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Faulty Engine
John from the UK has a question for readers:
Could you please offer advice (or could somebody on the site if you do not feel able to), on a problem with electric multiple units in OO gauge. This is a purely electrical question, a question which is based on the mysteries of Kirchoff’s Law.
If you have a double headed loco hauled train I have found by experiment that one transformer/controller unit will satisfactorily convey the current draw of the two locomotives without overheating and tripping out through being overloaded. My layout is absolutely flat. There are no gradients anywhere. This is deliberate because I want to run trains of reasonable length.
I am building a number of UK Southern Railway electric multiple units. Each unit consists of a trailer car and a motor car with one motor bogie based around a Mashima 10/24 motor. I intend to run a six coach train.
On the face of it, two pairs only will be motored as this is commensurate with the double headed loco hauled train which we know imposes a safe level of current load. The third unit pair will have a dummy motor bogie i.e. it is in fact two trailer cars only. Thus each motor bogie is hauling three cars each only.
Here comes the question.
If the third unit is motored there are still only six coaches total. So that means each motor bogie will only draw two-thirds of the current as each will only now be powering two cars instead of three. Therefore if I am interpreting Kirchoff’s Law correctly, then the transformer/controller unit will still not overload.
What do you think? I would respect your opinion – especially as American freight trains have as many as five locos in multiple. Please don’t tell me that it can only be done using DCC!!!
If that is your opinion I will cry tears of blood!!
To comment on John’s question, or to offer a solution, please use the green Comments link below.
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Dear John, perhaps I’m not understanding your situation because if you have two or more loco power units, you will be pulling two or more times the current in parallel from the 12 V supply except that slightly more volts will be lost across the PSU’s own internal resistance owing to the higher current pull. You must have run two or more locos on the same non-DDC track – this is the same. Whether the PSU trips or not depends on its own limit’s setting and any heating. With more loco power units, the current is multiplied not divided. However, with a third unit, you will probably have a better power-weight ratio so will not have to the PSU control so high – probably. Some loss of “resistance matching” may mean power losses in the circuit but you can’t help this so give it a try.
Please tell me what I have not understood!