Crankpin sparks?

[Nathan-1213561]

I'm relatively new to the model railroading scene, and just today got my first loco, coaches and track to work. I noticed that my loco (a second hand Hornby railroad Tornado) would produce visible electrical sparks from the leftside crankpin above speeds of 45. Would anyone be able to help me understand why?

[Chrissaf]

Would anyone be able to help me understand why?

 

 

Electric DC motors can technically do two things. The turn when a voltage is applied across them, but they can also act like a dynamo and generate a voltage when physically turned. Thus when a DC motor is turning under power it is also generating a voltage output at exactly the same time. This can be referred to as the Back EMF. Due to the nature of 'Inductors' [the coil of a motor is an inductor] the EMF voltages generated can be much higher values than the 0-12 volts DC applied to the track to operate the loco. Thus sparks can easily be generated when the electrical path from the track to the motor has very tiny micro breaks in the circuit. Loose parts or more commonly dirt on electrical contact surfaces create these micro breaks. These micro breaks are not usually long enough to prevent the motor from turning and bringing the loco to a stop, but the micros breaks have such small physical air gaps that the high voltage generated EMFs are sufficiently high enough to jump the gap and a visible spark is observed. These sparks can generate radio frequency noise to be emitted from the loco and is the reason that DC Analogue track power connectors have 'radio frequency' noise suppression capacitors in them.

It is not unusual to see a DC Analogue loco with some sparking from the wheels and/or areas where the pickups are located. In most cases, a badly sparking locomotive just needs a really good clean of the components that are being used to transfer current from the track to the motor. Some locos can also use the coupling rods as part of those electrical paths. Whenever the electrical path is reliant upon two physical surfaces rubbing one against the other, dirt and grime can allow these 'micro breaks' to occur. For example, wheels rolling on the rails. The pickups rubbing against the wheels, or any other moving parts that are part of the electrical path etc.

[ColinB]

I suspect that at high speeds the valve gear on that side is occasionally touching the chassis, not long enough though to trip the controller. I assume the side that is sparking is the side that has insulated axles.

[Going Spare]

It may be that the crank in the wheel has been pushed inwards and the back of it is momentarily touching the chassis without the contact being sufficiently heavy to cause the loco to hesitate. The end of the crank should not protrude beyond the back of the wheel. If you turn the loco on its back and apply power to the wheels, you may be able to check this with the wheels rotating slowly. On a loco as new as Tornado, all driving wheels will be insulated, not just on one side. If you find that is the cause, it should be possible to gently lever the crank back in to the wheel with a small bladed screwdriver. Also check the positioning of the pick-ups which should be bearing on the inside of each wheel rim.

[96RAF]

Not quite the same but I have seen sparks on an MN, however this was the motion rods touching the wheel rim. In this early loco the valve gear was live across the chassis so when the rods touched they effectively short side to side. Later valve gear has an insulator twixt sides curing the problem.

[Rana Temporia]

I have had this happen on older Tri-ang/Hornby locos where one of the wheels hasn’t been straight on the axle and as the wheel turns it momentarily loses contact with the pick-up or touches the chassis. The spark comes from the pin on the coupling rod where it fits into the hole in the wheel. Fixed by checking back to back measurements for the whole turn of the wheels, done a bit at a time by rotating the motor by hand.

A pushed in crank pin would have the same effect as it touched the chassis while rotating.