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an investigation of dual stator winding induction machines

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1.2.9 Induction Machine Drive---Vector Control<br />

The vector control method is also called the Field Orientation Control (FOC) method,<br />

in which the air-gap flux vector is kept orthogonal to current vector so that effective<br />

decoupling between flux <strong>an</strong>d current c<strong>an</strong> be obtained <strong>an</strong>d inst<strong>an</strong>t<strong>an</strong>eous torque control is<br />

achieved. The vector control method is divided into three categories: indirect vector<br />

control, direct vector control <strong>an</strong>d direct torque control.<br />

Indirect rotor field orientation is basically achieved by controlling the slip frequency<br />

to satisfy the following slip condition.<br />

rr<br />

Lmiqs<br />

ω s = (1.5)<br />

L λ<br />

r<br />

dr<br />

where, i qs <strong>an</strong>d λ dr are the q-axis current <strong>an</strong>d rotor d-axis flux linkage in synchronous<br />

reference frame. The synchronous speed or <strong>stator</strong> frequency is obtained by adding the slip<br />

frequency to the rotor speed. The disadv<strong>an</strong>tages <strong>of</strong> this method are the need for a speed or<br />

position sensor <strong>an</strong>d accurate knowledge <strong>of</strong> machine parameters. Unfortunately the<br />

machine parameters are subject to ch<strong>an</strong>ge for different operating conditions, so certain<br />

machine parameter estimation methods or adaptive parameters modification scheme are<br />

necessary for high perform<strong>an</strong>ce control which increases the complexity <strong>of</strong> the control<br />

structure. The slip calculation is critical in the control method. Unfortunately, the rotor<br />

time const<strong>an</strong>t <strong>of</strong> the machine, which is variable during different operation conditions, is<br />

involved in the slip calculation. Several adaptive schemes have been proposed to<br />

overcome this problem [10.4-10.7]. In [10.4], the rotor time const<strong>an</strong>t <strong>of</strong> <strong>an</strong> <strong>induction</strong><br />

machine has been measured by injecting a single-phase ac current <strong>an</strong>d observing the<br />

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