What are the symptoms of an SCR failure? Non destructive.
For converter grade SCRs, the type used on DC drives, I don't ever recall seeing a failure that was anything other than a short circuit from anode to cathode. You can check that with a simple multimeter on the Ohms range - with the power to the panel turned off, of course!
A shorted SCR during operation would blow the supply fuses or trip the circuit breaker. You get a short between phases when another SCR is turned on.
Customer has a 200hp dc motor that just decides to stop or looses speed control. The main contactor stays closed. I helped them a week or two?? ago replace the control board and run through the setup procedure and thought everything was fine. Apparently not. Motor will run fine for 20 minutes or so then become erratic or stop. Each start will result in shorter run times. They had found a loose connection at the brushes but no damage and no visual spark/arc via inspection window while it is running.
You are asking because there is still a problem after the loose connection at the brushes was found and fixed?
I assume that the SCR controller is in the motor armature circuit?
It sounds like the fault is thermally induced. The motor warms up, the fault occurs. On the next run, the motor maybe hasn't cooled down completely so the fault takes a shorter time to manifest itself.
What happens to the current and voltage from the controller when the motor stops?
The contactor, you say, stays in so that would indicate that the controller is still enabled. Or at least supposed to be. The correct sequence is that the contactor has to be closed prior to enabling the control electronics.
If the output voltage from the controller to the motor is high with no current being drawn, that would indicate an intermittent open somewhere in the motor armature circuit. That could be in the controller itself, the cabling to the motor, any intermediate disconnection points, or at/in the motor including the terminal box. Sticking brushes, lack of brush pressure, an internal cable joint becoming loose as the motor warms up?
If the voltage is low and the current is high, that could be a mechanical load related problem.
If both current and voltage go off when the motor stops, that would imply that control electronics is no longer being enabled. That could be dirty contacts on the auxiliary contacts of the main contactor or an interposing relay if there is one. We've had a few recently, mainly in water treatment works. Chemicals in the atmosphere....
I hope the above is of some help.
It's based on my experience of variable speed drives over a number decades. When I first started in that field it was mainly DC drives and SCRs were just starting to come into more common use but, at that time, there was still a lot of older kit that had to be supported and, being young, single, and willing to travel, I was lumbered with a quite a lot of it.
