Electrical news and engineering . ting current, however,such as its economy of transmission and cost have made itpreferred by most operators. Types of Motor Used The question as to what type of motor is best for cer-tain operations, is not definitely settled. It is quite usual tofind a mill where all the motors are of the squirrel cage typeand then to find others where slip rings are used even in thesmaller sizes. The tube mill is practically the only machinetliat has severe starting conditions. 120 to 130 per cent, nor-mal torque being required for starting; this is an inherentcharacteristic,
Electrical news and engineering . ting current, however,such as its economy of transmission and cost have made itpreferred by most operators. Types of Motor Used The question as to what type of motor is best for cer-tain operations, is not definitely settled. It is quite usual tofind a mill where all the motors are of the squirrel cage typeand then to find others where slip rings are used even in thesmaller sizes. The tube mill is practically the only machinetliat has severe starting conditions. 120 to 130 per cent, nor-mal torque being required for starting; this is an inherentcharacteristic, and somewhat prohibits the application ofsquirrel cage motors for the larger size. These are, how-ever, quite commonly used for tube mill drive up to the limitof 200 horse-power; in these cases they are either started froma high voltage compensator tap, or thrown direct on the linewith full voltage. Slip Ring Motors Slip ring motors are used mainly because of the possi-bility of speed variation, and because of their low starting. Motor Direct Connected to Swing Hammer Mill—Canada Cement Company the rings are not materially worn by the dust and hence thecurrent and small line disturbance. It has been found thatthe rings are not materially worn by the dust and hence the 34 THE ELECTRICAL NEWS use of the dust-proof enclosures is passing away. As men-tioned before, frequent inspection is necessary for motorsused in cement mills, and this inspection trip can serve aswell for the cleaning of the dust from the air ducts, by com-pressed air, and blowing the rings clear. The use of the compeb mill, which requires as high as500 horse-power, has had a marked effect on the type ofmotor used in the cement industry. This mill is a con-stant speed load and the synchronous motor with its unity i W^ 9 i Ki 19 Induction Motors, each 1V5 , 660 Volt, belted to tube Cement Co., Limited, Longne Pointe, Montreal. power-factor has been found to give a very satisfactorydrive; since
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