International Journal of Engineering Trends and Technology (IJETT) – Volume 7 Number 2- Jan 2014
High Torque Low Current Starter System for
Synchronous Motor
Dr.K.Ayyar #1, Ann Mary Abraham *2
#1
EEE Department, Associate Professor
EEE Department, PG Scholar, Anna University
Vivekanandha Institute of Engineering and Technology for Women, Tiruchengode, Tamilnadu,India.
*2
Abstract:
High Torque Low Current (HTLC)
Starter is the new innovative motor soft starter
system. This is a motor starting method for high
inertia loads using a low voltage drive with input and
output transformers to control motor torque and
limit current when starting a medium/high voltage
motor. The HTLC is a solution to motor starting
problems where Direct on Line (DOL) or “Across the
Line” starting is not feasible due to high in-rush
current causing problems on the distribution system
or where a reduced voltage starter cannot provide
enough torque to achieve breakaway and accelerate
the motor to full speed. The main features of HTLC
starter system are reduction of starting inrush
current from 600% to 10% and over 60% breakaway
torque available during starting, significantly less
costly than a fully rated VFD and multiple motors can
be started from a single HTLC. This innovative motor
soft starter system used in BPCL-Kochi Refinery for
starting of 6MW Synchronous motor.
Keywords: HTLC, Synchronous Motor, Starters,
Protection, Relays Starting Torque, Starting Current
.
I.INTRODUCTION
The HTLC (High Torque Low
Current) Starter is motor starting method for
high inertia loads using a low voltage drive
with input and output transformers to control
motor torque and limit current when starting a
medium/high voltage motor. The HTLC is a
solution to motor starting problems where
Direct on Line (DOL) or “Across the Line”
starting is not feasible due to high in-rush
current causing problems on the distribution
system or where a reduced voltage starter
cannot provide enough torque to achieve
breakaway and accelerate the motor to full
speed.
Developed from the combined experience
of Converteam, one of the world’s leading
manufacturers of large VFD’s and drives systems,
this unique soft-starter is aimed primarily at the Oil
and Gas industry for starting medium/high voltage
compressor motors. The HTLC starter solves these
ISSN: 2231-5381
problems with a combination of innovative
engineering and power electronics application and
design know how.
II.THEORY OF OPERATION
The HTLC is equipped with an input
isolation transformer to change the Medium
Voltage supply to a low voltage level, typically 690
Volts. The VFD is chosen to deliver the current
needed to start the motor and connected load. An
output transformer, connected as an autotransformer, is used to change the voltage back to
the same level as the utility supply. The motor is
supplied with the proper voltage and current to
accelerate it from rest to the frequency of the
electrical supply grid. The auto-transformer uses
three resistors connected in series with the
windings at low frequencies to add impedance to
the circuit and limit the transformer saturation
current. As the frequency increases the resistors are
removed from the circuit and the transformer then
operates normally.
The VFD operates at its own variable
frequency and will not be in synchronism with the
grid when the motor reaches full speed. In order to
match the grid frequency the inverter will typically
run at a frequency slightly above the grid so that
the VFD and the grid will match the phase rotation
and can accommodate any variations in the supply
frequency. The HTLC contains a synchronizing
relay that determines when the two are frequency
and phase matched. When the match is detected a
signal is sent to close the bypass circuit breaker or
contactor in the Medium Voltage Switchgear.
Once the breaker closes the inverter load
will increase as it is still running at a frequency
slightly higher than the supply but still in parallel
with it. At this point the HTLC Output Circuit
Breaker will open, the drive will shut off and the
motor is then operated solely from the utility
supply. The HTLC input power can be removed at
this point, as the starter is no longer required. A
typical HTLC schematic is shown in Fig.1
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International Journal of Engineering Trends and Technology (IJETT) – Volume 7 Number 2- Jan 2014
VOLTAGE RATING
AMPERES
HP
POWER RATING
RPM
MAKE
6600V
568A
8043
6000KW
333
CONVERTEAM
IV.SYNCHRONOUS MOTOR
STARTER
Fig.1 A typical HTLC
On a medium voltage supply the HTLC
would be fed by a separate fused disconnect or
contactor arrangement. A single HTLC starter can
be sequenced to start multiple motors off a
common bus. A separate isolating contactor is
required for each motor to be started. Shown in
Fig.2
Fig.2.HTLC with multiple motors
III.HTLC WITH SYNCHRONOUS MOTOR
This HTLC starter system here we are
using to start Synchronous Motor (VHC-2A/B)
with 6MW capacity, which is installed at
VGO/HSD plant for operation of MUG
Compressor. The details of motor are given below.
TABLE I
Motor Details
SWGR
PANEL NO
SERIAL NO
FRAME
ENCLOSURE
INSULATION
CLASS
SERVICE FACTOR
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SS1 HT-209
02
108182511
18L9338
IP55
F
S1
The standard Synchronous motor, those
designed for Line Supply and fixed speed, have
brushless dc excitation and use built in induction
motor features for starting. For the soft or weak line
condition (allowed current <100%) the > 400%
Direct On Line current will be unacceptable.
Reducing the voltage will reduce the starting torque
in proportion to the voltage squared, so there may
not be enough torque.
The HTLC system utilizes a VFD to ramp
up the voltage and frequency from start to 100%
speed. For a typical induction motor this allows a
very effective motor utilization achieving near 1
per unit torque / current at all speeds. The relatively
poor induction motor characteristics (small cage &
high slip) prevent the synchronous motor from
achieving good torque efficiency. For a given
motor the ratio of Torque / Stator current is
maximum of 0.4 at 90% speeds (10% slip). Even
with the benefit of a VFD providing the maximum
torque efficiency at all speeds the value of 0.4
would mean we need > 100% stator current for
44% breakaway torque.
The great benefit of the synchronous
motor (with variable stator frequency) is that torque
is proportional to the product of the stator and
excitation flux. If excitation can be 100% the
required stator current for 44% torque reduces to
44% or less. This solution requires an AC exciter.
A. Exciter Protection
A rapid change of flux (by abrupt load
angle change) induces a voltage into the exciter
winding. If the load is large enough the motor can
“pole skip” which induces a very high voltage.
Operating on the VFD the stator current is forced to
remain synchronized (the frequency or phase
voltage change) so the condition never occurs.
Once the motor is running on the Line Supply the
frequency and phase is fixed so “pole skip” may
occur. An overvoltage protective device must be
included in the excitation system that triggers from
excess positive voltage and short circuits the
excitation coils. The shorting circuit may be
comprised of 2 thyristors inverse parallel with 2
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International Journal of Engineering Trends and Technology (IJETT) – Volume 7 Number 2- Jan 2014
rectifiers of the exciter bridge. Once the fault
condition occur the stator and excitation power is
shut off. The Field Discharge Resistor typically
used in a DC exciter may be used but the function
of providing induction motor torque is not required.
with the stator connected to a fixed frequency and
voltage supply.
Typical HTLC connections shown in Fig.3
B. Starting Sequence with HTLC
Motor at zero speed, the exciter supply
applies voltage to the synchronous motor’s AC
Exciter. Main field current is applied through the
rectifier and the Exciter control loops are now
satisfied (balanced 3 phase currents above
minimum and below overload). Field current is
regulated to 100%. HTLC autotransformer is
energized with resistance in 3 neutral connections.
This allows very low frequency (even zero) with a
limit to transformer saturation current. In this mode
the VFD controller injects a current source (rated
VFD current) which pulls the motor in step and
accelerates as the frequency increases to ~ 5%.
Around 5% speed (and frequency), a significant
voltage is required for the current source (neutral
resistor and motor impedance in series). A
contactor shorts out the neutral resistor. Since the
controller is in “current source mode” no change or
step is realized by motor, but VFD voltage
decreases due to shorted neutral. The VFD control
system switches to Synchronous Motor Vector
Control. The inner loop controls current amplitude
and phase to maintain stability to accelerate the
motor and load to higher speeds. The outer loop
checks motor voltage versus speed (should follow a
linear volts per hertz to rated volts) and outputs a
field current reference for Exciter. Effectively these
control loops will achieve a VFD current providing
for torque and field current providing for VARS.
As the system approaches rated full speed;
 The frequency will be approaching 50Hz
(controlled by vector control)
 The voltage will be motor rated voltage
(controlled by field adjustment)
 The stator Power Factor will be unity
(controlled by vector control)
 The Synchroscope / Synch check relay will
close the transfer contact to initiate LINE
BREAKER CLOSE with motor, HTLC and
LINE all in parallel, line current will be small
(HTLC is supplying the motor)
 Once HTLC OUTPUT BREAKER opens the
LINE will pick up the motor current
 Exciter is controlled from the HTLC processor.
By operator choice the exciter will
maintain unity power factor within the motor limits
or will maintain constant field current. The motor
continues running as normal with the stator
connected to the LINE and the Exciter controlled
by the HTLC. Once the motor has started and
synchronized the VFD is now out of the circuit,
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Fig.3.Connection diagram of HTLC with
synchronous motor
V. HTLC DRIVE PROTECTION SYSTEM
A. Winding Temperature Monitoring
Input and output power transformer winding
temperature is monitored by nine RTD sensors. The
input transformer uses six sensors – one for each of
three delta phases and one for each of three star
phases. The output auto-transformer has three
RTDs – one in each of the three phases.
B. Transformer over temperature Protection
A trip lockout will occur if transformers T1
or T2 core temperature rises above set value and
the fault trip code 62 will be indicated. If the VFD
auto-reset function is enabled, the drive will make
3 auto reset attempts to restart. If the trip condition
is cleared within several seconds the VFD will
restart and continue to operate. However the
thermostat will reset only if the core temperature
falls below set value. If the VFD does not auto-start
it will require a manual reset once the fault is
cleared in order to restart the motor.
C. Input under voltage Protection
The 600 Volt AC Input power is
monitored as is the DC link voltage inside the
VFD. When the VFD DC link voltage drops below
450 Vdc it will trip and indicate fault trip code 4
(DC Under-volt). If the VFD auto-reset function is
enabled, the drive will make 3 Auto reset attempts
to restart. If the trip condition is cleared within
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International Journal of Engineering Trends and Technology (IJETT) – Volume 7 Number 2- Jan 2014
several seconds the VFD will restart and continue
to operate. In the event that the under voltage
condition does not clear then the VFD will require
a manual reset once the fault is removed.
D. Input Overvoltage Protection
The over voltage level is set within the VFD.
When the DC Link and the Input voltage are
exceeded the internal protection of the VFD will
trip and indicate fault code 3 (DC Overvoltage). Ifs
the VFD Auto-reset function is enabled, the drive
will make 3 auto reset attempts to restart. If the trip
condition is cleared within several seconds the
VFD will restart and continue to operate. In the
event that the overvoltage condition does not clear
then the VFD will require a manual reset once the
fault is removed.
E. Output over current Protection
When the VFD over loads beyond it’s over
current limit set point, then it will begin to start
timing and will trip and indicate fault code 5 (timed
over current). However, when there is
instantaneous over current then it will trip and
indicate fault code 7 (Instantaneous Over current).
If the VFD auto-reset function is enabled, the drive
will make 3 auto reset attempts to restart. If the trip
condition is cleared within several seconds the
VFD will restart and continue to operate. In the
event that the over current condition does not clear
then the VFD will require a manual reset once the
fault is removed.






Ground Fault
Overload
Current unbalance
Differential Protection
Over/Under Frequency
Motor cooling air inlet and outlet temperature.
A leak detector is installed inside the motor to
detect any coolant leaks and is connected to the
Multiline Motor Protection Relay.
A. Multiline 369 Motor Management Relay
The 369 Motor Management Relay is a
digital relay that provides protection and
monitoring for three phase motors and their
associated mechanical systems. A unique feature of
the 369 Relay is its ability to ‘learn’ individual
motor parameters and to adapt itself to each
application. Values such as motor inrush current,
cooling rates and acceleration time may be used to
improve the 369 Relay’s protective capabilities.
F. VFD Over temperature Protection
The VFD will trip when its internal heatsink
temperature overheats above its set point limit. This
will cause a trip and indicate fault code 6 (Over
Temperature). The drive will require a manual reset
after the trip cause is cleared. The operation of the
cooling system should be checked before a restart.
G. VFD Interlock Input
If any one of these above mentioned functions
fail then it will trip and indicate fault code 1
(Interlock Trip). The drive will require a manual
reset after the cause of the fault is cleared.
VI. MOTOR PROTECTION SYSTEM
A Multiline Motor Protection Relay is used to
monitor and protect against the following faults for
VHC-2A/B Motor:
 Under voltage
 Bearing over temperature (RTD)
 Stator over temperature (RTD)
 Short circuit
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Fig:4. 369 Relay
VII. M8100 SYNCHROSCOPE
The M8100 Synchroscope provides
illuminated indication of the actual phase
difference between generator voltage and bus
voltage. If the vector and the light spot turn
clockwise, the generator frequency is too high and
must be reduced. The light spot turning anti
clockwise indicates a lower generator frequency,
and consequently it must be increased. In case the
generator and bus bar voltage and frequency and
phase angle are within acceptable limits, the builtin check synchronizer relay will activate. The limits
of above parameters can be adjusted by a
potentiometer on the rear side of the unit.
For clearing of blackout situations the
M8100 Synchroscope can be set up to connect a
live generator to a dead bus bar (dead bus bar
closure function). When enabled, the M8100
Synchroscope will give a close command to the
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International Journal of Engineering Trends and Technology (IJETT) – Volume 7 Number 2- Jan 2014
generator breaker in case it detects that the bus bar
voltage is below dead bus bar voltage offset limit
and the generator has reached at least 80% of its
nominal voltage. The bus bar must be dead during
the entire dead bus bar delay time (this time delay
is also adjustable). The purpose of the dead bus bar
voltage offset is to give room for possible noise on
the bus bar voltage input of M8100 Synchroscope.
Typical connection of M8100 Synchroscope is
shown in below fig.5
[3]
[4]
[5]
[6]
[7]
[8]
[9]
Fig.5. Synchroscope
[10]
VIII. RESULT &CONCLUSIONS
The HTLC soft starter thus is used for
the smooth starting of synchronous motors. We
need to understand why or when to use the HTLC
starter:
 On weak power systems
 At the end of long T lines
 Power company voltage restrictions
 Power company flicker specifications
 High inertia loads
 When we can’t afford VFD losses or cost
.
The advantage of using HTLC starter
over VFD starter or DOL starter has already been
discussed.
The
protection m e t h o d s
e mplo ye d i n H TLC s t ar t er ar e al s o of
g r e a t importance in order to avoid system failure
or plant shutdown. Thus the HTLC starter is
useful in energy saving and has reduced cost,
making it a favourable method of starting.
[1]
H.H. Goh, M.S. Looi, and B.C. Kok (2009), “Comparison
between Direct-On-Line, Star-Delta and Auto-transformer
Induction Motor Starting Method in terms of Power
Quality, Proceedings of the International Multi
Conference of Engineers and Computer Scientists Vol. II
Javier Portos, Russell King, and Paul A. Gaynor (2009),
“The Influence of Magnetic Retaining Rings on AC
Motors for Boosting Torque and Reducing Starting
Current”, IEEE Transactions on Industry Applications,
Vol. 45, No. 3
Jian-Xin Shen, Peng Li, Meng-Jia Jin, Guang Yang
(2013), “Investigation and Countermeasures for
Demagnetization in Line Start Permanent Magnet
Synchronous Motors”, IEEE Transactions on Magnetics,
Vol. 49, No. 7
Jie Zhou, King-Jet Tseng (2002), “Performance Analysis
of
Single-Phase
Line-Start
Permanent-Magnet
Synchronous Motor”, IEEE Transactions on Energy
Conversion, Vol. 17, No. 4 .
Kazumi Kurihara, M. Azizur Rahman (2004), “HighEfficiency
Line-Start
Interior
Permanent-Magnet
Synchronous Motors”, IEEE Transactions On Industry
Applications, Vol. 40, No. 3
M. Azizur Rahman, Ali M. Osheiba, Kazumi Kurihara
(2012), “Advances on Single-Phase Line-Start High
Efficiency Interior Permanent Magnet Motors”, IEEE
Transactions on Industrial Electronics, Vol. 59, No. 3
Mark G. Solveson, Behrooz Mirafzal, Nabeel ,A. O.
Demerdash (2006), “Soft-Started Induction Motor
Modeling and Heating Issues for Different Starting
Profiles Using a Flux Linkage ABC Frame of Reference”,
IEEE Transactions on Industry Applications, Vol. 42, No.
4
Mircea Popescu, T. J. E. Miller,Malcolm McGilp,
Giovanni
Strappazzon,Nicolla
Trivillin,
Roberto
Santarossa (2005), “Asynchronous Performance Analysis
of a Single-Phase Capacitor-Start, CapacitorRun Permanent Magnet Motor”, IEEE
Transactions on Energy Conversion, Vol. 20, No. 1
[11] R. Nagarajan, , V.K Misra., (1991) “Starting Transient
Current Of Induction Motor Without and With Terminal
Capacitors”, Mine Electrical Laboratory.,Department
of Mineral Engineering.,90 WM 143-8.
[12] R.M. Hamouda A.I. Alolah M.A. Badr* M.A. Abdel-halim
(1999).,“ A Comparative Study on The Starting Methods
of Three Phase Wound-Rotor Induction Motors - Part I”,
IEEE Transactions on Energy Conversion, Vol. 14, NO.4
[13] Lin, C. Bi, Q. Jiang, H. N. Phyu (2007), “Analysis of
Three Synchronous Drive Modes for the Starting
Performance of Spindle Motors”, IEEE Transactions on
Magnetics, Vol. 43, No. 9
[14] Sol Kim, Jung-Ho Lee, Ju Lee.(2003), “A Study on
Hysteresis Analysis of Line Start Permanent Magnet
Motor Using Preisach Modeling”, IEEE Transactions On
Magnetics, Vol. 39, No. 5.
[15] Stephen Williamson, Andy M. Knight (1999), “Voltage
Regulation System during Large Motor Starting and
Operation”, IEEE Cement Industry Technical Conference,
pp. 109 - 119.
REFERENCE
[1] Ali Akbar, Damaki Aliabad, Mojtaba Mirsalim,Nima
Farrokhzad Ershad (2010), “Line-Start Permanent-Magnet
Motors: Significant Improvements in Starting Torque,
Synchronization, and Steady-State Performance”, IEEE
Transactions on Magnetics, VOL. 46, NO. 12
[2] Andrew M. Knight, Catherine I. McClay (2000), “The
Design of High-Efficiency Line-Start Motors”, IEEE
Transactions on Industry Applications, VOL. 36, NO. 6
ISSN: 2231-5381
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