Here the problem is with a frame 5 twin shaft machine's second stage nozzle with speedtronic Mark V TMR as its control system.

With all the outgoing cables to the servo valve to the field isolated I found a difference in the polarity in the servo commands between <R> ,<S> and <T> cores. The values measured are +10VDC in <R> and <S> cores and -10VDC in <T> core because of this one coil for servo valve is inversed in the field to get the right feedback.

Would like to know from experts the reasons for this difference, because of the above issue the servo current is quite high for all the coils in the Autocal page.

An immediate response would be highly appreciated.

Brian

 

Brian,

When did this problem start? After a servo-valve was replaced?

This is not a simple problem--at least from the description provided. I'm not really clear what the problem is. You seem to be saying that the turbine is not running and that you are measuring the voltage of the three second-stage nozzle servo-valve outputs, but it's not clear where you are measuring the voltages--at the QTBA terminal boards, or at a Marshalling cabinet or at the junction box near the second-stage nozzle servo-valve and -actuator.

And the issue of "inverted" coils is also extremely confusing.

If you are measuring the voltages at the QTBA terminal boards of the Mark V, then it's either a problem with the feedbacks to the servo-valve regulators of the three processors causing the three processors not to see the same position and output different currents <b>from</b> the three processors to the servo-valve coils.

OR, it's a problem with the TCQC and/or TCQA cards in one of the processors.

EITHER problem would likely be accompanied by Diagnostic Alarms. What Diagnostic Alarms are being annunciated by the Mark V? (Please provide the full text of the Diagnostic Alarm messages--not just the Drop Numbers.)

If you are measuring the voltages at the junction box nearest the second-stage nozzle servo-valve, then it's most likely there is some problem with the wiring between the QTBA terminal boards and the junction box where you are measuring the voltages. Because this is a two-shaft machine, it's most likely IS (Intrinsically Safe) barriers (active or passive) are used somewhere between the Mark V and the field devices, and this means there is a Marshaling Cabinet or enclosure where the wiring has to pass through the IS barriers, as well as other intermediate JBs (Junction Boxes) where wiring problems could be introduced. I've also seen IS barriers fail and cause all manner of unusual problems.

It could also be that there is a problem with the TBQC card the LVDT feedbacks are connected to, or with the ribbon cables/connectors between the TBQC and the three processors carrying the LVDT feedback voltages from the TBQC cards to the processors. Sometimes the pins/connectors of the ribbon connectors can develop corrosion which can impact the feedback/signals.

Without a LOT more information about:

1) when the problem started,

2)specifically where you are measuring the voltages,

3)precisely how you performed the servo-valve polarity check to determine that one coil's leads needed to be inverted to get the proper <b>ACTION</b> (not feedback, but action),

4) what Diagnostic Alarms are active and unresolved,

5) and what the second-stage nozzle feedbacks (both of them) to each of the three processors are

it's not possible to provide any more assistance.

Please write back with all of the information requested and to let us know how you fare with your troubleshooting.

 

Hi Brian,

What is the application of this particular servo? Does this occur to one of the servos in <T>? Have you checked on the feedback signals for this regulator in <T> whether it is correct?

Like CSA said, not enough information.

If this measurement is taken from the terminal board then I believe there must be something wrong on the feedback signals, but it was taken from marshaling, JB or at the server connector, then it could be wiring mistake or feedback error.

All the best.
Kevin Goh