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an investigation of dual stator winding induction machines

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Z<br />

real r = (11.10d)<br />

b<br />

where, v , i , ω <strong>an</strong>d r are per unit value <strong>of</strong> voltage, current, electrical speed <strong>an</strong>d<br />

resist<strong>an</strong>ce respectively; V real , I real , ω real <strong>an</strong>d rreal represent the actual values <strong>of</strong> voltage,<br />

current, electrical speed <strong>an</strong>d resist<strong>an</strong>ce respectively.<br />

11.5 Numerical Consideration<br />

The per unit model has been developed so that the s<strong>of</strong>tware representation <strong>of</strong> current<br />

is one when the actual current is equal to the base value <strong>of</strong> current. It should be noted that<br />

the actual current may be greater th<strong>an</strong> the based value during the overload condition or<br />

the starting process so that the overflow will appear in the s<strong>of</strong>tware representation <strong>of</strong><br />

current when all the 15 lower bit are used to represent the per unit current value. Similar<br />

condition is applicable for voltage, flux, rotor speed <strong>an</strong>d etc. So a proper definition <strong>of</strong><br />

numeric format in the fixed point DSP is necessary. It has been found from the<br />

experiences that the per unit value in the machine model is within the r<strong>an</strong>ge from -4 to 4<br />

at most <strong>of</strong> times. The selection <strong>of</strong> a demonstration r<strong>an</strong>ge <strong>of</strong> [-8; 8] ensures that the<br />

s<strong>of</strong>tware values c<strong>an</strong> h<strong>an</strong>dle each drive control qu<strong>an</strong>tity, not only during the steady state<br />

operation but also during the tr<strong>an</strong>sient operation. So the numeric format Q12 is used in<br />

this application, which me<strong>an</strong>s that the four highest bits are dedicated to the integer part<br />

including the representation <strong>of</strong> the sign <strong>of</strong> the number by the highest one bit while the<br />

twelve lowest bits are dedicated to the fractional part. This format is also called 4.12f in<br />

some other references. The representation r<strong>an</strong>ge <strong>of</strong> this format is from<br />

resolution for this format is:<br />

432<br />

3<br />

− 2 to<br />

3<br />

2 <strong>an</strong>d the

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