From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical discussion on the design and operation of motors for cranes, comparing AC and DC systems.
| Identifier | Morton\M6\ img089 | |
| Date | 4th June 1928 guessed | |
| hand applied brake strap or shoes. We first of all apply force to the hand lever operating this brake, then we put the controller handle on the first step afterwards decreasing the force applied until the requisite hoisting or lowering speed is obtained. Such creeping speed devices are in satisfactory operation on foundry cranes, and particularly on cranes making the usual domestic Cast Iron baths. I do not think the Motor Generator Set and the added Control Gear would appeal to Foundry Owners. (10) We have in many instances, fitted Motor Generator Sets to cranes, and even on Direct Current cranes. Such sets have usually been of sizes up to 10 K.W. for the purpose of supplying current to lifting magnets, and they have been mounted on a short length of platform on the opposite side of the crane to the long drive platform, and the control gear has been fitted into the crane cage. Where there is already an alternating current supply available customers will occasionally put down a Motor Generator Set, but they will not as a rule give a second thought to placing a small set on each crane. An equally effective, and a more economical arrangement is to use a mercury Arc rectifier, which has no moving parts, and which will give you an efficiency up to 93% at quarter loads. (11) Whilst still considering the question of A.C. and D.C. Cranes, we are faced with two very different problems in regard to the hoisting motors. With D.C. Cranes we usually use a hoisting motor having a full load speed of about 450/500 R.P.M. and on a 5-Ton Crane with two gear reductions we obtain a full load hoisting speed of 30 feet per minute. Our speed of lifting and lowering the light hook is normally 3 1/2 to 4 times this speed, or about 100/120 feet per minute. With an A.C. Motor running at 700 R.P.M. we have to use 3 gear reduction to get down to the 30 Feet per minute, and, therefore, you will realise that after making both the motors strictly interchangeable as regards bolt holes and shaft ends, we have not achieved anything very great. Our A.C. Crab with 3 gear reduction is always more expensive than the D.C. Crab with two reductions. If we put the D.C. motor up to a full load speed of 700 R.P.M. we shall have our pinion rattling round at 2500/2800 R.P.M. and still be compelled to use the expensive crab. On the other hand, if we decide to use the cheaper two gear reduction crab we shall have to use 12 pole A.C. motors, which will have a very low power factor. (12) You will see from the designs of A.C. Motors we are sending herewith that we propose to take the end thrust in one direction on the main bearing and in the opposite direction on the minor bearing. This method of construction is cheap and simple, and it is not necessary to dismantle the bearing in order to take the armature or rotor out. (13) We are proposing to make our slip rings from stainless steel pressed on to sleeves insulated with mica. | ||