Faraday disc voltage increase

[FusionJim]

Just a quick question. A Faraday disc - worlds first electric generator, since the time it was invented has been and stayed impractical due to the very low voltage that it produces. It has very low resistance/impedance and could in theory produce large amperage if the current path had low enough resistance which is practically unattainable with slip rings and carbon brushes.

But just for curiosity, lets say that in series with the disc we have a high current high voltage source, now thanks to the higher voltage a significant current can be passed through the disc, does this current cause the disc to become a motor or is it amplified in any way due to the mechanical motion of the disc?

Normally applying a voltage to the disc would cause current to run through it and it would work as a motor, but what if the disc is already spinning, normally the current that can be extracted from a spinning disc is limited by the resistance of the loop, but here we apply a higher voltage in series to overcome the resistance in the brushes, so a meaningful current can flow in the loop.

Could this work similarly to a current amplifier stage? Where voltage is amplified before and then current is amplified separately ?


The way I understand it is this, normally the spinning disc electrons experience a lorentz force that causes charge imbalance to form, this imbalance sets up a radial E field , by connecting a load to the disc the loop is closed and the continual deflection of electrons is what maintains the radial E field that then drives the current. The E field between the disc rim and center is due to the electron accumulation in either center or rim. But the E field is weak, so it cannot overcome the brush resistance effectively. Now if we apply a stronger E field from "outside" in series then the resistance is overcome and a meaningful current starts to flow.


I wonder how does this current then interact with the free electrons (disc) spinning in the magnetic field? Because the same electrons that are deflected in the disc are also now part of the current established due to the series high voltage source, so do the series current is in a way "amplified" due to the charge motion within the B field?

 

[Baluncore]

A Faraday disc - worlds first electric generator, since the time it was invented has been and stayed impractical due to the very low voltage that it produces. It has very low resistance/impedance and could in theory produce large amperage if the current path had low enough resistance which is practically unattainable with slip rings and carbon brushes.

You need to investigate the homopolar generator.
There are high current, 2 MA, discs. They use mercury or molten metal brushes.
https://en.wikipedia.org/wiki/Homopolar_generator#Homopolar_generator_development

 

[FusionJim]

Thank you @Baluncore for your response, I have been aware that it is possible to use liquid metal brushes to decrease the contact resistance. But my question was more theoretical than practical. I am interested to understand a situation where the current through the disc is started by a voltage source in series with the disc, what happens to this current that then passes through the disc if the disc is spinning ?

If the disc was stationary then passing current through it would generate a torque within the disc, but if the disc is already spinning (assuming the direction of rotation and B field is such as to cause current in the same direction as that from the voltage source) , now the voltage source generates a current that flows in a loop that also contains a spinning disc, is the current in the loop the sum of current from the voltage source + current from disc?


I wonder is it possible to know how much of the current in such a loop is caused by the disc and how much from the voltage source?
Is it solely dependent on the voltage drops across each?

Current in series electric parts is the same so if we assume the voltage source has a much higher potential difference than the disc then most of the current in the loop would come from the voltage source?

 

[Ivan Nikiforov]

Просто небольшой вопрос. Диск Фарадея — первый в мире электрический генератор — с момента своего изобретения был и остаётся непрактичным из-за очень низкого напряжения, которое он вырабатывает. У него очень низкое сопротивление/импеданс, и теоретически он мог бы вырабатывать большую силу тока, если бы путь тока имел достаточно низкое сопротивление, что практически невозможно при использовании контактных колец и угольных щёток.

Good afternoon! The low voltage in the Faraday disc is not related to the use of rings or brushes. The rings and brushes provide quite good electrical contact. The low voltage in the Faraday disk is explained by the principle of its operation. In such a generator, it is impossible to apply a winding and increase the voltage due to ampere turns. This is due to the fact that the electric field that appears when the disk rotates is electrostatic. For an electrostatic field, the integral of the closed-loop voltage is zero. The EMF during disk rotation is determined by the equation:

Thus, it is possible to increase the voltage in such a generator in the following ways:: 1) increase the speed, 2) increase the magnetic induction, 3) increase the length of the conductor (disc radius). As the disc radius increases, its speed on the outer brush increases, which leads to increased wear. To eliminate wear, liquid conductors are used.
If a high voltage source is connected in series to a rotating Faraday disk, a very large current will flow in the circuit, almost a short circuit. The fact is that the Faraday disc has a very low internal resistance. In addition, this connection will switch the generator to electric motor mode. The disk will start rotating in the opposite direction, overcoming the force of the primary motor (or simply stop if it does not overcome it).
When the disk rotates in generator mode, load current flows through it. An Ampere force acts on this current, which tends to stop the disk. If the load current is amplified by an external current, the Ampere force will exceed the force of the primary motor and the disk will either stop or start rotating in the opposite direction. In any case, the disk will stop generating and nothing will be added to the external voltage and current.