Variable speed drives are widely used in heavy-duty applications such as stone crushers, heavy-duty mixers, and pumps. These machines often have several hundreds of kilowatts up to several megawatts of power, and require special measures to counteract bearing currents, including isolated bearings and current and voltage filters at the inverter output. A type of leakage current known as circulating bearing current can cause electrical corrosion and damage to the bearings of these machines.

Working principle

Inverter-fed asynchronous induction machines with a shaft height greater than 300mm may exhibit harmful circulating bearing currents at low rotor speeds. The common-mode voltage of the 2-Level voltage source inverter drives a common-mode current via the motor cable, the motor itself, and back to the inverter. A portion of this common-mode current flows via magnetic coupling as circulating bearing current in the machine. It is not the rated power that matters, but the mechanical size of the machine.

Damaging effects

The electrical current that flows through the mechanical bearings of the machine can damage the bearing raceway surface. This can cause the rolling elements to jump over the unsmooth surface, resulting in noise and possible mechanical failure of the bearings. An electrical damaged bearing raceway surface looks like a washboard pattern or fluting.

The Solution

Our Blueferrite nanocrystalline cores have a high impedance to common-mode currents. By mounting the cores at the inverter output over the three motor phases U, V, W, they can reduce the common-mode currents and therefore also the circulating bearing currents to a harmless level. The core sizes, quantity of cores, and magnetic properties must be adapted to the specific application, which mainly depends on the rated power and motor cable length. Our selection guide and engineering team can help you find the right cores for your application.

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