A PULSEWIDTH MODULATED VOLTAGE-FED INVERTER VECTOR-CONTROLLED PERMANENT MAGNET SYNCHRONOUS MOTOR DRIVE

ABSTRACT

A  Pulsewidth  Modulated  Voltage-Fed  Inverter  Vector-Controlled   Permanent  Magnet Synchronous Motor (PMSM) Drive based on Hysteresis Current Control (HCC) is  presented in this work. A detailed conceptual dq modelling of the PMSM was undertaken in the rotor reference

frame for open loop studies, thereby setting the pace for the variable speed drive (VSD) of the PMSM which, inherently, is not capable of variable speed operation. Subsequently, vector control by Field Orientation  Control  (FOC),  is used to decouple the flux and torque  producing  stator current components of the PMSM thereby permitting independent and precise control of flux and torque as obtainable in separately excited dc machines. A  complete closed loop control system employing an outer PI speed controller and an inner hysteresis current controller was implemented to realize this speed-controlled  drive. Since torque can be made proportional to current either in the stationary or rotor reference frames and effective control of current gives effective control of torque, speed and position, the  HCC strategy is aimed at ensuring that the actual motor phase currents  track  their  respective  sinusoidal  references.  The  HCC  algorithm  was  developed  and employed for the logical firing of the power semiconductor switches of the inverter. The control algorithm was  optimised to obtain fast speed response, while maintaining  effective current and torque tracking for all practical speed inputs namely Constant, Step and RAMP reference speed inputs.  The  optimal  control  variables  were  identified  with  emphasis  on effective  current  and torque  tracking.  Four  quadrant  operation  of the PMSM  was  also  implemented  as  obtains  in numerous applications in industry where controlled starts and stops are required in both forward and  reverse  directions.  Compared  to the standard  AC6  of MATLAB  Simpower  systems,  the developed  model  achieved  rise time and settling  time of 0.0108  seconds  and 0.0143  seconds respectively  while  the  corresponding  values  for  AC6  model  are  0.1944  seconds  and  0.1984 seconds  respectively.  This,  clearly,  shows  that  the  developed  model  has  an  enhanced  speed response.

ONE

INTRODUCTION

1.0 Introduction

A Pulsewidth Modulated Voltage-Fed Inverter Vector-Controlled Permanent Magnet Synchronous Motor Drive is presented in this thesis based on the Hysteresis Current Control (HCC) Technique.

For the same output power, Permanent Magnet Synchronous Motors (PMSM) offer performance enhancement over the conventional induction and synchronous motors in terms of power factor, efficiency, power density and torque-to-inertia ratio [1,2,3]. This justifies the recent concentration of research efforts in the design, analysis and control of the PMSM. Unlike in low precision applications where fixed speed operation of the PMSM may be tolerated, Adjustable Speed Drives (ASD), depending on specific load requirements, significantly improves motor drive performance. This is the practice in industry where high precision in speed and torque is desired.

Since torque can be made proportional to current either in the stationary or rotor reference frames and control of current gives control of torque and speed, current  control  strategies  are  employed  in  ASD  to  ensure  that  stator  phase currents track their respective reference values.  One of  such  current control strategies is the Hysteresis Current Control (HCC).   When compared to other

current control strategies, HCC offers the advantage of varying operating frequency and eliminates the need for feedback loop compensation [4,5]. The problems  of  poor  load  transient  response  and  regulator  inaccuracy  have, however, consistently necessitated further research efforts to achieve optimal drives performance.

1.1 PMSM Compared to other AC Machines

The Permanent Magnet Synchronous Motor (PMSM), since its entry into the industrial application, has become a strong contender for servo applications [6,7,8,9] and  later  for aerospace actuation  [10], traction,  robotics, and other automotive applications[11,12,13].

The  PMSM  offers numerous advantages over other machines that  are conventionally used for ac drives. While the stator current of an induction motor (IM) contains magnetizing as well as torque-producing components, the use of permanent magnets in the rotor of the PMSM makes it unnecessary to supply magnetizing current through the stator to create airgap flux; the stator current need only be torque-producing. Hence for the same output, the PMSM will draw lesser current, operate at a higher power factor (because of the absence of magnetizing current) and will be more efficient than the IM [1,14,15].

The conventional wound-rotor synchronous machine (SM), on the other hand, must have dc excitation on the rotor, which is often supplied by brushes

and slip rings. This implies rotor losses and undesirable downtime periods due to regular brush maintenance. The development of the PMSM was to remove the foregoing disadvantages of the SM by replacing the later’s field coil, dc power supply, and slip rings with a permanent magnet [16].

The fact that the rotor field is created by permanent magnets instead of induced rotor currents reduces the loss of the machine. The predicted efficiency of the PMSM in the range 10-100kW is about 95-97% as compared to 90-94% for induction motors [17]. Other attractive properties of PMSM are high power density and torque-to-inertia ratio [17,18,19].

Recently, several efforts have been made toward the operational comparison  between  the  Permanent  Magnet  Synchronous  Motor  and  the Induction Motor   which, hitherto, is the workhorse of the industry. These comparisons have favoured the PMSM. The performances of three and single- phase Line Start Interior Permanent Magnet Synchronous Motors (LSIPMSMs) and Induction Motors (IMs) with equal squirrel-cage design were directly compared and evaluated which emphasized the favourable impacts of the permanent magnet breaking torque and reluctance breaking torque on the LSIPMSM performance in the asynchronous operation region [20].

Even for Variable Speed Drives (VSD),  employing measurement data from no-load tests, load tests, and temperature rise tests of the aforementioned motors and different cage materials, it was, furthermore, shown that the lower

loss of the LSIPMSM enables a significant increase of the VSD’s constant power range [21].

In the field of semi-hermetic drives, like in compressors, where heavy loading is obtained because of good cooling, a good comparison has been made between line-start interior permanent magnet synchronous motors (LSIPMSMs) and the Induction motors (IM) [22]. The study revealed the improvement of LSIPMSMs  characteristics  in  comparison  to  IMs  characteristics  and recommended for the immediate replacement of existent IMs with LSIPMSMs in the target semi-hermetic compressor application.

1.2 Characteristics of Permanent Magnet Materials

The properties of the permanent magnet material will affect directly the performance of the motor and proper knowledge is required for the selection of the  materials  and  for  understanding  PM  motors.  The  earliest  manufactured magnet materials were hardened steel. Magnets made from steel were easily magnetized. However,  they could  hold  very  low energy  and  it was easy to demagnetize. In recent years, other magnet materials such as Aluminum Nickel and Cobalt alloys (ALNICO), Strontium Ferrite or Barium Ferrite (Ferrite), Samarium  Cobalt  are  in  use.  First  generation  rare  earth  magnet,  Samarium Cobalt (SmCo5) and Second generation rare earth magnet, Neodymium Iron- Boron   (Nd2Fe14B), having superior B-H characteristics, have been developed

and used for making permanent magnets.  These types of magnets have a high energy density and high resistance for demagnetization.

The figure 1.1 below shows the B-H characteristics of common permanent

magnet materials [23].

Neodymium Iron Boron

Samarium  cobalt

Ferrite

Aln ico

1.25

1.00

0.75

0.50

B(T )

0.25

1.00

 0.75

 0.50

 0.25      0

H (MA / m)

Figure 1.1: B-H Characteristics of Common Permanent Magnet Materials

Neodymium Iron Boron (Nd2Fe14B) magnets are the most common rare earth magnets used in PM motors these days because of their superior B-H characteristics. Some potential limitations of Nd2Fe14B material in comparison with other high energy magnets are its relatively low temperature limit and vulnerability to corrosion [24]. Adverse effects of excessive temperature on the magnet property of rare earth materials were also discussed in [25].

Although permanent magnet synchronous motor (PMSM) with rare-earth

PMs  are  most  popular  for  automotive  applications  because  of  its  excellent

31

performance  such  as  high  power  density,  high  torque  density,  and  high

efficiency, the rare-earth PMs have problems such as high cost and are in very limited supply due to politics; with China supplying a large bulk of the rare earth magnets used globally. The unpredictability associated with procuring such materials, clearly, suggests the need for a competitive alternative.

Several research efforts are on-going to utilize ferrite magnets in place of Permanent magnets where the ferrites are expected to have competitive power density and efficiency of the rare-earth PMSM.

A structure for a high-power-density PM-assisted synchronous reluctance motor  involving  the  use  of  a  ferrite  PM  has  been  proposed.  The  structure prevents irreversible demagnetization of the PM even in the presence of heavy flux-weakening excitation or an inverter fault. The proposed structure achieved high-power and high-efficiency performance with suitable application areas as in Electric Vehicles (EV) and Hybrid Electric Vehicles (HEV) [26, 27, 28].

1.3 Classification of Permanent Magnet Motors

PM motors are classified on the basis of the flux density distribution and the shape of current excitation. They are Permanent Magnet Synchronous Motors (PMSM) and Permanent Magnet Brushless DC Motors (BLDC). The PMSM has a sinusoidal-shaped back EMF and is designed to develop sinusoidal back EMF waveforms. They have the following features:

a) Sinusoidal distribution of magnet flux in the air gap

b) Sinusoidal current waveforms

c) Sinusoidal distribution of stator conductors.

BLDC  has  a  trapezoidal-shaped  back  EMF  and  is  designed  to  develop trapezoidal back EMF waveforms. They have the following features:

a)  Rectangular distribution of magnet flux in the air gap b) Rectangular current waveform

c)  Concentrated stator windings [29, 30].

Our  interest  in  this  research  is  the  Permanent  Magnet  Synchronous  Motors

(PMSM)

1.4 Classification of Permanent Magnet Synchronous Motor (PMSM)

Within the PMSM family, two major groups based on the placement of the permanent magnets are known for purpose of analysis and adjustable speed operation;  they  are  the  Surface  Mount  and  Interior  Permanent  Magnet  type motors  [19,  31].  Surface  mounted  PM  motors  have  each  of  the  permanent magnets mounted  on the surface  of  the rotor, making it  easy  to  build,  and specially skewed poles are easily magnetized on this surface mounted type to minimize cogging torque. This configuration is used for low speed applications because of the limitation that the magnets will fly apart during high-speed operations. Surface mounted magnets will not withstand high rotational speed

due to high centrifugal forces which crack or separates the magnets from the rotor.

These motors are considered to have insignificant saliency, thus having practically equal inductances in both the q and d axes [32]. The ratio between the

quadrature  and  direct  axis  inductances  is  close  to  unity.    So  for  a  surface

permanent magnet motor,

Ld   Lq . The rotor has an iron core that may be solid or

may be made of punched laminations for simplicity in manufacturing [33, 34]. Thin  permanent  magnets  are  mounted  on  the  surface  of  this  core  using adhesives. Alternating magnets of the opposite magnetization direction produce radially directed flux density across the air gap. This flux density then interacts with currents in windings placed in slots on the inner surface of the stator to produce  torque.  Figure  1.2  shows  the  placement  of  the  magnet  in  surface mounted permanent magnet synchronous motors [35].

Figure 1.2: Surface Permanent Magnet Motor Rotor Assembly

Interior PM motors, on the other hand, have interior mounted permanent magnet rotor as shown in figure 1.3 [35]. Each permanent magnet is mounted

inside the rotor. Because of the internal positioning of the rotor magnets, it is good for high-speed operation.

The high ratio between the quadrature and direct axis inductances in the interior  PMSM  is  a  boost  for  the  electromagnetic  torque  augmentation  by

bringing in the reluctance torque. These motors are considered to have saliency

with q-axis inductance greater than the d-axis inductance,

Lq   Ld . The magnets

are very well protected against centrifugal forces [32, 36].

Figure 1.3: Interior Permanent Magnet Motor Rotor Assembly

Among  other  applications,  Interior  Permanent  Magnet  Synchronous Motors (IPMSM) have gained importance due to their high torque per volume ratio  particularly  for  hybrid  electrical  vehicles  [37].  A  tabular  comparison between Surface and Interior Permanent Magnet Synchronous Motors is given in Table 1.1.

Table  1.1:  Comparison  between  PMSMs  with  Surface  and  Interior  Rotor

Magnets [38].

Surface PMSMs	Interior PMSMs
Simple motor construction	Relatively complicated motor construction
Airgap magnetic flux density is smaller than the remnant flux density Br	Air gap magnetic flux density can be greater than the remnant flux density Br
Small armature reaction flux	Higher armature reaction flux
PMs not protected against armature field	PMs protected against armature field
Low PM flux leakage	Higher PM flux leakage
Poor flux-weakening capability	Large flux-weakening capability
Medium speed range	Wide speed range

Other  available  classifications  of  PMSM,  based  on  the  placement  of permanent magnets, are derived from these two basic classifications [39, 40].

1.5 Research Objectives

The  main  aim  of  this research is to  implement a  Pulsewidth Modulated Voltage-Fed Inverter Vector-Controlled PMSM Drive based on Hysteresis Current Control (HCC). Specifically, the objectives of this work are to:

  develop an open-loop model of the PMSM,

   develop and utilize HCC algorithm for Field Orientation Control (FOC)

of a complete closed loop speed-controlled PMSM drive system,

     optimize  the  HCC  algorithm  to  obtain  fast  speed  response  with  all possible reference speed inputs while maintaining effective current and torque tracking,

     identify the optimal control variables of the HCC on the PMSM with emphasis on effective current and torque tracking,

   compare drive performance with ramp and step reference speed inputs,

   achieve four quadrant operation of the PMSM, and

  compare the speed response of the developed model with the AC6  of MATLAB Simpower systems in terms of rise time, overshoot and settling time.

1.6 Thesis Arrangement

The thesis is divided into six chapters:

  Introduction: Chapter one introduces the objectives of Adjustable Speed Drives  (ASD)  and  its  superiority  over  the  fixed  speed  operation  of electric  motor,  permanent  magnet  materials,  the   permanent  magnet synchronous motors as compared to other AC motors, their classifications and features. The research objectives and thesis  arrangement were also made.

  Literature Review: Chapter two gives historical account of the existing literature on the broad area of PMSM drives. This includes very relevant research papers in diverse aspects of vector control of PMSM as well as a review of some selected research materials on the design of  PMSM. A thorough highlight was made on the contribution of each of the previous research papers.

  The   Permanent   Magnet   Synchronous   Machine:   Chapter    three presented, from first principle, the detailed dq modelling of the  PMSM yielding the dynamic equations suitable for open-loop digital simulation. The torque enhancement in the interior permanent magnet  synchronous motor was examined using the steady state equations. The line-start run- up characteristic of the motor from grid voltage was also  studied under load and no load condition.

  Features and Control of the Three Phase Voltage-Fed Inverter for AC Drives: Chapter four gives a background on the variable voltage variable frequency control of PMSM and the three phase inverter control which is the  backbone  of  the  Adjustable  Speed  Drives  (ASD)  in   industrial practice. The available modulation techniques were examined for merits and demerits. The current control strategy utilized in the drives as well as some harmonic elimination strategies was examined. Insight  was given

into the features and benefits of four quadrant operation of electric drives as obtained in industry.

  Permanent  Magnet  Synchronous  Motor  Control:  Chapter  five   is devoted to the implementation of the PWM Voltage-Fed Inverter Vector Controlled PMSM based on Field Orientation using Hysteresis  Current Control technique for the PMSM.  The HCC algorithm is developed and utilized for Field Orientation Control (FOC) of a  complete closed loop speed-controlled  PMSM  drive  system  under  full  load  stress  for  the Constant Reference Speed Input, Step Reference Speed Input, and RAMP Reference  Speed  Input.  Optimisation  of  the   control  algorithm  and identification  of  optimal control  variables  were  made.  Four Quadrant Operation of the PMSM was also  achieved.  Response comparison was made  between  the  developed  model  and  the  AC6  of  the  MATLAB Simpower systems in terms of rise time, overshoot, and settling time.

     Conclusion  and  Recommendations:  The  thesis  was  concluded  in Chapter  Six.  Recommendations  were  also  made  for  future  research works.

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