Eddy current brake

Figure 4-142. FCC string using an eddy current brake.

Figure 4-143 represents a computer-generated plot that demonstrates the effectiveness of eddy current brakes in preventing overspeed of the string. The lower curve assumes the butterfly valve characteristic is linear from 60° open to the closed position. The rate of closure is 3.25 sec. (Butterfly valves are normally used to throttle the expander inlet gas.)... 

The top curve of overspeed versus time demonstrates that the string will accelerate to 22% overspeed due to expansion of the gas trapped between the valve and expander nozzle ring. However, if an eddy current brake were part of the string, the acceleration of the string would be reduced to 11% overspeed. This would provide an important margin of safety. 

Synchronization Control. During startup, partial load is applied to the eddy current brake. The expander is then throttled to a speed below synchronous speed, and the brake voltage controller varied to lower the voltage of the coil, causing a small speed... 

Figure 4-143. Computer-generated plot showing effectiveness of eddy current brake.

Figure 4-144. Typical torque versus speed curves for eddy current brakes.

Figure 4-145. Power recovery string with eddy current brake.

Figure 4-146. Expander-driven generator can export power. Note the eddy current brake.

Figure 4-147. Elliott eddy current brake control schematic.

The eddy current brake controls for the power recovery string are designed for fail-safe operation of the eddy current brake. These controls provide all of the logic required to make the eddy current brake recovery string a viable contender for recovering power from previously wasted energy. 

The Lamboume abrader is essentially the Dunlop apparatus which used to be specified in BS 903, and which uses a wheel test piece with the degree of slip being controlled by an eddy current brake. An improved Lamboume machine is now being proposed for standardization in ISO TC 45. This instrument has both the test piece and the abradant in the form of wheels (type (c) of Figure 11.4) but both are driven at different speeds to give slip. 

The electric brake is an eddy current brake, which can simulate different aerodynamic drags, rolhng resistances, and driving cycles using a specific software developed in Labview. 

Diesel exhaust was generated by a diesel engine (1.8 1, 44kW) mounted on a test bed including an eddy current brake. For all smog chamber runs, similar test cycle conditions were used (see Wiesen (2000) for details). 

The more AND gates you use, the safer the system is. AND gates denote a fault tolerance for example, for a braking subsystem failure, both the primary brake AND the backup (emergency) eddy-current brake must fail. 

The main component of the back-drive assembly can be an eddy current brake, inverter motor or a DC motor. The Viscotherm Rotodiff hydraulic conveyor drive is a variable speed device, powered by a fixed speed hydraulic motor. 

Conventional back-drive systems on decanters perform a braking duty. As such many of them have the ability to regenerate power. Although an eddy current brake is unable to do this. AC. DC. and inverter motors, and hydraulic systems are. The braking process causes the electric motor to act as a generator and so returns power to the grid. In the case of the DC and inverter motor, the power regenerated is usually considered dirty unless electric filters are fitted which smooth out unwanted harmonics. 

Early decanters using eddy current brakes were not able to indicate conveyor torque continuously. A reading of brake speed, and another of brake current, had to be recorded and used when referring to a brake calibration chart, to obtain the brake torque. Conveyor torque could be obtained by multiplying this figure by the gearbox ratio. A typical eddy current brake calibration chart is shown in Figure 7.1. 

Figure 7.1. A typical Eddy Current Brake calibration.

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