Summary: A method to enhance motor control for low speed operations. By driving the motor at a supersonic frequency, it is less likely to cause extraneous noises from the motor or the body. This is an advanced DCC topic, and is not required to understand for operation of a DCC layout or multifunction decoder.
Decoders with this feature allow better slow speed operation without a distracting buzz from the motor.
High Frequency Multifunction Decoders
A Name is a Name
Let us get the marketing stuff out of the way. Various manufacturers have come up with various marketing names for their high frequency motor drives:
- NCE calls theirs Silent Running.
- Train Control Systems uses Quiet Drive.
- Digitrax calls their implementation SuperSonic Drive.
There are probably other names. No matter what the manufacturer calls their technology, it's mostly all the same technology - sending power pulses to the motor in a way so that people cannot hear it.
Under the Hood
Multifunction Decoders control the locomotive's speed through Pulse Width Modulation (PWM). A square wave is fed to the motor, and the duty cycle is varied to control the motor speed. Modern electronics can implement this method easily.
The Duty Cycle describes the relationship between ON and OFF during one complete cycle. For example, at 100 Hertz, a cycle is 1/100 S or 10mS (0.010 seconds) At 50%, the on/off relationship is 50/50, or 5mS on and 5mS off. As the duration of the ON pulse increases or decreases, the duty cycle changes with it.
When the frequency of these pulses are within the range of human hearing, it can cause a buzzing or humming sound in locomotives. The PWM can be increased to frequencies above the range of human hearing, so the buzz can not be heard by us mere humans. (Your dog may not like it...) Unfortunately, increasing the frequency reduces the motor's torque (the twisting force produced by the motor). Decoder manufacturers provide various methods for adjusting the PWM frequency without losing too much torque.
The buzz starts in the motor and is transmitted to the frame. The buzz appears to be more common in diesels than steam, and brass compared to plastic. There are a few ways to reduce the noise, but increasing the frequency to above what most people can hear makes it inaudible. There is also the possibility of resonance with the body, meaning that at a certain frequency the body will vibrate and amplify the sound. Various options are available to reduce the resonance of the body.
As the pulse frequency increases you lose motor torque. Some manufacturers of high-frequency decoders allow the user to adjust the frequency for a balance of torque and quiet operation. Other manufacturers set it for an optimum frequency then allow for minor adjustments from there - one way or another.
To help compensate for loss of motor torque, manufacturers have started implementing dithering technologies which further manipulates the PWM.
Running an analog locomotive (a unit not equipped with a DCC decoder) on a DCC layout can also produce this effect. This is a result of a DCC waveform driving the motor, and the Zero Stretching technique used to operate a direct current motor. The sound will change as the throttle is opened. Some direct current motors are a lot more sensitive to signals that are not pure direct current than others. See the page on Zero Stretching for important information and warnings.
But Mine Doesn't Buzz...
Not all locomotives suffer from the buzzing problem. Results can vary between the same manufacturer and model. There appears to be no reason or why one loco will, yet another won't. Some decoder motor control circuitry can send a very dirty signal to the motor, and no amount of tweaking can fix that.
Direct Current PowerPacks with Pulse Power
Many of the better Direct Current power packs offer a feature called "Pulse Power". Short high amplitude pulses are sent to the motor to get it moving, and allow realistic slow speed operations. This evolved into the high frequency Pulse Width Modulation used on DCC decoders to drive the motor for slow speed operations.
- Some Bachmann locos have a capacitor across the motor leads that will drastically affect the operation of high-frequency-type decoders. You must disconnect that capacitor. There may also be inductors in series with the motor connections, which also must be removed.
- These components are added to reduce Radio Frequency Interference, but are not compatible with pulse wave modulation. They are required for products sold in the EU and other countries, but not North America.
- Due to the global nature of the market, manufacturers like Bachmann build to meet EU requirements, allowing their manufactures to be sold anywhere.
- Multifunction decoders made for the EU are often designed to work with any RFI suppression circuits, as they are required in the EU.
- Capacitors decrease in impedance with frequency, an inductor's impedance increases with frequency. It is like a resistor across the motor terminals bleeding off energy, or additional resistance in the motor leads and windings. The right combination can be very interesting from an electrical standpoint, creating a tuned circuit with almost no impedance.
- Avoid operating vehicles equipped with a multifunction decoder on analog pulse power throttles/power packs. The dirty signal they create excessive voltage spikes which can damage the decoder.