Subject : Electrical Machines II (AC Machines)
Stator frequency control of induction motor
Stator frequency speed control of Induction motor:
- The expression for the synchronous speed indicates that by changing the stator frequency also it can be changed. This can be achieved by using power electronic circuits called inverters which convert dc to ac of desired frequency.
- Depending on the type of control scheme of the inverter, the ac generated may be variable-frequency-ﬁxed-amplitude or variable-frequency variable-amplitude type. Power electronic control achieves smooth variation of voltage and frequency of the ac output.
- This when fed to the machine is capable of running at a controlled speed. However, consider the equation for the induced emf in the induction machine.
where N is the number of the turns per phase, φ_{m} is the peak flux in the air gap and f is the frequency.
- Note that in order to reduce the speed, frequency has to be reduced. If the frequency is reduced while the voltage is kept constant, thereby requiring the amplitude of induced emf to remain the same, flux has to increase.
- This is not advisable since the machine likely to enter deep saturation. If this is to be avoided, then ﬂux level must be maintained constant which implies that voltage must be reduced along with frequency.
- The ratio is held constant in order to maintain the flux level for maximum torque capability. Actually, it is the voltage across the magnetizing branch of the exact equivalent circuit that must be maintained constant, for it is that which determines the induced emf. Under conditions where the stator voltage drop is negligible compared the applied voltage, eqn. above is valid.
- In this mode of operation, the voltage across the magnetizing inductance in the ’exact’ equivalent circuit reduces in amplitude with reduction in frequency and so does the inductive reactance.
- This implies that the current through the inductance and the flux in the machine remains constant. The speed torque characteristics at any frequency may be estimated as before. There is one curve for every excitation frequency considered corresponding to every value of synchronous speed. The curves are shown below.
- It may be seen that the maximum torque remains constant. This may be seen mathematically as follows. If E is the voltage across the magnetizing branch and f is the frequency of excitation, then E = kf, where k is the constant of proportionality. If ω = 2πf, the developed torque is given by