Voltage and Frequency Control Methods
In this control method the ratio of voltage to frequency is kept constant so that the flux remains constant. The voltage at variable frequency can be obtained from voltage source inverters or cycloconerters. There may be various circuit arrangement to obtain the variable voltage and frequency.
Initially the 3- phase AC may convert in DC by using phase controlled rectifier. The dc output voltage of phase controlled rectifier then fed to voltage source inverter. Inductor and capacitor may be used to inter connect the phase controlled rectifier and this circuit is shown in figure .
In figure instead of PWM inverter stepped wave inverter may be used. But an induction motor fed from a stepped wave invert suffers from the followings drawbacks.
a.Due to low frequency harmonics the motor losses are increase at tll speeds causing dreading of the motor.
b.Motor develops pulsating torques due to 5th ,7th, 11th, an 13th harmonics which cause jerky motion of the rotor at low speeds.
c.Harmonic content in motor current increases at low speeds. The machine saturates at light loads at low speeds due to high ration. These two effects overheat the machine at low speed thus the limiting lowest speed to around 40% of base speed.
Harmonics are reduced low frequency harmonics are eliminated associated losses are reduced an smooth motion is obtained at low speed also when inverter is operated as a pulse width modulated inverter as shown in figure .
In another circuit arrangement the output of phase controlled rectifier may step the ac down by using chopper then fed to the inverter. Again this may interconnect by inductor and capacitor. Inductor and capacitor are used to get smooth dc to the inverter.
In figure chopper is also used to reduce the harmonic injection into the ac supply. For the purpose of rectification dual converter may be used. In this scheme the control of the phase controlled rectifier and the voltage source inverter provide the desired voltage and frequency respectively to the stator of the induction motor.
The another way to obtain variable frequency and variable voltage cyclocnverter can be used. The output of cycloconverter s fed to the induction motor. Because of low harmonic content when operating at low frequencies smooth motion is obtained at low speeds. A motor with large leakage inductance is used in order to minimize derating and torque pulsation due to harmonics in capability. Full four quadrant operation is obtained by reversing the phase sequence of motor terminal voltage. Since cycleconverter employs large number of thyristors. It becomes economically acceptable only in large power drives. Cycloconverter fed induction motor has application sin high power drives requiring good dynamic response but only low speed operation. The low speed operation is obtained by feeling a motor with large pole numbers from a cycloconverter operating at low frequencies.