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an investigation of dual stator winding induction machines

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1.2.8 Induction Machine Drive---Scalar Control<br />

Generally speaking the variable speed control <strong>of</strong> <strong>induction</strong> machine c<strong>an</strong> be classified<br />

into three main categories: scalar control, vector control <strong>an</strong>d direct torque control.<br />

Scalar control is used in low cost drives <strong>an</strong>d there is no direct control <strong>of</strong> either air-gap<br />

flux or electromagnetic torque. The simplest <strong>an</strong>d most robust scalar control method is<br />

const<strong>an</strong>t Volts per Hertz (Const<strong>an</strong>t V/Hz), in which the magnitude <strong>of</strong> input <strong>stator</strong> voltage<br />

is adjusted in proportion to the frequency <strong>of</strong> input voltage. The <strong>stator</strong> flux linkage is kept<br />

approximately const<strong>an</strong>t by keeping this proportional relationship between the magnitude<br />

<strong>an</strong>d frequency <strong>of</strong> input phase voltages. In const<strong>an</strong>t V/Hz method, the speed <strong>of</strong> the rotating<br />

magnetic field <strong>of</strong> the machine is controlled by ch<strong>an</strong>ging the supply frequency. The<br />

electromagnetic torque developed in the machine depends only on the slip frequency. The<br />

general configuration <strong>of</strong> a speed control system using const<strong>an</strong>t V/Hz method is given in<br />

Figure 1.5.<br />

ω<br />

∗<br />

rm<br />

+<br />

-<br />

ω<br />

rm<br />

Slip<br />

controller<br />

ω s +<br />

-<br />

ω<br />

rm<br />

P 2<br />

*<br />

ωe<br />

Voltage<br />

controller<br />

24<br />

*<br />

Vs<br />

−1<br />

T ( θ )<br />

&<br />

11<br />

Sine-<br />

PWM<br />

S , S , S<br />

21<br />

31<br />

speed<br />

sensor<br />

Inverter<br />

Figure 1.5 The general diagram <strong>of</strong> a speed control system using const<strong>an</strong>t V/Hz method<br />

Motor

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