DC currents may induce disturbing voltages in other circuits
Mark Van Helvoort
Polaris Program Line Manager Ecosystem / Program Manager Public-Private Partnerships Magnetic Resonance Imaging
Direct current (DC) is the unidirectional flow of electrical charge, distinguishing it from alternating current (AC) where charges move back and forward in time. Though it is a rather common misconception this does not mean that DC currents are constant: currents which vary in time are DC currents as long as the direction of the charge flow is not alternated.
Since all currents are inherently associated with a magnetic field, time varying DC currents produce time varying magnetic fields. Time varying magnetic fields induce voltages in conductive circuits forming (near) closed loops.
In the Netherlands both train and tram systems employ DC traction. At regular distance the overhead lines are fed with DC which is created by rectifying 50 Hz mains in sub-stations. When a train or tram is between two subsystems in normal operation the train or tram is fed from both directions. When the train is coming closer to or moving further away from an observer in the between the substations, he will observe a variation in current due to different loading conditions. Furthermore the load will change during speed increase and decrease of the train or tram.
These time varying fields can be observed with a simple measurement device, called a ‘Tesla meter’. It consists of a pick-up coil, an integrator and an indicator. In this video I show that time varying fields in DC traction systems really exist.
The video was submitted to the Elektor TV Olympics and was awarded with the gold prize. Thank you for your support.
Mostly retired.
5 年You should tell VW, they seem to make a complete mess of fast DC charge in battery packs.
Technical Director at Momentum Trains (Pacific Partnerships)
7 年Umm - yes, the effects of ripple and stray currents have been known for about as long as electricity has been used for powering anything big, particularly trains. We design railways to have stray current protection, by a combination of measures such as: insulation at ground level, interruption of steel reinforcement track concrete, and provision of an attractive path for return current. London underground, for example, use a negative return conductor. We also shield underground rail lines near sensitive receivers such as hospitals, universities and the like to limit EM field transmission.
Not to mention stray currents...
Mega Infrastructure Project Management | Board Member | Design and Construction | Operational Readiness
7 年Yeah the Tesla indicator is likely to pick up any nearby fields, whether caused by trains, exisiting feeders or anything else creating a field. The trick is really to seperate background/baseline from the causal. In Sydney we generally don’t employ dual feeds, solves some problems but has effect on others.. standard!
Founder of the Direct Works
7 年Can we put some clever people together to find a way to introduce DC into new housing, supplemented with solar PV and batteries to save paying extortionate prices for electricity when Hinckley Point comes on stream?