|Magazine advertisement, McClure's Magazine, 1900|
|Cover of GE Bulletin No. 4755, dated June, 1910|
This point is of great importance and was one of a number of reasons for using the three-phase system. The motors will stand any amount of abuse and rough use.
This is important, particularly at starting. The three-phase motor will work to a three or four per cent greater coefficience of adhesion than a single-phase motor of fifteen cycles.
This is important, as it leads to a less cost and a better transformation of power-station apparatus; moreover, it is standard and the power supply can readily be used for other purposes as well as for traction; a commercial supply can be provided.
This is ordinarily stated as a disadvantage of the three-phase motor. But it is a distinct advantage in mountain service, particularly the limitation of the speed on down grades. It has also an advantage on up-grades: meeting points can be arranged with greater definiteness.
This matter has been discussed since the earliest days of electric traction, but has not been, up to the present time, put into practice. Although this result can be attained with other forms or motors, yet it is most perfectly attained by three-phase motors. There being no complications involved. This is of importance in reducing the power-house capacity required for a given space. Although no doubt the saving in power-house capacity will not be as great as indicated by theory, owing to the various emergencies that must be provided for; nevertheless, there will be a material saving.
There will be no necessity for the complication of the friction connection between the armature and driving wheels, as in the recent large direct-current locomotives.
Even when the wheel slips the speed remains constant. Therefore, the maximum stresses put on the motor are less and are more accurately known than with any other form of motor.
This is rather an advantage for this class of service.
The fact is that the motor is a constant power motor, and therefore requires the same power at starting and accelerating as at full speed.
In this particular motor the clearance is one-eighth of an inch, which is ample for all practical purposes.
To meet this, an adjustable resistance is included in the rotor of each motor. The motors are then balanced up and no further attention is required as long as the wear on the driving wheels is approximately the same.
This does not seem to be borne out by practice. The power factor, as shown by the switchboard instruments in the power house, is eighty-five per cent.
There is no doubt that two wires will cause more trouble than one, and in case of complicated yard structure, it might not be practicable to use two overhead wires.