Transcript Motor Start Theory

Motor Start Theory
ME00107A
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
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.
Motor Performance
Full load characteristics are well
understood with factors such as motor
speed,torque and efficiency being the
typical selection criteria.
A motor‘s 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
Typical Motor Data
A motor‘s start performance
can be identified by examining
the motor data sheet.
The table details selected
performance data for a range
of 110kW motors.
Sample Of Typical 110kW Motors
Motor Speed FLC
LRC
LRT
% FL Torque
(rpm) (amps) (%FLC) (%FLT) Ef’ncy @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
Start Current
The motor performs as a
transformer with current
induced in the rotor by the flux
in the stator.
Sample Of Typical 110kW Motors
Motor Speed FLC
LRC
LRT
% FL Torque
(rpm) (amps) (%FLC) (%FLT) Ef’ncy @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
LRC levels vary considerably
between motors
G
1480
190
780
200
94
29.6
H
1475
182
850
220
93.5
27.4
In the example, Motor H will
draw 55% more current at
start than Motor E.
I
1480
190
670
120
94
24
Maximum motor start current
under full voltage start
conditions is defined by the
motor‘s Locked Rotor Current.
(LRC) This is when the rotor
is stationary
LRC ranges from 550% to 850%
Torque-Speed Characteristic
 The Torque Speed Curve shows how the
motor’s 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 motor’s pullup torque is less than that required by its
application load , the motor will overheat and
eventually stall.
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.
Start Torque
Motor start torque performance
is indicated by the motor‘s
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.
Sample Of Typical 110kW Motors
Motor Speed FLC
LRC
LRT
% FL Torque
(rpm) (amps) (%FLC) (%FLT) Ef’ncy @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%
LRC & LRT Work
Together
Sample Of Typical 110kW Motors
Motor Speed FLC
LRC
LRT
% FL Torque
(rpm) (amps) (%FLC) (%FLT) Ef’ncy @3xFLC
LRC & LRT must be
considered together when
determining a motor‘s 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
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 developed at 3 x FLC
Reduced Voltage
Starting Amplifies
Motor Differences
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.
Sample Of Typical 110kW Motors
Motor Speed FLC
LRC
LRT
% FL Torque
(rpm) (amps) (%FLC) (%FLT) Ef’ncy @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
2
How To Calculate
Start Torque
Start Torque = LRT x
65.8% = 263% x
Follow the example and
calculate the start torque at
3 x FLC for motors B, C & D.
Motor LRC
LRT
(%FLC) (%FLT)
Current
( StartLRC
)
(
2
300%
600%
TORQUE
@ 3 X FLC
A
600
263
65.8
B
600
190
47.5
C
570
150
41.5
D
660
190
39.3
)
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.
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
250
600
500
200
400
150
300
100
200
50
100
0
100 90
0
80
70
60
50
40
SLIP (%)
30
20
10
0
CURRENT (%)
Current rises instantaneously
to LRC levels. This causes a
current transient that can have
undesirable effects on the
supply.
FULL LOAD TORQUE (%)
Full Voltage Starting
Typical torque falls from LRT
to Pull Out Torque before
rising to Breakdown Torque
just before full speed.
300
700
250
600
500
200
400
150
300
100
200
50
100
0
100 90
0
80
70
60
50
40
SLIP (%)
30
20
10
0
CURRENT (%)
Torque rises instantaneously
to LRT levels. This causes a
torque transient that can be
damaging.
FULL LOAD TORQUE (%)
Full Voltage Starting
3. Torque transient
4. Torque magnitude
Reduced voltage starting
attempts to overcome these
limitations by applying the
voltage gradually.
300
700
250
2600
500
200
400
150
300
100
200
50
0
100
13
100 90
4
80
70
60
50
40
SLIP (%)
30
20
0
10
0
CURRENT (%)
1. Current transient
2. Current magnitude
FULL LOAD TORQUE (%)
Full Voltage Starting
Limitations
Direct on Line
START
Line Contactor
Overload
% VOLTS
100
80
60
40
20
0
Run
Start
TIME
Reduced Voltage
Starters
Electromechanical
-- Primary Resistance
-Auto-transformer
- Star/Delta
Electronic
- Soft Start
Primary Resistance
RUN
CONTACTOR
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.
M
3~
LINE
CONTACTOR
START
RESISTORS
THERMAL
OVERLOAD
MOTOR
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
FULL LOAD TORQUE (%)
Set for 4 x FLC start current.
300
700
250
600
500
200
400
150
300
100
200
50
100
0
100 90
0
80
70
60
50
40
SLIP (%)
30
20
10
0
CURRENT (%)
Primary Resistance
Primary Resistance
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.
300
700
250
600
500
200
400
150
300
100
200
50
100
0
100 90
0
80
70
60
50
40
SLIP (%)
30
20
10
0
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.
FULL LOAD TORQUE (%)
Set for 3.5 x FLC start current.
Run Contactor
START
Line Contactor
Resistors
Primary
Resistance
Overload
% VOLTS
100
80
60
40
20
0
Run
Start
TIME
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.
Run
Contactor
(A) Start Contactor
3 Phase
Auto Transformer
Thermal
Overload
(B) Start Contactor
M
3~
Limitations:
- Limited voltage taps
- Limited number of starts per
hour
- Torque reduced at all
speeds
- Costly
FULL LOAD TORQUE (%)
60% Tap
300
700
250
600
500
200
400
150
300
100
200
50
100
0
100 90
0
80
70
60
50
40
SLIP (%)
30
20
10
0
CURRENT (%)
Auto-transformers
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.
FULL LOAD TORQUE (%)
50% Tap
300
700
250
600
500
200
400
150
300
100
200
50
100
0
100 90
0
80
70
60
50
40
SLIP (%)
30
20
10
0
CURRENT (%)
Auto-transformers
Autotransformer
Star Point
Contactor
START
Transformer
Contactor
% VOLTS
100
80
60
40
20
0
Line Contactor
Overload
Run
Start
TIME
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.
Main
Contactor
Delta
Contactor
Thermal
Overload
Motor
3~
Star
Contactor
Limitations:
- No adjustment possible.
- Open transition switching
between star and delta
causes damaging current
and torque transients.
300
700
250
600
500
200
400
150
300
100
200
50
100
0
100 90
0
80
70
60
50
40
SLIP (%)
30
20
10
0
CURRENT (%)
Insufficient torque to
accelerate this load in star
configuration.
FULL LOAD TORQUE (%)
Star/Delta
START
Star Delta
Star Point
Contactor
Delta
Contactor
% VOLTS
100
80
60
40
20
0
Line Contactor
Overload
Run
Start
TIME
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.
Trigger circuit
Phase Angle Control
A
N
Reduced Voltage
Starting
T
ST
(
= LRT x
I
2
ST
LRC
)
Current can only be reduced
to the point where the torque
output from the motor exceeds
the torque required by the
load.
300
700
250
600
500
200
400
150
300
100
200
50
100
0
100 90
0
80
70
60
50
40
SLIP (%)
30
20
10
0
CURRENT (%)
Reduces start torque by the
square of the current
reduction.
FULL LOAD TORQUE (%)
Reduces start current.
Below this speed the current
will step through to almost
LRC levels thus removing any
benefit from the reduced
voltage starter.
FULL LOAD TORQUE (%)
To be effective, a reduced
voltage starter must allow the
motor to accelerate to around
90% speed before applying
full voltage.
Small Reduction
at 50% speed
300
700
250
600
500
200
400
150
300
100
200
50
100
0
100 90
0
80
70
60
50
40
SLIP (%)
30
20
10
0
CURRENT (%)
Reduced Voltage
Starting
Large Reduction
at 95% speed
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.
M
3~
Contactor
AC Switches
Overload
Motor
300
700
250
600
500
200
400
150
300
100
200
50
100
0
100 90
0
80
70
60
50
40
SLIP (%)
30
20
10
0
CURRENT (%)
- Minimum possible start
current
- No current steps
- No torque steps
- Good start torque
characteristics
FULL LOAD TORQUE (%)
Soft Starter
Soft Starting
START
% VOLTS
100
80
60
40
20
0
Run
Start
TIME
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 +)
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.
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.