Cascaded H-Bridge Multilevel Inverter Fed Pmsm

Cascaded H-Bridge Multilevel Inverter Fed Pmsm

ABSTRACT

Permanent magnet synchronous motors are increasing applied in several areas such as traction,automobiles, robotics and aerospace technology. The power density of permanent magnet synchronous motor is higher than one of induction motor with the same ratings due to the no stator power dedicated to the magnetic field production. Multilevel inverters have drawn tremendous interest in the power industry. It is easy to produce a high- power, high voltage inverter with the multilevel structure because of the way in which device voltages stresses are controlled. The unique structure of multilevel inverters allow them to reach high voltages with low harmonics. Cascaded multilevel inverters has gained more interest because of its advantages over other MLI configurations like diode-clamped, flying capacitor inverter. In this paper high voltages required for Permanent Magnet Synchronous Motor which is used in traction, automobiles is fed using cascaded multilevel inverter andsimulation is done using MATLAB SIMULINK.

1. Introduction

Permanent Magnet Synchronous Motor consists of permanent magnets in rotor assembly generating a steady magnetic fieldyielded many advantages.

These advantages include

1. compact form with high torque density and less weight

2. higher continuous torque over a wider range of speeds

3. lower rotor inertia, higher dynamic performance under load

4. higher operational efficiencies with no magnetizing current

5. absence of heat due to current in the rotor

6. low torque ripple effect

Permanent magnet synchronous torque motors typically have 30%-60% higher torque capacity and 30% better torque utilization with faster acceleration and deceleration, compared to asynchronous induction type motors, hence used in traction and electric vehicles.

High power and high-voltage conversion systems have become very important issues for the power Bridges, each H-Bridge consists of four switches connected as in fig.1

electronic industry handling the large ac drive and electrical power applications at both the transmission and distribution levels. High power ratings is not possible for a two-level inverter (i.e. it gives +Vdc , – Vdc), as the semiconductor devices must be connected in series to obtain the required high- voltage operation, this can be achieved by summing the outputs of several two-level converters with transformers or inductors, or direct series connection, or by some topologies such as the diode clamped inverter and the flying capacitor inverter which are usually termed as multilevel voltage source inverters. The general structure of the multilevel converter, which has a multiple of the usual six switches found in a three-phase inverter, is to synthesize a sinusoidal voltage from several levels of voltages, typically obtained from capacitor voltage sources. The main motivation for such converters is that current is shared among these multiple switches, allowing a higher converter power rating than the individual switch VA rating would otherwise allow with low harmonics. As the number of levels increases, the synthesized output waveform, a staircase like wave, approaches a desired waveform with decreasing harmonic distortion, approaching zero as the number of levels increases.

There are three main types of transformer less multilevel inverter topologies, which have been received considerable interest from high-power inverter systems are the flying-capacitor inverter, the diode-clampedinverter, and the cascaded H-bridge inverter. In this paper we choose to work on cascaded H-bridge inverter due to its advantages:

1. It uses fewer components than the other types.

2. It has a simple control, since the converters present the same structure.

3. Soft-switching technique can be used to reduce switching losses and devices stresses.

   Because of these advantages, the cascaded inverter bridge has been widely applied to such areas as HVDC, SVC, stabilizers, and high-power motor drives.

   1. Cascaded H-Bridge MLI

      Figure 1. Single cascaded bridge

      The output generated by each H-Bridge is of three different levels i.e, +Vdc, 0, -Vdc by connecting dc source to the ac output side by different combinations of the four switches, S1,S2,S3,S4. Turning on S1,S4 gives +Vdc. Turning on S2,S3 yields Vdc. Turning off all switches gives 0V. In the same manner output at each level is obtained. The switching sequence for a single bridge is as follows, the firing pulse for upper switches S1,S3 has phase delay of 1800 . The lower switches are compliments firing pulse given through NOT gate. The same holds good for any no of bridges connected either in single phase or three phase. Here three phase cascaded MLI is simulated. For N-level output no of bridges required per phase is given by N=2n+1.

      Where n= no of bridges

      For 5 level we require 2 bridges per phase.

      Figure 2.Three phase 5 level MLI

      Controlling the conducting angles at different inverter levels can minimise the harmonic distortion of the output voltage. As the no of levels increases the output voltage tends to sinusoidal.

      2.1 Switching Technique

      Switching is implemented by sinusoidal pulse width modulation. In pulse width modulation the firing pulses required for semiconductor switches is obtained by comparing reference wave with carrier wave. In sinusoidal pulse width modulation technique sinusoidal wave is taken reference wave and triangular wave as carrier wave. The output of inverter i.e. amplitude and frequency can be varied by changing the reference wave amplitude and carrier wave frequency respectively. Amplitude modulation index is ratio of reference wave amplitude to carrier wave amplitude ma = Vr / Vc. The frequency modulation is defined as ratio of carrier wave frequency to reference wave frequency mf = fc /fr. In this paper the amplitude modulation is taken as ma = 1 and the frequency modulation mf

      =21. The pulses are generated as below in figure

      Figure 3.Pwm comparator

      Here the MLI is three phase the firing pulses are given with phase delay of 1200to each leg. The switches in a single leg are connected as shown in fig.4

      Figure 4. Single leg of three phase inverter

      The switch S1 and S2 has phase delay of 1800. Switch S1 and S5 has phase delay of 900. Switches S3,S4 are compliment for switch S1, S2 respectively and similarly S7,S8 are compliment to switch S5,S6. In the same the other two legs are connected and switching is done in the similar fashion.

      The switching pattern is tabulated below is for one leg of three phase inverter.

      Table.1. Switching patterns(1(1-On, 0-Off)

      S 5	0	0	0	1	1	1	1	1	0	0	0	0	0
      S 6	1	0	0	0	0	0	1	1	1	1	1	1	1
      S 7	1	1	1	0	0	0	0	0	1	1	1	1	1
      S 8	0	1	1	1 /td>	1	1	0	0	0	0	0	0	0

   2. Modeling of PMSM

      The dynamic model of the permanent magnet synchronous machine (PMSM) isderived using a two-phase motor in direct and quadrature (d-q) axes. This approach is desirable because of the conceptual simplicity obtained with only one set of two windings on the stator. The rotor has no windings, only magnets.The magnets are modeled as a current source or a flux linkage source, concentrating all its flux linkages along only one axis. The d- and q-axes stator voltages are derived as the sum of the resistive voltage drops and the derivative of the flux linkages in the respective windings as

      Vqs = Rsiqs + iqspLqq + Lqqpiqs + Lqdpids +ids pLqd + afpsinr

      Vds = Rs ids + iqspLqd + Lqdpiqs +Lddpids + ids p Ldd + afpcosr

      where

      pis the differential operator, d/ dt .

      vqs and vds are the voltages in the q- and d-axes windings .

      iqs and ids are the q- and d-axes stator currents .

      Rq and Rd are the stator q- and d-axes resistances. qs and ds are the stator q- and d-axes stator flux linkages.

      The surface mounted magnet machines in stator reference frames is derived initially in which the inductances are rotor position dependent. The rotor position dependency is eliminated by transformation. The PMSM model in rotor reference frames is obtained as

      S W It c h	0	3 0	6 0	9 0	1 2 0	1 5 0	1 8 0	2 1 0	2 4 0	2 7 0	3 0 0	3 3 0	3 6 0
      S 1	1	1	1	1	1	0	0	0	0	0	1	1	1
      S 2	1	0	0	0	0	0	1	1	1	1	1	0	0
      S 3	0	0	0	0	0	1	1	1	1	1	0	0	0
      S 4	1	1	1	1	1	1	0	0	0	0	0	1	1

      S W It c h	0	3 0	6 0	9 0	1 2 0	1 5 0	1 8 0	2 1 0	2 4 0	2 7 0	3 0 0	3 3 0	3 6 0
      S 1	1	1	1	1	1	0	0	0	0	0	1	1	1
      S 2	1	0	0	0	0	0	1	1	1	1	1	0	0
      S 3	0	0	0	0	0	1	1	1	1	1	0	0	0
      S 4	1	1	1	1	1	1	0	0	0	0	0	1	1

      Vr = + +

      + 0

      Vrqs, Vrds Stator voltages in rotor reference irqs, irds – Stator currents in rotor reference

      Where wr-rotor speed in electrical radians per second.

      The electromagnetic torque is Te=(3/2)(P/2)[af+(Ld-Lq)irdr]irqr (N.m)

      P- no of poles – flux linkage

      Ld,Lq- d,q axis inductances

      The rotor mechanical speed is obtained as wm= (( Te- Tl -Bwm)/ J) dt

      Where Te Electromagnetic torque Tl= load torque

      B – Friction coefficient J Inertia of motor

      Simulink model of PMSM

      Figure 5.PMSM Simulink block.

      The overall Simulink block consisting of Three phase five level MLI and PMSM

      Figure 6. Complete simulation block

   3. Simulation Results

      1. Firing pulses

         The firing pulses for the switches provided for single leg are as shown in figure.7

         Figure 7.firing pulses

      2. Output Voltages

         The output phase voltage waveforms of three phase 5 level of cascaded MLI are as shown

         Figure 8. Three phase voltage waveforms

      3. FFT Analysis of Output Phase Voltages Total harmonic distortion of three phase voltages are as shown

         Figure 9. THD of three phase voltage

         1. Simulation results of PMSM:

            Motor values are

            Stator Resistance Rs= 1.2 , B = 4.6752e-5 kg/m^2

            J = 2.0095e-5 N-m s

            Stator Resistance = 2.0357

            Direct axisInductance(Ld) =7.8e3H Quadrature axis Inductance(Lq) = 7.8e-3 H Flux () 0.154 mWb

            Poles=4, Vrms = 440 V

            1. Statorcurrent

               The quadrature axis and direct axis currents of PMSM

               Figure 12. Stator currents of pmsm

            2. Speed,Electromagnetic torque, Rotor position:

         Figure 13.speed,torque,rotor position

   4. Conclusion

      In this paper three phase cascaded MLI is simulated and three phase output voltages are obtained. This output voltage is fed to mathematically modelled permanent magnet synchronous motor. The motor characteristics like electromagnetic torque, Speed, Rotor position, Stator current are observed. Motor speed is maintained constant with change in load torque which is the inherent property of PMSM is observed. In extension to this work a suitable drive with appropriate controller can be done.

   5. References [1]SivaGangadhraraRao.V,Sneha.V,Sravani.M Mathematical modelling and simulation of permanent magnet synchronous motor International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering, Vol 2,Issue 8,August 2013.

1. R. Krishnan, Electric Motor Drives Modelling, Analysis and Control, Prentice Hall, 2001.

2. John Wiley & Sons, Parker, R.J., Advances in Permanent Magnetism, 1990.

1. NedMohan, Undeland, Riobbins Power electronic converter, applications and design ,Wiley Student Edition.

2. R. Krishnan, Permanent Magnet Synchronous and Brushless DC Motor Drives, CRC Press, 2010.