February 13, 1930. The Model Engineer and Practical Electrician.
PRACTICAL
PRACTICAL LETTERS from OUR READERS
35
Building Rotor for "The Model Engineer A.C. 1 h.p. Motor. DEAR SIR, In your issue of January 30 I notice a suggestion from one of your readers for building up the rotor of the 1 h.p. A.C. motor for which I contributed some notes recently. 'This method of brazing the rotor bars to a copper ring, either internally or externally, is never likely to result in such a sound electrical job as the way described in the original article. It is admittedly easier, but all who are out to make a really good job of the motor should beware of making any modi- fications of this nature, even though they may be easier to carry out. The performance characteristics of a squirrel-cage induction motor are so inseparably bound up with the question of rotor resistance that one has to be extremely careful over the question of " joint resistance" between rotor bars and end rings. If the maker of the motor in question should not feel competent to braze up his rotor with the bars through the end rings, the best alternative is to mill a 3/16th-in. slot across the two ends of each rotor bar to a depth of in. and drive copper end rings into these slots, brazing up the lot solidly after- wards. To render the method described by your correspondent satisfactory it would be necessary to alter the shape of the rotor bars to the section of an inverted letter -, utilising the flat at the bottom for brazing to the rings. Mechanical strength in these jobs is quite a secondary matter in comparison with electrical conductivity and a high-resistance rotor will give an unsatisfactory speed characteristic when the machine is on foad.-Yours, etc., 16 A. H. AVERY. Making Small Electric Motors Self-starting. DEAR SIR-Reference to the letter in THE MODEL ENGINEER issue dated January 23, page 96, by F. Helsmoortel, of Antwerp. I wonder if your corre spondent has tried sloping the slots in his armatures? Some time ago I had an order for a quantity of the Edison" type of model boat motor for 4-6 volts, one of the stipulations being that they must be self-starting. I designed these motors with eight- section drum armatures running in field magnets as per rough sketch (not reproduced), the tunnels being properly bored out (I have a few of the cast- ings still on hand). When mounting the stampings, I made the slots form a slight spiral, not straight, and parallel with the shaft. I found that about 1/16th of the circumference about correct, i.e., the tooth at one end of the armature being in the posi- tion of its next door slot at the other end of the armature. Both discs " and cylindrical commuta- tors were tried, but results were about equal, and, owing to restricted space, disc commutators were finally decided upon. It is taken for granted that all likely spots for friction received attention by Mr. Helsmoortel? I offer the above in case Mr. Helsmoortel has not tried this, and should be decide to do so, I should like to hear results.-- Yours faithfully, GEO. C. SMITH. DEAR SIR, the letter Replying to of Mr. F. Helsmoortel in your issue 167 of January 23, it is probable that the trouble of his small motors failing to start is due to the armature slots being straight, i.e., parallel to the centre line of the spindle, and would be over- come by staggering the slots in the manner shown in the accompanying sketch. Many eight-pole stampings for models have very large slot openings compared with the tooth width and are apt to lock. Should the above fail to effect a remedy, try one of the following (a) rock brushes slightly backwards, E Diagram showing Arrangement of Armature Stamping. (b) increase the air-gap between the armature and poles, or (c) put twice as many commutator seg- ments as slots and wind two coils per slot.-Yours faithfully, S. F. PHILPOTT, A.M.I.E.E. DEAR SIR, In reply to your Antwerp corre- spondent's letter on above in your issue of January 23 last if nine divisions (or some multiple of three) instead of the eight were used, the motors ought to be self-starting. Yours faithfully, CECIL R. F. EUSTACE. Magnalium For Engine Pistons, etc. DEAR SIR, Re M. Suzor's letter some time ago concerning the metal of his piston of his famous racing petrol engine. I think magnalium is the same as the electron he refers to, and should suit his engine. This is an alloy composed of magnesium and aluminium. It is lighter than aluminium, and is a stronger and tougher metal, its tensile strength ranging from 14 to 21 tons per sq. in., according to its composition. Good, sound, clean castings can be made from it, while it works well in rolling, filing or turning. Further, it can be soldered, The forged and welded. melting point ranges from 1,184° Fah. to, 1,4280 the Fah., according to alloy. It is a good.com- ductor of heat, the specific heat being 0.2185° Fah. The alloy can be turned and faced at the same speed as brass. The tools should be keen edged to do good work, and will work beter if kept lubri- cated with turpentine, petrol or vaseline. The alloy takes a high polish, silvery-white, with rotten stone, tripoli, or rouge, and will retain its polish. Yours respectfully, The 3-in. Scale Locomotive by Mr. G. G. Buick referred to on page 143 last week's, issue. Cylinders 2 ins. by 4 ins.; boiler pressure 200 lbs.; dia. drivers 12 ins.; bogies 6 ins.; length 7 ft. 8 ins. EDWARD F. EVANS.
