What is back EMF

[OffshoreAlan]

On several occasions in threads I have come across the term back EMF.

I've no idea what it is - can somebody please tell me in simple terms, or should I not need to worry about it ?

[Steven B]

When power is removed from a motor it keeps spinning due to inertia and whilst doing so turns into a dynamo and produces a small amount of electricity. Back EMF (electro motive force) is the name given.

By monitoring this power controllers can measure the load on the motor and adjust the drive up or down as needed to keep the train speed constant even with differing loads, gradients etc.

You don't normally need to worry about it.

Steven B

[ntpntpntp]

Yep, as Steven says it's the tiny amount of voltage generated by a spinning motor when the power is cut off.  Feedback controllers use this with "negative feedback loop" circuitry to attempt to keep the running speed reasonably constant. 

The main problem we have is most feedback controllers use circuitry designed for the amount of feedback a typical iron cored motor may generate.  A coreless motor generates far less back-EMF hence the controller tries to over-compensate leading to rough running. 

I have feedback controllers but modified so that the feedback can be switched out, and now I only use the feedback with old 1960s/70s/80s models which get some benefit from it.

Decent quality DCC Decoders usually have the ability to "tune" their feedback sensing to suit the type of motor being driven.

[OffshoreAlan]

Great thanks, I can actually understand that 

[devonjames]

There's a pretty good comprehensive explanation at:
https://dccwiki.com/Back_EMF

[chrism]

There's a pretty good comprehensive explanation at:
https://dccwiki.com/Back_EMF

That's more like it - back EMF is generated all the time that an armature is rotating within the magnetic field and is what regulates the motor speed according to the supply voltage and the load. It's not generated just when the supply power is cut, it's always there as long as the armature is moving. It's why a DC motor draws more current on initial startup and reduces the current drawn as the back EMF increases with the increase in speed.

Feedback controllers work out the motor "speed" by briefly cutting the supply power and measuring the back EMF voltage, then restoring the power before the motor has a chance to slow down too much.
They can then use that measurement to increase or decrease the supply voltage if they detect that the speed and, hence, the load has changed.

[GAD]

Back in my slot car racing gays, we used that EMF to provide a braking effect on the cars. Given, back in the 60s we were doing close to 44ft/sec (30mph actual, 960 scale mph) on the longer 20'+ straights we got a huge amount of retardation my shunting the back emf through the controller.
Rather than two wires needed to apply power there was a third which effectively shorted the track through the controller when the controller was 'off', So much so if you lost it, usually due to a broken wire, the car would crash at the next corner.