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Unread October 9th, 2019, 05:44 AM   #1
Prince
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DC Motor over-current problem in No load

I have a problem with a DC Motor of 250Kw. We try to process hot steel bars.

We are using these motors with speed control in a roughing chain of Rolls. Normally We see used power very low before we feed the material. As we expect we will see the full load when the material is fed to the chain of rolls and the current gets normal as it crosses each of the rolls.

The problem happens when the process finishes in the last roll. Then we see an overcurrent in the last DC motor. This shock is so high that it burns down the fuses on transformers. It seems very confusing as I have no load on the roll when I see the overcurrent.

Why is this happening? Can I solve this problem by Torque controlling the last roll? Does anyone have a suggestion for me
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Unread October 9th, 2019, 06:12 AM   #2
Mad_Poet
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Check the motor for a short to ground while the motor is hot?
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Unread October 9th, 2019, 06:57 AM   #3
Prince
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We normally Check them, The thing about this motor is that the problem happens just as the loads get out of it. I thought maybe there is a problem in its converter settings that can not go from full load condition to no-load, fast enough.
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Unread October 9th, 2019, 07:12 AM   #4
Gene Bond
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I am assuming the drive is regenerative.

It sounds like when the load comes off the roll, the drive mis-fires the SCRs, which looks like a short circuit to the line.

I would check the condition of the SCRs. It sounds like one of them is bad.
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Unread October 9th, 2019, 08:11 AM   #5
widelto
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Have you check field circuitry? , please check field voltage and field current.
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Unread October 9th, 2019, 10:10 AM   #6
GaryS
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I agree check the field current a weak field or no field will cause the motor to overload
always verify the current as voltage may be present with no current also make sure that you read the same current on F1 and F2 if they are not the same then it a good indication that the field is grounded
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Unread October 9th, 2019, 10:11 AM   #7
GaryS
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I agree check the field current a weak field or no field will cause the motor to overload
always verify the current as voltage may be present with no current also make sure that you read the same current on F1 and F2 if they are not the same then it a good indication that the field is grounded
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Unread October 11th, 2019, 03:59 PM   #8
Prince
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FYI, I just checked the HMI panel for all the rolls and I see that although the problem is harsher on the last rolls, this power increase happens in all. So it can not be a problem with one drive. I thought it might be of the effect of the billet shear on the billets
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Unread October 11th, 2019, 04:23 PM   #9
Aabeck
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Is it possible the steel leaving is kicking the roll and driving a regen back to the controller?
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Unread October 11th, 2019, 05:00 PM   #10
GaryS
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I would still look at the field supply Always check the current the voltage is not as important as the current.
without seeing the actual setup it's hard to tell but they could be using a single field supply for all the motors on the line the would control the speed of the motor by controlling the armature and field supply. the old rolling mills used a 500 VDC Buss in the plant to supply the power to both the armature and the field they just switch jumpers across the resisters in both the armature and the field
the start reducing the field current when the armature voltage reaches the full 500V
as I said before the armature current will go be what ever the motor needs to move the load. while the bar is still in the mill all the motors share the load as the load goes off the mill the load goes down . a DC motor with no load and an open field will draw more current and try to go into runaway I have seen this happen before be careful
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