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Title:  Low Switching Frequency Pulse Width Modulation for Induction Motor Drives 
Authors:  Tripathi, Avanish 
Advisors:  Narayanan, G 
Keywords:  Induction Motor Drive Pulsewidth Modulation Current Ripple Torque Ripple Voltage Source Inverter Pulse Width Modulation (PWM) Torque Harmonic Mitigation Torque Harmonics Space Vector Analysis Harmonic Torque Quarter Wave Symmetry Voltagesource Inverter 
Submitted Date:  2017 
Series/Report no.:  G28503 
Abstract:  Induction motor (IM) drives are employed in a wide range of industries due to low maintenance, improved efficiency and low emissions. Industrial installations of highpower IM drives rated up to 30 MW have been reported. The IM drives are also employed in ultra highspeed applications with shaft speeds as high as 500; 000 rpm. Certain applications of IM drives such as gas compressors demand high power at high speeds (e.g. 10 MW at 20; 000 rpm).
In highpower voltage source inverter (VSI) fed induction motor drives, the semiconductor devices experience high switching energy losses during switching transitions. Hence, the switching frequency is kept low in such highpower drives. In highspeed drives, the maximum modulation frequency is quite high. Hence, at high speeds and/or high power levels, the ratio of switching frequency to fundamental frequency (i.e. pulse number, P ) of the motor drive is quite low.
Induction motor drives, operating at lowpulse numbers, have significant loworder voltage harmonics in the output. These loworder voltage harmonics are not filtered adequately by the motor inductance, leading to high total harmonic distortion (THD) in the line current as well as loworder harmonic torques. The loworder harmonic torques may lead to severe torsional vibrations which may eventually damage the motor shaft. This thesis addresses numerous issues related to lowpulsenumber operation of VSI fed IM drives. In particular, optimal pulse width modulation (PWM) schemes for minimization of line current distortion and those for minimization of a set of loworder harmonic torques are proposed for twolevel and threelevel inverter fed IM drives.
Analytical evaluation of current ripple and torque ripple is well established for the induction motor drives operating at high pulse numbers. However, certain important assumptions made in this regard are not valid when the pulse number is low. An analytical method is proposed here for evaluation of current ripple and torque ripple in lowpulsenumber induction motor drives. The current and torque harmonic spectra can also be predicted using the proposed method. The analytical predictions of the proposed method are validated through simulations and experimental results on a 3:7kW induction motor drive, operated at low pulse numbers. The waveform symmetries, namely, halfwave symmetry (HWS), quarterwave symmetry (QWS) and threephase symmetry (TPS), are usually maintained in induction motor drives, operating at low switching frequencies. Lack of HWS is well known to introduce even harmonics in the line current. Impact of threephase symmetry on line current and torque harmonic spectra is analyzed in this thesis. When the TPS is preserved, there are no triplen frequency components in the line current and also no harmonic torques other than those of order 6, 12, 18 etc. While TPS ensures that the triplen harmonics in the threephase pole voltages are in phase, these triplen frequency harmonics form balanced sets of threephase voltages when TPS is not preserved. Hence, triplen frequency currents flow through the stator windings. These result in torque harmonics of order 2, 4, 6, 8, 10 etc., and not just integral multiples of 6. These findings are well supported by simulation and experimental results.
One can see that two types of pole voltage waveforms are possible, when all waveform symmetries (i.e. HWS, TPS and QWS) are preserved in a twolevel inverter, These are termed as typeA and typeB waveforms here. Also, QWS could be relaxed, while maintaining HWS and TPS, leading to yet another type of pole voltage waveform. Optimal switching angles to minimize line current THD are reported for all three types of pole voltage waveforms. Theoretical and experimental results on a 3:7kW IM drive show that optimal typeA PWM and optimal typeB PWM are better than each other in different ranges of modulation at any given low pulse number. In terms of current THD, the optimal PWM without QWS is found to be close to the better one between optimal typeA and optimal typeB at any modulation index for a given P . A combined optimal PWM to minimize THD is proposed, which utilizes the superior one between optimal typeA and optimal typeB at any given modulation index and pulse number. The performance of combined optimal PWM is shown to be better than those of synchronous sinetriangle (ST) PWM and selective harmonic elimination (SHE) PWM through simulations and experiments over a wide range of speed.
A frequency domain (FD) based and another synchronous reference frame (SRF) based optimal PWM techniques are proposed to minimize loworder harmonic torques. The objective here is to minimize the combined value of loworder harmonic torques of order 6, 12, 18, ..., 6(N 1), where N is the number of switching angles per quarter cycle. The FD based optimal PWM is independent of load and machine parameters while the SRF based method considers both load and machine parameters. The offline calculations are much simpler in
case of FD based optimal PWM than in case of SRF based optimal PWM. The performance
of the two schemes are comparable and are much superior to those of synchronous ST PWM
and SHE PWM in terms of loworder harmonic torques as shown by the simulation and
experimental results presented over a wide range of fundamental frequency,
The proposed optimal PWM methods for two levelinverter fed motor drives to minimize
the line current distortion and loworder torque harmonics, are extended to neutral point clamped (NPC) threelevel inverter fed drive. The proposed optimal PWM methods for the NPC inverter are compared with ST PWM and SHE PWM, having the same number of
switching angles per quarter. Simulation and experimental results on a 3:7kW induction
motor drive demonstrate the superior performance of proposed optimal PWM schemes over ST PWM and SHE PWM schemes.
The di_erent optimal PWM schemes proposed for twolevel and threelevel inverter fed
drives, having di_erent objective functions and constraints, are all analyzed from a space vector perspective. The threephase PWM waveforms are seen as a sequence of voltage
vector applied in each case. The space vector analysis leads to determination of optimal
vector sequences, fast o_ine calculation of optimal switching angles and e_cient digital
implementation of the proposed optimal PWM schemes. A hybrid PWM scheme is proposed
for twolevel inverter fed IM drive, having a maximum switching frequency of 250 Hz. The
proposed hybrid PWM utilizes ST PWM at a _xed frequency of 250 Hz at low speeds. This
method employs the optimal vector sequence to minimize the current THD at any speed in
the medium and high speed ranges. The proposed method is shown to reduce both THD as well as machine losses signi_cantly, over a wide range of speed, compared to ST PWM
Position sensorless vector control of IM drive also becomes challenging when the ratio
of inverter switching frequency to maximum modulation frequency is low. An improved
procedure to design current controllers, and a closedloop ux estimator are reviewed. These are utilized to design and implement successfully a position sensorless vector controlled IM drive, modulated with asynchronous third harmonic injected (THI) PWM at a constant switching frequency of 500 Hz. Sensorless vector control is also implemented successfully, when the inverter is modulated with synchronized THI PWM and the maximum switching frequency is limited to 500 Hz. 
Abstract file URL:  http://etd.iisc.ernet.in/abstracts/4558/G28503Abs.pdf 
URI:  http://etd.iisc.ernet.in/2005/3688 
Appears in Collections:  Electrical Engineering (ee)

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