Motors powered by alternating current are considerably different from those powered by direct current (DC). AC motors come in a variety of designs, but each has two major components: The stator or stationary parts and the rotor or rotating components. The stator is made of sheet-steel laminations. The slotted inner surface holds coil windings that induce the magnetic forces that turn the rotor.
Because brushless AC motors have no commutators or brushes, they require less maintenance than brushed DC motors. DC motors are controlled by varying voltage and current. With AC motors, voltage and frequency (along with the number of magnet poles) control the motor speed.
There are two fundamental types of AC motors: induction and synchronous.
In induction motors, the rotor turns in response to the induction of a rotating magnet field within the stator. The most common design for AC induction motors is the squirrel-cage configuration, consisting of two rings, one at each end of the motor, with bars of aluminum or copper connecting the two ends.
The properties of induction motors make them suited to several medical applications. For instance, they are simple, rugged, and easy to maintain. They also run at constant speed across a range of loads, from zero to full load. Their only drawback is that they are generally not amenable to speed control, although the availability of sophisticated, usually three-phase, variable frequency drives means that even induction motors can be speed-controlled.
Synchronous motors are so named because they run synchronously with the frequency of the source. The motor speed is fixed and does not change with load changes or voltage. These motor are mostly used where precision and constant speed are required. Most synchronous motor are used in heavy industrial applications.
Comparing AC and DC brushed and brushless motors, all three have power losses in the form of I-R losses. Because DC motors use permanent magnets, no energy is used to generate the magnetic field, as with AC motors. The energy used by AC motors to create the magnetic field decreases their efficiency compared to DC motors.
The frequently encountered National Electrical Manufacturer Assn. reference deserves a few words. NEMA and its classifications are a further way to characterize motors and size. For instance, a NEMA 1 motor is best used indoors and has some protection against falling dirt. A NEMA 13 refers to a motor enclosure constructed for indoor use to provide some protection to personnel against access to hazardous parts. NEMA 23 refers to a mounting area of 2.3 in. x 2.3 in.