Alternating current (AC) motors are a broad category of motors that include single / multiphase, universal, servo, induction, synchronous, and gearmotor types. The magnetic field, generated by AC motors, is produced by an electromagnet powered by the same AC voltage as the motor coil. The coils that produce the magnetic field are traditionally called the "field coils" while the coils and the solid core that rotates is called the "armature.” There are many advantages in the use of ac motors aside from the wide availability of AC power. In general, AC motors cost less than DC motors. Some types of AC motors do not use brushes or commutators. This eliminates many problems of maintenance and wear, and also eliminates the problem of dangerous sparking.  They are also particularly well suited for constant-speed applications. This is because its speed is determined by the frequency of the AC voltage applied to the motor terminals.