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slip ring motor problem


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#1 tahira

tahira

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Posted 29 August 2002 - 06:06 PM

i have replaced a slip ring motor of

48 kw slip ring motor
stator 440 v , 50 hz wiht 87 A in delta 950 rpm
rotor star 195 v , , 152 A

to
48 kw slip ring motor
stator 440 v , 50 hz wiht 87 A in star 1475 rpm
rotor star 195 v , 152 A


the motor in detal taking a 91 A at full load however now in star 158 A at full load ,

why it so , although the name plate shows
star connected stator 440v,
power 48 kw , with 87 A

the o.l trip continuosuly and under rated with this motor

plz help me

#2 marke

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Posted 29 August 2002 - 09:53 PM

Hello tahira

Welcome to the forum!!

To me, your problem is a very simple case of increased load due to increased speed.
There will be no difference due to one motor being delta connected and one being star, as this is what they are designed for.
You do not say what the driven load is, but you can be pretty sure that with the increase in speed, there will be a significant increase in load. If it was a pump or a fan, the load increase would be far more dramatic.
Remember that for constant power, an increase in speed results in a reduction in torque capacity. If the load required the same torque at the increased speed, then the power required would increase proportionally. i.e. the new power required would be 48 x 1475/950 = 75KW

In many cases, as the speed increases, the torque also increases resulting in a power increase proptional to the speed squred or even cubed.

The current that you quote is what I would expect with a constant torque load due to the increased speed.

If the machine is belt driven, I would suggest that you change the pullies so that the speed of the driven load is the same as it was before and the problem will go away.

Best regards,




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