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an investigation of dual stator winding induction machines

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Since the input qu<strong>an</strong>tities to be integrated are AC signals, drift <strong>an</strong>d saturation<br />

problems c<strong>an</strong>not be avoided if pure integrators are used. The reasons for these problems<br />

c<strong>an</strong> be addressed as: firstly, the starting point <strong>of</strong> integration is not always zero. It is not<br />

practicable to always start the integration when the integrated signal is exactly at zero.<br />

This is called the initial condition problem, which results in a DC <strong>of</strong>fset after integration.<br />

Secondly, the actual input signal for integration is not a pure AC signal <strong>an</strong>d it has a DC<br />

component. The DC component <strong>of</strong> the integrated signal is due to the distortion <strong>of</strong> the<br />

signal itself, the resolution <strong>of</strong> the sensors <strong>an</strong>d the difference between the desired reference<br />

DC voltage <strong>an</strong>d actual reference DC voltage. The DC <strong>of</strong>fset will drift the integration <strong>an</strong>d<br />

saturates the integrator. Finally, if the frequency <strong>of</strong> the integrated signal is low, the<br />

integration may be saturated even if the magnitude <strong>of</strong> the signal to be integrated is very<br />

small.<br />

To solve the problems, it has been proposed in [10.11, 10.12] that the pure integrator<br />

is replaced by a Low Pass Filter (LPF). The tr<strong>an</strong>sfer function <strong>of</strong> a LPF is<br />

302<br />

1<br />

while that<br />

s + a<br />

1<br />

<strong>of</strong> a pure integrator is given as . The effectiveness <strong>of</strong> LPF to solve the problems <strong>of</strong> pure<br />

s<br />

integration has been demonstrated by both simulation <strong>an</strong>d experimental results [10.10,<br />

10.11, 10.39]. By using the low pass filter, the small DC <strong>of</strong>fset in the integrated signals<br />

c<strong>an</strong>not cause saturation although the estimated values are not correct at the very<br />

beginning. The DC <strong>of</strong>fset in the outputs introduced by the initial values is eliminated.<br />

However, the introduction <strong>of</strong> a (cut-<strong>of</strong>f frequency) produces the magnitude <strong>an</strong>d phase<br />

errors in the integration. The error will increase when a is relatively large, which is

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