May 2022 Volume 4

EQUIPMENT & TECHNOLOGY

Bolted Connections and YOUR Induction Heater By Josh Carney

When I say power supply, what is the first thing that comes to mind? Some may imagine a mobile phone wall charger. Others may think of a 4,000-kilowatt induction heater. Well, if you are reading this, you possibly thought of both.

Delivering currents in the kilo-amp range requires conductors with large cross-sectional area to minimize heating and eddy current losses.The conductors are usually liquid-cooled and take the form of busbars which are fabricated and bolted together to connect various components forming a circuit.

Image 1: Power supplies come in many sizes and are all around us. Photo courtesy of Integral Power Technologies, LLC. The first and notably smaller power supplies have the significant advantage of being able to conduct all those little electrons mostly along paths of printed circuits interlaced with soldered connections. This yields a high degree of reliability thus tolerating physical and thermal stress without loss of mechanical integrity. When it comes to an induction power supply, the power circuits cannot be confined to circuit boards as in a phone charger. These circuits are replaced with large diameter copper cables, busbars, water-cooled bus tubing and bolted connections abounding. It is these bolted connections and their importance which is the focus of this article- as those little fasteners which join a power circuit together can be the cause for production bliss or significant monetary loss through parts/service costs and downtime. Bolted Connections and Their Importance in an Induction Heater To accomplish the high rate of heating required in hot forging, induction power supplies must command electrical currents in the range of several thousand amperes. To draw a comparison: • A household LED light bulb uses 0.1-0.2 amps • Stick welding process output 85-250 amps • Car engine starter motor uses 250-800 amps • Induction heater (parallel tuned load) outputs 800-10,000 amps, depending on conditions

Image 2: Example bolted connection busbar. Photo courtesy of Integral Power Technologies, LLC.

Image 3: Interface between bubars In Image 3, the interface between the two busbars is noted: this is the point of contact between the surfaces of the bus conductors. The bars are bolted together to provide adequate pressure which keeps the contact surfaces tight. If the connection is allowed to come loose, the contact resistance between the surfaces will increase which leads to local I2R, or ‘Joule’ heating. We will briefly examine three examples of bolted connection failures then look at why bolted hardware needs periodic attention.

FIA MAGAZINE | MAY 2022 10