Petrol-Electric Transmission for Road Vehicles.
Page 8
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Reply to the Discussion on Messrs. Hart 8/ Durtnall's paper.
The Ilart-llurtnall system is based on experience in carrying out large, eiectrical-power work in some of the largest factories in this country. It must be remembered that buses have to run about 16 hours a day, and, if the dynamo and motor are cut fine, to get the weight down, it would be impossible to run these continuous-current dynamos and motors without getting them exceedingly hot, and the possibility of the soldered connections at the commutator coming undone is very evident. As the HartDuitua11 system is on direct, mechanical drive for the major portion of the time the bus is running, it most be evident that, although the generator and motor are cut as low as is practically consistent with economy, the machines are cooling down all the time the bus is on direct drive.
Mr. Hopps says that the chief advantage, which continuouscurrent motors have over alternating-current motors, is the higher torque per ampere which can, undoubtedly, be got from a series-wound, continuous-current motor. But, as he says, the polyphase, induction motor has a characteristic very similar to that of a direct-current, shunt-wound motor. This is so, and, for further evidence of our knowledge in that respect, we will quote an authority, Professor Silvanus P. Thompson:— " Therefore, by raising the stator pressure, at the moment of starting, we are enabled to increase the starting torque of a three-phase motor far beyond that of a shunt-wound motor, and approximating to the starting torque of a series motor. This raising of the stator pressure, at starting, may, easily and economically, be accomplished by one of the following ways: by altering the pressure actnally sup plied to the motor . ."
We hope that makes it clear that sufficient starting torque can be, and is, provided for all ordinary purposes.
Our chassis, loaded with iron to a total weight of chassis, and load, of 6 tons, 5 cwt., can start, quite easily, silently, and very satisfactorily, up a grade of 1 in 12, on ordinary macadam road. By measuring the current at the moment of starting, we found that the torque required was about 290 footpounds ; the current in Our stator, at the moment of starting, was 210 amperes and we had 9a amperes to spare. You see we can get 432 foot-pounds starting torque, with about 300 am• pares in the stator ; it will be evident, therefore, that we still have something in hand, as regards starting torque, to deal with any extraordinarily rough roads. As soon as the bus had started, the current fell to 135 amperes, and this took place in about three to four seconds from the time the driver put his switch on No. 0 contact, with the engine running at 800 r.p.m. and it will perhaps, surprise you that the engine did not stop. It will be evident that, should the engine attempt to slow down to, say, 300 r.p.m., the exciter loses its field, and, as the excitation falls; SO will the magnetic clutch slip. It maybe of some interest to know that, as an experiment, we, purposely, left the exciter on the clutch, and, then, stopped the engine. \Ve re-started the engine again, and the magnetic clutch started the chassis, without any serious damage. Of course, we do not advocate this rather barbarous method of starting, but this operation can he performed without trouble. It was stated that, itt order to increase the torque limit of the induction motor, it is necessary to increase the weight of the same, We would point (Int that the raising of the voltage, momentarily, in the way we du, is, also, a good way to get over the apparent difficulty. Mr. Hopps' comparison of our system to the general adeption of the continuous-current motor for electric traction is not correct, as three-phase, alternating-current traction is quite common in Switzerland and Germany, but the real reason of its not being so popular as the continuous-current system is because of the fact that two overhead lines are required, instead of the single one in the continuous-current method, generally used. Engineers can quite understand that considerable trouble would be experienced in arranging the two overhead lines, especially when at a centre, say, like Hammersmith Broadway_ It will, possibly, be observed that the number of conductors, between our generators and our motors, does not matter, and, further, that the distance is only about 24 inches ; the argument, in that direction, must, therefore, in our opinion, fail. The very Iciest line, the one which is working so satisfactorily in the Simplon Tunnel, is a three-phase current railway_ The continuous current with its larger torque was not adopted, we daresay, for the same reason that we have adopted the polyphase-inductien motor on our bus, because sufficient starting torque can be obtained, and because of the great simplicity of construction, and the low cost of -mailitenatme,
e nolice that Mr. Hopps said "one of the least of the troubles of the continuous-current, electric-trernway engineer is due to eommutettirs and brush gear :" we know a few electric-tramway engineers who would be very glad to eispense with this really necessary evil, but we should draw your attention to the fact that electric trams run on very smooth roads, and the effect of sparking is not so much as it will be found when rnotorbusee are equipped with, say, 4tthp., continuous-current generatols, and motors.
After studying our diagram, he stated that we have two separate, and irmepenaent, winnings. tie is perfect:1y correct, and we are aware thr:,t a polypnase, induction motor can have its poles vared with one winding on the stator. We have tried Leis method, and, although a very good running power can be obtained, it is very inefficient, when connected with the smallest number of poles, as regards starting torque. isy putting two separate, and distinct, windings on the stator, the total diatneter of the machine is increased by one inch, which all engineers will agree, is not a very large amount. It is, however, not detrimental to the satisfactory application of that simple piece of apparatus, the polyphase, induction motor, for motorbus propulsion. Our field excitation is by hand regulation, although we can, very easily, make it automatic; we are of opinion that, by the simple method which we have adopted, in connection with the little, continuous-current exciter, with the switch arranged for on the steering wheel, it is more reliable, and is better understood, than any complicated method of automatic regulation. We appreciate Mr. Hopps' remarks that, with a continuouscurrent generator and motor, it is quite possible for a driver to leave his excitation full on, and, owing to the voltage rising More quickly than the starting and acceleration of the bus, that the engine would come to a dead stop, possibly, in crowded traffic. Mr. Hopps has, evidently, had this occur, and we do not mind admitting that we have had a similar experience on a 40h.p. petrol-electric, touring car fitted with continuous-current apparatus: this is not possible with the Hart-Durtnall system. Yes, we have a method of engine control. The total weight of the complete electrical equipment, including alternator, exciter, and motor with field switch, is just above ll cwt., and the following parts, which would not be required, should be put to the credit : the engine flywheel, the friction clutch, the gear box and oil, and all the gear wheels, spindles, etc., the reverse gear, foot levers, connecting rods, etc. The total weight of a 40h.p. Hiert-Durtnall bus is about 500 lb. above the weight of an existing, geared bus of similar power. We thank Mr. ItiVeelis for his remarks, and we believe we have, already, answered his question re starting torque, We were glad to see Mr. F. C. Make here, to represent Mr. Farrows' method for petrol-electric cars, as we believe that, for touring cars, there is a field for this class of apparatus, especially if extra precautions are taken to prevent the transmission cf concussion to the commutator and brush gear. We note that the batteries weighed 900 lb.; but, of course, batteries for a similar output have been very much improved during the last three or four years.
Mr. F. J. Field is perfectly correct in saying that, to a great extent, it depends on what voltage is employed on this or any motorbus. In order to get any sort of efficiency, it is necessary to work with low voltage, and to get as much copper in the slots as possible: we agree with him, that voltages of 6,000 would give immense trouble, and that short-circuiting, by the insulation of the stator coils giving way, would, possibly, occur. Mr. Field will, perhaps, be surprised that we have no trouble in that respect, owing to the fact that our alternating-current voltage, when running on the level, is about 30 volts, and, when climbing a hill, is never more than about 80 volts, and we desire to remind him that the voltage depends upon the distance the electrical power is to be transmitted: he will, now, we believe, understand that, as our main electrical transmission is about 24 inches, the voltage we use is quite ample.