Motor Start Theory

1
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 speed Starting Function
• To convert electrical energy into productive
  work output from the machine Work Function

3
Motor Performance

• Motors consist of two major sections The 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
  characteristics are determined
• The motor speed is 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.

4
Motor Performance
Full load characteristics are well understood
with factors such as motor speed,torque and
efficiency being the typical selection
criteria. A motors start performance
characteristics 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 consider
motor start characteristics when seeking to-
Minimise start current - Maximise start torque
5
Sample Of Typical 110kW Motors
Typical Motor Data
Motor Speed FLC LRC LRT FL Torque (rpm) (amps)
(FLC) (FLT) Efncy _at_3xFLC A 1470 191 600 263 9
3 65.8 B 1475 184 600 190 93.5 47.5 C 1475 191 5
70 150 92 41.6 D 1480 187 660 190 94.5 39.2 E 14
70 185 550 120 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
A motors start performance can be identified by
examining the motor data sheet. The table
details selected performance data for a range of
110kW motors.
6
Start Current
Sample Of Typical 110kW Motors
The motor performs as a transformer with current
induced in the rotor by the flux in the
stator. Maximum motor start current under full
voltage start conditions is defined by the
motors 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.
Motor Speed FLC LRC LRT FL Torque (rpm) (amps)
(FLC) (FLT) Efncy _at_3xFLC A 1470 191 600 263 9
3 65.8 B 1475 184 600 190 93.5 47.5 C 1475 191 5
70 150 92 41.6 D 1480 187 660 190 94.5 39.2 E 14
70 185 550 120 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
7
Torque-Speed Characteristic

• The Torque Speed Curve shows how the motors
  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.

8
Torque-Speed Characteristic

• Breakdown Torque is the greatest amount of
  torque a motor can attain without stalling.
• Full Load Torque is produced by a motor
  functioning at a rated speed and horsepower.
• Synchronous speed is the speed at which no
  torque is generated by the motor.This only occurs
  in motors that run while not connected to a load.
  

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Start Torque
Sample Of Typical 110kW Motors
Motor start torque performance is indicated by
the motors Locked 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.
Motor Speed FLC LRC LRT FL Torque (rpm) (amps)
(FLC) (FLT) Efncy _at_3xFLC A 1470 191 600 263 9
3 65.8 B 1475 184 600 190 93.5 47.5 C 1475 191 5
70 150 92 41.6 D 1480 187 660 190 94.5 39.2 E 14
70 185 550 120 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
10
LRC LRT Work Together
Sample Of Typical 110kW Motors
Motor Speed FLC LRC LRT FL Torque (rpm) (amps)
(FLC) (FLT) Efncy _at_3xFLC A 1470 191 600 263 9
3 65.8 B 1475 184 600 190 93.5 47.5 C 1475 191 5
70 150 92 41.6 D 1480 187 660 190 94.5 39.2 E 14
70 185 550 120 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 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
Torque developed at 3 x FLC
11
Sample Of Typical 110kW Motors
Reduced Voltage Starting Amplifies Motor
Differences
Motor Speed FLC LRC LRT FL Torque (rpm) (amps)
(FLC) (FLT) Efncy _at_3xFLC A 1470 191 600 263 9
3 65.8 B 1475 184 600 190 93.5 47.5 C 1475 191 5
70 150 92 41.6 D 1480 187 660 190 94.5 39.2 E 14
70 185 550 120 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 ¼ motor torque Motors B G
produce almost the same torque at full
voltage. Motor B produces 60 more start torque
at 3 x FLC.
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How To Calculate Start Torque
2
( )
x
65.8
Follow the example and calculate the start torque
at 3 x FLC for motors B, C D.
Motor LRC LRT TORQUE (FLC) (FLT) _at_ 3 X
FLC A 600 263 65.8 B 600 190 C 570 150 D 660 1
90
47.5
41.5
39.3
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Summary
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.
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Full Voltage Starting
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 current which
is always LRC.
300
700
600
250
500
200
400
FULL LOAD TORQUE ()
CURRENT ()
150
300
100
200
50
100
0
0
100
90
80
70
60
50
40
30
20
10
0
SLIP ()
15
Full Voltage Starting
Torque rises instantaneously to LRT levels. This
causes a torque transient that can be
damaging. Typical torque falls from LRT to Pull
Out Torque before rising to Breakdown Torque just
before full speed.
300
700
600
250
500
200
400
FULL LOAD TORQUE ()
CURRENT ()
150
300
100
200
50
100
0
0
100
90
80
70
60
50
40
30
20
10
0
SLIP ()
16
Full Voltage Starting Limitations
17
Direct on Line
START
Line Contactor
Overload
VOLTS
Run
Start
18
Reduced Voltage Starters

• Electromechanical
• - Primary Resistance
• Auto-transformer
• - Star/Delta
• Electronic
• - Soft Start

19
Primary Resistance
Resistors are connected in series with each
phase, between the isolation contactor and the
motor. The voltage drop across the resistors
results in a reduced voltage applied to the
motor, thus reducing start current and torque.
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Primary Resistance
Set for 4 x FLC start current.
300
700
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 inertia loads
600
250
500
200
400
FULL LOAD TORQUE ()
CURRENT ()
150
300
100
200
50
100
0
0
100
90
80
70
60
50
40
30
20
10
0
SLIP ()
21
Primary Resistance
Set for 3.5 x FLC start current.
300
700
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.
600
250
500
200
400
FULL LOAD TORQUE ()
CURRENT ()
150
300
100
200
50
100
0
0
100
90
80
70
60
50
40
30
20
10
0
SLIP ()
22
Primary Resistance
START
VOLTS
Run
Start
23
Auto-transformers
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 can be used to set the
start voltage. The motor current is reduced by
the start voltage reduction, and further reduced
by the transformer action resulting in a line
current less than the actual motor current.
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Auto-transformers
60 Tap
300
700

• Limitations
• - Limited voltage taps
• - Limited number of starts per hour
• - Torque reduced at all speeds
• - Costly

600
250
500
200
400
FULL LOAD TORQUE ()
CURRENT ()
150
300
100
200
50
100
0
0
100
90
80
70
60
50
40
30
20
10
0
SLIP ()
25
Auto-transformers
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.
300
700
600
250
500
200
400
FULL LOAD TORQUE ()
CURRENT ()
150
300
100
200
50
100
0
0
100
90
80
70
60
50
40
30
20
10
0
SLIP ()
26
Auto-transformer
START
VOLTS
Run
Start
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Star/Delta
The motor is initially connected in star
configuration and then, after a preset time, the
motor is disconnected from the supply and
reconnected in delta configuration. The current
and torque in the star configuration are one
third of the full voltage current and torque when
the motor is connected in delta.
Delta Contactor
Main Contactor
Thermal Overload
Motor3
Star Contactor
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Star/Delta

• Insufficient torque to accelerate this load in
  star configuration.

300
700
600
250
500
Limitations - No adjustment possible. - Open
transition switching between star and delta
causes damaging current and torque transients.
200
400
FULL LOAD TORQUE ()
CURRENT ()
150
300
100
200
50
100
0
0
100
90
80
70
60
50
40
30
20
10
0
SLIP ()
29
Star - Delta
START
VOLTS
Run
Start
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Open Transition Switching
Occurs when the starter goes through an open
circuit stage in the switching sequence. Stage
1 connection to the reduced voltage 2
disconnect from the reduced voltage (open
circuit) 3 connect to the full voltage. Open
transition starting causes severe current
torque transients that can be more detrimental to
the supply and the mechanical equipment than full
voltage starting. When the motor is spinning
and then disconnected from the supply, it acts as
a generator. Output voltage can be the same
amplitude as the supply. At the time of reclose
there can still be significant voltage present at
the motor terminals. Voltage generated by the
motor at the instant of reclose may be equal to
the supply voltage but exactly out of phase. This
equates to reclosing with twice the supply
voltage on the motor. The result is a current of
twice locked rotor current and a torque transient
of four times locked rotor torque.
31

Phase Angle Control
A
N
32
Reduced Voltage Starting
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Reduced Voltage Starting
300
700
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.
600
250
500
200
400
FULL LOAD TORQUE ()
CURRENT ()
150
300
100
200
50
100
0
0
100
90
80
70
60
50
40
30
20
10
0
SLIP ()
34
Soft Starter
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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Soft Starter
- Minimum possible start current - No current
steps - No torque steps - Good start torque
characteristics
300
700
600
250
500
200
400
FULL LOAD TORQUE ()
CURRENT ()
150
300
100
200
50
100
0
0
100
90
80
70
60
50
40
30
20
10
0
SLIP ()
36
Soft Starting
START
VOLTS
100
80
Run
60
Start
40
20
0
TIME
37
Summary
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 )
38
Summary
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.
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Why Use Soft Starters
Because
they reduce electrical and mechanical stresses
beyond the capabilities of electro-mechanical
reduced voltage starters. 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.