MOTORS

Electric motors have not always been used as the first choice for a compressor. This concept changed in the 1980s as the cost of fuel increased. Electric motors have high efficiency and reliability and are available to drive almost any configuration of centrifugal compressor. Many plants have moved from small steam turbine drives to electric motor drives. Since the mid-1990s large electric motors have become popular. In addition to the considerations of heat balance and investment costs, a third factor, the availability of feedstock, must be considered. Hydrocarbons are needed for many processes thus the burning of the hydrocarbons could be unproductive. Electric power generated from coal, hydro, or nuclear energy can be purchased with a resultant saving of feedstock to make more products.

All electric motors operate on the same basic principal, regardless of type or size. When a wire carries electric current in the presence of a magnetic field (at least partially perpendicular to the current), a force on the wire is produced perpendicular to both the current and the magnetic field. In a motor, the magnetic field radiates either in toward or outward from the motor axis (shaft) across the air gap, which is the annular space between the rotor and stator. Current-carrying conductors parallel to the axis (shaft) then have a force on them tangent to the rotor circumference. The force on the wire opposes an equal force (or reaction) on the magnetic field. It makes no difference whether the magnetic field is created in...