locos amps draw

[Aussie Fred]

How can I find out what each of my locos draw in amps?

[Chrissaf]

Perform a 'Stall Test' and measure it.

 

Note that this test must be performed without a DCC decoder fitted using a DC Analogue controller.

[Aussie Fred]

Perform a 'Stall Test' and measure it.

 

Note that this test must be performed without a DCC decoder fitted using a DC Analogue controller.

Thanks but that looks terrible complicated and knowing me, I probably would burn out the motor.

I would have thought there was a "standard" for each type of loco - like there is a fuel consumption calculation for cars - by the manufacturers.

[Chrissaf]

No it really isn't complicated unless you have already hard wired in a decoder.

 

I would have thought there was a "standard" for each type of loco

 

No there is no 'standard' as such, there are too many variables to consider. It really does depend upon the type of motor that is installed, and the type of motor is a variable based upon age of loco manufacture.

 

Some variables that need to be considered that will impact on current drawn.

 

Modern CAN motors draw less current than older motor types such as X03 / X04 and Ringfields, then Ringfields themselves have different versions in both 3 pole and 5 pole variations.

 

Then what current do you want to have information for. The average normal running current at a sedate speed or the current when hauling a rake of coaches and wagons or the current when the controller is set to max speed. The more work the motor has to do, then the higher the current drawn. No two similar locos will draw the same current unless performing the same task on the same layout pulling the same rolling stock behind it with the same controller set to the same speed.

 

Then the controller itself will have an effect on how much current a loco draws. Modern controllers have highly regulated outputs, not so with old transformer winding based controller technology circa 1950 to 1970.

 

A modern CAN motor might draw anything from 100mA to 300mA. An X03 / X04 could, if in good condition, draw double that. But an X03 / X04 motor with weak magnets can draw up to 1 AMP.

 

So motor type, age and condition impacts current draw too.

 

There just is no way that you can do any more than generalise with theoretical current draws, making assumptions on motor age and condition. The only way you will truly know how much current a loco draws is to measure the 'Stall' current, which will be the absolute maximum under maximum load conditions that the motor will be asked to undertake.

 

Just to demonstrate the effect of weak magnets on motor current draw [motor magnets do lose their magnetic strength over time] then watch this video.

[Fishmanoz]

No way around it.  Stall testing needs to be in the kitbag of everyone converting older locos, unless you prefer blowing decoders until you fit one with sufficient current handling. 

[96RAF]

Some DCC controllers provide a facility to show current draw but this is simply the draw on the controller i.e. the total track load, which is different to a motor stall test.

 

Trying to stall test a motor with a decoder in place is really only testing the decoder overload protection and not a true indication of its stall current. unplugging a decoder and fitting a blanking plug then doing a DC stall test is not onerous. Once the loco body is off the whole test can be achieved in a few minutes.

 

If you have a decoder test rig you can fit a blanking plug to that, connect a DC controller and ammeter to the input and do a stall current test of the rig motor to get your head round the procedure.

[ColinB]

Ok, if you want a rough value then measure the resistance between the two terminals of the motor. Then assuming maximum voltage is about 14 volts to the motor, then the stall current is 14 volts divided by the resistance you measured. I(current) = Voltage/ Resistance  which is ohms law. When the motor stalls there is no back emf, so the motor becomes a pure resistor. What you also have to do is is mesure the impedance of you multimeter leads. So just connect them together and read the value. Subtract this from the value you read across the motor. A stall test will give give the most accurate value, but this will be near enough. The other thing about a stall test is if you are not careful, you can burn out the motor (watch Sam's trains video), so be careful.

[96RAF]

All well and good Colin but that method takes no account of decaying magnet strength which has a huge effect on stall current in older motors.

[ColinB]

I did say it was only a rough way to measure it. I would have thought the magnetic strength should have no effect as the motor has stopped moving and you are feeding it with dc, but I may well be wrong. When I was doing the same test myself I looked it up and that is what it said. Either way, my way you will get a higher value for the current, which is better for worst case calculations.

[ColinB]

I did the test on one of my ringfield motors actual stall test was 0.5 amps, calculated value at 12 volts was 0.67 amps, so as I said the calculated value is higher which is the one to use as it is worse case.