Dynamic Model of Induction Machine MEP 1522 ELECTRIC DRIVES

WHY NEED DYNAMIC MODEL? In an electric drive system, the machine is part of the control system elements To be able to control the dynamics of the drive system, dynamic behavior of the machine need to be considered Dynamic behavior of of IM can be described using dynamic model of IM

WHY NEED DYNAMIC MODEL? Dynamic model – complex due to magnetic coupling between stator phases and rotor phases Coupling coefficients vary with rotor position – rotor position vary with time Dynamic behavior of IM can be described by differential equations with time varying coefficients

a b b’b’ c’c’ c a’a’ Simplified equivalent stator winding i as Magnetic axis of phase A Magnetic axis of phase B Magnetic axis of phase C i cs i bs DYNAMIC MODEL, 3-PHASE MODEL

stator, b stator, c stator, a rotor, b rotor, a rotor, c rr DYNAMIC MODEL – 3-phase model

Let’s look at phase a Flux that links phase a is caused by: Flux produced by winding a Flux produced by winding b Flux produced by winding c DYNAMIC MODEL – 3-phase model

Flux produced by winding b Flux produced by winding c Let’s look at phase a The relation between the currents in other phases and the flux produced by these currents that linked phase a are related by mutual inductances DYNAMIC MODEL – 3-phase model

Let’s look at phase a Mutual inductance between phase a and phase b of stator Mutual inductance between phase a and phase c of stator Mutual inductance between phase a of stator and phase a of rotor Mutual inductance between phase a of stator and phase b of rotor Mutual inductance between phase a of stator and phase c of rotor DYNAMIC MODEL – 3-phase model

v abcs = R s i abcs + d(  abcs )/dt - stator voltage equation v abcr = R rr i abcr + d(  abcr )/dt - rotor voltage equation  abcs flux (caused by stator and rotor currents) that links stator windings  abcr flux (caused by stator and rotor currents) that links rotor windings DYNAMIC MODEL – 3-phase model

Flux linking stator winding due to stator current Flux linking stator winding due to rotor current DYNAMIC MODEL – 3-phase model

Flux linking rotor winding due to rotor current Flux linking rotor winding due to stator current Similarly we can write flux linking rotor windings caused by rotor and stator currents:

DYNAMIC MODEL – 3-phase model Combining the stator and rotor voltage equations, p is the derivative operator, i.e. p = d/dt

DYNAMIC MODEL – 3-phase model Windings x and y, with N x and N y number of turns, separated by α x x’x’ y y’y’ Magnetic axis of winding x Magnetic axis of winding y It can be shown that the mutual inductance between winding x and y is

DYNAMIC MODEL – 3-phase model To get self inductance for phase a of the stator let N x = N y = N s and α = 0 If we consider the leakage flux, we can write the self inductance of phase a of the stator L as = L am + L ls L as Self inductance of phase a L am Magnetizing inductance of phase a L ls Leakage inductance of phase a

DYNAMIC MODEL – 3-phase model Due to the symmetry of the windings, L am = L bm = L cm, Hence L as = L bs = L cs = L ms + L ls The magnetizing inductance L ms, accounts for the flux produce by the respective phases, crosses the airgap and links other windings The leakage inductance L ls, accounts for the flux produce by the respective phases, but does not cross the airgap and links only itself

DYNAMIC MODEL – 3-phase model It can be shown that the mutual inductance between stator phases is given by:

DYNAMIC MODEL – 3-phase model The mutual inductances between stator phases can be written in terms of magnetising inductances

DYNAMIC MODEL – 3-phase model The mutual inductances between rotor phases can be written in terms of stator magnetising inductances Self inductance between rotor windings is when N x = N y = N r and α = 0

DYNAMIC MODEL – 3-phase model The mutual inductances between the stator and rotor windings depends on rotor position Self inductance between phase a of rotor and phase a of stator is when N x = N s, N y = N r and α = θ r

DYNAMIC MODEL – 3-phase model The mutual inductances between the stator and rotor windings depends on rotor position

DYNAMIC MODEL – 3-phase model

stator, b stator, c stator, a rotor, b rotor, a rotor, c rr DYNAMIC MODEL – 3-phase model

DYNAMIC MODEL, 2-PHASE MODEL It is easier to look on dynamic of IM using two-phase model. This can be constructed from the 3-phase model using Park’s transformation Three-phase Two-phase equivalent There is magnetic coupling between phases There is NO magnetic coupling between phases

It is easier to look on dynamic of IM using two-phase model. This can be constructed from the 3-phase model using Parks transformation stator, b rotor, b rotor, a rotor, c stator, c stator, a rr rr rotating Three-phase rr stator, q rotor,  rotor,  stator, d rotating rr Two-phase equivalent DYNAMIC MODEL – 2-phase model

It is easier to look on dynamic of IM using two-phase model. This can be constructed from the 3-phase model using Parks transformation rr stator, q rotor,  rotor,  stator, d rotating rr Two-phase equivalent However coupling still exists between stator and rotor windings DYNAMIC MODEL – 2-phase model

All the 3-phase quantities have to be transformed to 2- phase quantities In general if x a, x b, and x c are the three phase quantities, the space phasor of the 3 phase systems is defined as:, where a = e j2  /3 DYNAMIC MODEL – 2-phase model

All the 3-phase quantities have to be transformed into 2-phase quantities d q DYNAMIC MODEL – 2-phase model

The transformation is given by: For isolated neutral, i a + i b + i c = 0, i.e. i o =0 i dqo = T abc i abc The inverse transform is given by: i abc = T abc -1 i dqo DYNAMIC MODEL – 2-phase model

v abcs = R s i abcs + d(  abcs )/dt v abcr = R rr i abcr + d(  abr )/dt IM equations : rr stator, q rotor,  rotor,  stator, d rotating rr 3-phase v dq = R s i dq + d(  dq )/dt v  = R rr i  + d(   )/dt 2-phase

 dq where,   Express in stationary frame Express in rotating frame v dq = R s i dq + d(  dq )/dt v  = R r i  + d(   )/dt DYNAMIC MODEL – 2-phase model

Note that: L dq = L qd = 0L  = L  = 0 L dd = L qq L  = L  The mutual inductance between stator and rotor depends on rotor position: L d  = L  d = L sr cos  r L q  = L  q = L sr cos  r L d  = L  d = - L sr sin  r L q  = L  q = L sr sin  r DYNAMIC MODEL – 2-phase model

L d  = L  d = L sr cos  r L q  = L  q = L sr cos  r L d  = L  d = - L sr sin  r L q  = L  q = L sr sin  r rr stator, q rotor,  rotor,  stator, d rotating rr DYNAMIC MODEL – 2-phase model

In matrix form this an be written as: The mutual inductance between rotor and stator depends on rotor position DYNAMIC MODEL – 2-phase model

Magnetic path from stator linking the rotor winding independent of rotor position  mutual inductance independent of rotor position rotor, q rotor, d stator, d stator, q rr Both stator and rotor rotating or stationary The mutual inductance can be made independent from rotor position by expressing both rotor and stator in the same reference frame, e.g. in the stationary reference frame DYNAMIC MODEL – 2-phase model

How do we express rotor current in stator (stationary) frame? In rotating frame this can be written as: rr rr   irir In stationary frame it be written as:  qsqs dsds i rd i rq is known as the space vector of the rotor current DYNAMIC MODEL – 2-phase model

If the rotor quantities are referred to stator, the following can be written: L m, L r are the mutual and rotor self inductances referred to stator, and R r ’ is the rotor resistance referred to stator L s = L dd is the stator self inductance V rd, v rq, i rd, i rq are the rotor voltage and current referred to stator DYNAMIC MODEL – 2-phase model

It can be shown that in a reference frame rotating at  g, the equation can be written as: DYNAMIC MODEL – 2-phase model

DYNAMIC MODEL  Space vectors IM can be compactly written using space vectors: All quantities are written in general reference frame

Product of voltage and current conjugate space vectors: DYNAMIC MODEL  Torque equation It can be shown that for i as + i bs + i cs = 0,

DYNAMIC MODEL  Torque equation Ifand

The IM equation can be written as: The input power is given by: DYNAMIC MODEL  Torque equation Power Losses in winding resistance Rate of change of stored magnetic energy Mech power Power associated with  g – upon expansion gives zero

DYNAMIC MODEL  Torque equation

We know that  m =  r / (p/2),

but DYNAMIC MODEL  Torque equation

DYNAMIC MODEL Simulation Re-arranging with stator and rotor currents as state space variables: The torque can be expressed in terms of stator and rotor currents:

Which finally can be modeled using SIMULINK: