Siemens' dynamo (3D)
About this app
The dynamo-electric principle of 1866: how the machine excites itself from residual magnetism.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
The dynamo-electric principle
In 1866 Werner Siemens realised that a generator needs no permanent magnets. After every magnetisation the iron of the field magnets keeps a weak residual magnetism. When the armature turns in it, a small voltage arises. Its current flows through the field winding and strengthens the field, so voltage and current rise further. This feedback is called self-excitation; it only ends when the iron is saturated. His report was presented to the Berlin Academy of Sciences on 17 January 1867. The principle made large electrical power economical for the first time.
What the app shows
A schematic 3D reconstruction of the machine: double-T armature between two pole shoes, iron cores with field windings, commutator with brushes, drive wheel and an arc lamp as load. Field lines and moving current particles make field and current visible. A chart records how field, voltage and current grow during start-up, slowly at first, then faster and faster. The characteristic curve shows the operating point, the energy balance the counter-torque at the shaft. All numbers are model values for illustration.
What you can try
Start the machine and watch it build up until the lamp burns. Remove the residual magnetism from the iron: then nothing happens. Raise the load resistance above the critical value and the machine does not build up. Reverse the connection of the field winding and it de-excites itself. The Energy panel shows how much power has to be put in at the shaft and how many people would have to crank for it.
Frequently asked questions
- What is the dynamo-electric principle?
- A generator feeds its field magnets with its own current. A small residual magnetism in the iron provides the first voltage, whose current strengthens the field. In this way the machine builds itself up without a permanent magnet until the iron is saturated. Werner Siemens described this in 1866/67.
- Why does a self-excited generator sometimes fail to start?
- Without any residual magnetism the seed for the first voltage is missing. If the resistance in the circuit is too high or the speed too low, the circuit demands more voltage than the weak field delivers, and the current cannot grow. With a reversed field winding connection the current weakens the field.
- Does a generator create energy out of nothing?
- No. It converts mechanical work into electrical energy. The current in the armature produces a counter-torque that brakes the rotation and grows with the current. Every watt the lamp gives off has to be supplied at the shaft, plus losses through heat and friction.
- Is this the machine from the Deutsches Museum?
- It is our own simplified reconstruction of the principle, not a copy of the museum piece. The Deutsches Museum in Munich shows a replica of the 1866 machine. The values in the app are model values, not measurements of the historical machine.
Subject: Physics experiments (Deutsches Museum)
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