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auto transformer vs Star delta


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Hello herselman74

Welcome to the forum.


I would never recomend a star/delta starter!! To me, the star/delta starter is just a political starter that does more damage than Direct On Line starting.

The star/Delta starter operates by first connecting the motor in star connection. This will draw one third of Locked Rotor Current and produce one third of Locked Rotor Torque. One the motor has reached maximum speed, you open the star connection and then reconnect the motor in Delta.

Problems with the star/delta starter.

1. If there is insufficient torque in star to accellerate the motor to full speed, the motor will be changed to delta at part speed and this wil result in close to Locked Rotor Current. (no advantage over Direct On Line)

2. The transition from star to Delta is open transition. That is, the motor is effectively disconnected from the supply during the transition. During this open time, the rotor with it's magnetic field is spinning inside the stator and acting as a generator. When you reconnect the motor in delta, the supply voltage is effectively added to the generated voltage causing a very high transient current and torque. This transient is many times higher than DOL and is the cause of much electrical and mechanical damage.

Advantage of the Star/Delta Starter

1. It is cheap

2. It complies with the requirement for a reduced voltage starter, but it does not provide any operational benefit!!


I would definitely use the autotransformer option!!, or better still, I would suggest a solid state soft starter.


Also additional to above are all motors able to be made into 6 wire options for star delta connection???


No, some motors are designed to operate at their rated voltage in star. These can not be operated in delta and therefore can not be converted to 6 wire connections for star/delta starters.


Best regards,

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Hello herselman24,


I agree with everything marke has already said.


In my opinion the auto-tranformer will present you with some benefits that may not have been adequately covered thus far.


1. Auto-transformer starters and normally provided with 3 voltage tappings. The ability to select the tapping which best meets the starting torque requirement of the crusher will ensure the minimum possible electrical and mechanical stress.


2. Most auto-transformer starters these days are korndorfer connected. This provides 'closed transition' switching between starting and running ie without current and torque transients. Note however that incorrect voltage tapping selection will result in a current and torque step.


3. Although a electronic soft starter will provide superior starting performance, the auto-transformer is a very simplistic (and robust) device that can be easilly serviced by the average industrial electrician. Also as the conditions at most quarrie sites are less than ideal, the auto-transformer starter may be the best option.


I hope the above helps you with your selection.




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Hello homerjay


Yes, and no.

If you use a line and bypass contactor with a soft starter, the cost does climb, and in some cases, you need to do this.


The point that GGOSS was making I believe, is that the soft starter is a technical piece of equipment that can be missapplied or damaged and require "technical" people to sort it., whereas a Korndorfer autotransformer starter is a simpler device that almost any electrically qualified person should be able to sort. This can be an advantage.


Another advantage on a purely inertial load, is that the transformer action will reduce the voltage, and it will also reduce the current. for example, on the 50% tap, the voltage on the motor terminals is half line voltage, there fore the start current into the motor will be half the locked rotor current of the motor. The starting transformer has a turns ratio of two to one, therefore the starting current in the line will be half the start current into the motor, or 25% of the Locked Rotor current of the motor.

Where there is a purely inertial load, this can result in a useful reduction in start current relative to a soft starter, however, if the load exhibits a torque that increases with the speed, or the square of the speed, the start voltage required to accellerate the motor to full speed can be 80% to get through the top 20% speed increase. If this is the case, the soft starter can actually yield better results than the auto transformer starter.


The bottom line is that nearly all starters have advantages and disadvantages and should be selected/engineered to suit the individual installation, environment and personell.


Best regards,

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Thanks Marke for saving me some time.


Homerjay, just want you to know that I am in fact responsible for marketing a range of soft starters and have taken the company I represent to number 3 in its respective market. Having said that, where given the opportunity I will always promote the product that will provide the best 'overall' outcome for the customer, and sometimes it just isn't a soft starter.




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