Archive for the 'Education' Category

How Does a Rheostat Work?

The last post might look confusing without knowing what rheostats are and what they are used for. So this post will explain that without going into too much detail.

First, a simple reminder from the electric motor theory: any electric motor is a reversible machine, meaning that it can be used for both converting electricity into mechanical force and for converting mechanical energy back into electric energy.

Taking for example the train acceleration, as train starts to gain velocity its motors start acting as electric generators, generating counter-current that negates the externally applied voltage. If one would simply connect an electric motor to the DC power, it would simply spin up to a certain finite (and fixed) RPM value – at that point the generated voltage would be equal to external voltage.

The Ohm law applies for electric motors as well. A motor has very low internal electric resistance. Applying a high voltage would quite literally burn the motor. This is why when DC motor is starting, it requires only a fraction of voltage. One of the easiest way to adjust voltage is to dissipate the excess energy using extra resistance.

As train accelerates, the resistance should be decreased to compensate for constantly decreasing voltage (which is caused by motors generating electric voltage in the opposite direction of externally applied voltage). A rheostat is simply a variable electric resistance. The 81-717 train we simulate in Subtransit uses a rheostat controller to switch contacts through preset positions, resulting in parts of rheostat being shorted out and total current adjusted as needed (the schematic is a simplified version just for illustration):
rk_example1
(black square indicates closed contact, empty square indicates contact is open)

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