DC Motor (Separately Excited) 900kW

use one of these, less than $100

Let me see if I understand you. I have a few assumptions about what you are asking to do so let me sum up what I think you are asking and telling us.

You have a machine that has been working for the past thirty years. This machine is still working today. You believe this machine does not provide you enough diagnostic signals to keep this machine reliable. Now you wish to take this machine out of service and modify it to add two temperature signals to provide better diagnostics. You do not know how to perform this modification so you ask a group of anonymous people for ideas on how to do this.

Is this correct?

I'm all for the general concept of keeping things working for as long as possible. One of the general rules I follow to keep things working is if it is not broken, do not fix it. If thirty years of operation doesn't prove a good design then I don't know what will. Leave this motor alone. I works much better than you do.

Continuously monitoring of temperature of armature winding is almost impossible.

Continuously monitoring of Temperature of Field Coils is not a problem - a simple solution is to tie RTD on surface of each coil and connect to monitoring instrument - lot of digital and reliable instruments or scanners with adjustable alarm and trip set points are available in Indian Market and reasonable price.

Question is whether above is worth doing? I do not think it is - as the hot spot temperature of field coil shall be half way from yoke to surface (near yoke, the iron mass works as a good heat sink absorbing heat of winding. Similarly at surface coil is cooled by blast of air).

It will be more useful to monitor temperatures of cooling air at inlet and outlet of the motor - feed in to a PLC - set both limit of Delta T as well as absolute temperature of air outlet.

Monitoring Delta T - if this exceeds normal value (Normal value shall be available from factory test report or can be calculated from Cu and Iron losses and air flow), means problem inside motor. These can be Chocked air vents, dirty winding, faulty core etc.

Monitoring Outlet temperature - if it exceeds Normal value ( found above) means either higher ambient temperature (Room Ventilation not proper), or Over load of motor or recirculation of some of discharge air of motor back in to inlet.

By above you can maintain your plant at good health.

Best regards,

Ramesh