Today the most widespread type of electric drive with the alternating-current motor includes for itself a transformer on the basis of frequency transformer with the link of direct current and inverting with latitudinal impulsive modulation.

       In transformers with the link of direct current variable initial voltage is formed the autonomous inverting of voltage of direct current. As usually as a source of feed is used network of alternating current, the guided or out of control rectifier enters in the complement of frequency transformer. Such transformers on principle do not have a limit on the maximal value of initial frequency. The frequency transformers with the link of direct current subdivide on a transformer with the autonomous inverting of current and autonomous inverting of voltage. Presently transformers are most widely used with the autonomous inverting of voltage.

       It is possible to subdivide electric drive with synchronous motor into electric drives with synchronous motors which have an external control, and self-control synchronous motors.

       Noncommutator motors of direct current (NMDC) and servo motors (SM) is a synchronous motor in the closed system, realized with the use of sensor of position of rotor (SPR), transformer of co-ordinates (TC) and power semiconductor transformer (PST).

       The sensor of position of rotor (SPR), transformer of co-ordinates (TC) and power semiconductor transformer (PST), jointly form on the wounds of stator phases voltages U_A, U_B, U_C so that the resulting vector of voltage always was moved on a corner θ and immobile in relation to the axis of the magnetic field of rotor. In this case and the resulting vector of current will be moved and immobile in relation to the stream of rotor , that creates a moment on a shaft of motor.

       Mathematical description of synchronous motor will be realized like an asynchronous motor. Equalizations of voltages on the wounds of stator:

       ,

where L_S - phase inductance of stator, R_S - phase resistance of stator, - vector of stator voltages, - vector of stream of permanent magnets, - vector of currents of stator.

       Equalization for the torque looks like:

       ,

where Ì - is a torque of motor, Z_p - is a number of pair of poles.

       Equalization of motion looks like:

       ,

       ,

where J - is a moment of inertia of motor, - angular speed of the field of motor.

       Picture 1 - The model in absolute units of synchronous motor with permanent magnets on a rotor in the immobile system of co-ordinates