DRIVES, MOTORS + SWITCHGEAR
Fault-finding and redesign to rectify motor vibration Rob Melaia, Engineering & Technical Executive, Marthinusen & Coutts In early 2020 a large South African petrochemical company asked Marthinusen & Coutts (M&C) to establish the cause of excessive vibration in a 17 MW 11 kV 4-pole synchronous motor which is used to drive a gas compressor.The investigation process proved intriguing.
T he motor, designed and manufactured by a reputa- ble Europe-based OEM, had been in operation for 13 years. No vibration problems had occurred in the ear- ly stages of its operation, but first became noticeable about five years ago and deteriorated in subsequent years, even- tually becoming so severe that the motor could no longer be used and was sent to M&C for investigation and repair. Resonance anomaly The investigation started with several non-destructive tests, but these failed to indicate the cause of the problem. Subse- quent investigations however showed a resonance anomaly, as the motor’s resonance frequency was found, through spe- cialised resonance test procedures, to be extremely close to the motor’s operating speed, even to the extent of impinging into that range at times – depending on ambient conditions. The originally intended speed range for the motor was 1 200 to 1 500 rpm, and the resonance frequency reading was between 1 200 and 1 300 rpm. This clearly could not have been the intention of the OEM at the time of manufacture, as the resonance frequency of a motor is required to be well clear of its operating speed. In addition, the resonance frequency is supposed to be higher than the motor’s rated speed, which would have meant it should be around 1 800 rpm or more.
While conducting the investigations and tests that revealed this irregularity in the motor’s resonance frequency against its operating speed, M&C also identified during testing a looseness of the stator core supports to the main stator enclosure, which should not be present in a healthy motor. The next step was to remove the inspection covers of the motor for a close examination of the stator and its support structure to the main enclosure. A key discovery was find- ing that the OEM had applied a highly unorthodox method of fixing the stator to the main frame. Instead of following the normal procedure of employing a heat-shrink interfer- ence fit to bond the complete outer diameter of the stator to the frame, the OEM had used about eight small, welded brackets for this purpose. We deduced that the OEM, having discovered that the required resonance frequency could not be achieved if the stator was firmly fixed to the frame in the normal way, installed the brackets instead, as a more flexible way of doing this for the express purpose of changing the resonance frequency to ensure it would not coincide with the motor’s rated speed. Fortunately, the motor had never been operated under its originally intended variable speed capability. The measure that was applied failed to fix the resonance frequency higher than the motor’s speed range, but it at least achieved the necessary objective of fixing the resonance frequency sufficiently below the rated speed so as not to cause operational problems. A machine design fault We deduced further that the OEM’s failure to achieve the desired resonance frequency by means of the proper fixing procedure was due to a design flaw in the motor. The meth- od the OEM had adopted to circumvent this problem had, however, revealed its shortcomings over time. An additional discovery we made during our investiga- tions into the problem was that there were cracks in the sup- port structure between the stator and the main frame. We deduced that the cracks in the support structure – which, as noted above, consisted of the small welded brackets used as an improvised solution to address the resonance problem the OEM had apparently encountered during manufacture – had been caused by the welds of the brackets having been subjected to excessive strain during years of operation. We then carried out the necessary repairs to return the
Main motor frame/ enclosure
Gap between stator core and main frame
Welded support of stator to main frame
Stator core frame
Stator winding overhang
In this close-up view of a portion of the stator core fitted into the main motor enclosure, one of the welded connections as modified by M&C can be seen on the left.
12 Electricity + Control MARCH 2022
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