May 2022 Volume 4

EQUIPMENT & TECHNOLOGY

Where Failure Begins The Joule

Causes of High Contact Resistance Incorrect Assembly

To understand how critical bolted connections are, and why anyone at all should care about them, we will return to the illustration of a bolted connection.

During assembly, it is possible to have metal burs, swarf, or other debris sandwiched between the two contact surfaces; this is especially true in field assemblies and repair work. It is also not uncommon that fasteners are left loose following a breakdown repair and reassembly. Material Creep as a Factor The materials and fasteners used in large power supplies are often aluminum (aluminum for our European friends), brass, copper, and silicon-bronze. These are malleable metals, and when used in conjunction with slotted holes; as is the case for many power circuit conductors, they are subject to material creep. The fastener washer gradually flows into the slot or deforms slightly under the pressure of the fastener head. Even with the use of certain types of lock washers, the fastener clamp pressure is now reduced. The rate of creep decreases over time but is high during the initial stage as found with new fasteners and new power supply components. Vibration The induction power supply is oftenmechanically coupled to the rest of the production line via the concrete floor. Low amplitude/ high frequency vibration from the forging process can lead to loosening of fasteners.This is a common occurrence in forging and is especially prevalent where the power supply is not physically isolated from the source of vibration. Regular Maintenance is a Simple Solution It is desirable to prevent damage to bolted connections caused by loosening, arcing, and melting. A damaged busbar or bus component usually shuts down the entire induction heater and since busbar components are custom fabricated, replacements may have extended lead times. The large electrical disconnects used on induction heaters also cost several thousand dollars and lead-times for older switches can exceed one-month or longer. A more advantageous route is to implement a maintenance program which focuses on the unique operating conditions of the equipment. Understanding Vulnerabilities Just as every installation is unique, the maintenance needs of induction equipment are unique and vary with the situation. Determining the vulnerability of the equipment and effective countermeasures are important considerations. Image 9: Illustration of washer material creep.

FIA MAGAZINE | MAY 2022 12 This kind of heating leads to rapid oxidation of the connection, which leads to arcing, then melting. When conductor melting occurs, the ensuing electric arc creates conditions in the power supply which may result in subsequent failure of multiple electrical components: so much for adding insult to injury. Image 8: Bolted connection At the point of contact between the two bars, the ideal assembly condition will have a contact resistance of a mere 0.000002 (2.0x10 6) ohms. This resistance is infinitesimally small and could be disregarded under normal circumstances. However, with the addition of surface imperfections, debris, and oxidation to the connection, we find the contact resistance can increase by hundreds or even thousands of times, creating resistances of 0.002 to 0.440 ohms. These numbers are still exceedingly small, much less than 1.0. By comparison, a hot 100-watt filament light bulb has a resistance of approximately 140 ohms. But looking now at the electrical conditions in induction heating, we can do a little bit of math and find that dismissing minuscule values of resistance is a mistake. The formula for Joules law is I2R which give us electrical energy converted to heat, in joules.The Joule is also equivalent to a more common form of energy, the watt. Using the I2R calculation will give an indication how much heating can occur at a bolted connection. Assuming conservatively, an electric current of 1000 amperes: The Joule is also equivalent to a more common form of energy, the watt. Using the I 2 R calculation will give a indication how muc heating ca occur at a bolted connection. Assuming conservatively, an electric current of 1000 amperes: Current (amperes) contact resistance (ohms) Heating (watts) 1000 0.000002 2.0 1000 0.0002 200 1000 0.02 20,000 1000 0.2 200,000 Table 1 From table 1 it becomes obvious that a very small amount of resistance results in a large amount of generated heat. With a resistance of just 0.02 ohms, a bolted connection on an induction coil can become a 20-kW heating element! Table 1 From Table 1 it becomes obvious that a very small amount of resistance results in a large amount of generated heat. With a resistance of just 0.02 ohms, a bolted connection on an induction coil can become a 20-kWheating element! This kind of heating leads to rapid oxidation of the connection, which leads to arcing, then melting. When conductor melting occurs, the ensuing electric arc creates conditions in the power supply which may result in subsequent failure of multiple electrical components: so much for adding insult to injury.

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