This paper presents a novel contactless field excitation~(CFE) system based on wireless power transfer~(WPT), which utilizes the existing voltage source inverter~(VSI) of the motor drive for electrically excited synchronous motors~(EESMs). In conventional CFE systems, an extra high-frequency converter is required to excite the field winding.  In this paper, it is proposed to utilize existing voltage switching harmonics of the VSI for exciting the field winding while the low-frequency modulated component is used to drive the motor. In the proposed system, a novel variable carrier phase shift method~(VCPSM) is developed to achieve constant input excitation voltage for the WPT part independently from the motor operation. In addition, a hybrid frequency detuning control method is introduced to adjust the field current.  For experimental validation, a small-scale prototype with 100~V DC-link and 60~kHz switching frequency is established. It is observed that the field current could be kept almost constant at 5~A under different motor driving conditions operations regarding modulation index and fundamental frequency.  Also, it is shown that the field current could be reduced by detuning the switching frequency. In brief, without an additional active converter and only with a software update, a cost-effective CFE system for EESMs can be easily implemented.