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an investigation of dual stator winding induction machines

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2.2.4 Power Estimation<br />

From the general <strong>induction</strong> machine design point <strong>of</strong> view, the power <strong>of</strong> the machine<br />

is one <strong>of</strong> the design objectives that are known from the beginning <strong>of</strong> the design progress.<br />

The Essen's rule is used to calculate the import<strong>an</strong>t dimensions <strong>of</strong> the machine—inner<br />

diameter <strong>of</strong> <strong>stator</strong> D is <strong>an</strong>d the effective length <strong>of</strong> the <strong>stator</strong> core l e . However, in the<br />

design <strong>of</strong> the <strong>dual</strong> <strong>stator</strong> <strong>winding</strong> <strong>induction</strong> machine, the dimensions <strong>of</strong> the machine are<br />

known, hence the Essen's rule is used to estimate the power <strong>of</strong> the <strong>dual</strong> <strong>winding</strong> machine<br />

[2.3].<br />

The equation <strong>of</strong> Essen's rule c<strong>an</strong> be expressed as:<br />

2 ⎛ 2π<br />

⎞<br />

2<br />

Pmech = ⎜ ⎟<br />

⎜<br />

Ωsk<br />

1(<br />

Disle<br />

) Bg<br />

K s(<br />

rms)<br />

η gap cosφgap<br />

120 ⎟<br />

(2.24)<br />

⎝ ⎠<br />

where, s<br />

Ω is the rotor mech<strong>an</strong>ical speed; s(rms)<br />

K is the value <strong>of</strong> the surface current<br />

density in the <strong>stator</strong> <strong>winding</strong>; η gap is the air gap efficiency <strong>an</strong>d cos φgap<br />

is the air gap<br />

power factor.<br />

Although the gap efficiency ηgap is different from the terminal efficiency <strong>an</strong>d the gap<br />

power factor cos φgap<br />

is different from terminal power factor cos φ , the difference<br />

between both pairs, which depends on the parameters <strong>of</strong> the <strong>stator</strong> resist<strong>an</strong>ce r s <strong>an</strong>d<br />

leakage induct<strong>an</strong>ce L ls , is normally small. It is assumed that they are equal in the power<br />

estimation.<br />

Using Essen's rule for the power estimation, the only variable in equation (2.19) that<br />

is not clear is the surface current density K ) . The surface current density is based on<br />

s(rms<br />

the steady state value, which could be constrained by the ability <strong>of</strong> the cooling system to<br />

56

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