Document 55bRMnX3zxgbRXj9xE9pdrmD

1 J I j f ,I ( shou some oj the ad/u stable rultage control equipment t;/i i;; J/, il hr litliottjor each bridge. .1/ right is the control Jor the generators, which is located in the totallr-eaclo sett motor-generator set hnuw\ {tielnh ) is n portion oj the motor eoritrnl oil of which is located in the holler cab. ulnae the operator's cab. Panels shown set re ore bucket motors. with three master switchers and three font-operated pushbuttons. The switches control all motions of trolley, bucket, and bridge. One of the push buttons chanties the output of the two hoistin'; generators to the pier and shear let; trucks, while simul taneously releasin'; the wheel clamps. The other two pushbuttons control the trolley rails sanding equipment and the trolley brake. Adjustable voltage control The new Ohio Works ore bridges redect the trend toward d-c adjustable voltage operation. This type of elec trical system, under the proper cir cumstances, offers several significant advantages over the more traditional d-c constant potential method. In essence, the constant potential system powers the bridge from 250volt d-c generating equipment in stalled at the mill. This power supply is picked up by collectors mounted on the bridge and fed to 230-volt d-c motors which operate the various bridge motions. The largest standard mill motors presently available are rated at 200-hp continuous capacity at this voltage. Hence, the maximum ore-handling capacity of the bridge is limited, unless multiple motor drives are employed with consequent increase in bridge structural weight and cost. On the other hand, the adjustable voltage system employs a 2300-volt, 25-cycle a-c power supply, picked up by trolleys on the bridge and carried to the ac-to-dc motor-gen erator set mounted on the bridge pier structure. The generators in turn sup ply adjustable voltage d-c to the ap propriate drive motors. By operating the motors at double-rated voltage and double speed, their shaft horse power can be effectively doubled. Thus, using standard 600-series mill motors, much greater bridge ca pacity can be obtained without in crease in the size or number of the bridge motors. In the case of each the Ohio Works bridges, about I percent more capacity was achievt than would have been possible wi a constant potential system. In addition to the increased bridr capacity which is obtained thrum the adjustable voltage system, oth advantages suggest its use. For e ample, the increased bridge capaci is possible with no increase in exit ing plant d-c capacity. Or, as in tl case at the Ohio Works, conversit to adjustable voltage bridge ope ation reduces the load on preset plant d-c equipment, appreciab. increasing its service life. Further, the adjustable voltas system requires less maintenance, does not require frequent operatio of many large contactors. And tf inherently smooth acceleration i drive motors possible with adjustab' voltage does not impose shock loac on the mechanical partsand structun Speed control is improved, an increased hoisting speeds are possibh both tangible operating advantage Finally, over-all power consumptio is appreciably less than that of th constant potential system which d< pends upon dissipating unneedc energy in resistance. At the Ohi Works, all of these advantages wci obtained at only about a 9 percet increase in first cost over the cost t ncwconstant-potcntialdrivcn bridge 14 POWERFAJt, SUMMER 19!