BUS BAR CONNECTION BETWEEN SLIPRING & ROTOR WINDING IS MELTED, EACH PAHSE IS HAVING TWO PARALLEL  WITH TOTAL  FOUR BUS BAR(WIDTH 35 mm & THICKNESS 4.5 mm) DID ANYBODY FACE THIS KIND OF PROBLEM,PLZ SHARE YOUR VIEWS  NAME PALTE: 5300 KW,11 KV,323 A,1890 V,1690A RPM 99720171018_10444020171018_10444820171018_10450020171018_104532

Original Post

First off, this is a synchronous or wound rotor motor (not specified which). These are very different from induction motors. You treat both of them the same way.

First look for physical damage. Is any of the wiring stretched out of shape like it was ripped apart by something mechanically? If not, move on. This is rare but it does happen occasionally. It is very obvious because the wiring gets stretched and distorted, tie downs ripped apart, etc., with little heat damage.

Move on to checking the rotor controls. Check the resistor that is typically used for starting. It should be around the same resistance as the rotor, typically only a few ohms. If it's much higher on startup you will get extremely high voltages on the rotor since V=IR. How high? Thousands of volts. It easily overcomes the typically fairly low rotor insulation and destroys everything around it. Check the contactors. DC contactors on synchronous motors in particular tend to have fairly heavy arcing wear since they aren't assisted by zero crossings when it comes to breaking the arc. Check for AC out of the "DC" supply (burned up diode) which can wreak havoc on these systems. Check resistance of the rotor itself and compare to name plate. The name plate will only list voltage and current so R=V/I gives you a close approximation to the correct value. That's about all you can check. If you don't find anything wrong chances are you had a high resistance connection develop. It causes the same high voltage damage problem on startup and if it survives startup, then you just get simple ohmic heating (I^2*R) during operation that opens up the connection further causing more heating until eventually it fails. Easily found with infrared scans or annual low ohm resistance resistance testing, and insulation resistance testing to track contamination issues.

Usually people go off on the wrong track looking at this kind of failure and assume that it was excessive current that did the damage. But think about it. With wound rotors, the motor is basically a rotating transformer. With synchronous motors realistically even in the event that voltage regulation completely fails, how much voltage can you realistically get from a rectifier bridge? 145% of the input AC voltage, that's it. So you might get 145% of voltage and since resistance is fixed on the rotor, 145% of the current. It's high but not really enough to do the kind of damage you're looking at. It would show up with the entire rotor insulation darkened from overheating.

Once a year you need to be going in there to clean off all the carbon dust built up around the slip rings, checking the film pattern on the rotor, checking brush wear, and so forth, including resistance readings. At 11 kV you should be cleaning and testing the medium voltage side too. Cleanliness is critical to avoiding tracking damage in these systems. That's why motor shops require a "clean room" for winding. If you don't do this eventually you will flash over and cause significant damage to the slip ring system if nothing else. My field service group does this pretty routinely for customers that don't do it themselves or want a third party report for internal political reasons.

The failure you have is fairly common believe it or not. Rotor issues are much more rare compared to stator problems but that doesn't mean that they don't exist. They are getting more and more prevalent too simply because since DC motors are no longer prevalent, end users simply have no experience with commutators and slip rings and the required maintenance. Unlike an induction motor where the only mechanical part is the bearings, you have an extra mechanical part in the slip rings or commutator which requires just as much or more routine maintenance, some of which can only be done while de-energized.

I can sell you the fancy MCSA testing too (PdMA) but to be honest this is not as well studied as the much more common induction motor case.

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