This goes back a few years to the Class 421/423s- I distinctly remember on occasion when a unit pulled away, there was sometimes a "pop" from underneath the motor coach, accompanied by an electrical looking flash. This would be followed by a full brake application, then almost immediately the train would pull away this time successfully. Can anyone tell me what went on here?
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Class 421/423 EMUs- electrical "pop" on departure
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RailUK Forums
NSEFAN
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That's usually the sound of arcing. The unit takes power and accelerates, however there are sometimes gaps in the third rail near or at stations. The high current flow as the train is pulling away causes a big spark when the contact is broken, creating an audible pop and bright flash, as well as a bit of smoke sometimes.
satisnek
Member
I remember this too - it sounded like a circuit breaker operating, perhaps as a result of wheelslip or something? It was a controlled current interruption, unlike the 'splash' which occurs when the last shoe leaves the conductor rail at a gap.
Sounds more like the series to parallel changeover to me. The normal EMU DC motor drive is for pairs of motors to start connected in series, thereby reducing the power to be dissipated in the resistor chain. When the resistance chain was switched out, a contactor would momentarily short the return (negative) motor to ground then connect its positive terminal to its own resistor chain, at the same time leaving the other motor connected to ground and returning its resistor tap to the top. I am not on a computer with diagram drawing facilities but here is a simple diagram to show a typical scheme:
https://www.google.dk/search?q=seri...AUIESgB&biw=1280&bih=800#imgrc=rvz2ar47E0D07M:
The timing of the two series to parallel switches is critical as interrupting a heavy current will cause a large reverse voltage and potentially damaging arcing at switch contacts. To prevent this, a common technique used on BR EMUs was to short the most negative connected motor out with a fast contactor. This produces a short braking effect followed by a surge forward as the power is switched back on. The effect can become worse when the equipment needs servicing. Note the diagram refers to an OLE DC power supply but that operates exactlythe same as a 3rd rail system.
Most early ac EMUs tend to use transformer tap changing, so the changeover is not needed as the voltage is not controlled by a resistor chain. The exception was the GE 306 & 307 units both of which were converted to ac but retained their DC series/parallel circuits. If anybody here remembers the old class306 EMUs, they can probably recall the pop-pop-pop when the contactor had to make several attempts to changover to parallel, which when going up Brentwood Bank meant a few sharp jerks.
https://www.google.dk/search?q=seri...AUIESgB&biw=1280&bih=800#imgrc=rvz2ar47E0D07M:
The timing of the two series to parallel switches is critical as interrupting a heavy current will cause a large reverse voltage and potentially damaging arcing at switch contacts. To prevent this, a common technique used on BR EMUs was to short the most negative connected motor out with a fast contactor. This produces a short braking effect followed by a surge forward as the power is switched back on. The effect can become worse when the equipment needs servicing. Note the diagram refers to an OLE DC power supply but that operates exactlythe same as a 3rd rail system.
Most early ac EMUs tend to use transformer tap changing, so the changeover is not needed as the voltage is not controlled by a resistor chain. The exception was the GE 306 & 307 units both of which were converted to ac but retained their DC series/parallel circuits. If anybody here remembers the old class306 EMUs, they can probably recall the pop-pop-pop when the contactor had to make several attempts to changover to parallel, which when going up Brentwood Bank meant a few sharp jerks.
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