CHAPTER - 3 CONVENTIONAL SOURCE INVERTER FED INDUCTION MOTOR DRIVE 3.1. Introduction The objective of this chapter is to describe conventional source inverters, modes of operations and comparisons. DC to AC converters is lm~wn as hverters. The function of an inverter is to change a DC input voltage to a symmetrical AC output voltage of desired magnitude and frequency. The output voltage could be fixed or variable at a fixed or variable frequency. A variable output can be obtained by varying the input DC voltage and maintaining the gain of the inverter constant. These inverters generally use PWM control signals for producing an AC output voltage. An inverter is called a voltage fed inverter if the input voltage remains constant, a current fed inverter if the input current is maintained constant, and a variable DC linked inverter if the input voltage is controllable. 3.2. Types of Conventional Source Inverters Conventional source inverters are i). Voltage Source Inverter (VSI) ii). Current Source Inverter (CSI). The input of Voltage Source Inverter (VSI) is a stiff DC voltage supply, which can be a battery or a controlled rectifier. Both Single phase and three phase Voltage Source Inverters are used in industry. The switching device can be a Conventional MOSFET, Thyristor or a Power Transistor. Voltage Source Inverter (VSI) is the one in which the DC source has small or negligible impedance. In other words a Voltage Source Inverter (VSI)
36 93438 has stiff DC source voltage at its input terminals. A Current-fed Inverter or Current Source Inverter (CSI) is fed with adjustable current source. In Current Source Inverter, output current waves are not affected by the load. 3.2.1. Voltage Source Inverter (VSI) When the power requirement is high, three phase inverters are used. When three Single phase inverters are connected in parallel, we can get the three phase inverter. The gating signals for the three phase inverters have a phase difference of 120'. These inverters take their DC supply from a battery or from a rectifier and can be called as six- step bridge inverter. Fig 3.1 shows the three phase inverter using six MOSFETs and diodes. A large capacitor is connected at the input terminals which tends to make the input DC voltage constant. This capacitor also suppresses the harmonics feedback to the source. Therefore the Voltage Source Inverter (VSI) is only buck (step down) inverter operation for DC to AC power conversion or boost (step up) operation for AC to DC power conversion. For applications wherever drive is desirable and the available DC voltage is limited, an additional DC - DC boost converter is needed to obtain a desired AC output. The additional power converter stage increases system cost and lowers efficiency. The upper and lower devices of each phase leg cannot be gate on simultaneously either by force or by EM1 noise. Otherwise a shoot through problem by Electromagnetic interference noise's misgating.
Fig. 3.1 : Voltage Source Inverter (VSI) It is a major concern to the inverter reliability. Dead time to block both upper and lower devices has to the provide in the Voltage Source Inverter (VSI) which causes the waveform distortion, etc. An output LC filters are needed for providing a sinusoidal voltage compared with Current Source Inverter (CSI) which causes additional power loss and control complexity. 3.2.2. Current Source Inverter (CSI) A Current Source Inverter (CSI) is fed from a constant current source. Therefore load current remains constant irrespective of the load on the inverter. The load voltage changes as per the magnitude of load impedance. When a voltage source has a large inductance in series with it, it behaves as a Current Source. The large inductance maintains the current constant. The conventional three phase Current Source Inverter (CSI) structure is shown in Fig.3.2. A DC current source feeds the three phase main inverter
circuit The DC current source can be a relatively large DL uruurwr ra, uy a Voltage Source such as a battery or a rectifier. It consists of six switches and with anti-parallel diodes, This diode provides the bi-directional current flow and unidirectional voltage blocking capability. Current Source Inverter has the following conceptual and theoretical barriers and limitations. The AC output voltage has to be greater than the original DC voltage that feeds the DC inductor or DC voltage produced is always smaller than the AC input voltage. Therefore this inverter is a boost inverter for DC to AC power conversion. For applications where a wide voltage range is desirable, an additional DC to DC buck converter is needed. The additional power conversion stage increases system cost and reduces the efficiency. Fig.3.2: Current Source Inverter (CSI) At least one of the upper devices and one of the lower devices have to be gated on and maintained on at any time. Otherwise, an open circuit of the DC
inductor would occur and dcmoy the devices. The open circuit problem by EM1 noise's misgatting-off is a major concern of the converters reliability. A current source inverter is fed from a constant current source. Therefore load current remains constant irrespective of the load on the inverter. 3.3 Comparison of CSI with VSI The following table 3.1 gives the comparison of CSI and VSI Inverters. Table 3.1 : Comparison of CSI with VSI Current Source Inverter 1. As inductor is used in the DC link, the source impedance is high. It acts as a constant current source 2. A CSI is capable of withstanding short circuit across any two of its output Voltage Source Inverter 1. As capacitor is used in the DC link, it acts as a low impedance voltage source. 2. A VSI cannot accept the misfiring of switches. terminals. Hence complementary short circuit on load and misfiring of switches are acceptable. - 3. CSI is used in only buck or boost operation of inverter. 4. The main circuit cannot be interchangeable. 5. It is affected by the EMI noise. 3. VSI is used in only a buck or boost operation of inverter. 4. The main circuit cannot be interchanged here also. 5. It is also affected by the EMI noise
3.4. Modes of Operation Three Phase inverters are normally used for high power applications. Three single-phase half or full bridge inverters can be connected in parallel to form the configuration of a three phase inverter. The gating signals of single phase inverters should be advanced or delayed by 120" with respect to each other in order to obtain three phase balanced voltages. The three phase output can be obtained from a configuration of six switches and six diodes. Two types of control signals can be applied to the switches: 180 conduction or 120" conduction. 3.4.1 180' Conduction In these inverters each switch conducts for a duration of 180". Three switches remain on, at any instant of time. When switch-1 is switched on, terminal 'a' is connected to the positive terminal of the DC input voltage. When switch-4 is switched on, terminal 'b' is connected to the negative terminal of the DC source. There are six modes of operation in a cycle and the duration of each mode is 60".The switches are numbered in the sequence of gating the switches 1-2-3, 2-3-4, 3-4-5, 4-5-6, 5-6-1, 6-1 -2. The gating signals are shifted from each other by 60" to obtain three phase balanced voltages. 3.4.2 120' Conductioe In this conduction mode each switch conducts for 120". Only two switches remain on at any instant of time. The conduction sequence of switches is 6-1, 1-2, 2-3,3-4,4-5, 5-6, and 6-1. There are three modes of operation in a
half cycle and the equivalent circuits for wye connected load are shown in Fig 3.3. During mode 1 for 0 I at L x13 switches i and 6 conducts. During mode 2 for n13 5 ot 12x13, switches 1 and 2 conduct. During mode 3 for 2x13 1 at 231~13, switches 2 and 3 conduct. van = 0 v*, = - vs v, = --vs 2 2... 3.3 The line to neutral voltages can be expressed in Fourier series as given in equations 3.1 to 3.3. 03 vbn = 2 (5 n=1,3,5 nk cos 7 sinn (wt - The a to b line voltage is Vb = 43 Van with a phase advance of 300'. There is a delay of n/6 between the turning off of switch 1 and turning on of switch 4. Thus there should be no short circuit of the DC supply through one upper and lower switch. At any time, two load terminals are connected to the DC supply and the third one remains open. The potential of this open terminal will depend on the load characteristics and would be unpredictable. Since one switch conducts
120, the switches are less uiilized as compared to that of 180' conduction for the load condition. The control of output voltage is done using Pulse Width Modulation (PWM). The commonly used techniques are r Single Pulse Width Modulation. Multiple Pulse Width Modulation. r Sinusoidal Pulse Width Modulation. r Modified Sinusoidal Pulse Width Modulation. r Phase displacement control. 3.5. Simulation Results 3.5.1. Voltage Source Inverter PSI) fed Induction Motor Drive Fig.3.3: Voltage Source Inverter (VSI,)fid Induction Motor Drive 47
Voltage Source Inverter (VSI) fed Induction Motor Drive is designed and it is simulated in MATLAB 6alSIMULINK. The following Figure 3.4 shows its corresponding diagram. 3.5.2. Results of Voltage Source Inverter (VSI) fed Induction Motor Drive Rectifier output voltage is shown In Fig.3.4. (a). Motor Speed curve is shown in Fig 3.4. (b). FFT analysis is show11 ill Fig 3.4. (15). From the FFT analysis it is observed that the T.H.D value is 7.08%. " Timc (Set) Fig. 3.4 (a): RectiJier output voltage
Tic (Set) Fig 3 4 (h) Motor specvi I 1 1 1, Ftequenr y (Hz) Fig. 3.4. (c): FFT analvsis
3.5.3. Current Source Inverter (CSI) fed Induction Motor Drive Current Source Inverter (CSI) fed Induction Motor Drive is designed and it is simulated in MATLAB/SIMULINK, The following Figure 3.5 shows SIMULINK circuit in CSI fed Induction motor drive. 11 P $1 Unii I Fig.3.5: Current Source Inverter (CSI) fed Induction Motor Drive Rectifier output voltage is shown in Fig. 3.6. (a). Motor Speed curve is shown in Fig 3.6. (b). FIT analysis is shown in Fig 3.6. (c). From the FFT analysis it is observed that the T.H.D value is 8.12%.
3.5.4 Results of Current Source lnverter (CS1) fed Induction Motor Drive - Time (Scc) Fig. 3.6.(0): Rectifier outprrt volluge -- * Time (Sec) Fig. 3.6.0: Motar speed
f rcquenr y (I+?) Fig. 3.6. (c): FFT unuf~~,sis 3.6. Conclusions In this chapter the Convcntional Invcrtcrs both Voltagc Source Inverter (VSI) and Current Source Inverters (CSI) are discussed. Flow these inverters are connected to Induction nlotor is discussed. Comparison between these two concepts is also discussed. Later these designs are simulated in imai'lm 6d Simulinb. Simulatioll results and comparison are also presented in this chapter. Their comparison results are presented here in tablc. The main observation here the total harmonic distortion values are 7.08% and 8.12% in VSI & CSI fed Induction Motor Drive respectively.
Table 3.2: Comparison of VSI & CSI fed Induction Motor Drive Description AC Input (Volts) Rectified Output (Volts) T.H.D. (%) VSI 230 220 7.08 CSI 230 220 8.12