What is back EMF on a DCC decoder?

EMF stands for Electromotive Force and requires a fair knowledge of electrics to fully grasp what it is and the effects that it has on motors and circuits, the problem is that most explanations often contain quite advanced terminology or at the very least assume a certain amount of knowledge from the reader when trying to tackle the subject. From a modeller's perspective, the best way to understand EMF from a standing start is to talk about its effects rather than trying to figure out why it takes place, we can worry about the whys, hows and the correct terminology another day.

When the motor in your model engine is running a number of reactions take place. Firstly, the electric that your controller sends to your engine ends up in a kind of wheel (inside your engine's motor) that is covered in coiled wire, and the electric passing through those coils makes an electromagnet, in turn, the magnetised wheel is attracted to some permanent magnets placed around the motor's casing, causing the wheel to start to spinning. In essence that is how a motor turns electric into movement, but there is a side effect.

When the electromagnet spins a reaction takes place and it starts to generate a small amount of its own power, this self-generated power is what is known as EMF. The problem is this power will be naturally attracted the wrong way along the circuit effectively battling against the incoming power from your controller. The faster the magnet spins, the more EMF is produced and consequently less power can get through form your controller.

This sounds like a terrible thing to happen and eventually, the motor will produce so much EMF that no power will get through from your controller and the motor will stop. But that is not the case because as EMF increases and power from your controller is blocked, the motor inevitably slows down and therefore the amount of EMF being produced also reduces and once again lets more of your controllers power through. So now you are likely imagining your loco speeding up and slowing down as all of this takes place, but this all happens in microseconds so the naked eye is blissfully unaware that anything other than a nice smooth operation is taking place.

So why if we don't need to know about EMF is it even mentioned in our hobby? Because EMF is a valuable tool, it gives us a way to measure what your motor is experiencing, by simply turning your controller to a quarter, half or full we cannot tell whether your train is pulling a large load, tackling a gradient or struggling with dirty tracks or turnout frogs. The power is simply sent and your locomotive will perform the best it can with the power it's given. But reverse engineer this same scenario and imagine your train comes to an incline, the train will naturally slow down under the strain resulting in the motor slowing down resulting in less EMF being produced. A controller that can measure EMF will understand that your motor is struggling and send more power to compensate giving you a kind of cruise-control for your train.

It's not just this scenario where EMF can be a useful tool either, EMF can be used to help sound chips to create the correct engine noise or braking sound at the right time, along with lights, smoke or anything else that relies on feedback to be operated more prototypically, all of these applications can use EMF to get valuable feedback.