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Motor Start Theory
ME00107A
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Induction Motors Have Two
Prime Functions
To convert electrical energy into
mechanical energy in order to
accelerate the motor and load to
operating speedStarting Function
To convert electrical energy into
productive work output from the
machine Work Function
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Motors consist of two major sectionsThe Stator and the Rotor
The stator consists of magnetic poles and stator windings within the
frame of the motor.By variation of winding configuration and the
contour of the stator laminations , the full load characteristicsare
determinedThe motor speedis determined by the number of poles
The rotor consists of a cylindrical short-circuited winding around iron
laminations The rotor design affects starting performance.
The shape, position and material of the rotor bars affect the current
drawn and torque produced during motor starting.
Motor Performance
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Full load characteristics are well
understood with factors such as motor
speed,torque and efficiency being the
typical selection criteria.
A motors start performancecharacteristics are usually the least
understood but set the limits of what
can be achieved with either a full
voltage or reduced voltage starter.
It is especially important to considermotor start characteristics when
seeking to:
- Minimise start current
- Maximise start torque
Motor Performance
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A motors start performance
can be identified by examining
the motor data sheet.
The table details selected
performance data for a rangeof 110kW motors.
Typical Motor DataMotor Speed FLC LRC LRT % FL Torque
(rpm) (amps) (%FLC) (%FLT) Efncy@3xFLC
A 1470 191 600 263 93 65.8
B 1475 184 600 190 93.5 47.5
C 1475 191 570 150 92 41.6
D 1480 187 660 190 94.5 39.2
E 1470 185 550 120 92 36
F 1470 191 670 150 93 30.1
G 1480 190 780 200 94 29.6
H 1475 182 850 220 93.5 27.4
I 1480 190 670 120 94 24
Sample Of Typical 110kW Motors
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The motor performs as a
transformer with currentinduced in the rotor by the flux
in the stator.
Maximum motor start current
under full voltage start
conditions is defined by themotors Locked Rotor Current.
(LRC) This is when the rotor
is stationary
LRC levels vary considerably
between motors
In the example, Motor H will
draw 55% more current at
start than Motor E.
Start Current
Motor Speed FLC LRC LRT % FL Torque
(rpm) (amps) (%FLC) (%FLT) Efncy@3xFLC
A 1470 191 600 263 93 65.8
B 1475 184 600 190 93.5 47.5
C 1475 191 570 150 92 41.6
D 1480 187 660 190 94.5 39.2
E 1470 185 550 120 92 36
F 1470 191 670 150 93 30.1
G 1480 190 780 200 94 29.6
H 1475 182 850 220 93.5 27.4
I 1480 190 670 120 94 24
LRC ranges from 550% to 850%
Sample Of Typical 110kW Motors
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Torque-Speed Characteristic
The Torque Speed Curveshows how themotors torque production varies throughout
the different phases of its operation.
Starting Torque (LRT) is produced by a motor
when it is initially turned on. Starting torque is
the amount required to overcome the inertia
from standstill.
Pull-up Torque is the minimum torque
generated by the motor as it accelerates from
standstill to operating speed. If the motors pull-up torque is less than that required by its
application load , the motor will overheat and
eventually stall.
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Torque-Speed Characteristic
Breakdown Torque is thegreatest amount of torque amotor can attain without stalling.
Full Load Torque isproduced by a motor functioning
at a rated speed andhorsepower.
Synchronous speed is thespeed at which no torque isgenerated by the motor.This
only occurs in motors that runwhile not connected to a load.
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Motor start torque performance
is indicated by the motorsLocked Rotor Torque (LRT)
figure.
This is the measured torque with
the rotor locked and the rated
voltage and frequency applied to
the motor.Torque is a product of
force and the radius at which it
is applied and is measured in
Nm.
LRT levels vary considerably
between motors.
In the example, Motor A
produces twice as much torque
during start as Motor I.
Start Torque
Motor Speed FLC LRC LRT % FL Torque
(rpm) (amps) (%FLC) (%FLT) Efncy@3xFLC
A 1470 191 600 263 93 65.8
B 1475 184 600 190 93.5 47.5
C 1475 191 570 150 92 41.6
D 1480 187 660 190 94.5 39.2
E 1470 185 550 120 92 36
F 1470 191 670 150 93 30.1
G 1480 190 780 200 94 29.6
H 1475 182 850 220 93.5 27.4
I 1480 190 670 120 94 24
LRT ranges from 120% to 263%
Sample Of Typical 110kW Motors
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LRC & LRT must be
considered together when
determining a motors start
performance.
The example does this by
ranking the motors according
to the torque produced at
3 x FLC.
A good measure of
comparison between motors is
to divide the LRT% by the
LRC% - the bigger the
number, the better the result
LRC & LRT Work
TogetherMotor Speed FLC LRC LRT % FL Torque
(rpm) (amps) (%FLC) (%FLT) Efncy@3xFLC
A 1470 191 600 263 93 65.8
B 1475 184 600 190 93.5 47.5
C 1475 191 570 150 92 41.6
D 1480 187 660 190 94.5 39.2
E 1470 185 550 120 92 36
F 1470 191 670 150 93 30.1
G 1480 190 780 200 94 29.6
H 1475 182 850 220 93.5 27.4
I 1480 190 670 120 94 24
Sample Of Typical 110kW Motors
Torque developed at 3 x FLC
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Motor Speed FLC LRC LRT % FL Torque
(rpm) (amps) (%FLC) (%FLT) Efncy@3xFLC
A 1470 191 600 263 93 65.8
B 1475 184 600 190 93.5 47.5
C 1475 191 570 150 92 41.6
D 1480 187 660 190 94.5 39.2
E 1470 185 550 120 92 36
F 1470 191 670 150 93 30.1
G 1480 190 780 200 94 29.6
H 1475 182 850 220 93.5 27.4
I 1480 190 670 120 94 24
Torque is reduced by the
square of the current
reduction.Eg:- If you halve the current
the result will be motor
torque
Motors B & G produce almost
the same torque at fullvoltage.
Motor B produces 60% more
start torque at 3 x FLC.
Reduced Voltage
Starting AmplifiesMotor Differences
Sample Of Typical 110kW Motors
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Motor LRC LRT TORQUE(%FLC) (%FLT) @ 3 X FLC
A 600 263 65.8
B 600 190
C 570 150
D 660 190
Follow the example and
calculate the start torque at
3 x FLC for motors B, C & D.
How To Calculate
Start TorqueStart Torque = LRT x Start Current
LRC( )
2
( )2
300%
600%65.8% = x263%
47.5
41.5
39.3
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Selecting a motor with low
Locked Rotor Current (LRC)
and high Locked Rotor Torque
(LRT) will:
- Reduce start current.
- Increase start torque.
- Reduce soft starter cost.
Summary
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Current rises instantaneously
to LRC levels. This causes a
current transient that can have
undesirable effects on the
supply.
Current gradually falls as
motor speed increases.
Motor loading affects only the
time taken for acceleration,
not the magnitude of currentwhich is always LRC.
Full Voltage Starting
0
50
100
150
200
250
300
100 90 80 70 60 50 40 30 20 10 0
SLIP (%)
FULLLOADTORQUE(%)
0
100
200
300
400
500
600
700
CURRENT
(%)
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Full Voltage Starting
0
50
100
150
200
250
300
100 90 80 70 60 50 40 30 20 10 0
SLIP (%)
FULLLOADTORQUE(%)
0
100
200
300
400
500
600
700
CURRENT
(%)
Torque rises instantaneously
to LRT levels. This causes a
torque transient that can be
damaging.
Typical torque falls from LRTto Pull Out Torque before
rising to Breakdown Torque
just before full speed.
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0
50
100
150
200
250
300
100 90 80 70 60 50 40 30 20 10 0
SLIP (%)
FULLLOADTORQUE(%)
0
100
200
300
400
500
600
700
CURRENT
(%)
Full Voltage Starting
Limitations
1. Current transient
1
2. Current magnitude2
3. Torque transient
3 4
4. Torque magnitude
Reduced voltage starting
attempts to overcome these
limitations by applying the
voltage gradually.
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100
80
6040
20
0TIME
% VOLTS
START
Line ContactorOverload
StartRun
Direct on Line
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Electromechanical
-- Primary Resistance
-Auto-transformer
- Star/Delta
Electronic
- Soft Start
Reduced Voltage
Starters
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Primary ResistanceRUN
CONTACTOR
MOTOR
OVERLOAD
THERMALSTART
RESISTORS
LINE
CONTACTOR
3 ~M
Resistors are connected in
series with each phase,
between the isolation
contactor and the motor.
The voltage drop across the
resistors results in a reducedvoltage applied to the motor,
thus reducing start current
and torque.
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Set for 4 x FLC start current.
Primary Resistance
0
50
100
150
200
250
300
100 90 80 70 60 50 40 30 20 10 0
SLIP (%)
FULLLOADTO
RQUE(%)
0
100
200
300
400
500
600
700
CURREN
T(%)
Limitations:
- Difficult to change
resistance
- Dissipate a lot of heat- Limited number of starts per
hour
- Start characteristics change
between starts if resistors
have not totally cooled
- Hard to start high inertialoads
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Primary Resistance
0
50
100
150
200
250
300
100 90 80 70 60 50 40 30 20 10 0
SLIP (%)
FULLLOADTO
RQUE(%)
0
100
200
300
400
500
600
700
CURREN
T(%)
Set for 3.5 x FLC start current.
Start voltage is determined by
the resistors used. If the
resistance is too high there
will be insufficient torque to
accelerate the motor to full
speed.
The reduced voltage start
time is controlled by a preset
timer. If the time is too short,the motor will not have
achieved full speed before
the resistors are bridged.
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100
80
6040
20
0TIME
% VOLTS
START
Line Contactor
Run Contactor
ResistorsOverload
Start
Run
Primary
Resistance
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Auto-transformers
ThermalOverload
3 PhaseAuto Transformer
(B) Start Contactor
(A) Start ContactorRun
Contactor
3 ~M
The Auto-transformer Starter
employs an auto-transformer
to reduce the voltage during
the start period. The
transformer has a range of
output voltage taps that canbe used to set the start
voltage.
The motor current is reduced
by the start voltage reduction,
and further reduced by thetransformer action resulting in
a line current less than the
actual motor current.
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0
50
100
150
200
250
300
100 90 80 70 60 50 40 30 20 10 0
SLIP (%)
FULLLOADTO
RQUE(%)
0
100
200
300
400
500
600
700
CURREN
T(%)
60% Tap
Auto-transformers
Limitations:
- Limited voltage taps
- Limited number of starts per
hour- Torque reduced at all
speeds
- Costly
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0
50
100
150
200
250
300
100 90 80 70 60 50 40 30 20 10 0
SLIP (%)
FULLLOADTORQUE(%)
0
100
200
300
400
500
600
700
CURREN
T(%)
50% Tap
The initial start voltage is set
by tap selection, and the start
time is controlled by a timer.
If the start voltage is too low,or the start time incorrectly
set, the transition to full
voltage will occur with the
motor at less than full speed,
resulting in a high current and
torque step.
Auto-transformers
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100
80
6040
20
0TIME
% VOLTS
START
Line Contactor
Transformer
Contactor
Star Point
Contactor
Overload
Start
Run
Auto-
transformer
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Star/Delta
Motor
3~
Thermal
Overload
Star
Contactor
Delta
Contactor
Main
Contactor
The motor is initially
connected in star
configuration and then,
after a preset time, the
motor is disconnected from
the supply andreconnected in delta
configuration. The current
and torque in the star
configuration are one third
of the full voltage current
and torque when the motoris connected in delta.
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Limitations:
- No adjustment possible.- Open transition switching
between star and delta
causes damaging current
and torque transients.
Star/Delta
0
50
100
150
200
250
300
100 90 80 70 60 50 40 30 20 10 0
SLIP (%)
FULLLOADTORQUE(%)
0
100
200
300
400
500
600
700
CURREN
T(%)
Insufficient torque to
accelerate this load in star
configuration.
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100
80
6040
20
0TIME
% VOLTS
START
Line Contactor
Delta
Contactor
Star Point
Contactor
Overload
Start
Run
Star -
Delta
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Open Transition
Switching
Occurs when the starter goes through an open circuit
stage in the switching sequence. Stage [1] connectionto the reduced voltage; [2] disconnect from the reducedvoltage (open circuit); [3] connect to the full voltage.
Open transition starting causes severe current & torquetransients that can be more detrimental to the supplyand the mechanical equipment than full voltage starting.
When the motor is spinning and then disconnected fromthe supply, it acts as a generator. Output voltage can bethe same amplitude as the supply. At the time of reclosethere can still be significant voltage present at the motorterminals.
Voltage generated by the motor at the instant of reclosemay be equal to the supply voltage but exactly out ofphase. This equates to reclosing with twice the supplyvoltage on the motor. The result is a current of twicelocked rotor current and a torque transient of four timeslocked rotor torque.
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A
Phase Angle Control
Trigger circuit
N
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0
50
100
150
200
250
300
100 90 80 70 60 50 40 30 20 10 0
SLIP (%)
FUL
LLOADTORQUE
(%)
0
100
200
300
400
500
600
700
CURRENT(%)
Reduced Voltage
Starting
IST= LRT xT
ST LRC( )2
Reduces start torque by the
square of the current
reduction.
Current can only be reduced
to the point where the torque
output from the motor exceeds
the torque required by the
load.
Reduces start current.
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Reduced Voltage
Starting
To be effective, a reduced
voltage starter must allow the
motor to accelerate to around
90% speed before applying
full voltage.
Below this speed the current
will step through to almost
LRC levels thus removing any
benefit from the reduced
voltage starter.0
50
100
150
200
250
300
100 90 80 70 60 50 40 30 20 10 0
SLIP (%)
FULLLOADTORQUE(%)
0
100
200
300
400
500
600
700
CURRENT
(%)
Small Reduction
at 50% speed
Large Reduction
at 95% speed
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Soft Starter
MotorOverloadAC SwitchesContactor
M3 ~
Soft Starters control the
voltage applied to the motor
by the use of solid state AC
switches (SCRs) in series with
the supply to the motor.
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- Minimum possible start
current
- No current steps
- No torque steps
- Good start torque
characteristics
Soft Starter
0
50
100
150
200
250
300
100 90 80 70 60 50 40 30 20 10 0
SLIP (%)
FULLLOADTORQUE(%)
0
100
200
300
400
500
600
700
CURREN
T(%)
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% VOLTS
100
80
60
40
20
0
TIME
Soft StartingSTART
Start
Run
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Motor characteristics set the
limits of what can be achieved
with a soft starter.
Pay special attention to motor
characteristics when:
- it is important to minimise
start current
- it is important to maximise
start torque
- dealing with large motors
(200kW +)
Summary
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Soft start is technically the
best reduced voltage starting
system.
Star/Delta starting is the
cheapest and most commonly
employed reduced voltage
starting system. However its
performance characteristics
are damaging.
Summary
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Because;
they reduce electrical and mechanicalstresses beyond the capabilities of
electro-mechanical reduced voltagestarters.
This further reduces machine downtime,
increasing plant productivity.
Note however, that the level of performance is dependant upon
the design of the soft starter and functionality it offers.
Why Use Soft Starters
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