The principle of an electrical machine is that the conduction of electric current is the basic condition for its operation. In order to conduct this current continuously through the various coils, there are electrical connections that connect one coil to another, both in the rotor and in the stator. These connections are made by soldering or brazing.
In brazing, the connection established between two metallic elements is made by means of an addition metal, which is in the liquid state and whose fusion temperature is higher than 450ºC and lower than the fusion temperature of the base materials, and the base metals do not participate in the constitution of the joint.
Welding, on the other hand, is a weak brazing, in which the melting temperature of the filler metal is below 450ºC. Normally, the filler metal penetrates by capillarity between the surfaces of the parts to be joined. Uniform heating of the surfaces should be promoted, because with the heating, the base material molecules get farther apart and the filler metal in liquid state penetrates between these molecules.
The goal to be achieved in the connections is that in this bridge, which connects the coils, there is no bad contact generating a high ohmic resistance value and consequent heat losses and Joule effect. The mechanical part is also important, because in addition to the Joule effect (electrical losses) there are centrifugal forces and thermal movements that generate stress in these connections, reasons that cause wear and malfunctions. Predictive and preventive maintenance is essential to identify the integrity of these connections and thus avoid unscheduled failures in them.
As an illustration, in an 8 160 hp, 13 800 V, 1 800 rpm motor, the client had a history of failures in the damping cage and requested a detailed inspection of the equipment for preventive evaluation and intervention, considering the need for high reliability of the equipment for operation. Through tests on the rotor circuit we concluded that the connections were intact, which enabled the return in operation, restoring peace of mind to the customer.
winding rotor connections
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| Inspection of electrical connections of a high voltage wound rotor | Wound rotor test to evaluate electrical connections |
In another case, a 66 760 kVA, 13 800 V, 257.14 rpm hydro generator, a partial winding was performed on the stator winding with bar change. Since this generator had already failed due to problems in the brazing of the series connections of the bars, as a preventive measure, a general evaluation of all the connections was made before releasing the machine for operation.
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| Brazing process on electrical connection | Test to evaluate problems in the brazing of serial connections of the bars |
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