ScottyUK
Electrical
- May 21, 2003
- 12,915
I got asked to give a second opinion on a motor which we have at our overhaul contractor's works. The machine is a form-wound 3300V 250kW type on a 355 frame. It suffered a failed DE bearing which cooked itself, hence it was removed from site and sent for overhaul. The stator was untouched during the rebuild and we don't believe there has ever been a rewind, so we are reasonably confident that the stator connections are correct. The physical construction means that inadvertently swapping polarity of a winding could only occur during a rewind. The stator passed a Baker test.
Following rebuild the motor is noisy with a continuous low-frequency noise, not a growl or buzz, more of a rumble. The motor accelerates very slowly on an open shaft test where there is only windage and bearing friction to overcome. The rotor is mechanically free and coasts for a long period once moving. On this basis we think there is a gross lack of torque during acceleration rather than any mechanical drag. If the bearings were dragging to the point of stalling a 250kW machine they would be torn to bits, and this isn't happening. During a start attempt the motor crawled up to speed over about a minute or so, and the peak current captured by the clamp meter was just over running current, i.e. abnormally low for a DOL start.
The motor load is a centrifugal pump which currently presents very little load due to process conditions requiring very low flow, so it is entirely possible that it has been running in this state for a while without causing any apparent problem at the load..
My opinion from the symnptoms is that there is something wrong with the rotor rather than the stator. My current theory is that there is a fracture in the squirrel cage end-ring itself, and that some current is passing into a relatively high resistance steel retaining ring located inside the copper end-ring. As a result the currents in the squirrel cage are much smaller than expected, causing loss of torque and keeping the line current low. There a few electrical burns which indicate passage of current and perhaps light arcing. The brazes where the bars meet the ring, and the ring itself, are obscured by a heavy film of varnish and overspray. There are no visible cracks, but I'm getting it cleaned down to bare metal in prep for dye penentrant and/or ultrasonic NDT on the end ring and the brazing.
Has anyone seen a similar set of symptoms before? Am I on the right track, or am I totally off into the weeds? Any thoughts on other possible sources of the problem?
I am not certain whether the copper ring was cast and machined, or hewn out of solid plate: can anyone with experience of manufacturing this type of machine offer an opinion which is most likely? It may be of significance to the NDT guys, plus it will influence whether repair is viable or not.
Following rebuild the motor is noisy with a continuous low-frequency noise, not a growl or buzz, more of a rumble. The motor accelerates very slowly on an open shaft test where there is only windage and bearing friction to overcome. The rotor is mechanically free and coasts for a long period once moving. On this basis we think there is a gross lack of torque during acceleration rather than any mechanical drag. If the bearings were dragging to the point of stalling a 250kW machine they would be torn to bits, and this isn't happening. During a start attempt the motor crawled up to speed over about a minute or so, and the peak current captured by the clamp meter was just over running current, i.e. abnormally low for a DOL start.
The motor load is a centrifugal pump which currently presents very little load due to process conditions requiring very low flow, so it is entirely possible that it has been running in this state for a while without causing any apparent problem at the load..
My opinion from the symnptoms is that there is something wrong with the rotor rather than the stator. My current theory is that there is a fracture in the squirrel cage end-ring itself, and that some current is passing into a relatively high resistance steel retaining ring located inside the copper end-ring. As a result the currents in the squirrel cage are much smaller than expected, causing loss of torque and keeping the line current low. There a few electrical burns which indicate passage of current and perhaps light arcing. The brazes where the bars meet the ring, and the ring itself, are obscured by a heavy film of varnish and overspray. There are no visible cracks, but I'm getting it cleaned down to bare metal in prep for dye penentrant and/or ultrasonic NDT on the end ring and the brazing.
Has anyone seen a similar set of symptoms before? Am I on the right track, or am I totally off into the weeds? Any thoughts on other possible sources of the problem?
I am not certain whether the copper ring was cast and machined, or hewn out of solid plate: can anyone with experience of manufacturing this type of machine offer an opinion which is most likely? It may be of significance to the NDT guys, plus it will influence whether repair is viable or not.
