Motor Start Theory Motor Start Theory ME00107A
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Motor Start Theory
Motor Start Theory
ME00107A
Motor Start Theory
Low Slip v Full Slip
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(%)When applying AC speed
drives the motor‘s low slip performance characteristics are important.
In contrast, when applying soft starters the motor‘s performance at all levels of slip is important.
Soft Start
Inve
rter
When applying AC speed drives the motor‘s low slip performance characteristics are important.
Motor Start Theory
A motor‘s start performance characteristics 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
Motor Performance
Motor Start Theory
Rotor design affects starting performance.
Stator design affects full speed performance.
The shape, position and material of the rotor bars affect the current drawn and torque produced during motor starting.
Motor Performance
Motor Start Theory
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.
Typical Motor DataMotor Speed FLC LRC LRT % FL Torque
(rpm) (amps) (%FLC) (%FLT) Ef’ncy @3xFLCA 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 27.4I 1480 190 670 120 94 24
Sample Of Typical 110kW Motors
Motor Start Theory
Maximum motor start current under full voltage start conditions is defined by the motor‘s Locked Rotor Current (LRC).
LRC levels vary considerably between motors.
In the example, Motor H will draw 55% more current at start than Motor E.
Start CurrentMotor Speed FLC LRC LRT % FL Torque
(rpm) (amps) (%FLC) (%FLT) Ef’ncy @3xFLCA 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 27.4I 1480 190 670 120 94 24
LRC ranges from 550% to 850%
Sample Of Typical 110kW Motors
Motor Start Theory
Motor start torque performance is indicated by the motor‘s Locked Rotor Torque (LRT) figure.
LRT levels vary considerably between motors.
In the example, Motor A produces twice as much torque during start as Motor I.
Start TorqueMotor Speed FLC LRC LRT % FL Torque
(rpm) (amps) (%FLC) (%FLT) Ef’ncy @3xFLCA 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 27.4I 1480 190 670 120 94 24
LRT ranges from 120% to 263%
Sample Of Typical 110kW Motors
Motor Start Theory
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.
LRC & LRT Work Together Motor Speed FLC LRC LRT % FL Torque
(rpm) (amps) (%FLC) (%FLT) Ef’ncy @3xFLCA 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 27.4I 1480 190 670 120 94 24
Sample Of Typical 110kW Motors
Torque developed at 3 x FLC
Motor Start Theory
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 27.4I 1480 190 670 120 94 24
Torque is reduced by the square of the current reduction.
Motors B & G produce almost the same torque at full voltage.
Motor B produces 60% more start torque at 3 x FLC.
Reduced Voltage Starting Amplifies Motor Differences
Sample Of Typical 110kW Motors
Motor Start Theory
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 Torque
Start Torque = LRT x Start CurrentLRC( )
2
( )2
300%600%
65.8% = x263%
47.5
41.5
39.3
Motor Start Theory
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
Motor Start Theory
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.
Full Voltage Starting
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Motor Start Theory
Full Voltage Starting
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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.
Motor Start Theory
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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.
Motor Start Theory
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Reduced Voltage Starting
IST
= LRT xTST LRC( )
2
Reduces start torque by the square of the current reduction.
Current can only be reduced to the point where torque output from the motor exceeds the torque required by the load.
Reduces start current.
Motor Start Theory
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
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Small Reductionat 50% speed
Large Reductionat 95% speed
Motor Start Theory
Electromechanical- Auto-transformer- Primary Resistance- Star/Delta
Electronic- Soft Start
Reduced Voltage Starters
Motor Start Theory
Auto-transformers
ThermalOverload
3 PhaseAuto Transformer
(B) Start Contactor
(A) Start ContactorRunContactor
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 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.
Motor Start Theory
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60% Tap
Auto-transformers
Limitations:- Limited voltage taps- Limited number of starts per
hour- Torque reduced at all
speeds- Costly
Motor Start Theory
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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
Motor Start Theory
Primary ResistanceRUN
CONTACTOR
MOTOROVERLOADTHERMALSTART
RESISTORSLINE
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 reduced voltage applied to the motor, thus reducing start current and torque.
Motor Start Theory
Set for 4 x FLC start current.
Primary Resistance
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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
Motor Start Theory
Primary Resistance
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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.
Motor Start Theory
Star/Delta
Motor3~
ThermalOverload
StarContactor
DeltaContactor
MainContactor
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.
Motor Start Theory
Limitations:- No adjustment possible.- Open transition switching
between star and delta causes damaging current and torque transients.
Star/Delta
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Insufficient torque to accelerate this load in star configuration.
Motor Start Theory
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.
Motor Start Theory
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.
Motor Start Theory
- Minimum possible start current
- No current steps- No torque steps- Good start torque
characteristics
Soft Starter
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Motor Start Theory
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
Motor Start Theory
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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