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an investigation of dual stator winding induction machines

an investigation of dual stator winding induction machines

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where, P is the number <strong>of</strong> poles, λ af is the flux produced by the field current that links<br />

the armature <strong>winding</strong>, i a is the armature current. The difference between the field flux<br />

<strong>an</strong>d the flux linked with the armature <strong>winding</strong> is the leakage flux, which c<strong>an</strong> be<br />

represented by a leakage induct<strong>an</strong>ce.<br />

The excitation flux axis <strong>an</strong>d the axis <strong>of</strong> MMF due to the armature current are<br />

orthogonal to each other, which are enforced by the structure <strong>of</strong> dc <strong>machines</strong>. The<br />

adv<strong>an</strong>tage <strong>of</strong> this orthogonality removes the possible coupling interaction between the<br />

excitation flux <strong>an</strong>d the MMF from the armature current. Since the field flux c<strong>an</strong> be<br />

controlled independently by adjusting the value <strong>of</strong> the field current while the armature<br />

current <strong>of</strong> the dc machine is independently controlled, the torque control <strong>of</strong> dc machine is<br />

simple <strong>an</strong>d good dynamic response c<strong>an</strong> be achieved. For example, when the flux linking<br />

the armature <strong>winding</strong> is kept const<strong>an</strong>t, the output torque <strong>of</strong> dc machine will linearly<br />

depend on the armature current so that the torque control <strong>of</strong> the dc machine becomes a<br />

simple control <strong>of</strong> the armature current.<br />

However, the same control method <strong>of</strong> dc machine is not applicable to ac <strong>machines</strong>,<br />

since the field <strong>an</strong>d armature current do not exist <strong>an</strong>ymore. Before this problem was<br />

solved, the speed control <strong>of</strong> <strong>induction</strong> machine was obtained by simple const<strong>an</strong>t V/Hz<br />

control. The emergence <strong>of</strong> the field orientation control or vector control solved the<br />

problem <strong>an</strong>d brought us the idea that the independent torque control <strong>of</strong> ac <strong>machines</strong> is<br />

similar to what is known for dc <strong>machines</strong>. High perform<strong>an</strong>ce independent torque control<br />

<strong>of</strong> ac <strong>machines</strong> is obtained by external control algorithms although the method is more<br />

complex th<strong>an</strong> that <strong>of</strong> dc <strong>machines</strong>.<br />

344

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