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. Small synchronous motors are used in timing applications such as in synchronous clocks
in appliances, tape recorders
and precision servomechanisms
in which the motor must operate at a precise speed; speed accuracy is that of the power line frequency
, which is carefully controlled in large interconnected grid systems.
Synchronous motors are available in self-excited
sub-fractional horsepower sizes to high power industrial sizes. In the fractional horsepower range, most synchronous motors are used where precise constant speed is required. These machines are commonly used in analog electric clocks, timers and other devices where correct time is required.
In higher power industrial sizes, the synchronous motor provides two important functions. First, it is a highly efficient means of converting AC energy to work. Second, it can operate at leading or unity power factor
and thereby provide power-factor correction. Read more...