The remedial design of the stator of the gearless mill drive (GMD) of the 40 ft SAG mill at Cadia Hill was the project that defined the success of EAnD and the careers of its two engineering managers, Bill Lai and Chris Meimaris. The Cadia work led on to many other projects including some of the largest projects in the world such as Antamina, Cerro Verde and Collahuasi.
During the selection of equipment on the Collahuasi project in 2001, Kvaerner’s lead engineer for the grinding area kept repeating that Cadia had “dodged a bullet”. This hyperbolical phrase reflected the fear that had been generated in the mineral processing industry due to the events at Cadia. The mine owners’ and engineering contractors’ reactions on Collahuasi and several other projects were extreme, as many would not consider a Siemens motor even though the Cadia motor had been repaired and had been operating for more than 3 years. In most cases, the die had been cast prior to any tendering and ABB cornered the market. Unfortunately for Collahuasi (and others), their desire to avoid “the bullet” despite the fact that the Cadia issue had been resolved, resulted in a much greater problems. What is worse though is that we can definitely say with hindsight that there was no bullet to dodge at Cadia.
The original Cadia stator design was only just too flexible. It wasn’t “floppy” and impossible to manage or control. The maximum amplitude of the vibrations during a vibration excursion was about 7 mm. This was a horizontal deflection at the top of the frame. The greatest reduction of the air-gap was about 5 mm at ±45º to the horizontal. Also, the stator was metastable, not unstable. It switched from one stable state (normal operation) to another stable state (albeit with higher vibration levels) and back again. When the motor vibrated, the vibrations were self-limiting due to the magnetic forces in the air-gap, unlike resonant vibration where only the material damping limits the vibration level. There was physical evidence that the vibrations were metastable. The stator never touched the rotor during the early days of operation despite several extended periods of vibration excursions that had occurred, usually on cold nights. The vibrations did not increase and decrease proportionally with speed. The onset of vibration was sudden at a particular speed range unlike the gradual increase that would be expected if the motor was resonating as a linear system. Once the vibrations had started, the motor had to be stopped or its speed had to be reduced dramatically before they would cease. Again, this was not a resonant behaviour. We did not completely understand the metastable nature of the vibrations at the time. This understanding was developed once we solved the mathematical equations that governed the stator motion. Whilst we had the mathematical model of the vibrations in 1998, we were only able to solve it approximately because the equations were highly nonlinear. More refined solutions that enabled assessments of new designs were developed after the Strongback* was installed at Cadia. The solutions to the mathematical models show that the vibrations quickly increase from a very low level to about 300 mm/s and remain at that level until the speed of the motor is raised or lowered outside the critical speed range around 9 rpm. The vibrations then quickly reduce to the normal operating levels for both lower and higher speeds.
There is no doubt that the situation at Cadia was worrisome for Newcrest and Siemens. It was quite scary to see the motor suddenly change its behaviour. The forces generated by the vibration were sufficient to shake the entire grinding plant. However, quite soon after the behaviour was detected, Siemens worked out that the motor could be wedged to the foundations to prevent the vibrations from occurring at all. This was implemented by the plant’s engineering team and it worked very well as a temporary measure to prevent vibrations and hence any significant loss of production until it was agreed that the stiffener should be installed.**
So, there was no bullet to dodge at Cadia, as the stator was never a danger of collapsing onto the rotor. A more appropriate metaphor would have been that a gun had been waved around when the motor was commissioned and this was frightening because no-one knew that it wasn’t loaded. The danger of a catastrophic motor failure was perceived rather than real.
A Design Tool for Motors
Whilst the most exciting part of the work at Cadia was developing the repair, the most beneficial technical output of the work to our business was developing a design tool to assess new motor installations. It was evident early on that the nonlinear forces in the air-gap resulted in a sub-harmonic vibration. Sub-harmonic excitation could be modelled using Hill or Mathieu equations but knowing this did not help much in assessing new machines, i.e., the knowledge that sub-harmonic vibrations could occur does not help in predicting if they will occur. So,we initially came up with the 3 Hz design rule that related the operating natural frequency of the motor with the maximum pole passing frequency, i.e., the operating natural frequency had to be 3 Hz greater than half the pole-passing frequency. This was a robust tool but it resulted in a conservative assessment. Later, we developed an accurate mathematical model that simulated the stator behaviour at Cadia without any arbitrary adjustment of the input variables. This was used successfully on many large Siemens drives including the largest drive at Sino Iron (40 ft, 28 MW). The design tool requires input from the vendor in the form of the magnetic pull curve for each pole. The accuracy of these curves defines the accuracy of the model and it was evident from experiments at Cadia that the magnetic pull curve developed during design a motor can be substantially different from the actual curves and so a safety factor has to be included in the calculation. Despite this, the design tool has produced reasonable assessments of new drives and is considered sufficiently accurate that Siemens asked that it be used to assess the Sion Iron design as an independent design check of their modelling. The development of the design tool is the focus of this special interest topic and will be described in the next post.
* – The stiffener EAnD developed and Siemens implemented to resolve the vibration issue.
** – Siemens tried several other options before accepting the Strongback solution