Design choices for running an Electric train (and network)?

In summary, the designers of electric trains historically and current opted for AC over DC because it is cheaper and more reliable.
  • #1
tim9000
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Hi,

I'm going to pose my questions with a number of assumptions, so please pick me up on any that are wrong.
I want to know the design choices behind electric trains historically and current, this will mostly require some context of induction motors and inverters. I don't know exactly how EMUs work but I have some theories.

I noticed that 25 kVAC is becoming popular, I assume they rectify and invert this to control an induction motor to drive the train(?) and I was wondering why they don't rectify it for HV DC overhead, to save the train from having to invert it? Now that we can control the frequency easily with solid-state technology (inverters) I assume they use squirrel cage induction motors controlled by frequency because they are really robust?

I assume the switch to AC motors over DC is based around efficiency, or is this also because changing commutators all the time would be expensive?
I know that some older networks use 1500 VDC, did/do these invert then use AC motors, or do/did they always use DC motors? If they pretty much always inverted to use AC motors did they ever have to use wound rotor induction motors (in the old days before you could vary inverter frequency so easily), to vary the resistance of the rotor to get maximum torque for start up? Or was there some problem like maybe it was too inefficient or they couldn't get resistors big enough.

Thanks
 
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  • #2
Distributing AC is cheaper than DC because an arc will not extinguish with DC and the insulation is more reliable with AC than with DC.

Use high voltage because the distribution wires can be lighter and will cost less in copper. Transform the 25kV AC down to a lower voltage so it can be controlled.

Some old systems used DC distribution and motors because regenerative braking was possible using simple technology. Multiple DC traction motors could be grouped in series or parallel and regulated by adjusting the field current.
 
  • #3
Thanks or the reply
Baluncore said:
Distributing AC is cheaper than DC because an arc will not extinguish with DC and the insulation is more reliable with AC than with DC.
That is something I didn't think of, I presume it is because AC has a zero crossing. Although I didn't know that AC insulation was more reliable than DC though.
Baluncore said:
Some old systems used DC distribution and motors because regenerative braking was possible using simple technology. Multiple DC traction motors could be grouped in series or parallel and regulated by adjusting the field current.
I remember a bit about series and compound DC motors. So they did use DC motors in the past, I'm wondering how long ago did they stop, or if older DC trains still use them.
You don't have any idea why in older systems they didn't just use AC in the first place and use wound rotor induction motors to vary the field current (torque characteristic) with a resistor, do you?

Thank you
 
  • #4
tim9000 said:
You don't have any idea why in older systems they didn't just use AC in the first place and use wound rotor induction motors to vary the field current (torque characteristic) with a resistor, do you?
There was one contact above the train and one to the rails below. That limited power to two connections, so two or three phase systems were not possible. Wound rotor motors for single phase AC would be more difficult than DC. Regenerative braking was easier with DC systems.
Before the advent of the national grid, synchronisation would be required if several steam AC alternator plants were distributed in the region. The early DC supply to the regional rail network did not need to be synchronised.
 
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  • #5
Baluncore said:
There was one contact above the train and one to the rails below. That limited power to two connections, so two or three phase systems were not possible. Wound rotor motors for single phase AC would be more difficult than DC. Regenerative braking was easier with DC systems.
Before the advent of the national grid, synchronisation would be required if several steam AC alternator plants were distributed in the region. The early DC supply to the regional rail network did not need to be synchronised.
Ah, of course; how silly of me not to have thought of that. So as far as motors go, I suppose in years gone by the purpose of using a wound rotor induction over a squirrel-cage would have been advantageous, but the fact that you can't really get three phases onto the EMU in the first place was the problem. So now days rectifying and inverting to three phases to drive a squirrel-cage motor would be more or less a piece of cake.

So some of those 1500 VDC trains from the 1980s still in use today, you reckon they'd still be using DC motors?

Thanks
 
  • #6
tim9000 said:
So some of those 1500 VDC trains from the 1980s still in use today, you reckon they'd still be using DC motors?
Plant is continuously maintained and retrofitted with new equipment when it is economic to do so.
The answer to your question will depend on what country you refer to. We have no electric trains here.
 
  • #7
Baluncore said:
Plant is continuously maintained and retrofitted with new equipment when it is economic to do so.
The answer to your question will depend on what country you refer to. We have no electric trains here.
That's a fair point. Thanks
 

1. How are electric trains powered?

Electric trains are powered by electricity that is supplied through an overhead power line or a third rail. This electricity is then converted into mechanical energy to power the train's motors, which turn the wheels and move the train forward.

2. What is the main advantage of using electric trains?

The main advantage of using electric trains is that they are more environmentally friendly compared to diesel trains. They produce zero emissions and help reduce air pollution and greenhouse gas emissions.

3. How do design choices impact the efficiency of an electric train network?

Design choices such as the type of power source, train materials, and track design can greatly impact the efficiency of an electric train network. For example, using renewable energy sources such as solar or wind power can reduce the network's carbon footprint, while using lightweight materials for trains can improve their speed and energy efficiency.

4. What safety measures should be considered in the design of an electric train network?

Safety is a crucial aspect of designing an electric train network. Some important safety measures include insulation of power lines to prevent electrocution, regular maintenance checks on trains and tracks, and implementing emergency protocols in case of accidents or power failures.

5. How can design choices affect the cost of running an electric train network?

The design choices for an electric train network can greatly impact its cost. For example, using more advanced technology and materials may increase the initial cost, but can result in long-term cost savings through improved energy efficiency and reduced maintenance needs. Additionally, the location and layout of the network can also affect its cost, as some areas may require more infrastructure and resources for efficient operation.

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