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Super Smooth Running with BEMF

by Don Fiehmann

Progress Over the Years

The motors used in our locomotives have progressively become better over the years. Furthermore, the way we control these motors has also continued to improve. The old rheostat gave way to solid state electronics with better motor control. Then came DCC with a decoder that had direct motor control. Decoders have continued the progress by significantly improving motor control. The major improvement has been the use of Back-Electro Motive Force (BEMF) to control motor speed. BEMF has been likened to cruise control for locomotives.

Older decoders used Pulse Width Modulation (PWM) motor drive operating in frequencies of about 30 to 200 Hz (cycles per second). PWM creates a variable output voltage to the motor by quickly turning on then off the full motor voltage. Unfortunately, these frequencies were low enough to cause a hum or buzz in the locomotive. Most decoders today use a high frequency drive of 15kHz to as high as 43kHz and are above the range of hearing the buzz, hence the term silent running and supersonic. One of the problems with the high frequency drive is with slow starts. With high frequency drives the motor tends to act like it is on DC power and is subject to stiction, where the motor does not start smoothly. This is where BEMF significantly helps to overcome the problem. Also, some manufacturers use CV65 as a variable kick start that can help or a process called dither.

What is Back-EMF

“What is the difference between a motor and a generator?” The answer is there is very little difference between a motor and a generator. If you apply power to the motor, the shaft turns. Conversely, if the motor shaft is turning, it will generate a voltage. When a BEMF decoder applies power to a motor as pulses (PWM), there is a time between pulses when no voltage is applied to the motor. During this time the decoder can “read or sense” the voltage produced by the motor. If the motor slows, the BEMF voltage drops, if speed increases, the voltage also increases. This is how the decoder’s logic determines the speed of the motor and can sense any changes in motor speed.

With BEMF and CV2 adjusted correctly, an engine should start to creep at speed step 1. If you place a finger in front of the engine to stop it, the wheels should continue and slip on the rails indicating the decoder is applying more voltage to maintain constant speed. This is especially beneficial for steam locomotives at slow speed that tend to run rough.

One of the dilemmas with BEMF control occurs when operating in a consist. When the first locomotive with BEMF starts up a grade, more power is applied to the motor. The second locomotive then cuts back its power. Then the second starts up the grade and increases its power, the first locomotive sees a drop in the amount of “pull” needed and cuts back. This sets up a situation where the two locomotives keep changing power levels resulting in an accordion-like action between the two locomotives.

To solve this BEMF decoders have various CVs to adjust a cut-off speed for BEMF.

The upgraded QSI sound units have progressed another step further by implementing a feature called RTC - Regulated Throttle Control. RTC self-adjusts for BEMF locomotives in a consist.

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