Why Soft Starts?

The 3-phase induction motor is over 100 years old and obviously many design changes and variations have appeared over the years. However certain fundamental characteristics remain the same and it is the problems that these inherent features cause that electronic soft start aims to solve.

1. Direct-On-Line

Reduced Voltage Starting:
2. Star/Delta
3. Auto Transformer
4. Soft Starts

1. Direct-On-Line
The main method of starting the AC motor is direct-on-line starting. This simply means an electro-mechanical switch is opened and closed to stop and start the motor.

-High inrush current (typically 6 x full load which can cause several problems)
-Necessities over sizing of installation (particularly important on generator and UPS fed supplies)
-Limits Expansion
-Reduces service life of electrical components
-Excessive applied starting torque (typically 2.5 x full load)
-Increases wear on drive chain components
-Reduces service life of mechanical components

2. Star/Delta
This method requires both connections for each phase (six in all) to be taken to the starter. Three contactors are used to first connect the motor in star and then to delta after a given time. Connecting the motor in star reduces the voltage applied to each winding to about 60% of the line voltage. This reduces the starting torque and current (typically 3.5 x FLC). After a given time the motor is switched to delta connection and then runs as if direct-on-line. Its main advantages are that it is relatively simple and low cost. The major problem with this method is that the reduced voltage level is in a single stage and is fixed. sometimes this voltage is not ideal, the torque it produces (65% of full load torque) may be too small and the motor stalls or does not give complete acceleration, or if it is too great the motor still starts with a pronounced snatch. The star/delta transition will produce a second current and torque peak which is almost the equivalent of having two direct-on-line starts. On some loads the motor sometimes almost stalls during this transition time. This method of starting does however have the advantage of being a low cost and simple solution if its limitations can be tolerated.

-Low cost and simple

-Torque too high – causes snatch
-Torque too low – motor stalls
-Transition peak up to 20 x In
-Motor can stall in transition

3. Auto Transformer
This method uses transformer action to reduce the voltage applied to the motor and current seen by the supply. An improved torque/amp ratio is achieved and starting current is typically 3 x FLC, depending on the voltage rapping selected. Normally the voltage is applied to the motor in voltage steps through the transformer with the taps being selected through contactors. Typical tappings are 50%, 70%, followed by full voltage being applied to the motor. The major disadvantages are size and cost, and of course the mechanical snatch at switch on is not controllable and can still cause problems. Also once the tappings have been selected, it may be necessary to change them according to changes in load parameters.
-Simple operation

-Poor controllability
-Very Expensive
4. Soft Starts
The soft start is designed to apply an adjustable voltage to the motor and increase this voltage gradually over a user-selectable acceleration period. The acceleration time being dependent on the application and desired characteristics. The added advantage of this method of reduced voltage control is that the motor can also be stopped gradually by slowly reducing the output voltage to the .‘Soft Stop’ feature offers a smooth stop in many process industries such as pumps, where fast stops can result in ‘water hammer’ and mechanical damage.

-Reduced starting current
-Reduced starting torque
-Less mechanical stress
-Improved control of acceleration and deceleration


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