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SRD has great potential for mining applications.

Switched Reluctance Drives (SRD) are relative newcomers to the field of variable speed drives. However, now that they are available, British Jeffrey Diamond (BJD) sees great potential throughout the mining industry. Among the SRD's main features are that the rotor has no bars or windings, and little heat is generated in the rotor. It is a low inertia rotor which reduces mechanical problems if the drive is suddenly stalled, but the SRD can remain in a stalled condition for long periods without damage. It offers high starting and accelerating torques with extremely low current demand, and no limit on the number of starts per hour. This provides the basis for a very flexible drive. SRD is suitable for operation direct from a 1,100 V supply with no transformer. There is a high power factor throughout the speed range and the SRD's high efficiency, higher than any other variable speed system, reduces the cooling requirement. Finally its wide speed range is better than 20:1.

BJD has developed a switched reluctance variable speed drive for a shearer haulage application. The 35 kW continuously rated, water-cooled motor has speed variation from 0 to 1,500 rev/min and overspeed at rates up to 2,000 rev/min.

Tests carried out by running the shearer into a solid stop show that after 1,000 successive tests no deterioration was found in the drive or gearbox. The ability of the motor to remain in stall conditions for long periods without damage to it or the power electronics is an important characteristic for shearer applications where obstructions in the track can occur at any time. The absence of commutator and brushes has eliminated the problem, associated with totally enclosed flameproof dc motors, of the build-up of carbon and the degradation of insulation.

BJD is developing SRDs for general use in hazardous areas. Two development drives are 150 kW and 300 kW. Both are water-cooled and suitable for operation from a 1,100 V, three-phase, 50 Hz supply. Both motor and controller are flameproof.

One of the major problems with AFC drives for coal mining is to start up a fully loaded conveyor. The common use of two-speed induction motors started direct on line on slow speed and then switched to high speed causes severe current demands with voltage reduction at the motor and resultant drop in motor torque. In some cases conveyors fail to start up and trip out the electrical supply. Dedicated supply systems designed to reduce voltage drop have to be employed, in some cases resulting in additional cost.

The SRD is particularly suited to producing high starting and accelerating torques and eliminating shock loading on the conveyor system during start-up with extremely low current demand from the AC supply. The low inertia of the SR motor rotor is another important feature. The load on the AFC can vary considerably and is very 'peaky' in nature. Jamming of the AFC is not uncommon, allowing the inertia energy of the motor rotor to be transmitted to the AFC gearbox and chain. This reduction in inertia will have a beneficial effect on the gearbox and AFC chain load. The low inertia characteristic will also help to reduce the over-run time when stopping the AFC.

It is possible to vary the acceleration rate and torque output to suit applications. For example for belt conveyors where load inertia may be substantial, a reduction in acceleration and output torque can be beneficial, resulting in less load on the belt and the elimination of the fluid coupling.

In order to show in practice the features of the SRD for AFC use, BJD has arranged a demonstration conveyor 16 m long driven by two 35 kW SR motors via 18:1 gearboxes. The diagram below shows the demonstration motor full output speed torque curve and the programmed speed torque curve to suit the conveyor demonstration.

BJD, Thornes Moor Rd., Wakefield WF2 8PT, England. Ph: +44 924 375133, Fx: 361740.

Circle RSC No. 201
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Title Annotation:switched reluctance drives from British Jeffrey Diamond
Publication:Mining Magazine
Date:May 1, 1992
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