# Automatic grade compensation



## MichaelE (Mar 7, 2018)

I was checking the DigiTrax website and MRC and did not find anything about this, but do any of the DCC systems available out there offer automatic grade compensation for locomotive speed control?

I would think that the controller can sense when a locomotive is trying to draw more current going up a grade or drawing less when coming down.


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## gregc (Apr 25, 2015)

many decoders monitor back electro magnetic force (BEMF) which is the voltage produced by a coil of wire passing thru a magnetic field and which is proportional to the speed of the motor. 

My understanding is that the decoder momentarily isolates the motor from the drive circuits and measures the voltage (BEMF) across the motor windings.

the decoder adjusts the motor voltage to obtain a BEMF that is proportional to the throttle setting. If the motor is under load or going up a grade and starts slowing down the decoder compensates by increasing the voltage to maintain the BEMF and speed.


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## MichaelE (Mar 7, 2018)

Thanks. That's what I wanted to know.

So it's not so much in the DCC system as it is in the decoder for the locomotive then?


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## Mark VerMurlen (Aug 15, 2015)

Yes, it’s a function of the decoder. The side benefit is that for locomotives with sound, some decoders will also increase/decrease the rpm of the prime mover of a diesel or the chuff intensity of a steam locomotive based on load. That increases the realism of the sound system.

Mark


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## MichaelE (Mar 7, 2018)

The more I read about modern HO model equipment the better I like it.


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## D&J Railroad (Oct 4, 2013)

The problem with BEMF is when you are running several locomotives in MU. They will fight each other if their speed is slightly off.


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## MichaelE (Mar 7, 2018)

I won't be using multiple units on the same train.


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## Stumpy (Mar 19, 2013)

Thread necro alert!



gregc said:


> If the motor is under load or going up a grade and starts slowing down the decoder compensates by increasing the voltage to maintain the BEMF and speed.


Not a single decoder I have compensates for grade. Train goes up a grade, train slows down. 

Is this a decoder setting that you have to enable?


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## gregc (Apr 25, 2015)

not all decoders had BEMF features.


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## gunrunnerjohn (Nov 10, 2010)

D&J Railroad said:


> The problem with BEMF is when you are running several locomotives in MU. They will fight each other if their speed is slightly off.


Not nearly as much as a flywheel tach controlled speed system, those will really put up a fight if you have any speed difference. Back-EMF is actually quite forgiving of small speed variances.


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## MichaelE (Mar 7, 2018)

I have found that ESU, Zimo, Viessman, and Märklin decoders all have this feature. Sound of the electric motors can be adjusted for load or unload thresholds as can the one Diesel I have.


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