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).
Typed memorandum discussing design considerations for A.C. motors, particularly for crane and industrial applications.
| Identifier | Morton\M6\ img087 | |
| Date | 4th June 1928 guessed | |
| users and customers do not ask for easily renewable races, so why spend the money and time on exacting cones when there is no necessity. (4) We have hardly arrived at the stage of one A.C. voltage and one speed only. When the national scheme is completed then 400 volts, 3 phase, 50 cycles will be the standard for 75% of the motors at least, so far as crane motors are concerned we only have one speed, chiefly viz., 750 R.P.M. Synchronous speed and this we should make our standard for crane motors. For industrial motors, of which we shall certainly sell a number the speed will be 1000, 1500 and occasionally 3000 R.P.M. these latter being in frequent demand for pump driving. (5) We always put on the stator current at the same time as putting on the current to the brake magnet. (6) We cannot see any advantage, but we can see very definite disadvantage in making taper armatures or rotor cores. We prefer to keep them truly cylindrical and we can obtain all the endways pull by definitely placing them out of centre, i.e., keep them true concentrically, but place them definitely approximately 3/8 to 1/2 and inch out of centre axially, and then we should obtain all the end pressure necessary to release any plate brake against the action of the springs. We must use a spring or springs in this type of brake, and my own chief objection to the use of springs is that the brake fails should the springs break. Our armature or rotor will have no end pull or pressure when the stator current is off, and, therefore, the braking actions must be provided by some other means. When we put on the stator current then we can obtain a strong pull equaly to a solenoid pull by means of which we can release a brake. An unfortunate feature would, of course be introduced by the incorporation of a multi-plate brake actuated by movement in the rotor shaft, in that it would necessitate the establishment of 2 standard ranges of motors. This would be particularly troublesome in the cases of crane work where customers so often desire that one or more of the travelling motors should be a duplicate of the hoisting motor. (7) On all A.C. Cranes we can lower loads by supplying current to the motors, and the A.C. motor acts just the same as a shunt D.C. motor, and acts as a generator feeding back on to the line when the load is sufficiently heavy to pull the motor round. (8) We have eliminated all slipper drives from the long travelling motion of our cranes for some years past, and you obtain the same braking results on the A.C. motor as you do on the D.C. motor, if you reverse the motor and keep your controller on the first step. It is equivalent to what they term "plugging" in rolling mill practise to skid the ingots on the live rolls. (9) We have a very effective and simple device for obtaining creeping speeds on A.C. Cranes, and the same device can be equally well applied to D.C. Cranes, and occasionally is. cont... | ||