Machine

A machine is a physical system arranged to transmit or transform forces, motion and energy to perform a task. A lever changes the relation between applied and delivered force; an electric motor converts electrical energy into rotation; a pump transfers mechanical work to a fluid.

For an ideal slowly moving mechanism with no stored-energy change or dissipation, input and output work agree. If constant forces act through distances $d_{\mathrm{in}}$ and $d_{\mathrm{out}}$, then $F_{\mathrm{in}}{d}_{\mathrm{in}}=F_{\mathrm{out}}{d}_{\mathrm{out}}$. A larger output force therefore accompanies a smaller output displacement. Real machines also dissipate energy through friction, vibration and heating, so motion geometry and energy dissipation must be assessed separately.

An electric motor converts electrical energy into mechanical work, usually rotation, through electromagnetic forces. For a coil with $N$ turns, area $A$ and electric current $I$ in a uniform magnetic field $B$, the torque magnitude iswhere $\theta$ is the angle between the coil's normal and the magnetic field. Repeated rotation requires the current or effective field arrangement to keep supplying torque in the intended direction. Electrical input power must cover useful output plus losses.

A coil with $N=100$, $I=0.5\mathrm{A}$, $A=0.01{\mathrm{m}}^2$ and $B=0.2\mathrm{T}$ has maximum torque $0.1\mathrm{N}\mathrm{m}$. At angular speed $\omega$, instantaneous mechanical power is $P=\tau \omega$; the maximum-torque value alone does not specify a motor's sustained output.

Michael Faraday's 1821 rotation experiment made the conversion visible: a current-carrying wire moved around a fixed magnet while a pool of Mercury maintained electrical contact. The Royal Institution preserves his 1822 apparatus, consisting of a wire, glass vessel and fixed bar magnet connected through the conducting liquid. Continuous rotation, rather than a single deflection, established a route from steady electric current to repeated mechanical motion.^[ref]

A turbine extracts mechanical work from a flowing fluid through interaction with a rotating set of blades. Steam, hot gas or water changes its momentum while passing through the rotor, producing torque on the shaft. A steam turbine can drive an electrical generator in a thermal power station.

For steady axial through-flow in an ideal control-volume model, shaft torque is $\tau =\dot{m}(r_1{v}_{\theta 1}-r_2{v}_{\theta 2})$, the rate at which fluid angular momentum is removed. Here $v_{\theta }$ is the tangential velocity component and $r$ the radius. Shaft power is $P=\tau \Omega$ at angular speed $\Omega$. The relation connects blade-induced changes in flow direction with useful output; losses and operating conditions determine actual conversion efficiency.

A steam turbine extracts mechanical work from flowing steam as it expands through stationary passages and moving blades. The moving blades change the flow's angular momentum and receive a corresponding torque. A shaft transfers the resulting power to a generator or another load. Unlike a reciprocating steam engine, the working stream passes continuously through the machine.^[ref]

For steady approximately adiabatic operation with negligible changes in inlet-to-outlet kinetic and gravitational energy, the first law of thermodynamics giveswhere $\dot{m}$ is mass flow rate and $h$ is specific enthalpy. An actual enthalpy drop of $500\mathrm{kJ}\mathrm{k}{\mathrm{g}}^{-1}$ at $2\mathrm{kg}{\mathrm{s}}^{-1}$ therefore gives $1\mathrm{MW}$ of shaft power under these assumptions. If the reversible adiabatic drop to the same outlet pressure were $625\mathrm{kJ}\mathrm{k}{\mathrm{g}}^{-1}$, the turbine's isentropic efficiency would be $500/625=0.8$.^[ref]

A performance test measures inlet and outlet pressure and temperature, flow rate, shaft torque and rotational speed. If the exhaust is a liquid-vapor mixture, pressure and temperature alone do not determine its specific enthalpy; the vapor fraction or an additional energy balance is needed. Condensing exhaust and supplying useful process steam are different operating arrangements, so turbine efficiency and whole-plant fuel efficiency must also be distinguished.^[ref]