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time for star- delta motors


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#1 taknavaz

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Posted 13 October 2005 - 09:55 PM

hello
what is the formula for calculation the time of the timer for star- delta motors?
thanks.

#2 marke

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Posted 13 October 2005 - 11:09 PM

Hello taknavaz

While it is possible to calculate the time in star for a star delta starter, it is easier to use trial and error.
To calculate, you need to know the speed torque curve of the motor in star and the speed torque curve of the load, plus the inertia and full speed of the driven load.
To set empirically:
Set the time longer than you expect, and measure the time taken for the motor to reach full speed. This is the timer setting you require.
If the motor can not reach full speed due to insufficient torque, then the timing is not so relevant as the motor will draw almost locked rotor current after switching to delta. The best setting is to change over when the motor ceases to accelerate and further.

Best regards,

#3 bob

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Posted 14 October 2005 - 04:06 AM

Hi Marke ,

I have come across two interesting phenomenons in start-delta applications. We had on site a 55 k W 400 V motor driving a fan. This motor took around 20 seconds to reach full speed in star position. This motor burned for some reasons and a similar motor !!! 55 k W 400 V was used to replace the burned on. The new one was never able to reach full speed in star position and obviously pumped the LRC on switching to delta.
Basically, the characteristic of this motor is such that it could not develop the required torque in star position to reach the full speed of the load.

Regards.

Bob

#4 marke

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Posted 14 October 2005 - 04:22 AM

Hi Bob,
Yes that is a common occurance. There are major variations between motors that are the same size.
Why did the first motor fail?? was it a rotor failure or a stator failure?
If it was a rotor failure, it could indicate that the rotor was not suitable for the inertia of the load, or that the starter remained in star for too long.

Best regards,




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