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an investigation of dual stator winding induction machines

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phenomenon as c<strong>an</strong> be seen in (8.19-8.20). It is desirable to operate the generator <strong>an</strong>d<br />

control the dc voltage when the total copper loss <strong>of</strong> the machine is minimized. Since the<br />

machine operates as two independent <strong>machines</strong> at steady state, the total copper loss is<br />

minimized when the copper loss due to each <strong>stator</strong> <strong>winding</strong> is minimized. This condition<br />

is obtained qu<strong>an</strong>titatively, when the derivative <strong>of</strong> the expression <strong>of</strong> the copper loss for<br />

each machine with respect to the rotor slip is equated to zero (8.21).<br />

s<br />

*<br />

i<br />

= −<br />

ω<br />

r<br />

2<br />

ri<br />

2<br />

sirri<br />

1<br />

2 2 ( r L + r L ) −1<br />

si<br />

ri<br />

ri<br />

mi<br />

, i = 1 or 2<br />

(8.21)<br />

For the control scheme, the rotor flux reference is selected to achieve minimum total<br />

copper loss. The reference <strong>of</strong> the rotor flux linkage obtained from (8.11-8.14) as:<br />

λ<br />

qdri<br />

=<br />

− B<br />

λqdsi<br />

, i = 1 or 2<br />

(8.22)<br />

Ari<br />

− j<br />

1−<br />

ωri<br />

ri<br />

*<br />

si<br />

*<br />

si<br />

For the control scheme, instead <strong>of</strong> choosing the rotor flux reference to be a const<strong>an</strong>t<br />

value, it c<strong>an</strong> be adjusted on-line to further reduce the total loss including copper <strong>an</strong>d core<br />

losses.<br />

Figure 8.3(a) shows the relationship between the magnitudes <strong>of</strong> the modulation<br />

indexes ( 1<br />

M , 2<br />

M ) <strong>of</strong> the two boost rectifiers <strong>an</strong>d the coefficient ( ) K + = 1 1 α for a<br />

const<strong>an</strong>t output power when the rotor speed is const<strong>an</strong>t, the magnetizing flux λ m is set to<br />

a const<strong>an</strong>t value <strong>of</strong> 0.25 Wb <strong>an</strong>d the rotor slips are chosen to achieve minimum total<br />

copper loss. When α = 1, the 2-pole ABC <strong>winding</strong> set delivers all the dc power while in<br />

the extreme case the 6-pole XYZ <strong>winding</strong> set delivers the dc power alone when α = 0.<br />

Three different load resist<strong>an</strong>ce values—60 Ω, 80 Ω <strong>an</strong>d 100 Ω, are chosen to show how<br />

290

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