SpecMate: Hit the Brakes

A common DC-bus configuration is a good alternative if the expected amount of regenerated energy doesn't exceed the energy absorption requirements of the other common drives.

The growing discussion of regenerative braking techniques in the design of hybrid electric vehicles, where a portion of the vehicle's kinetic energy is reclaimed and stored in on-board batteries, reminds us that regenerative braking is still a widely used method for industrial machinery that has rapid and prolonged speed ramp ups and ramp downs as a normal part of its operating profile.

First, a quick review from the engineers at the Reliance Electric group at Rockwell Automation (http://www.rockwellautomation.com). When the rotor of an induction motor turns slower than the speed set by the applied frequency, the motor is transforming electrical energy into mechanical energy at the motor shaft. When the opposite is true--when the rotor turns faster than the synchronous speed set by the drive, the motor will transform mechanical energy from the shaft into electrical energy. This may result from a ramp command to stop, a reduction in the command speed, or "an overhauling load that causes the shaft speed to be greater than the synchronous speed. In any case, the condition is referred to as regeneration."

Consider a heavy-duty centrifuge that, as part of its operating sequence, rapidly speeds up and slows down or stops several times to de-water its contents and eventually discharge its contents. In each rapid motor speed reduction, regeneration causes AC power from the motor to flows backward through the DC bus to the inverter bridge diodes, which are there to convert AC building power to DC. They can't handle energy in this reverse direction. So, the energy needs a place to go or, if there's too much of it, it will trip the drives bus overloads and the motor will coast to a stop. A drive that operates at 460 VAC input has, according to Rockwell Automation, a nominal DC bus voltage of 650 VDC and is designed to tolerate up to 800 VDC. Beyond that, the energy must be dealt with.

The experts at Advance Technical Sales (http://www.advance-technical-sales.com) agree that there are three energy-absorbing methods to consider.

Dump the energy as heat through via shunt regulator dynamic brakes with energy absorption resistors. This is a series circuit placed across the DC bus and is also called a "chopper" or dynamic brake.

Other methods reuse the extra energy. Line-regenerative braking uses converter amplifiers that have the ability to change the DC-bus energy into fixed-frequency power to send back to the power line source.

Energy-conserving braking is another. By taking several other drives and tying the DC-bus connections together, the regenerative energy from one drive can be used by another that needs to convert electrical energy to mechanical energy or, simply stated, needs to speed up.

Advance Technical says a common DC-bus configuration is a good alternative if the expected amount of regenerated energy doesn't exceed the energy absorption requirements of the other common drives and are operating in a sequence where the some drives can use power when others are in a regenerating mode. If not, one of the first two methods must accompany the shared DC bus configuration.

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