DOL, Star-Delta, VFD, Soft- Starters, Auto Transformer Starters advantages and disadvantges



The advantages/disadvantages of different types of motor starters like :-

1. Direct online starter
2. Star/ delta,
3. Korndorfer Auto transformer starter
4. Primary resistance/reactance
5. Shunt capacitance
6. Slip ring
7. VFD
8. Soft Starter

DOL (Direct Online Starter) 
Advantages:-
1. Simple start up 
2. Very Low Maintenance cost.
3.  Ease of operation.
4. Very Simple Circuit
5. Very Low initial cost.
Disadvantages:-
1. Very high voltage drop during Starting.
2. Very High starting current
3. Very low starting period duration
4. Less power saving.
5. Not feasible for motors above 10 KW. 

Star/delta decrease the starting current to 1/3 but required many relays for control change between start connection then to delta. 

Slip ring starters are used for starting slip ring motors and these consists of separate slip ring for starting. Main disadvantage associated with these starters is their very high maintenance cost.


Auto transformer used for MV application by connect less voltage on motor during start up then change to nominal voltage on motor terminals after start up where reflect to decrease the starting current and decrease the voltage drop but increase the starting duration (very expensive where required 3 MVCB one for motor and one for primary of auto and one for secondary of auto).

VSD or VFD used for application where its worked with inlet guide van or valve (variable load) to make power saving.  Although they save the power but main disadvantage associated with VFD/VSD is their initial cost.
Visit for more knowledge about V/f control in VFD's:-
http://electrialstandards.blogspot.com/2016/01/vf-control-in-induction-motors-volts.html

Soft-Starter usually used for limiting the current to prescribed limit these starters limited the current as per settings in starter and thereafter soft-starter get bypassed and motor kept on running at line. Very less maintenance required in these starters.
But main disadvantage associated with soft-starters is their initial cost of soft-starters.


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