Synchronous Motor Troubleshooting

[dans1999]

I am new to this site, looking for some help with one of our old Westinghouse Motor Starters. We have an 1200 HP Synchronous motor with brushes. We have been experiencing exciter problems quite often when we have to restart or after a disturbance within our mill. The current problem we are experiencing is the mill was offline due to low tonnage through the mill, it was taken down normally. When we went to restart, it tripped in locked rotor current, we confirmed the current by using handheld ammeters on the CT wiring to the motor protector. Upon inspecting the exciter package we found all three incoming fuses to the exciter open, found 2 shorted SCR and all the resistors on the snubber boards were open.


We have found this situation before, replaced the damaged parts and the mill runs good, then occasionally this happen again. Does anybody have a idea where what may be the culprit causing these issues?

Thanks, Dan

 

[Besoeker]

I am new to this site, looking for some help with one of our old Westinghouse Motor Starters. We have an 1200 HP Synchronous motor with brushes. We have been experiencing exciter problems quite often when we have to restart or after a disturbance within our mill. The current problem we are experiencing is the mill was offline due to low tonnage through the mill, it was taken down normally. When we went to restart, it tripped in locked rotor current, we confirmed the current by using handheld ammeters on the CT wiring to the motor protector. Upon inspecting the exciter package we found all three incoming fuses to the exciter open, found 2 shorted SCR and all the resistors on the snubber boards were open.


We have found this situation before, replaced the damaged parts and the mill runs good, then occasionally this happen again. Does anybody have a idea where what may be the culprit causing these issues?

Thanks, Dan

Is it a static exciter that has the SCRs?

 

[iceworm]

I am new to this site, looking for some help with one of our old Westinghouse Motor Starters. We have an 1200 HP Synchronous motor with brushes. ...

Just curious:
Would this be the motor driving the chipper in Sitka?

ice

 

[dans1999]

Yes it is, the excitation transformer (4160VAC/240VAC) is fed from the main contactor, then to the exciter package. I will attach a diagram.

 

[dans1999]

No, not a chipper, this is actually a ball mill at Red Dog Mine

 

[Besoeker]

Yes it is, the excitation transformer (4160VAC/240VAC) is fed from the main contactor, then to the exciter package. I will attach a diagram.

Unfortunately the diagram resolution made it a bit difficult to read. However, it is the control of the exciter field which is a very inductive load. The diagram doesn't show details of SCR snubbers or suppression across the field.

My best guess at this stage is high transient voltages resulting from interrupting the inductice cirrent.

 

[iceworm]

Yes it is, the excitation transformer (4160VAC/240VAC) is fed from the main contactor, then to the exciter package. I will attach a diagram.

No, not a chipper, this is actually a ball mill at Red Dog Mine

Ah. For a bit I had visions of zombies crawling out of graves. :blink:


I can't see the date on the drawing - I'm guessing early 80's
Which fuses open? Must be the 240V side.

Q: Doesn't the rotor get shorted out (maybe a small resistance) and it starts as an induction (wound rotor) motor?

Just thinking out loud - If the starting SCR (I can't read the number - but the one associated with D1) were to stick on, that would short out the exciter, which would take out (short) two SCRs, (one on each side of the bridge) then the fuses would open right after.

I'm guessing the SCR is run by the Sync and Control Circuit - Could there be an intermittent causing the SCR to stay on?

ice

 

[dans1999]

Thanks for your comments, we ended up just replacing the burnt out components and brought in online. Everything looks normal to our eyes but I fear this will rear it's ugly head again too soon.

dan

 

[Besoeker]

Thanks for your comments, we ended up just replacing the burnt out components and brought in online. Everything looks normal to our eyes but I fear this will rear it's ugly head again too soon.

Seems quite probable if things just go back as they were.
I'd suggest puting a large surge suppression device across the alternator field.

 

[Besoeker]

Seems quite probable if things just go back as they were.
I'd suggest puting a large surge suppression device across the alternator field.

Should read exciter field. The winding controlled by the SCRs.

 

[ATSman]

I am new to this site, looking for some help with one of our old Westinghouse Motor Starters. We have an 1200 HP Synchronous motor with brushes. We have been experiencing exciter problems quite often when we have to restart or after a disturbance within our mill. The current problem we are experiencing is the mill was offline due to low tonnage through the mill, it was taken down normally. When we went to restart, it tripped in locked rotor current, we confirmed the current by using handheld ammeters on the CT wiring to the motor protector. Upon inspecting the exciter package we found all three incoming fuses to the exciter open, found 2 shorted SCR and all the resistors on the snubber boards were open.


We have found this situation before, replaced the damaged parts and the mill runs good, then occasionally this happen again. Does anybody have a idea where what may be the culprit causing these issues?

Thanks, Dan

I am by no means a motor man but we got involved with the motor starter part of this. It was a 5,000HP, 12KV motor driving a compressor. After sitting Idle for some time (a year?) when they tried to start it similar damage occurred within the exciter; blown SCR's and resistors (this was a brushless exciter.) What they concluded was that after sitting idle for that long and due to it's heavy weight, the armature rotor shaft developed a flat spot on the bottom. This produced enough resistance on start-up to cause a locked-rotor condition. That is why, I am told, they manually rotate the rotor with a chain-pole before starting after an extended down-time to reduce the resistance. I would guess that they make sure the compressor is unloaded as well.

 

[GoldDigger]

The manual rotation would also renew an oil film that might have been squeezed out of the bearings during the long down time, allowing some metal to metal binding.
A flat spot on the shaft would not have had much of an effect unless there was a matching flat on the bearing surface.

Tapatalk!

 

[Besoeker]

I am by no means a motor man but we got involved with the motor starter part of this. It was a 5,000HP, 12KV motor driving a compressor. After sitting Idle for some time (a year?) when they tried to start it similar damage occurred within the exciter; blown SCR's and resistors (this was a brushless exciter.) What they concluded was that after sitting idle for that long and due to it's heavy weight, the armature rotor shaft developed a flat spot on the bottom. This produced enough resistance on start-up to cause a locked-rotor condition. That is why, I am told, they manually rotate the rotor with a chain-pole before starting after an extended down-time to reduce the resistance. I would guess that they make sure the compressor is unloaded as well.

I come across similar situations elsewhere. Paper making machines producing tissue have one very large drying cylinder. It is referred to as the MG cylinder or Yankee dryer.
Typically around 15ft diameter and with a steel shell it is very heavy. Normal operation in production is typically several thousand feet per minute. When production is shut down, say for maintenance, there can be bearing and shaft problems if the MG is stationary for any length of time. It is common to have what's called a "Sunday Drive". It is very much lower power and fed from a different supply. It keeps the MG just rotating at a very low speed.