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Characteristics of DOL Starting Method Initial starting current Initial starting torque
Advantages
Disadvantages
Usual starting time Typical application
4 to 8 times running current 0.6 to 1.6 times running torque Simple and inexpensive starter High starting torque
Does not permit slow and progressive starting
Very high starting current 0.2 to 3 seconds Small motors , starting on full load
Ryan , let us move on to star delta starting :
A star delta starter starts the motor in the star connection . As a consequence the starting torque and current are reduced . The motor gradually gathers momentum and only once the motor gains around 80 % of its maximum speed will the starter switch over to delta .
It is interesting that even then the final 20 % increase is achieved very quickly in delta and at that time slightly loose v belts will squeal . The change over is determined by the setting on a timer which has to be correctly set for the application on commissioning or replacement .
• When a three-phase motor is started on the star-delta method , it means that when the motor is started , a reduced voltage is applied to all the windings ( lower starting torque ).
• This is done by connecting two windings in series in the star configuration . By doing this , the voltage across each winding is only 230 volts .
• It must be noted that the motor construction is the same as the motor for DOL starting .
• For this method of starting a six terminal motor is essential . A motor with only three terminals cannot be started with the star-delta method .
• In order to start a star-delta motor it must be changed over from the star connection to the delta connection , either manually or as in most applications , automatically .
• This change over must happen when the motor has accelerated to approximately 80 % of its final speed .
• The automatic timed change over is achieved by means of three contactors .
The power and control circuits for a Star-Delta started motor .
The figures below show both stages of starting , ie stage 1 ( star connection ) and stage 2 ( delta connection ).
Characteristics of Star-Delta starting method :
Initial starting current
Initial starting torque
Advantages
1.3 to 2.6 times running current
0.2 to 0.5 times running torque Relatively inexpensive starter
Good torque / current performance Low starting torque
Disadvantages
Break in power supply to motor when changing over
Six terminal motor required
The legend for the starter below
Usual starting time
Typical application
Six core cable required
3 to 7 seconds but can be longer on large equipment
Motors starting with no / low load , such as fans and centrifugal pumps .
www . refrigerationandaircon . co . za RACA Journal I January 2023 43