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The dc chopper-controlled car, offering numerous merits, is expected to introduce many advantages. However, from the beginning there has been apprehension that the chopper-controlled car may develop harmonic currents in the catenary system and inductively interfere with other electric equipment. Harmonics in an overhead wire system caused by dc chopper-controlled cars was examined, and the relationship of circuit element values and chopper operation to harmonic currents was theoretically analyzed. Results of inductive interference tests with a prototype chopper-controlled car nearly agreed with the result of this theoretical analysis. Practical conditions wherein dc chopper-controlled cars were fully adopted and run in accordance with standard running curves were further simulated, and a program to obtain harmonics in an overhead wire system by using a computer is developed. It was found that the simulated equivalent interference current, even under the worst condition, is only 20 percent larger than the maximum equivalent interference current of one train and can be sufficiently suppressed by appropriate filtering.