John Bird looks at the processes involved in delivering a fully refurbished commutator
Several industries - including mining, steel, rail and power generation - rely on large motors equipped with commutators to produce the power required by their applications. However, without precision maintenance, these vital components can become inefficient and eventually fail.
The first, and possibly most important step, is communication with the customer to ensure that all the necessary information has been received; especially the history of the commutator, which will have a bearing on the type of refurbishment to be carried out. The best way to illustrate the process is using an example that was completed by Sulzer’s engineers recently.
Initially the customer sent over a set of drawings and asked for a quote to reinsulate a 1.9m diameter commutator that was used in the steel industry. This is quite a common task and involves dismantling the component, cleaning and then reusing the original copper plates with new mica insulation.
The in-house commutator production facility at Sulzer's Birmingham Service Centre is able to refurbish almost any commutator or slip ring assembly and Sulzer is the only facility capable of building new Pollock commutators. The design capability combined with machining expertise and supported by a dedicated team of experienced engineers enables Sulzer to deliver commutator and slip ring projects to budget and on time.
To reinsulate or not?
When the job arrived in the workshop, a previous Sulzer job number was noticed on the component and the records showed that it had already been reinsulated once before.
Sulzer recommends that commutators are only reinsulated once before being rebuilt with new copper. This is because the job requires an amount of copper to be machined off the plates and if this is done more than once it can weaken the plate and may cause cracking.
Based on the information available, Sulzer advised the customer that it would be better to completely refurbish the commutator, including the manufacture of new plates. Following this advice, the customer agreed and the project got underway.
The next step was to measure all of the dimensions and compare them to the original drawings and with Sulzer's own drawings from the previous refurbishment job. This commutator consisted of 816 copper bars and 816 mica segments that separate the copper. Overall the component weighed in at close to 4 tonnes.
For every job a jig is required to arrange the copper bars and mica segments in the correct shape. Since this particular commutator had been previously repaired by Sulzer, the correct jig was already available. In the more extreme cases a brand new jig has to be manufactured by the Sulzer workshop, such as the one that was created for a 3.5 metre commutator which contained 2,400 copper bars.
Once all the measurements have been recorded, the original copper is stripped out and the steel frame is cleaned and primed ready for the new copper bars that are manufactured in house.
This involves starting with a flat rectangular piece of copper and milling an angle onto one surface so that the plate is very slightly wedge-shaped. The angle is very carefully calculated so that when all the segments are assembled they form a perfect circle. Therefore, the larger the diameter of the commutator, the smaller the angle on the plate.
The copper plates are then assembled, with the mica sheets between each plate for insulation, into the prepared jig, which consists of a number of sections that are all bolted together. At this point it is crucial to be certain that the correct number of bars have been assembled and so they are counted and recounted, just to be sure.
The process of consolidation involves the bolts of the jig being tightened so the sections are drawn together compressing the copper bars and mica sheets together. This starts with the bolts being torqued to an initial value before the complete assembly is placed in a stove to soften and consolidate the mica. Once out of the stove the bolts are tightened again and the process repeated until the required bolt torque is achieved.
With the mica consolidated, the entire jig assembly is mounted in a large lathe to allow the machining process to be completed. This is where the rectangular plates are machined to the exact shape and dimensions required by the drawings. In essence, two 'V' profiles are machined at each end of the bars to allow them to be located in the steel frame that has been previously stripped and cleaned.
While the machining is being completed, the steel frame is reinsulated to allow the new commutator to be positioned on the bottom section before the top section is added and bolted down. At this point the run-out is measured to ensure that the commutator will run true when it is reinstalled. On this particular component, the specification was for a maximum run-out of 30 thousandths of an inch or 0.76mm over the 1.9m diameter.
The bolts are then tightened progressively and the entire component is replaced in the stove before the bolt torque is increased again. This process is repeated until the steel frame holds the copper in place and the jig can be released.
Once the commutator has been reassembled and checked it moves to the testing bay where high voltage tests, insulation resistance tests and bar to bar resistance tests are completed. With a clean test sheet, the commutator is placed back on the lathe for the final machining process to produce the finished external diameter. All of the electrical tests are then repeated to ensure complete integrity of the insulation after which the component is packaged and shipped back to the customer.
John Bird is Commutator Supervisor at Sulzer's Birmingham Service Centre, UK.