how to resolve Armature over current tripping in DCS 800 connected ENTRY REEL Motor in 4HI Cold reversing mill
4HI Mill Entry Reel Tripping in Regenerative Mode (Forward Rolling) particularly at low speeds at the end of the Pass OR when Normal stop is applied even in the Middle of the Pass.
Entry Reel tripping in 3 rd , 5 th , and 7 th pass at the end of the pass when only few wraps are left on the mandrel leading the wraps getting loose on the mandrel
And Roll Gap opening due to emergency stop generation.
It was surprising as the Reel Motor was showing No signs while at High speeds whether its in coiling mode (i.e. Motoring Mode) and uncoiling ( regeneration mode). All basic tests were done to ascertain the Motor healthiness.
Motor Insulation Values are as follows:
Commutator ( Brushes raised) to Gnd.: 70 Meg Ohms
IP to Gnd.: 2.5 Meg ohms
Field to Gnd.: 200 Meg Ohms
Armature to Field: 150 Meg ohms
Motor and DC cables condition confirmed to in order.
Electronic cabinet checks
1. First replaced power supply unit. No improvement
2. Suspected reverse bridge thyristor malfunctioning. Identified the connections on pulse board and disconnected reverse bridge thyristor pulse connections. No improvement.
3. ETR Pulse tachos and Pulse distribution boards in AGC Panel Interchanged. No improvement.
4. Finally replaced Con 4 board with spare one.
5. DOWNLOADED the parameters and Monitoring of Conv Current and Motor speed feedback was done. Though there is smoothness was seen the speed feedback, Drive was Still tripping with armature overcurrent in Odd passes like earlier.
6. This erratic behavior was suspected to be due to synchronizing supply distortion especially in inversion control Changed the Tap position of ETR transformer which remained the in the same tap position since 1988.
7. Also measurement cards of ETR DTR interchanged expecting the problem to shift if the issue is with this board.
We interchanged all Boards with Healthy Delivery Tension Reel and Replaced 5 out of 6 Thyristors with New ones,
Field exciter Module too has been interchanged with DTR.
Snubber capacitors checked with oscilloscope and confirmed to be fine. These ones and CTs have not been Replaced / Swapped
DC Motor has been inspected once again thoroughly, Tacho inspected. Found to be in order.
Last action was Input transformers swapped i.e. Drive connected back to original transformer and tap changed to 2 from 3. Next day the Rolling went on well for 3 coils but in 4 th coil the problem resurfaced.
Today again for first 4 coils No tripping experienced. In 5 th coil at the end of 3 rd pass Mill almost come to halt only 2 wraps left on the mandrel and ASD auto stop command is about be released ETR tripped with
Over current CDP indicates 2222 Amps Field current 1.4 Amps but drive monitor doesn’t indicate any spike even with 10ms selection. 5 th and 6 pass rolling went well without any incidence.
We even tried operating ETR with only Forward bridge (disconnected reverse bridge GK wires from Pulse board) to eliminate the doubt of malfunctioning. In PLC Drive Master Low tension selection after pass end has been disabled as the drive was tripping at standstill condition during pass changeover.
Let me elaborate the above case
When Mill comes to halt after 4 th pass. PLC automatically triggers tension drop by half with LOW TENSION SELECTION signal generated at Mill Zero speed. Then Operator selects Direction change followed by Thread Command. At this instance Tension start to Pick up to the set value. Noticed ETR tripping at this point instantly. Defeated this interlock to avoid this rapid drop of tension followed by rapid rise which is thought to be the reason. To certain extent we succeeded by doing this as the tripping during pass changeovers is a thing of past. But the 3 rd pass end tripping is still sporadic and still exists.
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