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 5^th ,7^th, 11^th, an 13^th   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.

Posted Date: 4/3/2013 2:40:35 AM | Location : United States

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