CV 177
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NMRA CV 178 - BEMF Power Stroke Limit
CV 177 limits the response time of the BEMF algorithm. If the locomotive is overshooting its target speed (going too fast or oscillating speed significantly downhill for example), try decreasing the value in this CV.
In order to enable this feature you MUST first set bit 4 in CV 182.
Geek Speak: Motors build up BEMF voltages at different rates based on the motor response time, which is dependent upon the type of motor (coreless or not, etc), the load on the motor, and the number of poles. The target speed the decoder is expecting will change the decoders output to the motor as the BEMF level changes. If this level is too low, the decoder will begin increaing the duty cycle for the motor and watch the response over a period of time to see if the speed gets closer to the target. If the motor's response time is longer than what the decoder is expecting, the decoder will think the motor needs a bigger voltage swing to get it to change to the desired speed. As a result, it will continue adding to the duty cycle over and over; often, when the motor does start to respond, it will then overshoot its target speed - The end result of which is jerkiness and oscillations in speed.
This CV adjusts for how long the BEMF algorithm can increase the duty cycle to the motor. A lower value more strictly limits this time. The consequence of constricting this window can be undershoot and lack of pulling power, because now you are not allowing the BEMF algorithm to deliver the power the motor NEEDS to compensate for a load.
By default, this setting only affects the motor control when above Speed Step 32. If you would like to always have this option enforced at all speeds, you can Set Bit 4 of CV 182.
| !!!WARNING!!! This CV should not be modified unless you are trying to correct a significant issue with slow speed motor control. We do not suggest modifying this CV unless absolutely necessary. |
This "responsiveness" refers to the motor control itself, not necessarily the sound functions such as chuff intensity or prime mover RPM. Refer to our section on Prototype Throttle Mode for guidance on BEMF calibration for sound function.
| NOTE: This CV was introduced at different times in different decoders. This CV will not do anything if programmed with a value on a decoder which does not support the feature. |
Only current-generation decoders (V4.5 Diesel, V4.6 Steam, V90+ Mobile) support this CV and featureset.
