuit n Inverter Motor - fly- Control Unit Figure 3. Block diagram of frequency converter. The first part on the supply side is a rectifier. In the intermediate circuit the pulsating d.c. voltage produced by the rectifier is filtered in an LC low-pass filter or in a smoothing choke. The third part is an in verter which uses the d.c. current or voltage from the intermediate circuit to produce an a.c. current or volta ge of the desired frequency. The control unit oversees the operation of the frequency converter. Frequency converters can be classified into two main types on the basis of the construction of the inter mediate circuit. If the intermediate circuit consists of a smoothing choke alone, the converter is said to be cur rent controlled (Fig. 4a). A frequency converter of this type functions as a current source that supplies the mo tor with a current such that the desired voltage is pre sent at the motor terminals. The amplitude of the current is determined by a rectifier. Current controlled frequ ency converters are used in single-motor drives. Frequency converters that have an LC low-pass fil ter in the intermediate circuit are said to be voltage controlled. In frequency converters of this type the amplitude of the output voltage is adjusted either by - vxix - Q-tff- -w- -Jf- C-Tj^ d-#- Figure 4. Alternative frequency converter designs. a) Current-source intermediate circuit. b) Voltage-source intermediate circuit, thyristor bridge controlled. c) Voltage-source intermediate circuit, chopper controlled. d) PWM frequency converter. controlling the intermediate circuit voltage (Fig. 4a and b) or by altering the output voltage waveform (Fig.4d). The latter method is called Pulse Width Modulation (PWM). The voltage controlled frequency converters are suitable for use both in single and in multiple motor drives. The PWM frequency converter is the most commonly used of the alternative designs shown. It differs from other voltage controlled types with respect to its control speed and effects on the mains. The PWM frequency converter has a high control speed, as voltage control is effected by means of an in verter. If the intermediate circuit voltage is thyristor bridge controlled or chopper controlled, the control spe ed is poor, as the intermediate circuit capacitor has to be charged or discharged when speed is changed. Thanks to the diode bridge, the power drawn from the mains supply by the PWM frequency converter consist almost entirely of active power. Other frequency converter types, except converters having a chopper in the interme diate circuit, require a large amount of reactive power because of their controlled rectifiers. In PWM frequency converters the intermediate cir cuit voltage is constant. The task of the inverter is to use this d.c. voltage to produce a symmetrical, three- phase voltage whose magnitude and frequency can be cont rolled. ix - The operation of the inverter can be illustrated by the switch model shown in Fig. 5. In practice, semicon ductor switches are used. \ E U \ 1 W Figure 5. Switch model of an inverter. The switches U, V and W are controlled as approp riate to produce a three-phase voltage at the motor termi nals. As the inverter operates, each switch is in either of the positions shown. A switch may only assume the in termediate position when being turned, this is assumed to take place instantaneously. The way in which the switch position control sig nals are produced is called modulation. Side effects ca used by pulse-type voltage on a squirrel-cage motor depond on the shape of the voltage curve, i.e. on the modulation. A good overall result will be obtained when the modulati on method is properly chosen. The modulation can be based either on comparison of two waveforms or on ready-calculated pulses. In both cases, the voltage pulses generated must produce a symmet rical, three phase system. The most frequently used modu lation methods are based on sine-triangle and plane-trian gle comparisons. The most common method of implementing the sine modulation is to use a sine-triangle comparision for cont rolling the inverter switches. The sine-triangle modulation the sine wave of each phase is compared to the triangular wave in a comparator Fig. 6 shows the principle of the sine-triangle comparison, and the resulting phase-to-phase voltage V^. From Fig. 6. the following results are obtained;. When the sine wave is more positive (larger) than the triangular wave the switch of the phase con cerned is controlled to the plus position; in the opposite case to the minus position. x - Figure 6. Principle of the sine-triangle comparison, and the resulting phase-to-phase voltage VAB'. The ratio between the amplitudes of the sine wave and the triangular wave is used to control the pulse widths, and thus also the magnitude of the frequency converter output voltage.. The sine wave frequency is the frequency of the frequency converter output voltage.. The triangular wave frequency, on the other hand, determines the switching frequency of the inver ter. The modulation method, which is used in this the sis, is based on triangle and a constant d.c. value com parison. The pulse train, which has been obtained the re sult of this comparison, has been applied to the eprom. The outputs of the eprom has been obtained with respect to the input of the address. Hence, by switching transis tors in the inverter the pulse trains have been obtained. Since the resulting pulse trains have been synchronized to each other, the torque pulsations have not been shown. On the other hand, the advantage of this method is to make optimum the output waves.