Design and Voltage Supply of High-Speed Induction - Aaltodoc

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66 6 VOLTAGE MODULATION FOR THE HIGH-SPEED INDUCTION MOTOR In this chapter, effects of different voltage modulations on the performance of the high-speed induction motor drive are studied. Preference for different voltage waveforms is derived theoretically and compared against the measurements. The back-to-back measurement setup for the 60 kW 60000 1 /min high-speed motor drive is used in the measurements. The setup was presented in Chapter 5, where a comparison of different rotors for the high-speed motor was reported. The motors studied here have copper coated solid steel rotors. The motivation for this part of study was to see how different voltage modulations affect the power loss in the high-speed motor and the frequency converter feeding the motor. The study concentrates on the type of a frequency converter that has a voltage source inverter feeding the motor. Fig. 6.1 shows the frequency converters used to feed the high-speed machines with an appropriate voltage and frequency. Current source inverters are not considered in this study. Three phase frequency converter Model VACON NX5 Current 205 A Voltage 380 – 500 V Input frequency 45 - 66 Hz Output frequency up to 7200 Hz Switching frequency up to 16000 Hz Input side choke + diode rectifier Intermediate link DC-voltage capacitor Output side IGBT - inverter Output signal PWM voltage Modulation type synchronized carrier, sinetriangle comparison with added 3 rd harmonic Fig. 6.1 A photograph and data of the frequency converters used to supply the 60 kW 60000 1 /min high-speed machines in the back-to-back test setup. Converters in the picture are fitted side by side in the rack, power cables above connecting the DC-links.
6.1 Comparison of PAM and PWM for 60 kW 60000 1 /min drive In comparison of modulation techniques, different performance criteria can be selected. For example, torque, speed and position precision are important in machine tools and the modulation should be optimized in that respect. In the application used in this study, the more important criteria are efficiency and utilization. For the high-speed induction motor studied, the utilization depends on the temperature rise of the stator winding. The harmonic currents produce extra loss in the winding and the winding loss is the most important loss component for the temperature rise (Saari 1995). From this point of view, the voltage waveform should be selected so that the harmonic content of the current is the smallest. If skin effect is neglected, the power loss depends on the rms-value of the harmonic currents but not on the frequency of the harmonic components. Iron loss due to the harmonics is frequency dependent. Much of the power loss caused by harmonics are induced in a copper coating of a coated solid steel rotor. Due to skin effect this loss is frequency dependent. Thus, if the efficiency of the motor is to be maximized, both the rms-value and the frequency spectrum of the harmonic current must be considered. In PWM modulation, increasing the switching frequency increases the number of the pulses per half a cycle. According to the assumption that the harmonic currents mainly see the leakage reactance as a load, the higher the switching frequency the better. This is because the harmonic content of the voltage is moving upwards in frequency scale as the switching frequency goes up. Thus, the amplitude of harmonic currents decrease. The downside is then the increased switching loss that is proportional to the switching frequency. The switching loss in the IGBT bridges can become a major loss component for the frequency converter at high load and switching frequency. This will be seen in the measurement results later on. In the next section, a theoretical comparison between different voltage waveforms is made. The comparison considers the harmonics loss in the motor. It is assumed that the loss is relative to the rms-value of the harmonic current. The comparison is not influenced by the load impedance of the motor. This is made possible by comparing the different voltage waveforms against PAM. The comparison method was used by Holtz (1994). After the theoretical comparison, the results of the actual measurements are reported. At this point, the main issues are the utilization and power loss of the motor and the power loss in the inverter. The relation between harmonics current and harmonics loss for the different waveforms is studied. 67
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El 108 ACTA POLYTECHNICA SCANDINAVI
Page 3 and 4:
PREFACE This thesis is the result o
Page 5 and 6:
5 Comparison of rotors for 60 kW 60
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LIST OF SYMBOLS a Number of paralle
Page 9 and 10:
Rn Resistance of a parallel conduct
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1 INTRODUCTION The need for high ro
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Conventional (high-speed) drive Hig
Page 15 and 16: Chapter 5 reports the comparison of
Page 17 and 18: Some penetration depth values are c
Page 19 and 20: 2.2 Stator windings for high-speeds
Page 21 and 22: ⎛ π ⎞ sin⎜ν ⎟ ⎛ π ⎞
Page 23 and 24: Table 2.3 Air gap characteristics o
Page 25 and 26: Cooling of the stator could need ax
Page 27 and 28: f sw N p = (2.9) fs The switching f
Page 29 and 30: 3 ROTOR CONSTRUCTIONS Starting from
Page 31 and 32: and eddy-currents traveling on the
Page 33 and 34: support the squirrel cage winding,
Page 35 and 36: 3.1.3 Rotor loss and friction loss
Page 37 and 38: Table 3.1 High-speed induction moto
Page 39 and 40: critical speeds are passed relative
Page 41 and 42: Table 3.2 The free-free natural fre
Page 43 and 44: Table 3.3 The free-free natural fre
Page 45 and 46: 4 COMPARISON OF ROTORS FOR 65 KW 30
Page 47 and 48: Temperature [ºC] 110 109 108 107 1
Page 49 and 50: Temperature rise [K] 120 100 80 60
Page 51 and 52: improve the laminated design, more
Page 53 and 54: The copper coated solid steel rotor
Page 55 and 56: As reported in Lähteenmäki et al.
Page 57 and 58: Temperature rise [K] 100 80 60 40 2
Page 61 and 62: 6. The rest of the loss is defined
Page 63 and 64: Temperature rise [K] 100 80 60 40 2
Page 65: 5.2.7 About the mechanical robustne
Page 69 and 70: h d = , (6.2) I I h, PAM where Ih,P
Page 71 and 72: Fig. 6.4 Phase-to-phase voltage and
Page 75 and 76: Machines loss [W] 12000 10000 8000
Page 77 and 78: considerably higher than expected.
Page 79 and 80: In order to improve the performance
Page 81 and 82: l = + , (7.2) sλs 2leλ e l wλ w
Page 83 and 84: gap of 1 mm would yield an effectiv
Page 85 and 86: Table 7.1 Calculated effective leng
Page 87 and 88: Input current [A] 170 160 150 140 1
Page 89 and 90: 8 CIRCULATORY CURRENTS IN A STATOR
Page 91 and 92: Table 8.1 Circulatory current loss
Page 93 and 94: Fig. 8.4 Strand currents in the two
Page 95 and 96: 8.2 Impedance of the circulatory cu
Page 97 and 98: Because Φ l is approximately at th
Page 99 and 100: Table 8.2 Circulatory current loss
Page 101 and 102: Power loss [W] 3500 3000 2500 2000
Page 103 and 104: k cc 600 500 400 300 200 100 0 0 50
Page 105 and 106: 9 OPTIMIZATION OF SOLID STEEL ROTOR
Page 107 and 108: whole solution space determined by
Page 109 and 110: the average time saving is 39 %. Th
Page 111 and 112: Fig. 9.3 Cross sections of the 16,
Page 113 and 114: Height of bar slot [mm] 8 6 4 2 0 R
Page 115 and 116: The different rotor topologies were
Page 117 and 118:
possible. The coating practically v
Page 119 and 120:
Table 9.3. Optimization results for
Page 121 and 122:
Fig. 9.11 Initial (left) and opt. (
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high-speed induction machine. Accor
Page 125 and 126:
Foelsch K. 1936. Magnetfeld und Ind
Page 127 and 128:
Pyrhönen J. and Kurronen P. 1994.
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APPENDIX 129
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currents in the solid iron flow mor
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where the C sw E is the loss coeffi
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B. CALCULATION OF THERMAL CHARACTER
Page 137 and 138:
Axial cooling duct Frame R fr3 Stat
Page 139 and 140:
Linear equalities Ax = b, (C3) wher
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Design and Voltage Supply of High-Speed Induction - Aaltodoc

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