What is a DC generator?
Working principle
A DC generator converts the mechanical power of a prime mover, such as an engine or a turbine, into direct-current electrical power. The armature conductors cut the flux of the field poles and induce an EMF E = k·Φ·ω, so the voltage is proportional to both the flux and the speed. The commutator and brushes reverse the connection of each coil as it passes the neutral axis, which turns the alternating EMF of the coils into a one-directional voltage at the terminals.
Excitation and self-excitation
The field can be fed from a separate source, or the machine can feed its own field. A shunt generator starts from the small residual flux left in the poles: it induces a few volts, these drive a field current, the current raises the flux, and the voltage builds up until the open-circuit curve meets the field-resistance line V = R_f·I_f. This happens only if R_f is below the critical resistance, the slope of the open-circuit curve, if the speed is above the critical speed, and if the field aids the residual flux.
Load characteristic and regulation
Under load the terminal voltage is V = E − I_a·R_a − the brush drop, and armature reaction weakens the flux a little. A separately excited generator therefore loses only a few percent. A shunt generator loses more because the falling voltage also weakens its own field, and beyond a peak current it collapses to a small short-circuit current. A series generator rises with load because its field carries the load current, and a compound generator combines both to hold the voltage almost flat.
Losses and prime-mover torque
The prime mover must supply the electrical output plus the losses: armature and field copper loss, brush loss, and friction, windage and iron loss. The shaft torque is that power divided by the angular speed, a little more than the electromagnetic torque. Efficiency is typically 85 to 92 % at rated load. Alternators with rectifiers have largely replaced DC generators, but the same principles live on in dynamos, tachogenerators and the regenerative braking of DC drives.
Frequently asked questions
- Why does a shunt generator not build up its voltage?
- Four causes are common: there is no residual magnetism, the field is connected backwards so its current erases the residual flux, the field resistance is above the critical value, or the speed is below the critical speed. Try each one in the simulator: the warning names the cause and the build-up chart stays flat at the residual voltage.
- What is the critical field resistance of a DC generator?
- It is the slope of the open-circuit curve at the origin, in volts per ampere of field current. A field-resistance line steeper than that slope never meets the curve except near zero, so the voltage stays at the residual value. The slope is proportional to the speed, so a lower speed lowers the critical resistance and raises the critical speed for a given field circuit.
- How does the voltage change with load in different DC generators?
- In a separately excited generator it drops slightly, through the armature resistance and armature reaction. A shunt generator drops more and can collapse under a heavy overload. A series generator starts near the residual value and rises with load, and a compound generator is nearly flat. Switch the connection in the simulator and compare the load characteristics on the chart.