Decoder Motor Drive

Background

As one knows from the DC power days, by varying the DC voltage on the track which was directly connected to the DC motor, one can vary or control the speed of the train.  The key point is the voltage on the track determined motor and hence train speed.

With the change to DCC, 

1) The DC motor is no longer directly connected to the track power.  It is now feed power from the motor control circuits built into the decoder.

2) The track voltage is now set to the maximum allowed (think full speed) by the scale of the layout and is constantly on.    Power is always present on the track.
TYPICAL SCALE TRACK VOLTAGES:  12V for N scale, 14V for HO scale and 18V for G scale.  Let just use 12V from here on to keep it simple.

3) The decoder is now responsible for controlling the speed and direction of the motor.  The speed and direction is set by the DCC commands it gets that originate from the throttle used to control the train.

This means the decoder stands between the motor and full track power.  Given the decoder must no be allowed to dissipate any serious amount of heat, the most EFFICIENT way for the decoder to vary the speed of the motor is to use "DC Pulse Power".   This allows the decoder to simply use very efficient ON and OFF switches that connect or disconnect the full voltage going to the motor.  The switches are arranged such that they can also control the polarity of the DC pulses being applied to the motor.

The use of Pulse Power in throttles is not new.  Often the most advanced DC throttles used pulses of power to allow it to extend the low speed performance of the motor while blending in a varying level of pure DC.  However, DCC Decoder do not provide any pure DC to the motor but rely 100% of the use of pulses for speed control.

What is a pulse?

There are two type of pulses:  "ON Pulse" and "OFF pulse" used in motor control.

A ON pulse is defined as full track voltage (12V) being applied to the motor for a specified period of time.
A OFF pulse is defined as no voltage (0 Volts) being applied to the motor for a specified period of time.

Note that 12V and 0 volts are two fixed voltage values that never change.   The only variable is the TIME PERIOD that each exist.

What is Pulse Power?

The two type of pulses are alternatively applied to the motor and do so continuously. 

OFF - ON - OFF - ON - OFF - ON - OFF --->forever as long as the motor needs to run.

So the motor speeds up when the motor sees a 12V pulse and then slows down when it sees a 0V pulse.   

But we know we do not see OBSERVE this speed up and slow down with our eyes.  Why?  Because this cycling of the OFF & ON pulses is down at a very high rate or frequency.  Faster than we can see.

What is the Pulse Rate or Frequency?

If you add up the time the pulse is on and the time the pulse is off, we have what is called a complete period that consist of one of each type of pulse.  

PULSE ON + PULSE OFF = PULSE PERIOD

This is also called a pulse cycle since both together form a complete set of pulses of 1/each.  

Frequency is a term the describes the rate something repeats itself relative to a unit of 1 Second.   The cycle rate.   A 1 hertz (1 Hz) rate means something repeats itself again every second.  When we talk AC power transformers often used to power our layouts, we often come across the term 50Hz or 60Hz.  60Hz is saying the AC repeats it cycle 60 times a second.

We can figure out the cycle frequency of something if we know the total time it takes to complete a single cycle.

Hertz = 1 / Period.  

So if we figure out the decoders motor pulse cycle period, we can figure out the decoders motor PULSE FREQUENCY.

Decoder typically talk in terms of the pulse frequency as part of the specification relating to the motor drive.

Typically early generation Decoders used low frequencies in the 30 to 60Hz rate to get the motor to move very slowly.  The down side was you could easily hear this low frequency as a motor buzz sound and you could also feel the motor vibrate too.  This can become a big distraction especially with sound decoders. Modern decoder use very high frequencies almost beyond the hearing range so you cannot hear the buzz or feel the motor vibrate.  Typically we are talking pulse frequencies above 10,000Hz or 10KHz.

With a decoder choosing a fixed pulse frequency, it has by definition chosen a fixed pulse cycle period.

How do the pulses vary the Motor Speed?

It is the ration of the ON pulse time to the OFF pulse time that determines the speed of the motor.  This ratio goes by the term "Duty Cycle".  So we talk in terms of how long is the ON time relative to the total time.   

If the ON pulse time = OFF pulse time, we say the duty cycle if 50%.



IF we keep the pulse rate or frequency constant, but increase the on pulse time by taking that time away from the off pulse time, we say the duty cycle is increasing.

Decoder use a special trick or  technique called "Pulse Width Modulation" to step down the voltage going to the motor.