Tram regenerative braking..

[colly0410]

When the Nottingham tram system fired up we got a booklet about it, one part explained that when braking or going downhill it fed DC power back into the overhead line for other trams on same section to use, if no other trams are pulling power on section then it was fed back into AC mains via substation. AFAIK diode type rectifiers can't convert DC to AC, so presume they are using some other clever gubbins. Anyone know how it's done?

[ppppenguin]

The problem is similar to inverters for solar panels, except that power has to be able to flow from AC to DC as well as DC to AC.

The technology is now commonplace.

[boater sam]

I understand that under sea transfers of power are also DC and converted at both shores.

[ppppenguin]

For undersea (and all other) DC transmission links there are often bidrectional converters at both ends. At transmission power and voltage levels (100s of MW and 100s of kV) this is serious and expensive technology. For a tram system the power levels would be comparable to a medium size solar farm. DC potential would be hundred of volts, not thousands.

[boater sam]

I had a battery electric sit on train with Curtiss controller which had regenerative braking and over-run. It was so good that reverse could be selected to stop forward motion without any ill effect.

[Ed_Dinning]

Hi Folks, either plug braking or later regen braking has been used on electric vehicles since the 30's (mainly fork trucks etc).
A skilled driver could bring his vehicle to a halt without use of the friction brakes.
Plug braking returns no energy to the supply and can cause motor overheating. Earlier Regen systems would return a small percentage of power to the battery, more modern systems are much better

Ed

[PerdioPal]

I bet they do the timetables to time starts with stops, keeping the whole thing "local" to reduce the need for big DC to AC converters. There are usually big dump resistors to reduce brake wear and "just in case" too. Some of the older London Underground stations are raised a bit so the trains go uphill when stopping and downhill when starting, very clever.

[ppppenguin]

A lot of modern trains, and possibly trams too, have AC traction motors and variable frequency drives. The AC motors are more efficient, more reliable and need less routine maintenance. If you want regen braking the VFD has to be bidirectional too.

Some tube stations are raised but it's far from universal. The headway on lines that have modern signalling is sometimes only a minute or so. One train leaves the platform and the next one arrives within seconds. Running that frequency of service needs good acceleration too. I've lived near the Northern Line for much of my life. At one time it was often called the misery line but now it's pretty impressive most of the time. Unlike the Central Line which has huge problems with availability of trains. On the Northern and other lines you can hear the whine of VFDs. Not like the comforting click-click-click of old fashioned motor controls.

[Refugee]

Two way AC DC converters come in all sizes.
There is a big one at Folkestone.
https://en.wikipedia.org/wiki/ElecLink
DC regenerative brush motor systems are common on mobility scooters.
I sometimes go round the flea market in the company of a mate on a scooter.
It is a small one that can be taken down for loading into a car.
It climbs up the hill from the parking to the cobbled market with three LEDs of power still showing and goes down to one LED after going round the cobble stoned market square and then back up to three LEDs after going back down to the hill to the parking.
Trams and trains with overhead electrics use an on the fly system most likely with a brush-less motor.
They use a more exotic converter that creates variable AC from fixed AC in drive and the other way round during braking.

[Doodlebug]

I do like the reference to the' Valve Hall' at
HVDC sites, where the giant Thyristors are hung from
the roof. Name has a nostalgic ring to it.
That looks to be a man standing at the back of the hall, giving an
idea of the size