also safe to assume that in the list of motors taken, a large per cent of the motors were controlled with the manual controllers. I say this because it has not been common practise until recently to put automatic controllers on motors of 50 h.p. or under. With automatic control on all of the motors in the table above, the number of new armature coils required could have been greatly reduced, if not eliminated.

There is also the question of the electric brake that enters into the question of repairs on a motor. The d-c. brake is a much simpler one than the a-c. brake. A long-stroke plunger can be used, and this gives plenty of leeway and clearance in designing the brake. The d-c. brake consists of a steel casting, a winding and a steel plunger. The a-c. brake has a short-stroke plunger and is made up of laminated pieces, which chatter and easily get out of order.

The d-c. motor is the only right motor for auxiliary drive. The d-c. motor has the advantage over the a-c. motor of speed control and dynamic braking, and the most important advantage of all in that it will lift above its capacity until it burns out. This characteristic of the d-c. motor is most important in steel mill work where it is often cheaper to burn out an armature rather than wreck a more expensive machine or kill a man.

A. G. Ahrens: In connection with Mr. Treat's criticism of the commutating pole motor, that it is not able to stand heavy overloads of torque, I do not think Mr. Treat had in mind the mill motor. Mr. Lanier pointed out that the mill motor on ordinary loads is under-commutated, so to speak, so that on extreme overloads it is found that it commutates at its best. I have seen mill motors under a test with the special object of obtaining data as to their commutating ability, and I remember one test in which the motor was rated at 25 h.p., mill rating, which is equivalent to an armature current of 113 amperes, and under 400 amperes load, which is equivalent to over 350 per cent of normal load, that motor was sparking slightly, a condition of commutation which would have been called good on any industrial motor.

Brent Wiley: In regard to Mr. Whiting's question concerning reduced voltage, this subject has been considered from the standpoint of average conditions for motors operating with widely and rapidly fluctuating loads. This average condition is assumed to be such that the average voltage during the entire day, where 24hour service is required, is one-half normal line voltage. For example, if the normal line voltage is 230 volts, the average voltage is figured at 115 volts for a cycle in which the motor is operating at full voltage approximately 40 per cent of the total time; and the heating of the motor is calculated on the basis of the equivalent continuous current at this reduced voltage. Operation with reduced voltage at armature terminals, due to insertion of resistance in series, for a greater percentage of time, would give equivalent results.

For a large majority of the applications for which the mill motor is particularly suitable, it is practically impossible to predetermine the exact cycle of operation, including time and load. It has been determined, however, that by averaging the data. and conditions for various installations in steel mills, the actual operating period of the motor is approximately 40 per cent of the total period, and the table of ratings given has been developed on the basis of average voltage at the motor terminals equal to one-half normal line voltage.

It would be of advantage to take the motor characteristics into consideration when calculating the heating effect of a varying load for those cases where the load curve can be predetermined accurately; and further investigation of this point would be of value. It is questionable, however, if the attempt to apply such a close theoretical analysis would be of practical value for the general application of mill type motors.

Mr. Treat has questioned the ability of the commutating-pole mill motor to meet successfully the severe conditions of steel mill work.

The particular function of the commutating pole feature is to give better commutation over a wider range of operating conditions than can be obtained by the non-commutating pole motor. With the conditions to be met well established, there are no reasons why the proper commutating pole features cannot be included and better results obtained. It is true that, until a comparatively recent period, the theory of commutating pole design was not well established, and its application to motor design was therefore somewhat limited; but the unqualified success of the commutating pole railway motor is a forceful demonstration that for even severe, intermittent and widely varying load conditions, commutating poles are of great advantage.

Mr. Treat's criticism of the commutating pole motor for use in heavy-duty reversing service seems to be based on some particular design. It has come to be recognized that motors must be designed especially for this service, electrically as well as mechanically. No one conversant with conditions would apply a motor in this service having the same mechanical design as a motor suitable for, say, printing press drive. It is important to have a liberal electrical design and the use of the commutating pole permits this without going to proportions of armature that would make the machine excessively large. It is perfectly practicable to so proportion motors of the largest sizes required in this service that they will commutate the heavy overloads sparklessly, at the same time giving sparkless commutation on full load and lighter loads.

The time lag referred to between the current inrush and the building up of the flux is very much less than might be supposed, since the ampere-turns on the pole are ample to force the flux, not only through the pole, but also through the gap. As the ampereturns required for the gap are many times those required for the

iron part of the circuit, there is a very high m.m.f. forcing the rapid building up of the flux.

Results actually secured with commutating pole motors in this service show that no injurious results follow from this very slight time lag.

There are many applications of the commutating-pole mill motor being made on the mill machinery referred to by Mr. Treat, and from the preliminary tests that have been made, improved commutating conditions, as compared with those obtained with the older types of non-commutating pole motors, can be assured.

The reference which was made in my paper to improvements in control apparatus has a more significant meaning than has been brought out in the discussion. The point is that these improved conditions make it possible in many cases to increase the working capacity of the motor by the use of commutating poles. The function of series relays and series switches is to limit the accelerating and braking current to a predetermined amount. In the majority of applications, rapid acceleration and retardation are desirable-limited, however, to such values as are necessary to protect machinery and motor. With commutating pole motors these values of the current will be more dependent on the limits imposed by the machinery rather than by the motor. As the commutation limit has been raised, it means that the working capacity of the motor has been raised. It becomes more a question of heating limitations and, as stated previously, fireproof windings permit a much higher safe rise of temperature than can be obtained with the older types of motors.

Regarding the question of the relative merits of the alternating-current and the direct-current mill motors, as mentioned by Mr. Treat and Mr. Fishback, this is a very broad subject and it is not within the scope of this paper to give the various points proper discussion. There is no doubt that, for the most severe service, such as screw drives and reversing tables, the directcurrent series motor has more advantageous characteristics. This is equally true of the hoist motion of cranes; but the question whether one type of motor or the other should be used should not be answered on this basis alone. With the increased attention which is being given the question of economies, there is good reason to believe that the application of the alternating-current mill motor will be made in accordance with the saving that it will insure. Much progress has been made regarding the design of an alternating-current mill motor with suitable features for this severe duty, and a careful study of the gradual applications by the designing and the field engineer will insure further progress in the successful application of this type of motor.

American Institute of Electrical Engineers,
Pittsburgh, Pa., April 25, 1912.

Copyright, 1912. By A. I. E. E.

ELECTRIC BRAKING OF INDUCTION MOTORS

BY H. C. SPECHT

For various classes of work-for cranes, hoists, rolling mills, etc.-it is generally required that means be provided for stopping the motor quickly. When the electric power supply to the motor is taken off, the motor speed will gradually slow down, due to the friction load; however, the time required to bring the motor to a standstill by this method is generally too long, and other means have to be applied, stopping by brakes mechanically, electrically, or hand-operated, electric braking by alternating or direct current, etc.

The object of this paper is to discuss electric braking by alternating current and by direct current. If the braking is to be effected by alternating current, it is necessary to reverse the rotating field in the motor. This is ordinarily done on a three-phase motor by reversing two of the primary leads, and on a two-phase motor by reversing the two primary leads of one phase. At the moment of reversal, if the motor is running very near synchronous speed, a frequency is obtained in the secondary approximately twice that of the primary, because the secondary frequency is equal to primary frequency multiplied by the slip, which in this case is approximately two. Further, the secondary voltage changes in the same ratio as the secondary frequency, therefore, if the secondary is running with double frequency, the secondary voltage will be twice the voltage at standstill. This necessitates either that the secondary winding be insulated for the double voltage or that only half voltage be supplied to the primary for braking. On small machines the secondary voltage at standstill is so low that the insulation is strong enough

to withstand the double voltage under the required operating conditions.

On large slip ring motors, however, the secondary has to be designed for rather high voltage, in order to avoid too high currents. A higher voltage requires more insulation, while a greater current requires larger and more expensive switches, heavier leads and larger collector, etc. In order to meet the best conditions, it is necessary to make a fair compromise between the above advantages and disadvantages.

If the rotor is star-wound, the voltage to ground can be reduced by grounding the neutral point, thus allowing a decrease of insulation to ground. However, the insulation between phases cannot be reduced, because the voltage between phases does not change by grounding the neutral point.

On very large motors, of several thousand horse power, it sometimes becomes more desirable to apply half voltage to the primary or to use direct current for braking. In the case of application of half voltage to the primary, auto-transformers and extra switches are required, which entails some extra expense and complication. The braking by direct current may be accomplished by connecting the direct-current supply circuit to the primary as shown in Figs. 1a, 1b, 1c, to 1h.

The connections of Fig. 1a for three-phase star, 1d for threephase delta and 1g for two-phase, are generally used on account of their permitting the simplest switching. By using some of the other connections, a little can be gained in regard to the field form and to the least power requirement.

From the preceding, it is to be noted that there are various