About Direct Start of Induction Motor
with Two-Stage Reactive Power Compensation as a Part of the
Auxiliary Drive of Electric Locomotive
MIKHAIL PUSTOVETOV
Department of Electrical and Electronics Engineering
Don State Technical University
344000, Rostov region, Rostov-on-Don, Gagarin sq., 1
RUSSIA
Abstract: The auxiliary electric drive of electric locomotives is often built on the basis of three-phase induction
motors. In a number of auxiliary drive circuits, direct starting of motors is practiced, which is accompanied by
significant starting currents. This imposes increased requirements on the installed power of the supply
converter. A method for reducing starting and operating currents through the use of three-phase capacitor banks
is considered. The simulation results are presented.
Key-Words: induction motor, reactive power compensation, direct start, compressor, autonomous voltage
inverter, capacitor, computer simulation
Received: November 21, 2022. Revised: August 19, 2023. Accepted: September 19, 2023. Published: October 16, 2023.
1 Introduction
The power supply circuit of three-phase induction
motors (IM) for auxiliary needs of an electric
locomotive from electromechanical phase splitters
(Arno converters) or capacitive phase splitters
provides for direct-on-line start-up of both motor-
fans and air motor-compressors (MC) [1-5].
Also, the direct start of auxiliary electric drives is
provided by the static phase splitter based on IGBT
proposed in [6], and based on GTO proposed in [7].
The modern ideology of building auxiliary static
converters (ASC) of electric locomotives provides
for the presence of several parallel channels [8, 9].
The basis of the channel is a frequency converter
based on an autonomous voltage inverter (AVI).
One of the reasons for the multi-channel ASC is the
presence of loads of different nature on board the
electric locomotive. For example, frequency
regulation of the speed of rotation of IM driving
fans, depending on temperature or current of
traction motors, provides significant energy savings
[10]. The MC operates at a constant speed, has a
significant starting torque (especially with a piston
compressor [11, 12]), which may be aggravated in
cold operating conditions. To simplify the ASC
control system, it is permissible to connect the MC
to an unregulated channel with a fixed output
frequency and voltage, i.e. direct-on-line start of
MC.
2 Problem Formulation
Direct-on-line start of IM is associated with
significant starting currents, multiples of the
nominal value. Starting currents are consumed from
the phase splitter or from ASC. The installed power
of these devices should be able to withstand IM
start-ups, including protracted ones (at low voltage
in the overhead wire, at low ambient temperature).
The IM current, especially during start-up, contains
a significant reactive (inductive) component, the
compensation of which would make it possible to
facilitate the operation of the supply converter, to
reduce its installed power.
3 Problem Solution
One of the relatively simple ways to compensate for
the reactive component of the IM current is to install
three-phase capacitor banks in the power circuit.
The amount of reactive power to be compensated is
different in the starting and rated modes, therefore,
either compensation should be applied only in the
starting mode, or two-stage compensation, changing
International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2023.5.16
Mikhail Pustovetov
E-ISSN: 2769-2507
157
Volume 5, 2023
the value of the connected capacitance depending on
the operating mode.
Let the IM, which drives the MC, receive power
from an unregulated channel of the ASC, the
schematic diagram of the power part of which is
shown in Fig. 1. In Fig. 1 the diodes connected in
opposite parallel to the IGBT are not conventionally
shown as part of the AVI.
The structure of the ASC includes a two-level
AVI with hard switching, a sine-wave filter, a three-
phase transformer. The IM is connected to the
secondary winding of the transformer. In parallel
with the secondary winding of the transformer two
three-phase capacitor banks are connected, which
provide the process of direct-on-line starting of the
IM with two stages of reactive power compensation.
In our case, the sine-wave filter capacitors with a
capacity of 450 μF per phase are connected
according to the “wye” circuit, the sine-wave filter
inductances are 2 mH per phase, the transformer
windings have a /Y connection scheme (neutral is
connected to the secondary winding to be able to
connect single-phase loads), the capacitors in each
three-phase battery (start-up capacitors and run
capacitors) are connected in a star configuration. In
the further calculation and in computer simulation,
the parameters of the NVA-55 type IM were used,
the characteristics of which for the rated mode are
presented in the Table.
The capacity of the phase of the run capacitors is
calculated by the equation [13, 14]:
2
(tg tg )
2π
ph rated
ph run ini des
ph rated
P
CfV
, (1)
where (
tg tg
ini des
) - the conversion factor;
tg 0.75
ini
- initial value corresponds to the
value of the power factor
cos
= 0.8;
tg 0
des
- the desired value corresponds to the value of the
power factor
cos
= 1.0);
Figure 1 - Schematic diagram of the power part of the unregulated ASC channel
Table. Comparison of calculated and experimental characteristics of the nominal mode of operation of 4-
pole IM NVA-55
Phase-to-phase
voltage RMS (V)
Stator phase
current RMS (A)
Active power
consumed (kW)
Power on the
shaft (kW)
Electrical power
losses in stator
winding (W)
Electrical power
losses in rotor
winding (W)
Iron power
losses (W)
Efficiency (%)
Power factor
(p.u.)
Rotor speed
(rotation per
minute)
Torque on the
shaft (Nm)
Test results
379
116
62.2
55.8
1930
2310
1600
89.7
0.817
1441.5
369.8
Simulation results obtained by means of OrCAD
380
113
58.9
55.0
1635
2229
1531
93.4
0.792
1442.4
364.4
International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2023.5.16
Mikhail Pustovetov
E-ISSN: 2769-2507
158
Volume 5, 2023
21000
ph rated
P
W - the accepted value of
the active power consumed by one phase of IM
NVA-55 at rated mode;
220
ph rated
V
V - RMS value of the rated
phase voltage;
50f
Hz - the frequency of the fundamental
(1st) harmonic of the supply voltage.
2
21000 0.75 1000 F
2π 50 220
ph run
C
.
Equation (1) can be written in another way:
2
1( )
2π
ph run ph rated ini ph rated des
ph rated
C Q Q
fV
, (2)
where
ph rated ini
Q
and
ph rated des
Q
the initial
and desired values of reactive power.
When combining compensating capacitors
according to the “delta” scheme, the equation (1)
can be written as follows:
2
(tg tg )
2π
ph rated
ph run ini des
ph ph rated
P
CfV
,(3)
where
- ph ph rated
V
- RMS value of the rated
phase-to-phase voltage.
The value of the per-phase capacity of the start-
up capacitors is selected according to the criterion
for minimizing the current through the phase of the
secondary winding of the transformer in the process
of starting-up of IM and in our case amounted to
7000 μF per phase. From the formula (2) we can
calculate that when both stages of capacitors are
turned on (8000 μF per phase total), compensated
reactive power of the phase is about 122 kVAr. The
time of shutdown of the start-up capacitors (point t1
in Fig. 2) is selected so that the current in the phase
of the secondary winding of the transformer is less
than the current of the phase of IM during the entire
launch time.
Using OrCAD [15, 16], the computer model was
compiled that simulates the operation of the circuit
presented in Fig. 1. Mathematical models of three-
phase IM and transformer are described in [1, 17,
18]. Information about sine-wave filter simulation is
presented in [19].
In Fig. 2 designation Mcconst means reactive
load torque of a constant magnitude. Designation
Mcstart means the part of the load torque exists at
low speeds of the shaft. Designation Mc0 is the
instant load torque value without taking into account
it’s dependence on the angle of rotation. But the
designation Mc is the instant load torque value
which takes into account its dependence on the
angle of rotation of the piston compressor
mechanism (simplified) [2]. In Fig. 2 designation n2
is the rotor speed of IM (rotation per minute); Fi2 is
the angle of rotation of rotor of IM or compressor
mechanism (radian). The dependence of the load
torque of piston compressor on the angle of rotation
is described in more detail, for example, in [11, 12].
The simulation results for the direct start of IM
NVA-55, which drive the piston compressor of
double compression, with two-stage compensation
of reactive power are shown in Figs 3-6. Fig. 3
shows the acceleration process of IM. The
beginning of the acceleration process is presented in
Fig. 4.
In Fig. 3 and in Fig. 4 indicated: 1 - the rotor
speed of IM; 2 - load torque on the shaft of IM; 3 -
phase current of the IM stator; 4 - phase current of
the secondary winding of the transformer; 5 - phase
voltage of IM.
The steady-state mode at the end of the starting
process is shown in Fig. 5, the accompanying graphs
of the phase-to-phase voltages on terminals of sine-
wave filter are shown in Fig. 6.
The simulation results indicate the effectiveness
of the use of capacitive compensation of reactive
power during direct start and in the steady-state
mode of operation of the IM, including in the case
of power from the ASC of an electric locomotive. In
the steady-state mode, the fundamental harmonic of
the current of the secondary winding of the
transformer is about 38 % of the fundamental
harmonic of the IM current; during acceleration of
IM, this share is approximately 65 - 70 %.
It should be noted that in our computing
experiment in the IM steady-state mode, the most
pronounced 5th temporary harmonic of the current
of the secondary winding of the transformer is 10%
of the fundamental. In the spectrum of the phase of
IM, the 5th harmonic is 4% of the fundamental. In
this regard, it must be borne in mind that the
parameters of the sine-wave filter have a significant
impact on the harmonic composition of the currents
and voltages of the auxiliary electric drive.
International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2023.5.16
Mikhail Pustovetov
E-ISSN: 2769-2507
159
Volume 5, 2023
For example, a decrease in the sine-wave filter
inductance to 0.85 mH per phase would be quite
acceptable in the absence of reactive power
compensation in the capacitors. But in the presence
of reactive power compensation significant
distortions of the shape of curves of currents and
voltages are occurred in case of 0.85 mH: so, in the
steady-state mode of IM, the 5th harmonic of the
current of the secondary winding of the transformer
will be already 147% of the fundamental. For the
phase of IM, the similar ratio will be 60%.
Figure 2 - Piston type compressor load simulation segment of the start-up computer model of the IM type
NVA-55
Figure 3 - Process of acceleration of IM. t1 - moment of time to turn off the starting step of the capacity
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Figure 4 - Initial section of the acceleration process of IM
Figure 5 - Steady-state mode of IM at the end of the start-up process
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From Fig. 6 it is clear that the current of the
secondary winding of the transformer has a phase
shift somewhat advantageous with respect to the
phase of IM. We can see the overcompensation of a
reactive power of IM, the capacity of the run
capacitors is somewhat overstated. This happened
because the power of MC is below the rated power
of the NVA-55, while the capacity calculation of the
run capacitors is made for the rated mode.
4 Conclusion
Through computer simulation, it is shown that in the
ASC on board of electric locomotive it is possible to
use an unregulated channel to power the motor-
compressor during its direct starts and further
operation. At the same time, the system of two-stage
reactive power compensation with capacitor banks
makes it possible to effectively reduce the amount
of current consumed by the induction motor from
ASC.
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DOI: 10.37394/232027.2023.5.16
Mikhail Pustovetov
E-ISSN: 2769-2507
163
Volume 5, 2023
