The variable voltage inverter (VVI) uses an SCR converter
bridge to convert the incoming AC voltage into DC. The SCRs
provide a means of controlling the value of the rectified DC
voltage from 0 to approximately 600 VDC. The L1 choke and
C1 capacitor(s) make up the DC link section and smooth
the converted DC voltage. The inverter section consists of
six switching devices. Various devices can be used such as
thyristors, bipolar transistors, MOSFETS, and IGBTs. The
following schematic shows an inverter that utilizes bipolar
transistors. Control logic (not shown) uses a microprocessor
to switch the transistors on and off providing a variable voltage
and frequency to the motor.
This type of switching is often referred to as six-step because
it takes six 60° steps to complete one 360° cycle. Although the
motor prefers a smooth sine wave, a six-step output can be
satisfactorily used. The main disadvantage is torque pulsation
which occurs each time a switching device, such as a bipolar
transistor, is switched. The pulsations can be noticeable at low
speeds as speed variations in the motor. These speed variations
are sometimes referred to as cogging. The non-sinusoidal
current waveform causes extra heating in the motor requiring a
motor derating.
bridge to convert the incoming AC voltage into DC. The SCRs
provide a means of controlling the value of the rectified DC
voltage from 0 to approximately 600 VDC. The L1 choke and
C1 capacitor(s) make up the DC link section and smooth
the converted DC voltage. The inverter section consists of
six switching devices. Various devices can be used such as
thyristors, bipolar transistors, MOSFETS, and IGBTs. The
following schematic shows an inverter that utilizes bipolar
transistors. Control logic (not shown) uses a microprocessor
to switch the transistors on and off providing a variable voltage
and frequency to the motor.
 |
| Variable Voltage Inverter |
it takes six 60° steps to complete one 360° cycle. Although the
motor prefers a smooth sine wave, a six-step output can be
satisfactorily used. The main disadvantage is torque pulsation
which occurs each time a switching device, such as a bipolar
transistor, is switched. The pulsations can be noticeable at low
speeds as speed variations in the motor. These speed variations
are sometimes referred to as cogging. The non-sinusoidal
current waveform causes extra heating in the motor requiring a
motor derating.
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