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DC motor tripping on Over & under excitation simultaneously

06/13/2009 5:03 AM

A 5MW DC motor working as finishing stand in a Hot Strip mill of a Steel plant having Siemens Simadyn drive having a base speed of 185 rpm with armature voltage control upto 1000V and upto a speed of 460 rpm through field weakening. Drive has been tripping with signals of 1. Field overcurrent 2. Field failure 3. Overspeed. Drive used to run smooth upto an armature voltage of 500V but when armature voltage was increasing beyond this for increasing the speed, flashover between the rocker rings used to occur and DC breaker used to trip with abovementioned signals. Can anybody suggest reason for this unusual phenomena ?

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#1

Re: DC motor tripping on Over & under excitation simultaneously

06/13/2009 5:16 AM

What's the rated working voltage of the motor?

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#2
In reply to #1

Re: DC motor tripping on Over & under excitation simultaneously

06/13/2009 5:23 AM

1000 volts

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#3

Re: DC motor tripping on Over & under excitation simultaneously

06/15/2009 3:30 AM

Need to give more information pls.

Independant shunt motor? Rating of the shunt (field), ie volts and current. Base speed 185rpm, whats the max?(460rpm), min speed? Power rating of the motor at the various speed?

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#4
In reply to #3

Re: DC motor tripping on Over & under excitation simultaneously

06/15/2009 9:24 AM

Yes, it's independent shunt motor. Field rating is 125 Volts, 128A ( full field). It reduces after base speed of 185 rpm and goes to almost 32A at max speed of 460 rpm. Motor power is 4478KW, armature current is 4700A & armature voltage of 1000V.

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#5
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Re: DC motor tripping on Over & under excitation simultaneously

06/17/2009 3:11 AM

What's the supply to the field doing (volts), when going into field weakening? The current seems to drop very drastically from the current drawn at 185rpm (from 128A to 32A). I believe your problem lies in the supply to the shunt and hence the overspeed/field shunt trips.

The shunt current (for max speed, 32Amps) is only 25% of the original value at base speed (128Amps), which is to severe. According to my calculations the shunt current (for a max speed of 460rpm) should be approximiately 50amps.

Also from a table comparing (Hughes electrical technology), shunt current with the flux densities, a 25% flux density will result in half the current drawn on the shunt.

Check the supply to the shunt during fieldm weakening..

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#6
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Re: DC motor tripping on Over & under excitation simultaneously

06/19/2009 4:55 PM

See, excitation current of 128A or 32A are design parameters. But our problem does n't happen during field weakening regime. Problem happens much before field weakening starts. Excitation current at that time remains at full 132A. And armature voltage is around 500 volts at the time of problem and field weakening starts at 1000 volts.

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#7
In reply to #6

Re: DC motor tripping on Over & under excitation simultaneously

06/22/2009 8:41 AM

What is happening to the actual shunt voltage when increasing the speed above base speed (185rpm) ? I assume it must be dropping off from the 125v at full field? Do you apply an actual 125v to the field at 185rpm? What is the voltage applied to the armature at this speed?

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#8
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Re: DC motor tripping on Over & under excitation simultaneously

06/24/2009 2:08 AM

In healthy condition, excitation current above base speed of 185 rpm starts falling upto 32A and speed gets to 465 rpm. Armature voltage remains constant at 1000 V during field weakening period.

Now this problem is happening much below base speed and armature voltage is 500 V at that time. Field current remains constant to full field till that period.

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#9
In reply to #8

Re: DC motor tripping on Over & under excitation simultaneously

11/26/2011 10:24 AM

I forgot to post the final result. Later on after detailed checking, found out root of problem was repeatedly low insulation between motor armature's rocker rings.

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