Control of motor voltage in a.c. traction

The paper describes and compares methods of varying the voltage applied to commutator-type traction motors in single-phase railway vehicles. Reasons are given to explain briefly why a single regulating device to vary the voltage in a stepless manner from minimum to maximum is not commonly adopted and why, therefore, transformer tappings are used. Numerous methods of tap changing are discussed, together with their relative effects upon severity of rupturing duty. Examples of practical circuits, using the various methods for h.v. and l.v. tap changing, are compared as regards choice of tap-changing method and as regards the apparent transformer-winding power required. Comparison is made of practice in various countries and tentative deductions are made as to possible developments in h.v. and l.v. control, methods of interstep progression and elimination of notches.