HO Scale Operational Hi-Rail

Discussion in 'HO Scale Model Trains' started by Freelancer, Sep 1, 2008.

  1. Freelancer

    Freelancer Member

    I was wondering if that was possible, but after pgandw's explanation I figured that would not work. Plus that would make it too simple, and that just isn't possible. I will have to look further into the link you provided. In the mean time here is an update.

    I should have done this earlier as mentioned, but no that would have been smart. So I did a bench test today. Ya ya, I hear it now...."I told ya so!"hamr

    Anyway, the electronics shop is closed so I wasn't able to get the 50 ohm resistor so I decided to try the resistors that I have on hand. I tried 1k, 750, and a 560 ohm 1/4w resistors and had no luck. It was not responding to any of them. I decided to then use two of each thus taking them to 1/2w, did I figure that right? Anyway, with the two resistors it finally responded.

    The 1k's were too much and it did not respond very much to speed changes, however the motor and resistors stayed pretty cool. The 750's allowed the motor to respond better, the motor stayed relatively cool but the resistors warmed up a bit. Nothing that I couldn't hold in my hand though. The 560's allowed for more response and higher speed, again the motor temperature seemed to stay somewhat the same, but the resistors heated up. Again, nothing that was unbearable, but then that was only after a minute or so of running so I do not know what would happend after a few minutes/hours of operating.

    I like the responsiveness of the 560's, but obviously liked the cooler operating temp of the 1k's. If I were to get resistors of the same ohm value, but higher wattage, would that keep the resistors from over heating? But will the increase of wattage in the resistor cause the motor to heat up more? Also, since I combined the two 1/4w resistors to get the 1/2w would I then need to get a lower rated ohm when upgrading to the 1 or 2 watt?

    These tests were performed without the decoder so the next step is to try it with a decoder and also see how it handles under a load.
  2. beamish

    beamish HO & Steam Engineer

    Same Ohm value ,but higher wattage will help to dissipate some of the extra heat.
    The voltage drop over the resistor multiplied by the current passing through it will give you the power. The greater power should not add any heat to the motor as your current going into it should not change.

    If I am understanding you correctly you have 2 560Ohm resistors connected in parallel with each other. This will reduce the actual resistance of the circuit to 280Ohms.

    So the equivalent 1 component solution to what you have is 1 280Ohm 1/2watt resistor.

    It may be hard to find a resistor that has more than 1/2W rating that is an appropriate size for your application. It may be easier to stick with multiple resistors in a parallel configuration.

    For a single resistor I would suggest using something like the Panasonic ERJ-1TNF2800U (280Ohm,1W,2512 package), and for a 3 in parallel configuration I would try Panasonic ERJ-8ENF8450V (845ohm,1/4w,1206 package). For datasheets and info on these parts try searching the part number at www.digikey.com

    Hopefully this helps you to clarify what is going on in your circuit and give you some suggestions to try.

  3. pgandw

    pgandw Active Member

    Unfortunately, neither CVs nor DCC throttle settings directly control voltage to the motor.

    Pulse width modulation (PWM) means that the motor's speed is controlled by fixed voltage pulses of variable duration. The shorter the duration of the pulses, the lower the average voltage of the motor, and hence motor speed.

    It is impractical to directly regulate motor voltage in a decoder because of heat dissipation issues. PWM is merely stopping the current flow for periods of time - a relatively and efficient task to implement in digital control. At full throttle, the decoder is providing continuous DC (rectified from track power) of about 12-14 volts to the motor.

    The CVs literally tell the decoder what the pulse duration is at maximum and minimum throttle - not the actual maximum and minimum actual voltage to the motor. But these have the net intended effect of setting the maximum and minimum average voltage to the motor.

    In this particular case, a 1.2 volt rated motor is being used. The motor is not likely to tolerate pulses of 12-14 volts, no matter how short the duration, without overheating. The resistance of the windings determines what voltages the motor can tolerate before the current causes excess heat. A motor rated for a higher voltage will be wound with finer wire or more turns (more resistance).

    With a motor rated at 8 volts or better, DCC is probably a very good way to control speed. The caution is that the very smallest, most efficient motors don't like PWM either, because of its inherent heat generation compared to pure DC (like a battery puts out). Which is why I suggested that if going to a coreless motor (can be made very tiny, and are very efficient, but don't handle heat well), use one rated for 24 volts.

    yours in electrical and electronics theory and practice
  4. cidchase

    cidchase Active Member

    Hi Fred, I don't want to seem argumentative, :eek: but I don't think that's true, because... The motor must dissipate whatever power is not converted. This is a function of both voltage and current (and mechanical loading). With short duration pulses, the average power delivered to the motor is relatively low. If the motor is not overspeeding, (and not overloaded) then it will be within its power range. The motor has thermal mass and the temp of the motor (and wire) won't respond to the short duration current pulses. But again, it's easy enough to test out! :mrgreen: I don't know if the CVs can limit the voltage, but as you say, they can limit the output, so that max throttle corresponds to a shorter pulse, reducing the average power delivered to the motor.:thumb:
  5. cidchase

    cidchase Active Member

    Yes , the wattage rating adds when you parallel the resistors, so two 560 ohm 1/4 watt will give you 280 ohm half watt. It's getting on down closer to the wild a***d guess of 50-100 ohms. And as you have found, you get more power in the resistors as the resistance drops and the current goes up. If the resistance is too low, things will start to heat up. Setting the cvs to limit the output at max throttle will both protect the motor and give you a wider range of control. Sound like you are getting somewhere, keep on truckin'!!:thumb:
  6. Freelancer

    Freelancer Member

    I got the resistors today and they worked great! However, once I put the motor under load it did not have enough umph to do anything. :cry: So I tried another resistor of a lower value, here comes another "I told ya so" from cidchase:mrgreen: I used a pair of 22ohm 1/4w resistors and it had enough zip to get the wheels movin'. Unfortunately I learned the hard way of what you mentioned above, the low resistance caused it to heat right up the resistors started poppin' and smokin'. :eek: So, back to square one. Would you suggest a 1w or 2w? 22ohm or 50ohm? That n scale motor is seeming awfully tempting right now, either that or where is the best place to score an affordable 24v coreless micro motor?
  7. pgandw

    pgandw Active Member


    I don't like to argue either. The only reason I am posting instead of PM is because the reasons for motors getting hotter on pulse power are not intuitively obvious, and worthy of discussion. I had to review why all experiments end up with the same results - our DC motors run hotter on pulse power.

    Let's start with generalities with filtered DC. A motor is a non-linear device. The current drawn increases with the load, to a maximum when the motor stalls, appropriately called stall current - all without any change in the supply voltage. The limit on stall current is the resistance of the armature windings. Most motors will let the magic smoke out very quickly at stall current - Tortoise and a few special-design (usually very low torque) motors have a safe stall current rating.

    The voltage controls the RPM. Voltage and current are almost independent of each other in normal operation. As you increase voltage, motor RPM increases, but current increases very little, if at all (there is a very slight added friction from the higher RPM). As stated, current is a function of load, RPM is a function of voltage. At a certain point, a motor receiving an excess voltage, but within its current limits, will fly apart from too high an RPM - but it won't overheat.

    How can this be so? BEMF (back electro-motive force) provides some of the answers. As a permanent magnet motor turns, it also acts as a generator, generating a voltage opposite in polarity to the input. Since the impedance (resistance) of our power supplies are normally very high, the back current is negligible for the sake of our discussion. The BEMF voltage effectively reduces the voltage at the motor. If the power supply voltage is increased, the motor increases RPM, which increases BEMF to establish a new equilibrium.

    The second part of the answer is that the coils in our motor serve as an inductor - an inductor which increases impedance as the motor turns faster.

    This is why average current is highest when the motor is stalled. The increased impedance from the motor turning has disappeared, and so has the back emf. Actual voltage across the motor is at a maximum, and impedance of the motor is at a minimum. I know, technically, back emf and inductance are part of the same phenomena, but it helps to separate them in this discussion.

    As you said, energy not transformed into rotational motion is lost as heat in our motor. The formula for energy lost in a resistive impedance is:

    Energy=(current)squared times impedance times duration or
    Power=I(squared)R where energy is power over a time duration.

    In a linear circuit, E (voltage)=IR so one can write P=IE (for a linear circuit!). We can't use that substitution for our motors because we don't have values for the BEMF to calculate the real voltage at the motor.

    Now, to look at our non-linear motors on pulse power. Heat is proportional to the current squared times the resistance (some power is transformed to motor rotation). Let's assume we are running at a slow steady speed on pulses with about 25% duty cycle (the pulse is on 25% of the time). If the pulses were truly rectangular, the average supply voltage would be 1/4 the pulse voltage.

    Heat is proportional to the current squared, so a pulse generates around 16 times the heat (4 squared) as a steady state voltage equivalent to an averaged 25% duty cycle pulse. But that heat is only being generated 1/4 of the time period, so the average effect is only 4 times the heat.

    Open frame motors dissipate heat far better than an enclosed motor. The downside is that their lower efficiency (larger magnetic air gap, magnetic flux concentrated at only 2 points of the rotation) means more heat produced to begin with (shown by the higher operating current).

    A can motor is much more efficient, but the can enclosure limits heat dissipation. Still, the iron core of the armature is a great heat sink.

    The coreless motor has no iron mass, is enclosed, and has just plastic to hold the armature together. Great efficiency, terrible heat dissipation.

    A small motor has less mass to dissipate the heat into.

    In the '60s and '70s, when research was being done to get our open frame motors to run slowly and smoothly on various forms of DC, pulses of different shapes were tried and applied. Pulsed power was at its best in overcoming the cogging of our motors at very low RPMs. Filtered DC was found best for maximum power with minimal heat. Eventually, it was determined that superimposing pulses on a DC base was the right answer. At very low RPMs, the pulses dominated and provided tie-creeping speeds. As RPM increased, the benefits of pulses disappeared. So in the better throttles, the DC base was steadily increased until it overwhelmed the pulses.

    Pulse Width Modulation is very easy to implement in today's electronics. A power-handling electronic switch (triac, SCR, etc) is controlled by a timer. The timer adjusts the pulse duration, and the SCR conducts or is shut off. The pulse shape can be adjusted by introducing capacitance, and the pulse frequency can be part of the timing. On old power packs, "pulse power" was simply half wave rectified AC from our transformers. Pulses were 12 volt sine wave, 50% duration, with a 60 Hz frequency. Motors running on this pulse power ran hot and noisy, and could only achieve 1/2 speed.

    As pulse generator circuits became easier to implement, pulse frequency went up. Today's DCC decoders "silent drives" use a much higher pulse frequency and much shorter duty cycle to get rid of the noise and some of the heat. The double or quadruple efficiency of today's can motors also help keep the heat reasonable. Coreless motors are still iffy on DCC due to the heat of PWM.

    I hope this makes sense. It took me way too many years to understand it properly.
  8. cidchase

    cidchase Active Member

    I don't know if you are hooked up on DC or DCC. I still am not sure what the normal draw on the motor is at 1.2v, but I think it's maybe more than what I was guessing? :confused: And yeah, the z-scale motor would be easier! But I think you are tryin' to keep down the cost? However, I'm thinking your resistor is getting too big for the truck, sounds like you need to dissipate 3 or so watts! :curse: If that's the case, then that motor won't work unless you build a truck body out of aluminum to heat sink :eek::eek: the resistor,... hmmm, see how Rube Golberg got started?? :mrgreen::mrgreen::mrgreen:

    The 12v/24v motor sounds better and better...:yep: But you know, this sort of experimentation is never time wasted!! :thumb: (wo de er fen)
  9. Freelancer

    Freelancer Member

    Muhaha! It's alive!:twisted: Well...sort of.

    I am still trying to figure out the direction to take with the motor so I decided to go back to the first one I started. I mounted the wheels and wired in the decoder and took it for a test run. The first few minutes went flawlessly, but after a few minutes of operation it started to lose traction and now it spins its wheels a lot. It definitely needs more weight and the use of slotcar gearing probably does not help the situation any. None the less it is running! Hurray!:grin: Hopefully pics will follow shortly.

    You are right, it is definitely not a waste of time. This has been a learning experience and I am sure that this will come in handy later on down the line.
  10. sgtcarl

    sgtcarl Member

    Too bad I didn't study electronics in the Army. I'd know what you guys were talking about. Instead, I learned to push dirt around with a bulldozer. :eek:

Share This Page