A small steam turbine is an auxiliary device used in the steam turbines of thermal power plants. Simply put, a 600-MW power turbine requires a boiler with a capacity of over 1,000 tons, and this boiler is supplied with water by two feedwater pumps. The capacity of these two feedwater pumps needs to exceed 12 MW. So, what exactly is a small steam turbine?

　　 What is a small steam turbine?

　　A small steam turbine is a rotary prime mover that converts the thermal energy of steam into mechanical energy. It belongs to the machinery industry and serves as the primary power equipment in modern thermal power plants. It is also used in the metallurgical industry, the chemical industry, and marine propulsion systems.

　　The basic principle of a small steam turbine is as follows: Steam at a certain pressure and temperature enters the turbine and flows through nozzles, where it expands, gaining high rotational speed. The high-speed steam then passes over the moving blades on the turbine rotor, doing work in the process. Even when the moving blades are unloaded, the reactive power generated by the expansion of steam inside the blades still enables them to perform work. As a result, the moving blades drive the turbine rotor to rotate at a constant speed. This is the fundamental operating principle of a steam turbine.

　　From the perspective of energy conversion, the thermal energy of steam is converted into kinetic energy of the steam flow within the nozzle, and the moving blades convert this kinetic energy into mechanical energy. In reaction-type blades, the steam directly converts its thermal energy into mechanical energy in the expansion section of the moving blades.

　　 What is the working principle of a small steam turbine generator set?

　　A steam turbine generator is a power-generation device in which a steam turbine serves as the prime mover to rotate the rotor, thereby converting mechanical energy into electrical energy via the principle of electromagnetic induction. When a DC current is introduced into the rotor winding of the generator, a magnetic field is established in the rotor. This magnetic field, known as the main magnetic field, rotates along with the rotor of the steam turbine generator. The magnetic flux emanates from one magnetic pole of the rotor, passes through the air gap, the stator core, and another air gap, and then enters the adjacent magnetic pole of the rotor, thus forming the main magnetic flux circuit. As the generator rotor rotates along with the steam turbine, the magnetic poles of the generator complete one full rotation, during which the magnetic field lines of the main poles sequentially cut across the three-phase windings (or coils) labeled U, V, and W installed in the stator core. According to the law of electromagnetic induction, this induces three-phase alternating electromotive forces of different phases in the stator’s three-phase windings.

　　Suppose the rotor of a steam turbine generator has a pair of magnetic poles—namely, one north pole and one south pole. When the rotor of the steam turbine generator rotates at high speed coaxially with the rotor of the steam turbine, for example, if the steam turbine rotates at 3,000 revolutions per minute, the generator rotor will rotate at a constant speed of 50 cycles per second. As a result, the polarity of the magnetic poles will reverse 50 times. Consequently, the induced electromotive force in the stator windings of the generator will also reverse 50 times. At the same time, three-phase alternating electromotive forces of different phases will be induced in the three-phase windings of the stator, each with a frequency that is 50 times higher. At this point, if the ends of the three-phase windings of the generator stator (i.e., the neutral points) are grounded, and the leads from the three-phase windings are connected to electrical equipment, a current will flow through the circuit. This process represents the conversion of mechanical energy input by the steam turbine rotor into electrical energy.