page_title: Synchronous motor
section_title: DC-excited motors
hierarchical_section_title: Synchronous motor / Type / DC-excited motors
caption_attribution_description: Fig. 238 Westinghouse Type C belt-drive synchronous motor with a direct-connected exciter. See other images from this source in 'Electrical Machinery'
context_page_description: A synchronous electric motor is an AC motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Synchronous motors contain multiphase AC electromagnets on the stator of the motor that create a magnetic field which rotates in time with the oscillations of the line current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field of any AC motor. A synchronous motor is termed doubly fed if it is supplied with independently excited multiphase AC electromagnets on both the rotor and stator.
  The synchronous motor and induction motor are the most widely used types of AC motor. The difference between the two types is that the synchronous motor rotates at a rate locked to the line frequency since it does not rely on current induction to produce the rotor's magnetic field. By contrast, the induction motor requires slip: the rotor must rotate slightly slower than the AC alternations in order to induce current in the rotor winding.
context_section_description: Usually made in larger sizes (larger than about 1 horsepower or 1 kilowatt)  these motors require direct current (DC) supplied to the rotor for excitation.  This is most straightforwardly supplied through slip rings, but a brushless AC induction and rectifier arrangement may also be used.  The direct current may be supplied from a separate DC source or from a DC generator directly connected to the motor shaft.
caption: DC-excited motor, 1917. The exciter is clearly seen at the rear of the machine.
exif: {}