Back EMF

I recommend you read the High Frequency Motor Drive page (CLICK HERE) before you read this page. That page provides a bit of background knowledge for what is discussed here.

Back EMF (called BEMF) sensing decoders use the motor's intrinsic ability to function as a generator to sense what speed the motor is running.

When you are running at less than full speed, the decoder only has the power turned on to the motor part time (roughly the percentage of full speed that you are running).

The decoder senses the BEMF generated at less than full speed and adjusts the following pulse lengths to compensate for changes in the motor speed.

Note that nothing in this relates in any way to how fast the pulses are being applied (i. e. whether the decoder is normal or high frequency - with or without Dither).

The response time and memory of the BEMF system determines whether it is an "auto goose" to help the loco past stiction (motor starting frition), a binding mechanism or tight track work issues (short time constant) or "cruise control" to keep the train running at the same speed uphill and down.

One BEMF item which is NOT frequently discussed is the number of bits of resolution. Lenz, as an example, uses a 12 bit system, which will allow the decoder to sense speed variations as small as 0.02%. Other manufacturers may be as coarse as 8 bits (0.3%). The more bits, the smoother the control.

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