STEADY-STATE BY THE COMPLEX VARIABLE MODEL

By IM steady-state we mean constant speed and load. For a machine fed from a sinusoidal voltage symmetrical power grid, the phase voltages at IM terminals are

clip_image001 Va,b,c = V 2 cos⋅ ω −1t (i−1)23π ; i =1,2,3 (13.39)

The voltage space phasor Vsb in random coordinates (from (13.27)) is

clip_image002 Vsb = 2 (V ta ( )+aV tb ( )+a V t e2 c ( )) − θj er

3

From (13.39) and (13.40),

(13.40)

clip_image003 Vsb = V 2 cos[ (ω −θ1t b )+ jsin(ω −θ1t b )]

Only for steady-state,

(13.41)

θ = ωb bt +θ0

Consequently,

(13.42)

clip_image004 Vsb = V 2ej[(ω −ω1 b )t+θ0 ]

(13.43)

For steady-state, the current in the space phasor model follows the voltage frequency: (ω1 – ωb). Steady-state in the state space equations means replacing d/dt with j(ω1 – ωb).

Using this observation makes Equations (13.30) become

Vs0 = R is s0 + ω Ψj 1 s0; Ψs0 = L isl s0 + Ψm0;

clip_image005 Ψr0 = L irl r0 + Ψm0; im0 = is0 +ir0 (13.44)

clip_image006= R ir r0 + jSω Ψ1 r0; S = (ω −ω1ωr r ); Ψm0 = L im m0 Vr0

So the form of space phasor model voltage equations under the steady-state is the same irrespective of the speed of the reference system ωb.

When ωb, only the frequency changes of voltages, currents, flux linkages in the space phasor model varies as it is ω1 – ωb.

No wonder this is so, as only Equations (13.44) exhibit the total emf, which should be independent of reference system speed ωb. S is the slip, a variable well known by now. Notice that for ωb = ω1 (synchronous coordinates), d/dt = (ω1 – ωb) = 0. Consequently, for synchronous coordinates the steady-state means d.c. variables.

The space phasor diagram of (13.44) is shown in Figure 13.2 for a cage rotor IM.

cage rotor

image

Figure 13.2 Space phasor diagram for steady-state

From the stator space Equations (13.44) the torque (13.31) becomes

clip_image001[4] Te = 23 p1 jΨr0 r0i*  = 32 P1Ψr0 r0i (13.45) Also, from (13.44),

clip_image002[4]clip_image003[4]Ψr0 ir0 = −jSω1 (13.46)

Rr With (13.46), alternatively, the torque is

Te = 3 p1 Ψr02 Sω1 (13.47)

2 Rr

Solving for Ψr0 in Equations (13.44) lead to the standard torque formula.

3p1 Vs2 RSr

Te ≈ ω1 Rs +C1 RSr 2 +ω12 (Lls +C L1 lr )2 ; C1 = +1 LLmls (13.48)

Expression (13.47) shows that, for constant rotor flux space-phasor amplitude, the torque varies linearly with speed as it does in a separately excited d.c. motor. So all steady-state performance may be calculated using the spacephasor model as well.

EQUIVALENT CIRCUITS FOR DRIVES

Equations (13.30) lead to a general equivalent circuit good for transients, especially in variable speed drives (Figure 13.3).

clip_image005[4]clip_image006[4]clip_image007 Vs = R is s +(p+ ωj b )L isls +(p+ ω Ψj b ) m (13.49)

Vr = R ir r +(p+ j(ω −ωb r ))L irl r +(p+ j(ω −ωb r ))Ψm

The reference system speed ωb may be random, but three particular values have met with rather wide acceptance.

• Stator coordinates: ωb = 0; for steady-state: p Æ jω1

• Rotor coordinates: ωb = ωr; for steady-state: p Æ jSω1

• Synchronous coordinates: ωb = ω1; for steady-state: p Æ 0

image

 

 

image

Figure 13.3 The general equivalent circuit a.) and for steady-state b.) (continued)

Also, for steady-state in variable speed drives, the steady-state circuit, the same for all values of ωb, comes from Figure 13.3a with p Æ j(ω1 – ωb) and Figure 13.3b.

Figure 13.3b shows, in fact, the standard T equivalent circuit of IM for steady-state, but in space phasors and not in phase phasors.

A general method to “arrange” the leakage inductances Lsl and Lrl in various positions in the equivalent circuit consists of a change of variables.

clip_image001[6]clip_image002[8] iar = i /a; r Ψma = aLmis +iar (13.50)

Making use of this change of variables in (13.49) yields

clip_image003[6]clip_image004[8]clip_image005[6] aVr = a R i2Vsr =ar +R i(sps++j((ω −ωp b+ ωj br)()) (La aLs −aLrl mL)ims )+iar(p++ ω Ψ(pj+ bj()ω −ωbam r ))Ψam (13.51)

An equivalent circuit may be developed based on (13.51), Figure 13.4. The generalized equivalent circuit in Figure 13.4 warrants the following comments:

• For a = 1, the general equivalent circuit of Figure 13.4 is reobtained and

clip_image006[6]am = Ψm : the main flux

• For a = Lm/Lr the inductance term in the “rotor section” “disappears”, being moved to the primary section and

clip_image007[4]clip_image008clip_image009 aΨam = LLmr Lmis +ir LLmr  = LLmr Ψr (13.52)

image

 

 

image

Figure 13.6 Steady-state equivalent circuits

a.) rotor flux oriented b.) stator flux oriented

• For a = Ls/Lm, the leakage inductance term is lumped into the “rotor section” and

clip_image001[8]clip_image002[10]clip_image001[9] aΨam = LLs Lm is +ir LLms  = Ψs (13.53)

m

• This type of equivalent circuit is adequate for stator flux orientation control

• For d.c. braking, the stator is fed with d.c. The method is used for variable speed drives. The model for this regime is obtained from Figure 13.4 by using a d.c. current source, ωb = 0 (stator coordinates, Vr = 0, a = Lm/Lr) The result is shown in Figure 13.5.

For steady-state, the equivalent circuits for ar = Lm/Lr and ar = Ls/Lm and Vr=0 are shown in Figure 13.6.

Example 13.1. The constant rotor flux torque/speed curve.

Let us consider an induction motor with a single rotor cage and constant parameters: Rs = Rr = 1 Ω, Lsl = Lrl = 5 mH, Lm = 200 mH, Ψr0’ = 1 Wb, S = 0.2, ω1 = 2π6 rad/s, p1 = 2, Vr = 0. Find the torque, rotor current, stator current, stator and main flux and voltage for this situation. Draw the corresponding space phasor diagram.

Solution

We are going to use the equivalent circuit in Figure 13.6a and the rotor current and torque expressions (13.46 and 13.47):

Te = 3 p1 Ψr02 Sω =1 3 21clip_image003[8]2 0.2 2 6⋅ π = 22.608Nm

2 Rr 2 1

Ψr0 = −0.2 2 6⋅ π 1 = −7.536A

Ir0 = − ωS 1

Rr 1

clip_image004[10] The rotor current is placed along real axis in the negative direction. The rotor flux magnetization current Ir0 (Figure 13.6a) is

clip_image005[8] I0r0 = −j(−Ir0 )LRSr2LLmr = −j(Ir01L)Rm r = −j0.2 2 6 0.2clip_image009[4] j5A

ω1 Lmr

The stator current

clip_image010clip_image005[9] Is0 = −Ir0 Lr + I0r0 = 7.536 0.205 − j5 = 7.7244− j5.0A

Lm 0.2

clip_image011 The stator flux Ψs is

clip_image012 Ψs0 = L Is s0 + L Im r0 = 0.205(7.7244− j5.0)+0.2(−7.536)= 0.076− j1.025 The airgap flux Ψm is

clip_image013 Ψm0 = Lm (Is0 + Ir0 )= 0.2 7.( 7244− j5.0−7.536)= 0.03768− j1.0

The rotor flux is

Ψr0 = Ψm0 + L Irl r0 = 0.03768− j1.0+0.005(−7.536)= −j1.0 (as expected)

image

The voltage Vso is:

clip_image001[12] Vs0 = ω Ψj 1 s0 + R Is s0 = j2 6 0.π ( 076− j1.025)+1 7.( 7244− j5.0)= 46.346− j2.136 The corresponding space phasor diagram is shown in Figure A.

Leave a comment

Your email address will not be published. Required fields are marked *