Pulse Width Modulation is the method
used by decoders to control the speed of the loco.
With traditional DC Analog control,
the speed of the loco is controlled by the amount of voltage put on
the track. For example, if you want the loco to go at half speed,
you would put 6 volts (HO scale) on the track. With DCC, the decoder
provides full voltage 50% of the time. That is, a pulse of full voltage
for a short time, then no voltage for the same amount of time.
This pulsing happens so fast that
the motor tends to see the average voltage. One benefit of this is
that when the pulse happens at full voltage, it provides more power
to help overcome the "stiction" of the motor and drive train.
Therefore, locos (especially ones that are not of the highest quality)
tend to run better with DCC than with DC analog - especially at slower
speeds. The major reason PWM is used is that it generates less heat
than actually regulating the amount of voltage. This could be done,
but the decoder would be much larger and need a large heat sink.
One of the problems with PWM is that
it can cause buzzing. The buzzing starts in the motor, caused by the
power cycling on and off. If the on/off frequency matches the resonant
frequency of the loco's shell, the shell can amplify the buzzing enough
to be heard. There are several ways to reduce or eliminate this buzzing.
First, be sure that the shell is
secured to the chassis tightly. A loose shell can rattle.
Another solution is sticking thin
lead weights to the inside of the shell. Lead will change the resonant
frequency of the shell and it will not vibrate. This is one of my
favorite because it also adds weight to the loco. If there's not room
for thin lead, use duct tape - anything to change the resonant frequency
of the shell.
The magnets in Athearn motors are
not secured. They rely upon their magnetism to hold them in place.
The result can be a magnet that vibrates. The solution is to take
the motor apart and secure the magnets, each with a small drop of
silicon-based glue, such as Goo.
Another help is using Conducta
Lube on all moving metal parts that pass electricity: axles, bolsters,
even the motor's commutator if you can get to it. This won't eliminate
the buzzing, but it can reduce it in some cases.
Some decoders have a feature that
allows you to modify the frequency of the pulses, using CV9. By modifying
the frequency so that it doesn't match the resonance of the shell,
it won't be as noisy. Other decoders use a frequency that is so high
it is inaudible. Train Control Systems, for example, calls this Quiet
Drive. Digitrax calls it SuperSonic. NCE calls it Silent Running.
Whatever they call it, they all do it the same way - high
frequency. |
