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

Some Aspects Of Pulse Voltage Regulation For Induction
Motor Soft Starting And Braking In Electric Drives
Of Crane Travel Mechanisms
Vasilyev Dmitry, Belorussian National Technical University
(01.12.2006, prof. Firago Bronislav, Belorussian National Technical University)
Abstract
In this paper the application of pulse voltage
regulation for soft starting and braking of induction
motors of crane traveling mechanisms is discussed.
Shortcomings of phase voltage regulators - thyristor
softstarters - and advantages of pulse voltage
regulators (PVR) are analyzed. Three types of PVR
power circuit structures are described and compared.
Quasi-frequency control at underspeed operation of
a crane induction motor supplied with PVR is
considered. The research of soft starting and braking
modes as well as quasi-frequency control of a crane
induction motor has been performed with the help
of Matlab simulation model of a thyristor softstarter
and PVR based on three mains power switches and a
shunt diode bridge.
1. Introduction to the problem
Crane equipment is widely used in various fields
of industry, for example, in metallurgical production,
port terminals, warehouses and etc. A certain field of
application determines crane mechanism
construction features, functional characteristics and
also operating requirements. However crane
equipment has some common features and therefore
can be defined by a number of general requirements.
Since all the technological operations of crane
mechanisms are performed by means of electric
drives, crane equipment functionality requirements
first of all refer to crane electric drives. Basic
requirements include high production efficiency,
reliability, fail-safe operation and maintenance
simplicity.
These basic requirements can be extended further
to comply with the following technical conditions:
1) Operation at steady underspeeds which can
be achieved with stiff speed-torque characteristics of
the electric drives,
2) Soft starting and breaking with set
acceleration and deceleration during operation at any
speed of working range,
3) Efficiency of starting/breaking modes and
speed regulation,
XI International PhD Workshop
OWD 2009, 17–20 October 2009
403
4) Reduction of impact torques and starting
currents in mechanical gears.
Nowadays frequency converters with the
induction motors are widely used in the variable
speed electric drives of crane mechanisms – crane
traveling and hoisting mechanisms. Such a controlled
electric drive undoubtedly meets most of the
mechanism’s operating requirements but on the
other hand is considerably more expensive than the
uncontrolled one because of the frequency converter
price which is three-five times higher the price of an
induction motor.
When there is no need in full conformity with the
above mentioned technological requirements (such
as speed control ability) for a certain crane
mechanism, for example, crane trolley traveling
mechanism, cheaper and technologically simple soft
starter device can be used as an alternative to a
frequency converter.
2. Phase voltage regulation of an
induction motor
Thyristor softstarter operation is based on phase
regulation of an induction motor supply voltage at
constant mains frequency (Fig.1).
Fig.1. Phase regulation of an induction motor supply
voltage (phase A).
With the help of such a softstarter a value of
stator voltage is changing during starts and stops in
relation to a certain law, for example, linear or
exponential law.
A typical circuit of a softstarter consists of three
pairs of inverse-parallel connected thyristors and

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.

Fig.4. PVR with three power switches and a shunt
diode bridge.
Further research and simulation of the induction
motor soft starting and breaking as well quasifrequency
control study is based on the latter PVR
power circuit structure.
For a more detailed description and comparison
of possible PVR power circuit structures please refer
to [3].
4. Quasi-frequency control of an
induction motor supplied with
pulse voltage regulator
Functionality of a pulse voltage regulator is
extended with the application of an induction motor
quasi-frequency control ability. Such an improved
soft starter device would allow not only the
reduction of impact torques and starting currents but
also provide the operation of an induction motor at
underspeed. PVR with this feature can be used in the
crane electric drives for smooth stopping and precise
positioning of the crane traveling mechanisms
instead of an expensive frequency converter.
405
Thyristor softstarter output voltage curve during
quasi-frequency control of an induction motor is
shown in Fig. 5. The formation of the induction
motor supply voltage consists of a series of positive
and negative supply mains voltage half-waves, which
are formed according to the alternating sign
switching function.
Fig.5. Thyristor softstarter output voltage curve
during quasi-frequency control.
The sign of this switching function defines
output voltage curve polarity during the quasifrequency
control. The period of the switching
function sets the number of positive and negative
supply mains voltage half-waves in it.
The formation of the PVR output voltage curve
for phase A during quasi-frequency control is shown
in Fig. 6. According to the PVR power circuit
structure in Fig. 4 transistors T1, T2 take part in the
formation of the PVR voltage pulses for phase A,
while transistor T7 performs shunting of the motor.
Fig.6. PVR output voltage curve during quasi-frequency control.

Transistor T7 operates only as a shunt when either
T1 or T2 is in off state so that the circuit current
could decrease to zero. The same switching
algorithm during quasi-frequency control is applied
to the remaining power switches of phase B and C.
In general the formation of any PVR output
voltage curve during quasi-frequency control of an
induction motor is performed with a certain
switching sequence of the mains and shunt IGBT
transistors according to the desired quasi frequency.
To variate quasi-voltage value one should change the
supply mains transistors pulse width.
5. Simulation results
The research of the induction motor soft starting
and braking for phase and pulse voltage regulation
has been performed via Matlab simulation models.
PVR simulation model has been developed
according to the power circuit with three power
1
Usa
2
Usb
3
Usc
2
T2a_pulse
1
T1a_pulse
3
T1b_pulse
4
T2b_pulse
6
T4a_pulse
5
T3a_pulse
7
T3b_pulse
8
T4b_pulse
10
T6a_pulse
9
T5a_pulse
11
T5b_pulse
12
T6b_pulse
g
C
g
C
g
C
T1 T2
E
E
T3 T4
E
T5 T6
E
E
E
g
C
g
C
g
C
406
switches and a shunt diode bridge (Fig. 7). The
parameters of the special crane induction motor
4MTKF160LB8 (11kW, 380/220V, 40% duty cycle)
designed for intermittent operation mode have been
used during the simulation.
Pulse voltage regulation of the crane induction
motor 4MTKF160LB8 has been performed
according to the linear and exponential 1st harmonic
voltage variation.
PVR output voltage and current curves are
presented in Fig. 8. The induction motor
4MTKF160LB8 speed curves for various starting
current limitation settings are presented in Fig. 9.
The authors have also performed simulation of
the crane induction motor 4MTKF160LB8
underspeed operation at quasi-frequency of 8,33 Hz
for the same PVR power circuit structure.
The results of quasi-frequency control simulation are
presented in Fig. 10.
Tm
LOAD
U1a
U1b
U1c
Shunt IGBT pulses
OR
Shunt diode bridge
Tm
A
B
C
Crane Induction Motor
4MTKF160LB8
A
Shunt IGBT T7
Fig.7. Matlab simulation model of PVR with 3 power switches and a shunt diode bridge.
To provide steady underspeed operation of the
induction motor at the reduced frequency it is
imperative to maintain flux linkage at constant level
by decreasing the quasi-frequency control voltage
proportionally to the reduced frequency. Otherwise,
an oversaturation of the induction motor magnetic
system will occur.
Quasi-frequency control results in the motor
underspeed fluctuations (Fig. 10) because of the
pulse nature of stator current and motor torque
during this operating mode. Underspeed fluctuations
lead to the induction motor vibrations. Moreover
with the relatively high motor torque (close to its
nominal value), current amplitudes during quasifrequency
control of the induction motor increase up
to 200% and even higher. Therefore quasi-frequency
control should be applied for the induction motor
low speed operation for a short period of time.
g
C
+
B
E
-
C
m

Fig.8. Output phase voltages (a) and currents (b) of PVR with 3 power switches
and a shunt diode bridge.
Fig.9. 4MTKF160LB8 induction motor speed curves for 3 starting current
limitation settings.
407

Fig. 10. 4MTKF160LB8 induction motor speed curve with underspeed operation
at 8,33 Hz during quasi-frequency control.
6. Conclusions
Pulse voltage regulator based on a power circuit
with 3 power switches and a shunt diode bridge has
a number of obvious advantages over the thyristor
voltage regulator such as:
- reduced energy losses in the induction motor
transients due to symmetrical power circuit structure
and lack of output voltages and currents phase
asymmetry;
- lowered impact on supply mains due to the
better harmonic content of the PVR output voltage
and current curves;
- increased power factor and lack of 1st harmonic
current shift angle;
- quasi-frequency control ability for the induction
motor steady operation at underspeeds.
PVR with quasi-frequency control ability if
compared to thyristor voltage regulator can be
considered an improved softstarter device and is a
cheaper alternative to a frequency converter
provided there are no strict speed control
requirements for a certain technological mechanism.
For example, it would be reasonable and costeffective
to apply such PVR in the crane electric
drive of a trolley traveling mechanism.
Bibliography
[1] Firago Bronislav, Vasilyev Dmitry, Pawlaczyk
Leszek: Regulirovanie naprjazenija asinchronnogo
dvigatelja impulsnymi metodami dlja mjagkogo puska i
tormozenija, II mezdunarodnaja naucnotechniceskaja
konferencija ″Organizacionnotechniceskoe
upravlenie v mezotraslevych
kompleksach″, 20-21 nojabrja 2007, Belorusskij
Gosudarstvennyj Technologiceskij Universitet,
Minsk, s. 213-220
408
[2] Firago Bronislav, Vasilyev Dmitry, Pawlaczyk
Leszek: Zastosowanie impulsowego regulatora napięcia
dla miękkiego rozruchu i hamowania silników, prace
Naukowe Instytutu Maszyn, Napedow i
Pomiarow Elektrycznych, nr 62, Studia i
Materialy nr 28, s. 378-386, Politechnika
Wroclawska, Wroclaw, 2008
[3] Strzelecki Ryszard, Supronowicz Henryk:
Wspólczynnik mocy w systemach zasilania pradu
przemiennego i metody jego poprawy, oficyna
Wydawnicza Politechniki Warszawskiej,
Warszawa, 2000. – 452 s
Authors:
PhD student
Vasilyev Dmitry
Belorussian National
Technical University
F. Skaryna 65
Minsk 220027
Belarus
email: dmy@tut.by
Prof. Firago Bronislav
Belorussian National
Technical University
F. Skaryna 65
Minsk 220027
Belarus
email: dmy@tut.by