The invention discloses a transformer, an excitation device, a synchronous generator and a synchronous motor. A first part of an iron core of the transformer is basically perpendicular to a third part, one end of the first part is connected to a second part, the second part is connected to the third part, an interval is formed between the other end of the first part and the third part, and an open hole is formed in the third part. A rotary shaft is arranged in the open hole. A tubular support is provided with a tubular part and a tube bottom, the tubular part is arranged on the first part in a sleeved mode, the tube bottom is located in the interval, and the rotary shaft is fixedly connected to the center of the tube bottom. A first coil is wound around the iron core. A second coil is wound around the tubular part of the tubular support. The transformer provides exciting current for the excitation device of the synchronous generator or the synchronous motor, an electric brush does not need to be used, a high-mass iron core does not need to be additionally driven either, only the light coils and the light support need to be driven, and little mechanical energy is consumed.