Feedback Controllers

Art,

I use a GaugeMaster Type W hand held and it does not have feedback. I got it specially to work with coreless motors. You might want to try that controller instead.

Jim.

The noise comes because the controllers use PWM (pulse width modulation) at a frequency which is audible. This is a system where the full voltage (12 volts) is applied for a percentage of the time to effectively give a lower voltage.(eg 12 volts for 50% of the time is equivalent to 6 volts, 12 v for 25% of the time is equivalent to 3 volts etc) 12 volts gets across rail/wheel gaps better than 3 volts. It's the system of motor control used in DCC. Cheaper motors tend to vibrate at the pwm frequency due to sloppy motor bearings. The frequency tends to be around 50-100 cycles per second because that is where our motors work best and because higher frequency transistors (used to be) more more expensive. That frequency creates a "growl" while higher frequencies create whines or whistles. It is now easy to design pwm controllers where the frequency is above the human audible range and many DCC decoders advertise this feature.

Feedback control is another refinement which reads the voltage generated by the motor and so can boost the voltage to a stalled motor or reduce the voltage for an overspeeding motor. The two situations where this is effective for us is in slow speed shunting, and at higher speeds for constant speed uphill and down. IMHO feedback controllers are only important for slow speed shunting.

Hope that helps,

Greg.P. NZ

Agreed. In fact if you're going up a steep hill, the speed _should_ drop, just as it would if you were driving a car. If you want to maintain a constant speed, you need to be aware of the "train and the terrain" and turn the throttle up or down.

Running at low speeds, though, the cheap motors and drivetrains in model locomotives tend to stall when there's a minor obstacle. That's when feedback control is worth having--the voltage goes up and the engine gets a little boost of power to clear the obstruction.

Not engine stalls--you turn up the throttle engine stalls--you turn up the throttle engine stalls--you turn up the throttle engine goes Zoooooooom.....

Thanks Jim, I might just try that. Kent Panel Control also have a hand held controller without feedback. But I still have two questions:

1. Why might a controller without feedback work better with coreless moters?

1. And what is meant by a coreless moter?

Also thanks for the other very informative replies.

All the best,

Art

A coreless motor is one that has no iron in the armature (the rotating part). This means that it has very low inertia and inductance, hence current can build up almost instantaneously and the motor can accelerate very rapidly. This is desirable for high-performance servos, but it's really not useful for model railroads. If anything, we'd be likely to add a flywheel rather than pay extra money for a lightweight motor! The major drawback of this type of motor is that on a pulsating waveform, it can take large current surges which cause overheating, and along with its low mechanical mass it has a low thermal mass--the armature can heat up very quickly. As a result coreless motors have a reputation for being easily burned out. I have to say, though, that that's more rumor than attested fact, as most of us have no experience with them.

Thank you very much.

Are there, other than DCC, feedback controllers using PWM at frequencies above the human audible range?

All the best,

Art

Sorry, but I don't know the answer to that as I make my own.

John Brouewer, Fidelity Models does make one for the NZ market.

Regards, Greg.P. NZ