1. Motor Start Theory
ME00107A

2. 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 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

5. Motor Start Theory (ME00107A)
Sample Of Typical 110kW Motors
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.
Motor Speed FLC LRC LRT % FL Torque
(rpm) (amps) (%FLC) (%FLT) A B C D E F G H I

6. Sample Of Typical 110kW Motors
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 motor‘s 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) A B C D E F G H I
LRC ranges from 550% to 850%

7. 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 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.

9. Sample Of Typical 110kW Motors
Start Torque
Sample Of Typical 110kW Motors
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.
Motor Speed FLC LRC LRT % FL Torque
(rpm) (amps) (%FLC) (%FLT) A B C D E F G H I
LRT ranges from 120% to 263%

10. Sample Of Typical 110kW Motors
LRC & LRT Work Together
Sample Of Typical 110kW Motors
Motor Speed FLC LRC LRT % FL Torque
(rpm) (amps) (%FLC) (%FLT) A B C D E F G H I
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
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) A B C D E F G H I
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.

12. ( ) ( ) How To Calculate Start Torque Start Torque = LRT x
Start Current
LRC
( ) 2
How To Calculate Start Torque 2
( )
300%
65.8%
= x
263%
Follow the example and calculate the start torque at
3 x FLC for motors B, C & D.
600%
Motor LRC LRT TORQUE
(%FLC) 3 X FLC A B C D
47.5
41.5
39.3

13. 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.

14. 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 (%)
150
CURRENT (%)
300
100
200 50
100
100 90 80 70 60 50 40 30 20 10
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 (%)
150
CURRENT (%)
300
100
200 50
100
100 90 80 70 60 50 40 30 20 10
SLIP (%)

16. Full Voltage Starting Limitations50
100
150
200
250
300 90 80 70 60 40 30 20 10
SLIP (%)
FULL LOAD TORQUE (%)
400
500
600
700
CURRENT (%)
1. Current transient 1
2. Current magnitude 2 4
4. Torque magnitude
Reduced voltage starting attempts to overcome these limitations by applying the voltage gradually.
3. Torque transient 3

17. Direct on Line % VOLTS 100 80 60 Run 40 Start 20 TIME Line Contactor
Overload
% VOLTS
100 80 60 40 20
TIME
Run
Start

18. Reduced Voltage Starters
Electromechanical
- Primary Resistance
Auto-transformer
- Star/Delta
Electronic
- Soft Start

19. Primary Resistance
RUN CONTACTOR
MOTOR
OVERLOAD
THERMAL
START 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 reduced voltage applied to the motor, thus reducing start current and torque.

20. Primary Resistance Set for 4 x FLC start current. Limitations:
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 (%)
150
CURRENT (%)
300
100
200 50
100
100 90 80 70 60 50 40 30 20 10
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 (%)
150
CURRENT (%)
300
100
200 50
100
100 90 80 70 60 50 40 30 20 10
SLIP (%)

22. Motor Start Theory (ME00107A)
Primary Resistance
START
Line Contactor
Run Contactor
Resistors
Overload
% VOLTS
100 80 60 40 20
TIME
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.
Thermal
Overload
3 Phase Auto Transformer
(B) Start Contactor
(A) Start Contactor
Run
Contactor
3 ~ M

24. Auto-transformers 60% Tap Limitations: - Limited voltage taps
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 (%)
150
CURRENT (%)
300
100
200 50
100
100 90 80 70 60 50 40 30 20 10
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 (%)
150
CURRENT (%)
300
100
200 50
100
100 90 80 70 60 50 40 30 20 10
SLIP (%)

26. Motor Start Theory (ME00107A)
Auto-transformer
START
Line Contactor
Transformer
Contactor
Star Point
Overload
% VOLTS
100 80 60 40 20
TIME
Run
Start

27. 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

28. Star/Delta
Insufficient torque to accelerate this load in star configuration.
300
700
600
250
Limitations:
- No adjustment possible.
- Open transition switching between star and delta causes damaging current and torque transients.
500
200
400
FULL LOAD TORQUE (%)
150
CURRENT (%)
300
100
200 50
100
100 90 80 70 60 50 40 30 20 10
SLIP (%)

29. Motor Start Theory (ME00107A)
Star - Delta
START
Line Contactor
Delta
Contactor
Star Point
Overload
% VOLTS
100 80 60 40 20
Run
Start
TIME

30. 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. Trigger circuit
Phase Angle Control A N

32. ( ) 2 Reduced Voltage Starting I = T LRT x LRC Reduces start current.
Reduces start torque by the square of the current reduction.
Reduced Voltage Starting
Reduces start current. 50
100
150
200
250
300 90 80 70 60 40 30 20 10
SLIP (%)
FULL LOAD TORQUE (%)
400
500
600
700
CURRENT (%)
Current can only be reduced to the point where the torque output from the motor exceeds the torque required by the load.

33. Reduced Voltage Starting
Large Reduction at 95% speed
Reduced Voltage Starting
Small Reduction at 50% speed
300
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.
700
600
250
500
200
400
FULL LOAD TORQUE (%)
150
CURRENT (%)
300
100
200 50
100
100 90 80 70 60 50 40 30 20 10
SLIP (%)

34. Motor Start Theory (ME00107A)
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.
Motor
Overload
AC Switches
Contactor M
3 ~

35. 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 (%)
150
CURRENT (%)
300
100
200 50
100
100 90 80 70 60 50 40 30 20 10
SLIP (%)

36. Motor Start Theory (ME00107A)
Soft Starting
% VOLTS
100 80 60
Run 40
Start
TVR Soft Start Animation
Continued from previous slide. 20
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.

39. Motor Start Theory (ME00107A)
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.