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What kind of DC is best?


mikesndbs

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Hi everyone.

 

For some time I used my Safety Minor with its variable voltage output via old selenium rectifiers.

This was fine for years and years.

Never really bothered with the half wave rectification option.

 

Then I was instructed that really and truly motors needed smooth DC for best performance and longevity!

So I replaced the old selenium rectifiers with nice new silicone bridge ones and added a 470uf capacitor and bleed resistor of 470 ohms.

Things seemed fine for a while but I got fed up with the extra push the capacitor provided and used to just use the bridge rectifier without it.

 

Then dad purchased a load of Portescap motors and again the debate started about 'smooth dc' again LOL

 

So the old controller was replaced with a Orbit Supertroller (anyone still got one? I have) 

 

Reading further it seems that in fact motors give thier best performance when there is a little noise on the DC such as you might get from the simple full wave bridge rectifier and nothing else.

Apparently they integrate the spikes and perform better.

 

Now of course we have the common Darlington pair controllers and even LM317 based ones.

 

So I'd be fascinated to read what others think about all this?

 

 

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Under ideal conditions a motor will run well on Pure DC. The problem is that mass market RTR models are made down to a price and do not have perfect mechanisms. That's where a pulsed supply comes in useful to overcome the initial friction when starting.

 

The reason a DCC decoder, for example, uses a pulsed output is to minimise the heat dissipated by the decoder.

 

Andrew

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I like safety Minors, vastly better than the Clippers and Duettes which are variable resistance, you could still reinstate the half wave option with a dpdt switch and a simple diode as an alternative to the rectifier.  

I am surprised yoi did not persevere with the capacitor, sounds like a good idea, I must try it!

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...Then dad purchased a load of Portescap motors and again the debate started about 'smooth dc' again LOL...

A simple experiment with some batteries will give you a benchmark for what performance is available from various drive designs when supplied with a 'pure' DC.

 

Current RTR designs that borrow much from what was previously finescale mechanism practise (efficient can motor, free running multistage reduction, sensible gear ratio, bearings for the driven axles) perform very well, as naturally enough do kit or scratch built mechanisms of similar spec.. Older RTR with a less efficient motor, single stage reduction, draggy 'bearings' rather more variably: generally the 'lumpy' supply from unsmoothed rectification naturally coaxes them into a smoother start than can be achieved from pure DC.

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And if you get a shiny new-age DC controller you can get PWM (Pulse Width Modulation), which ends up looking not totally different to half-wave rectification. The benefit being you can control the size of the 'on' pulse. It can help to start sticky motors, but ultimately the frequency of the pulses is far faster than the dynamic response of the motor.

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Thanks guys.

Not sure about PWM, puts a lot of heat to the motor I'd think?

 

I am surprised at just how many people use and rave about the Honby R965 controller, I've always regarded it as being rather crude.

Think its a PWM type, but never checked.

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Smooth DC is the best, my battery powered OO garden railway diesels are uncannily smooth quiet and falter free when operating which is especially noticeabe on an indoor layout.

I do not like the "Growling" of PWM or Half wave but it does undoubtedly keep sticky locos moving probably at the cost of some motor life, peaks of 14 volts cannot be as motor friendly as constant 4.5 volts, but the rough bpower delivery does stop gear trains binding up which is a big plus when trying to get a smooth stop, indeed I drive my X04 powered locos down the steep gradient on the layout on full power and half wave to stop this gear binding which results in horrible surging enough to cause wagons to derail, hence my suggestion of having a diode as an alternative to the bridge rectifier.

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Thanks guys.

Not sure about PWM, puts a lot of heat to the motor I'd think?

 

I am surprised at just how many people use and rave about the Honby R965 controller, I've always regarded it as being rather crude.

Think its a PWM type, but never checked.

 

Its actually better than just a PWM, it is a quite sophisticated feedback controller - that is in the 'off' part of the PWM pulse, it measures the back emf (voltage generated) of the motor which is directly proportional to its speed. It therefore knows exactly how fast the motor is turning and automatically increases/decreases the pulse to maintain constant speed.

 

Just check out the review here (about half way down):

 

http://www.scottpages.net/ReviewOfControllers.html

 

Very useful if you run heavy trains around sharp curves or steep hills, and probably the cheapest option for a feedback controller you can get, although the quality probably matched the price...

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