Some Aspects Of Pulse Voltage Regulation For Induction Motor Soft ...

allows controlling softstarter output voltage by
changing the thyristor firing angle. Power circuit of
such a thyristor softstarter is presented in Fig. 2.
Fig.2. Power circuit of a typical thyristor softstarter.
As a result of the thyristor softstarter application
a negative impact of electromagnetic transients on
the induction motor performance is weakened with
the reduction of impact torques and starting
currents. However due to the delayed start-up of the
induction motor supplied with a softstarter and its
extended total starting time energy losses during the
induction motor transients tend to increase if
compared with its direct starting with nominal mains
voltage [1].
Other shortcomings of a thyristor softstarter
involve substantial distortions of a sinusoidal voltage
waveform supplying the induction motor and a lag
shift angle for the first current harmonic. The output
voltage curve of a thyristor softstarter contains
higher harmonics leading to the power factor
decrease [1].
Better harmonics content of the output voltage,
absence of a lag shift angle in the first current
harmonic waveform, less energy losses and therefore
greater number of switching frequency can be
obtained with the use of a pulse voltage regulator
(PVR) operated on full-controlled power switches
such as IGBT.
3. Pulse voltage regulation of an
induction motor
Pulse regulation of the induction motor supply
voltage is carried out by changing the width (in
relation to a certain law) and number of voltage
pulses, i.e. IGBTs switching frequency. Half-period
of PVR output voltage curve formed with 5 pulses is
shown in Fig. 3. From [2] one can draw a conclusion
that better harmonics content of the PVR output
voltage can be achieved by increasing the number of
voltage pulses. The higher harmonics amplitudes are
decreased significantly with growth of the switching
frequency. The author’s research in [2] also proves
that the application of PVR provides less energy
404
losses in the induction motor transients than during
its phase voltage regulation.
Fig.3. A half-period example of PVR output voltage
curve (formed with 5 pulses).
Power circuit of a pulse voltage regulator consists
of a certain number of supply mains power switches
and shunt power switches. The latter are used for
shunting of the induction motor stator windings
during the off-state of the supply mains power
switches. Each power switch is a pair of inverseparallel
connected IGBT transistors with bypass
diodes.
Depending on the number of power switches
used for pulse voltage regulation and induction
motor shunting various operational PVR power
circuits can be built. The authors of this paper have
considered three variants of PVR power circuit
structures.
The first one consists of six power switches:
3 supply mains power switches, 3 shunt power
switches - one for each induction motor winding.
Such a power circuit is completely symmetrical in
relation to the output phase voltages and currents
although rather expensive due to the increased
number of power switches.
The second type of PVR power circuit includes
four power switches (2 supply mains power switches,
2 shunt power switches for interphase motor
windings shunting) and therefore is more costeffective
than the previous one. This power circuit
has asymmetrical structure and therefore causes
output phase voltages and currents asymmetry.
The third type of PVR power circuit consists of
three supply mains power switches (one for each
supply mains phase), a diode shunt bridge with one
unidirectional transistor to operate the shunting
process. This type of PVR power circuit has
combined the advantages of the previous two
structures. It is symmetrical but at the same time
consists of a fewer number of power switches hence
is less expensive.
The power circuit of the pulse voltage regulator
with three power switches (transistors T1-T6), a
shunt diode bridge D1-D6 and one unidirectional
transistor T7 is shown in Fig. 4. Operation of the
supply mains transistors T1-T6 provides energy
exchange between the load and the mains. When
they are switched off transistor T7 shunts the
induction motor windings with the diode bridge.