Electric Propulsion System Frequency Converters Principles January 2014

Electric Propulsion System Frequency Converters Principles January 2014

ê Agenda Electro-magnetic Field Electric Machines Frequency converters Principles | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 2

ê Electro-magnetic Field The magnetic force is one of the four fundamental forces of the Universe (with gravitation, nuclear strong and nuclear weak forces). The origin of the magnetic force is in the tiny electric currents, at atomic level, of the spinning of electrons, thus it can be named the electro-magnetic force. At a natural stage it can be detected around naturally magnetic materials, or at the level of the earth magnetic field (itself due to the circulation of the magma, in the heart of the earth). As a force it exerts an attraction or a repulsion on any material subject to an electric current, such as another magnet, or any electric circuit. At our macroscopic level, in the natural stage, it is a small force compared to gravitation. It can be detected with the magnetic needle of a compass. To obtain a stronger magnetic force we must use much greater currents than those of the spinning of electrons. | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 3

ê Electro-magnetic Field It is easy to obtain a « magnet » with an electric circuit by simply « stacking » the effects of a current circulating (rotating) into a series of coils, wound together, in a common winding. Natural field lines B E If the value of the current is constant, the resulting magnetic field will be constant, in space as well as in time. The force itself can be represented in each point of the space by a vector, B, called « induction » , measured in Tesla Field lines guided by magnetic circuit The magnetic field can be guided within a magnetic circuit, composed of piled-up magnetic steel sheets. This is done in transformers and electric machines. Magnetic circuit | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 4

ê Electro-magnetic Field - Flux As any force the induction is represented by a vector, it has an intensity, and a direction, it is oriented. North B The orientation of the induction in each point of the space, gives the orientation of the field lines, the lines to which the induction vector is tangent, in each point. The intensity of the induction corresponds to a strength of the magnetic field. Because of its orientation and its strength, the magnetic field going through a closed circuit, or through a winding, can be seen as a flow. South In the same manner as a flow of water going through a pipe. The « flow » of the magnetic field is called the flux. Its strength, or intensity, is measured in Weber. | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 5

ê Electro-magnetic Field Magnetic to magnetic n Magnetic attraction between opposed poles : North and South Ø This is the principle used in synchronous machines n Magnetic repulsion between poles of the same type : North/North or South/South Electric current Magnetic to electric Magnetic field n An induced force is applied to any electric circuit, with a current, submitted to a magnetic field Ø This is used in induction machines n A variation of its surrounding magnetic field will induce a voltage in any electric circuit Ø This is used in transformers and in machines, for generator mode | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 6

ê Electro-magnetic Field If we have a variable current, in time, the magnetic field will also vary, in time. With a sinusoidal current, we obtain a sinusoidal variation of the induction B I t t The magnetic field will vary in time, but not in space : the forces lines remain in the same place, only the induction, the direction and intensity of the force, will vary. | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 7

ê Electro-magnetic Field Rotating magnetic field Three fixed windings, positioned at 120 ° of each other, fed with a three-phase sinusoidal current, will produce a resulting rotating field The three fields combine together to result in a unique field, rotating in space ! This is what we do with the stators or both : North pole South pole The B vector, above, corresponds to the field of a two-pole (rotating) magnet. Therefore this illustrates a two-pole machine - Synchronous machines - Asynchronous (induction) machines If we use more than three coils, six, or nine, etc, equally distributed on the stator circumference, then we shall have a multi-pole machine | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 8

ê Frequency Converters Principles Electro-magnetic Field Electric Machines | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 9

ê Electric Machines Stator The stator is the same for a synchronous or induction machine Its active parts consist in : An electric circuit : Made of a number of coils, where threephase stator current will circulate, creating the stator rotating magnetic field A magnetic circuit Made by stacked magnetic steel sheets, which role is to guide the magnetic field lines One electric coil Magnetic circuit | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 10

ê Electric Machines Stator Winding the stator coils For a two-pole machine, the simplest way is just to use three coils, as explained before, to obtain the rotating field. In this case the coils will be distributed as shown on this figure We can represent the coils on a cut and flattened stator, seen from above In order to obtain a better distribution of the field, each coil will be made by a number of parallel coils, distributed in several slots and connected together Example with three parallel coils per phase | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 11

ê Electric Machines Rotor Synchronous machine The rotor of a synchronous machine consists in magnets, each with two poles, North and South The number of poles at rotor side must be the same as the number of poles at stator side S N Here we have a four-pole machine (or two pairs of poles) S N The coiled rotor poles (magnets) must be fed by DC current. This is done by the « exciter » and excitation circuit (rotating diodes) | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 12

ê Electric Machines Rotor Induction (asynchronous) machine The rotor of an induction machine consists in an electric circuit, generally made of a simple « squirrel cage » , embedded in a magnetic circuit When submitted to the variable stator magnetic field, the rotor electric circuit will generate induced rotor currents We will explain the functioning later on | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 13

ê Electric Machines – Synchronous Motor Principles The rotating stator field causes the rotor poles to follow its rotation, at same speed, simply by magnetic attraction between opposite poles If there is no resisting torque on the shaft (no load), the poles will be in front of each other (north facing south) If there is a resisting torque on the shaft (load to be driven), there will be an angle between the axis of the rotor poles and the axis of the stator poles, but stator field and rotor will remain at the same speed The maximum possible torque corresponds to about 90° angle between stator poles axis and rotor pole axis. If the load increases beyond this limit the motor will drop out and stop | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 14

ê Electric Machines – Synchronous Motor Torque – Internal angle Characteristics Motor mode T Generator mode 150 % At excitation 100 % 50 % -180° -90° 180° | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS δ | January 2014 | 15

ê Electric Machines – Induction Motor Principles The stator of an induction motor is the same as that of a synchronous motor, with the same rotating stator field But the principle to drive the rotor is different The rotor is a simple squirrel cage with bars short-circuited at each end by a ring The variation of the stator field, due to its rotation, will induce a current to circulate in the rotor circuit (in the rotor bars) Each rotor bar, having a current, and submitted to a magnetic field, will be subjected to a force, perpendicular to the bar axis. The result will be an « induced » torque applied to the rotor cage | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 16

ê Electric Machines – Induction Motor If there were no resisting torque on the shaft (no load at all), the induced torque would start the rotation of the rotor and bring it to the same speed as that of the stator rotating field. If there is a resisting torque on the shaft (load to be driven), it will slow down the rotor, and there will be a speed difference between stator field and rotor, a slippage The heavier the load, the bigger the slippage | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 17

ê Electric Machines – Principles of functioning Let us first illustrate the principles of synchronous and induction machines with two animations Synchronous Motor Induction Motor A magnet is installed on a swivel A disk is installed on a swivel at the intersection of the coils axes It will follow the rotating field, at the same speed It will follow the rotating field, at a different speed, slipping (synchronous) (Asynchronous) see the red spot speed, and the field speed And now let us review some features of the induction motor, and its use with variable speed drives. | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 18

ê Electric Machines – Induction Motor Torque – Speed Characteristics T At variable F & constant U/F Rated curve Constant torque at At constant voltage & frequency different speeds T Motor mode Generator mode N Fr / Fs Frotor / Fstator (p. u) 0 1 At variable F & decreasing U/F T Rated curve Decreasing torque Fr / Fs | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 19

ê Electric Machines – Mechanical reminders Every electric machine is reversible, which means it can work either in motor mode or in generator mode Generator mode The machine receives mechanical energy from a prime mover, diesel engine or turbine, connected to its rotor, and converts it into electrical energy delivered to an electrical network connected to its stator Motor mode The machine receives electrical energy from the network, and converts it into mechanical energy available at the shaft of its rotor, to drive a load Every electric machine can rotate in both directions clockwise, counterclockwise | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 20

ê Electric Machines – Mechanical reminders The two mechanical parameters to be controlled in the use of a machine are : Torque (m. N) Speed (rpm or Rd/s) The mechanical power is the product : P (W) = Torque (m. N) X Speed (Rd/s) The convention for a motor is to note : Motor mode : P > 0 P = T. N Generator mode : P < 0 P = T. N Every electric machine can work in the four quadrants of a torque / speed diagram | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 21

ê Electric Machines – Mechanical reminders TM Generator TR N torque (m. N) T TM TR N Motor N TM TR TR N Motor TM N speed (rpm) Generator TM Motor torque TR Resisting torque N Shaft speed | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 22

ê Electric Machines – Speed control How to control the speed of the synchronous or induction motor ? This is easy The rotor shaft speed depends directly on the speed of the stator rotating field, which in turn directly depends on the frequency of the current or voltage applied to the stator circuit We will apply a voltage or current of the desired frequency to the stator circuit, to obtain the desired rotor speed For a synchronous motor this frequency directly corresponds to the rotor speed For an induction motor we shall have to add a frequency correction (ΔF), to compensate for the slippage : Fstator = Frotor + ΔF | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 23

ê Electric Machines – Speed control Example : For a synchronous motor, of nominal characteristics 720 rpm / 60 Hz : If we need to run it at 30 % of its nominal speed, the corresponding stator current will be a sine wave at 18 Hz = 30% x 60 Hz For an induction motor, of nominal characteristics 585 rpm / 60 Hz, with nominal slippage 2, 5 % (at nominal torque) : If we need to run it at 30 % of its nominal speed, still at nominal torque, the corresponding stator current will be a sine wave at 18, 45 Hz 18 Hz = 30% x 60 Hz 2, 5 % x 18 Hz = 0, 45 Hz | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 24

ê Electric Machines – Torque control How to control the torque of the synchronous or induction motor ? This is more complicated … The torque of an electric machine is given by vector product of : vector representing total flux, stator and rotor flux instant vector representing the three-phase stator currents Torque will be maximum when and But and are kept perpendicular to each other are not independent, they are linked together « Vector control » relates to the computation of equations linking the components of these vectors, in adequately chosen axes, so that they can be controlled separately The components are linked through a matrix representing a model of the motor | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 25

ê Electric Machines – Torque & speed relation Torque and Speed relation n Torque and speed are linked together : Ø Statically : you cannot reach the desired speed if you do not produce a sufficient torque Ø And dynamically : if the motor torque is greater than the resisting torque (propeller torque), you will finally reach the speed, but how long will this take ? n This is given by the relation : Motor torque (m. N) Resisting torque (propeller) Shaft line inertia (kg. m²) Speed variation acceleration / deceleration (rd/s) | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 26

ê Electric Machines – Importance of torque control Why bother so much about torque ? In industry (metal, paper, etc): torque means tension in the product Ø Quality of torque control (precision, dynamic response) means quality of product In propulsion (marine, trains, vehicles, etc): torque means dynamic response Ø Quality of torque control means manoeuvrability The torque of an electric machine is given by is the total flux, stator and rotor flux This is why for a synchronous motor the control of the excitation is not enough (rotor flux) Torque will be maximum when and are kept perpendicular to each other Vector control will deal not only with magnitude but also with phase, so that both vectors will be kept perpendicular at all times, including transient periods | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 27

ê Frequency Converters Principles Electro-magnetic Field Electric Machines Frequency Converters Principles | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 28

ê Frequency Converters Principles There are many sorts of variable speed drives, for synchronous motors as well as for induction motors The constant evolution of technology, for power electronic devices and for control devices, has rendered some solutions obsolete. So we will concentrate only on the up-to-date, most efficient, solutions, for the electric propulsion of ships We can distinguish two aspects : Ø Power architecture Ø Control architecture | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 29

ê Frequency Converters Principles Power Architecture AC Network In motor mode Machine Bridge Network Bridge R e c t i f i e r Propulsion Transformer DC Link I n v e r t e r M Basically we can define two families of drives Ø Current fed inverters synchroconverter Ø Voltage fed inverters pwm converter We will revert to each subject separately | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 30

ê Frequency Converters Principles Control Architecture Control architecture relates to the computation of : Ø Measurements Ø Motor model parameters and variables in order to achieve the required quality and performance for : Speed control Torque control | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 31

ê Frequency Converters Principles Control Architecture Speed Ref Motor Speed Ref Ramps Mot. Freq. meas. N. B Limited Torque Ref ε N + - Speed Control Loop Motor model Torque Ref Speed meas. M. B I U Selection M PLS Network frequency limitation Network voltage limitation < Sequential limitation Process limitation | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 32

ê Frequency Converters Principles Control Architecture We can distinguish between two control methods, applicable to both current and voltage fed inverters, to both synchronous and induction motors : Ø Scalar control : only the magnitude of vector variables is controlled Ø Vector control : magnitude and phase are controlled The difference is to be found in the motor model and computation method. Voltage (or current) and frequency are the two control variables. But both torque and flux depend on voltage (or current) and frequency. And moreover torque also depends on flux. So torque and flux cannot be controlled separately with scalar methods. The result of scalar methods is poor dynamic response and poor torque control (with the exception of « Direct Torque Control » , of which we will talk later on) But Flux Vector Control will de-couple, and separately control, torque and flux. | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 33

ê Frequency Converters Principles Basic principles The torque of an electric machine is given by vector product of : But and When we touch vector representing total flux, stator and rotor flux instant vector representing the three-phase stator currents are not independent, they are linked together ! … we also touch The principle is then to « break » ! ! into two components : Im and Ia which we can control independently Im is the magnetising component, which will control the flux Ia is the active component, which will control the torque This will require a lot of calculations, to be done at each scanning period, but it is now possible with modern digital micro-controllers | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 35

ê Frequency Converters Principles Instant current vector For a synchronous motor is the rotating vector, sum of the three instantaneous stator currents I(t) Iu(t) Iw(t) Iv(t) For an induction motor Iu(t) IW(t) IV(t) Two instant current vectors : Ø One for stator currents Ø One for rotor currents | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 36

ê Frequency Converters Principles Basic principles … some more details The three-phase machine (synchronous or induction), is first mathematically transformed into a two-phase equivalent machine Iu(t) Iβ(t) IW(t) IV(t) Iα(t) | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 37

ê Frequency Converters Principles We can still have the rotating field with the two-phase machine (only the equations are different) Iβ(t) Iα(t) | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 38

ê Frequency Converters Principles This transformation is applied to the instant current vector, Is, which is then expressed in the fixed reference axes α, β This new instant current vector is further expressed in a new reference axes m, a, rotating with the flux β Is a m Ia Im α The components of the instant current vector, in this rotating system of axes … Are the magnetising and active components we were looking for ! | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 39

ê Frequency Converters Principles We can now take a closer look inside the vector control architecture Synchronous motor case Speed Ref Torque Control Torque Ref + Ref Im - Ia Control Loop Im Control Loop m , a α , β I α Ø Meas Im Ref Im = f(N) N. B Firing angles computation M. B Model 3 ph Meas I Meas U Flux Control Meas Ia Firing angles computation Ref I Dynamic 2 ph I β Motor U m , a α , β Speed Meas. (computation) Instant vector 2 ph I computation 3 ph Ø Flux computation Speed Ref M Iex Firing angles computation | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS Excit. | January 2014 | 40

ê Frequency Converters Principles Let us now take a look into the vector control for the induction motor The same principle will apply : Ø Transforming the three-phase machine into a two-phase equivalent machine We have two instant current vectors, one for stator, one for rotor But : ü If we choose the rotating reference system (m, a) linked to the rotor flux Then only the stator current vector will intervene in the motor’s equations Also, of course, the motor dynamic model will be different | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 41

ê Frequency Converters Principles A closer look inside the vector control architecture Speed Ref Torque Control Torque Ref + Ref Im - Ia Control Loop Im Control Loop Flux Control m , a α , β Ø rotor Meas Im Motor Ø stator 2 ph I β Ref I α , β Speed Meas. (computation) N. B M. B Model Meas I Meas U m , a Firing angles computation Dynamic 3 ph Ø M / (1+ p. Tr) rotor Meas Ia Ref Im = f(N) I α Induction motor case U 2 ph 3 ph Ø stator Stator Instant vector I computation I M Stator Flux computation | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 42

ê Frequency Converters Principles This concludes with our presentation of the principles of frequency converters Further detailed presentations are available for : Synchro-Converters PWM Converters | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 43

This concludes our presentation Thank you for your attention | CUSTOMISED TECHNOLOGY FOR CUSTOMER SUCCESS | January 2014 | 44