SCIENCE 



[Entered at the Posi-OfBce of New York, N.Y., as Second-Class Matter. J 



A WEEKLY NEWSPAPER OF ALL THE ARTS AND SCIENCES. 



Eighth Year. 

 Vol. XVI. No. 391. 



NEW" YORK, August 1, 1890. 



Single Copies, Ten Cents. 

 $3.50 Per Year, in Advance. 



A SLOW-SPEED ELECTRIC MOTOR. 



A NEW automatic low-speed electric motor, designed by Frank 

 A. Perret, electrician of the Elektron Manufacturing Company of 

 Brooklyn, is shown in the accompanying illustrations, Fig. 1 being 

 an end view, and Fig. 2 a section showing the magnetic circuit. 

 It is of the multipolar type, and is designed to run at from 500 to 

 600 revolutions per minute. For special work the armature may 

 be wound for a speed of 350 revolutions. While the speed is com- 

 paratively low, the machine is not heavy, as is the case of many 

 motors designed for slow speed for special work. 



The practical advantages of low-speed motors are many. In 

 ordinary machine-shops, wood-working shops, and printing-ofSces, 

 for example, the shafting is commonly run at from 200 to 300 



heavy load is thrown suddenly on, as is often the case in elevator- 

 work and hoisting. 



These larger motors retain the distinctive feature of laminated 

 field-magnets which were characteristic of the smaller Perret 

 machines, as, it is claimed, the results secured would be practically 

 impossible with any other construction. The armature is a ring 

 of comparatively large diameter, with longitudinal channels on 

 its peripiiery, in which the conductors are wound. They are thus 

 embedded in the iron, which is in such close proximity to the 

 iron pole-pieces that there is practically no gap in the magnetic 

 circuit. 



The field consists of three separate magnets, arranged at equal 

 distances around the armature, each taagnet having two pole- 

 pieces, the winding being such as to produce alternate north and 



FIGS. I AND 2. — PERRET MULTIPOLAR ELECTRIC MOTOR. 



revolutions per minute; and it is a simple matter to belt directly 

 to it from a motor running at 500 or 600 revolutions, thus dis- 

 pensing with extra shafting and belting. These motors have 

 recently been applied by direct gearing to pumps and to coal-cut- 

 ting machines in mines, and also to the operation of coal-cutting 

 machines by means of rope transmission from the motor to the 

 cutter. Their slow speed also makes them well fitted for the 

 direct driving of large exhaust fans and blowers, and for operating 

 hoists and travelling cranes. In addition to the advantages of 

 low speed in the special cases mentioned, there is, of course, a 

 general advantage in the avoidance of the rapid wear and deteriora- 

 tion often connected with high speed. 



These motors are built with a 6-pole field, and with armatures 

 of large diameter. A powerful torque and great momentum of 

 armature are secured, which are decided advantages when a 



south poles. The magnets are built up of plates of soft charcoal 

 iron, which are shaped as shown in the diagram, the magnet thus 

 produced being readily wound in a lathe. A non-magnetic bolt 

 passes through a hole in each pole-piece, and the plates are 

 clamped together between washers and nuts. The bolts also 

 serve to attach the magnets to the two iron end frames, which are 

 of ring-shape, and are bolted to the bed-plates of the machine. 

 The magnetic circuit is of unusually low resistance, by reason of 

 its shape, its shortness, which is shown by the diagram, and the 

 superior quality of iron used. 



There is, it is claimed, no loss of magnetism in the frame or in 

 the shaft of the machine, as the magnets are supported at some 

 distance from the former by means of the non-magnetic bolts, and 

 the armature is mounted on the shaft by spiders of non-magnetic 

 metal. The whole machine is enclosed by a shield or case of sheet 



