Do trains use AC or DC motors?

[100andthirty]

Typically these days trains have large regulated power supplies producing 440V 50hz three phase. These will be connected to the traction supply on DC sysyens and to the transformed/rectified supply on AC vehicles. The 440V is used to power the large power consumers such as heaters and air conditioning. The 440V also supplies battery chargers for the DC circuits on the train ( usually 24V or 120V) used fot train control and for circuits that need uninterruptable power supplies

 

[RUFJAN15]

There is even a DC link between Willesden and Kingsnorth in Kent which allows parts of the South-East to run out of synchronism with each other in an emergency.

There was a DC link between Kingsnorth and Croydon/Willesden. It was shut down many years ago and there is little physical evidence left of the three terminals.

It was never the intention to run the two parts of the grid out of synchronism. I believe that the objective was to back-off power flows in the parallel AC network so as to improve voltage conditions in London. The experiment was apparently not a great success.

The concept of running DC in parallel with the AC network is being resurrected and a new link is currently under construction between Scotland and England; see http://www.westernhvdclink.co.uk/ for more details.

 

[Elecman]

Typically these days trains have large regulated power supplies producing 440V 50hz three phase. These will be connected to the traction supply on DC sysyens and to the transformed/rectified supply on AC vehicles. The 440V is used to power the large power consumers such as heaters and air conditioning. The 440V also supplies battery chargers for the DC circuits on the train ( usually 24V or 120V) used fot train control and for circuits that need uninterruptable power supplies

Are you sure it's 440 volt 3 phase as that is a very strange non standard voltage for any equipment. Do you mean 400/415 volt 3 Phase?

 

[edwin_m]

Yes, the difference can be substantial. The AC emu is effectively carrying round its own substation with it. There are a lot of downsides to this which are generally glossed over by the anti-third rail faction, but a 12-car AC emu is carrying round three substations under the motor unit frames.

You've answered a question that hasn't been asked. The actual question related to comparing a traditional DC with a modern AC traction package on an EMU supplied by (DC) third rail.

For an EMU running of the 3rd rail, is there much difference in the weight of the traction package between an AC package and a DC package?

And you've also thrown a rather contentious accusation in the direction of the railway electrification industry.

The question of low voltage DC versus high voltage AC supply has been debated many times on here. In a nutshell, low voltage is more efficient for short-distance services with lots of stops and also reduces safety concerns on street, so is universal for tramways and almost universal for metros. Longer journeys with more widely-spaced stops favour higher voltages, despite the extra weight on the train, and the energy consumption is readily measured for an existing railway or calculated for a future scheme so there's nothing uncertain about it. 25kV is pretty much accepted as the best compromise for main line rail, everywhere except where extending an existing electrification with a different voltage.

 

[AM9]

Are you sure it's 440 volt 3 phase as that is a very strange non standard voltage for any equipment. Do you mean 400/415 volt 3 Phase?

Not necessarily. 440Vac between phases is the equivalent of 254Vac phase to neutral and it may be a legacy of electric train design that power was distributed to heaters and auxiliaries at that voltage. Those loads were special for trains anyway. Lighting was traditionally 120V or lower and passenger power outlets weren't an issue.
That would be similar to low voltage domestic supplies where 250V was not uncommon, (before 240V became the universal nominal) and the current European standard of 230V is still met with 240V nominal voltage. I remember that the machine shop lathes etc., at school in the '60s had 415/440V ratings plates on them.
As a former British colonial outpost, Cyprus still has an actual 250Vac domestic voltage.

 

[100andthirty]

Are you sure it's 440 volt 3 phase as that is a very strange non standard voltage for any equipment. Do you mean 400/415 volt 3 Phase?

Yes, Elecman....you are right......momentary lapse,

 

[AM9]

... The question of low voltage DC versus high voltage AC supply has been debated many times on here. In a nutshell, low voltage is more efficient for short-distance services with lots of stops and also reduces safety concerns on street, so is universal for tramways and almost universal for metros. Longer journeys with more widely-spaced stops favour higher voltages, despite the extra weight on the train, and the energy consumption is readily measured for an existing railway or calculated for a future scheme so there's nothing uncertain about it. 25kV is pretty much accepted as the best compromise for main line rail, everywhere except where extending an existing electrification with a different voltage.

Because of the UK directive for all new EMUs to be convertible to ac, the weight penalties of ac power are virtually non-existent anyway. The suspension of pantograph/transformer cars needs to be stiffer for correct ride dynamics and can be ballasted to ensure that DC only versions do not need further type approval testing. As modern HV transformers are getting lighter with successive generations despite higher power ratings, this difference is diminishing as well.
As has been explained many times here, getting a constant 750VDC on a heavy traffic line is virtually impossible so even with relatively short distances between stops, the more reliable supply from ac OLE augmented by more efficient regeneration on braking makes ac OLE operation cheaper than 3rd rail DC with only marginally higher one-off infrastructure capital costs.

 

[Nym]

An example of this is the class 378/1s designed for DC only compared with the 378/2s which are dual voltage with a transformer. The 378/1s were lighter and had a slight performance edge over the 378/2s. Another example is the 375/6s which have AC equipment vs other their sub classes. Again there is a slight difference.

Does anyone know what the advantage is of the new permanent magnet motors and in particular why they are able to produce more power under lower supply voltages than older types ?

I was referring to AC vs DC motors, not supply.

 

[notadriver]

I was referring to AC vs DC motors, not supply.

That's a really helpful answer. Thank you.

 

[AM9]

There isn't a fair comparison, you'd need to compare an IGBT DC Chopper to an IGBT Invertor package, the former isn't really made.

Don't the class 319s have an IGBT DC chopper to control the drive to DC motors instead of the simple resistance regulators of the earlier MKIII EMUs, e.g. 455s?

 

[Domh245]

Don't the class 319s have an IGBT DC chopper to control the drive to DC motors instead of the simple resistance regulators of the earlier MKIII EMUs, e.g. 455s?

I think it was a GTO chopper, rather than an IGBT, but yes - DC motor control by PWM rather than resistances. Much more efficient

 

[edwin_m]

I think it was a GTO chopper, rather than an IGBT, but yes - DC motor control by PWM rather than resistances. Much more efficient

319 was the only class on the BR network to have a DC chopper, apart from (I think) a few 455s that were fitted experimentally. Other railways introduced them earlier but the Southern preferred to stick with the camshaft control they knew and understood. By the time of the Networkers, advances in power electronics meant that inverters powering AC motors were preferred.

Bot 319s and Networkers used GTO devices (later replaced by Hitachi on some Networkers). IGBTs didn't come in until the post-privatisation designs, but allowed the inverters to run at much higher frequency which reduces the signalling interference problem significantly.

 

[coppercapped]

Because of the UK directive for all new EMUs to be convertible to ac,...

What directive is that? Do you have a reference or a URL 'cos I'd like to read it? Ta!

 

[notadriver]

Don't the class 319s have an IGBT DC chopper to control the drive to DC motors instead of the simple resistance regulators of the earlier MKIII EMUs, e.g. 455s?

It's interesting that the latest DC motored units such as 317-322 all have motors concentrated in one coach whereas modern AC motored units tend to have more distributed traction. Perhaps good acceleration wasn't considered to be important for such units ?

Is it fair to say AC powered units are quieter ?

 

[Domh245]

319 was the only class on the BR network to have a DC chopper, apart from (I think) a few 455s that were fitted experimentally. Other railways introduced them earlier but the Southern preferred to stick with the camshaft control they knew and understood. By the time of the Networkers, advances in power electronics meant that inverters powering AC motors were preferred.

Bot 319s and Networkers used GTO devices (later replaced by Hitachi on some Networkers). IGBTs didn't come in until the post-privatisation designs, but allowed the inverters to run at much higher frequency which reduces the signalling interference problem significantly.

What did the 321s use then? I'm pretty sure they aren't camshafted, and for that matter, neither are the rest of the AC mk3 emus?

 

[cactustwirly]

What did the 321s use then? I'm pretty sure they aren't camshafted, and for that matter, neither are the rest of the AC mk3 emus?

Thyristors like the 315, I'm pretty sure the 456s also have a GTO chopper

 

[375610]

It's possible to control the voltage of an AC powered unit or locomotive with thyristors where as it is impossible to use them to control the voltage of a DC powered unit or locomotive.

This is because once a thyristor is turned on it cannot be turned off until the input voltage to it reaches zero. This obviously happens 50 times per second on an AC unit, so efficient voltage control is possible with thyristors, as proven on 87101

However to 'chop' DC in a similar way with thyristors there has to be way to turn them off, this became possible with the development of the 'Gate Turn-Off Thyristor' or GTO.

These were first used to 'chop' up the voltage to drive DC motors, such as in the 319 and 1992 TS. But later they were used to drive 3 Phase AC motors using VVVVF drives, such as in Networkers, 1996 TS, Eurostars, Class 92s etc. Switching frequency is quite low using GTO Thyristors and as the technology became available insulated-gate bipolar transistors or (IGBTs) became the norm.

 

[contrex]

This is because once a thyristor is turned on it cannot be turned off until the input voltage to it reaches zero. This obviously happens 50 times per second on an AC unit

Doesn't it happen 100 times with single phase 50 Hz AC? Twice each cycle?

 

[AM9]

Doesn't it happen 100 times on 50 Hz AC? Twice each cycle?

There are commutation techniques for turning thyristors off even with DC so 6 phase ac can be used if available.

 

[375610]

Doesn't it happen 100 times with single phase 50 Hz AC? Twice each cycle?

You're quite right, my apologies!

 

[gimmea50anyday]

No, in AC a cycle or one single hertz is for example nil to +250v back to nil then to -250v and back to nil. 50 hertz means this cycle repeats 50 times a second. It does however peak at the maximum voltage 100 times a second but that is 50 times at the + and 50 times at the -

 

[edwin_m]

No, in AC a cycle or one single hertz is for example nil to +250v back to nil then to -250v and back to nil. 50 hertz means this cycle repeats 50 times a second. It does however peak at the maximum voltage 100 times a second but that is 50 times at the + and 50 times at the -

By using a bridge rectifier configuration but with thyristors instead of diodes, it is possible to feed power in both positive and negative parts of the AC cycle. 87101 and classes 314-321 excluding 319 used this control system. No unit operating only off AC power supply had camshaft control - 313s have it because of their dual-voltage capability.

Older AC units and all AC locos up to class 87 were fitted with tap changers which switched parts of the transformer coils in and out of the circuit to vary the voltage to the motors. This is a little like a camshaft but there are no resistances involved, so traction control wastes less energy and can run indefinitely on any power setting without overheating.

 

[gimmea50anyday]

I have always wondered what causes the 323's distinctive sound like a formula one car when accelerating

Siemens stock have a distinctive warble which also makes them fascinating

 

[Class172]

I know very little electronics, even less so relating to use in trains, but my guess is perhaps it originates with some sort of frequency changing (from the thyristor?) to get the best performance from the traction motors at a particular speed? I may be completely wrong; I am basing my assumption on a sort of anologue to a mechanical gearbox!

 

[edwin_m]

I have always wondered what causes the 323's distinctive sound like a formula one car when accelerating

This is basically the GTO thyristors producing a waveform which increases in frequency as the motors accelerate. The motor need to see something approximating to a sine wave at a frequency and peak voltage appropriate to the motor speed and the power demanded.

Analogue switching to produce a varying voltage would waste a lot of power and burn out the electronics, so it is actually done by switching much more quickly than the motor speed and varying the ratio of on to off so as effectively to vary the voltage the motor is getting. This produces the sine wave at a frequency a bit more than the actual rotation speed, as this is necessary for an asynchronous induction motor to produce any torque.

The frequency of the sine wave increases as the motor speeds up, but after a short time the rate of switching reaches the maximum the devices can achieve. They then reduce their switching rate to generate a less accurate sine wave with fewer switching events per cycle. This explains why the sound suddenly drops to a lower frequency and starts ascending again.

Although electronic filters are fitted on the inputs to the drive, some current at the traction inverter operating frequency can be present in the overhead or third rail supply and running rail return, so the traction package must avoid operating at any frequency that might cause problems to the signalling.

The above all happens in reverse during dynamic braking.