MAVR null balance meter oscillation

[hallmg]

Hi,

On one of my gas turbine control panels the 'null balance meter' for the modular AVR unit is steadily fluctuating about 5 degrees either way.

The AVR unit is pretty old probably been in situ over 30 years, is made by John Brown, Type B9602941.

On the other panels this meter is not fluctuation, needle stays still at the zero mid point.

As the turbine is running it is currently not possible to open up the unit to investigate, any help would be much appreciated.

 

[ScottyUK]

That's not a John Brown AVR, it's a Brush design. Worryingly, I can recognise a Brush drawing number years after leaving the site! I have some drawings of a MAVR system although ours were slightly non-standard with an augmented power output stage - if they're any use I'll see if I can find them. I might have copy of some training notes too.

Have you checked the motorised pot (typically accessible on the front of the MAVR rack) and the follower (sometimes hidden away out of sight) for linearity? These definitely wear, especially if your machine operates consistently at the same excitation current. The DCCT power supplies to the field current transducers had a habit of aging and having messy failures - I became rather adept at repairing them. Generally the rack runs very hot and components age quite quickly. You'll see the signature browning / blackening on the circuit boards.

Brush Electric Machines at Loughborough still had some knowledge of these old units fairly recently, although the guy who designed them retired a while ago. Service or Training are worth a call.

 

[hallmg]

Thanks for the info, we will probably get brush to take a look at it next time the machine is shut down. hopefully it will keep running until then.