Perpetual Energy?

[Barc 1]

Something occurred last night that has me really scratching my head over.

I have digital volt meter to each lane that gives me a constant readout of the voltage to the rails. So as I move the trigger the voltage will go up accordingly. This is a no load voltage, that is no car is on the track drawing any current. So when I put a car on the track the load from the motor will draw this voltage down. So if the track voltage is set to 12 volts no load, when a car is put on the track this voltage will drop to 10 if the car has the tires lifted and is free wheeled. This makes sense to me as the motor is drawing current . This fact is constant with every car I tried . Makes sense right? . However one car caused the voltmeter to go up. Track,voltage was set at 10 volts no load, but when this car was placed on the track and free wheeled the voltage went up to 12?? How is this possible. I fully understand a motor used in reverse is a generator, but this is defying physics . How can motor generate more power then it is being given? Think I should phone NASA.?🤪

[Bal r 14]

You must have a really odd power supply or volt meter. Voltage should not change. What changes is amp draw. My cars will change between 0 - 1.75 amps during a race but voltage never changes.

[Barc 1]

That is because your power supply has the wattage(P] to deliver what ever the amperage draw on the motors is at 12 volts

[Barc 1]

I am using digital chips to power each lane, but the equation of P=V*I is a law that works for all electrical power. With P being the absolute in this case as that is all the power supply can deliver. If the current changes(I) then with P being fixed then the V must change to keep this happy. For all my other cars the law is being adhered to . However this one motor is not following this law. In order to satisfy this equation, with the power being fixed, and the observation that the voltage increases means the current drawn is dropping.How can a motor draw negative amps.? It is a real head scratcher because all my other cars are producing expected results, and with all other things being equal this one motor is not.

[HO RacePro]

Dan,

This can happen if you are sending a chopped waveform to your car. It could be your power supply is doing the chopping, or maybe your controller. Instead of supplying a steady 10 volts the output switches between 12 volts and zero, many times per second. How long it stays at 12 volts and at zero averages out to the same power that would be supplied by a steady 10 volt source.

Now would I be surprised if there are capacitors wired across the motor terminals of your car? Not at all. Most 1/32nd scale cars come with them. They provide electrical noise suppression. Might that one car have bigger capacitors than your other cars? Yup, could be. And maybe what your voltmeter is reading is the charge in those capacitors.

But there's another way to see a higher voltage than what you are inputing, again with chopped power. But I'm tired. Nearly bedtime. Maybe I'll share that with y'all later. It's an interesting story!

[Barc 1]

Thanks Ed. What you are saying does make sense. Digital chips do in fact supply chopped wave power, and that car does have a capacitor across the terminals ,. I wired it in myself as this particular motor was causing enough noise to affect my timing system,. Never thought of this source of stored energy as being the culprit.

[Bal r 14]

Now I am curious why you use digital chips? Is there some inherent advantage to that method?

[Cgyracer]

Noisy long can motors in particular will cause EMI spikes that will result in what you are seeing. Ive seen it so bad it can affect your race management system and produce ghost laps. I have seen only two motor do this I might add.

[Barc 1]

Digital chips used to power each track results in affordable digital throttle utilizing all the program functions of a Carrera digital system for analog racing. Works very well. I can adjust power to the track, and braking. Even the digital fuel management system can be employed, This system has been used on two tracks over the past five years.

[Bal r 14]

Thanks for the explanation. This is very interesting.

[slothead]

Sounds like what Ed is talking about is Pulse Width Modulation (PWM). To provide 6 volts to run a motor (or anything) an alternative to maintaining a steady 6 volts could be to provide 12 volts switched on and off so it's on half the time. If the unit of time being used is a second, 12v being supplied for half a second then turned off for half a second results in an average of 6v being supplied. However using this on-off methodology would be very crude creating a harsh sounding jerky performance due to the low frequency of switching. More on-off cycles per second provide better performance.

Also, the type of adjustable power supplies slot car tracks typically use provide a steady voltage output at the selected setting. The amps vary as needed depending on the motor and load, limited by the power supply's maximum output.

The results BARC 1 (Dan) is reporting must be due to how that voltmeter is being used and the possible influence of a rapidly charging and discharging capacitor.

[HO RacePro]

Okay, I'm awake, refreshed and ready to provide that explanation I promised...

And yes, slothead, Pulse Width Modulation is exactly what I am talking about. Very popular with the model railroading crowd because it allows very low speed operation without stalling. See, those full-voltage blips provide enough kick that it will keep a DC motor running despite the low average power.

If you don't understand why model railroaders prize reliable low-speed operation, well, yeah, we're all slot racers here aren't we?

Anywise, I discovered something very interesting using a model railroad power supply to test HO motors for my Rattler slotcars. I set that supply to half-power and hooked up a motor and a voltmeter in parallel across the terminals. Running the motor at half-power I found my voltmeter would measure anything from less than 9 volts -- for a truly lousy motor -- to over 11 volts for a great one. This despite the fact that the output of the power supply without a load was only a bit more than 7 volts.

'Course, because we're talking about Pulse Width Modulation, the actual voltage output of the power supply was bouncing between about 14 volts and zero.

What was happening is that half the time the voltmeter was seeing the 14 volt output of the power supply, and the other half of the time it was seeing the reverse EMF of the motor as it coasted, which was more than zero. Time averaging everything together the voltage looked to my voltmeter like more than 7 volts. Usually a bit more than 10 volts for a good motor.

(What is that "reverse EMF" I called out just now? Turns out a slotcar motor generates a voltage when coasting. We use that for our brakes. Look it up. You're on a computer. Interesting stuff.)

I found out that I could reliably weed out sub-par motors with this test. If I ran a motor at the half-power setting of the power supply and my voltmeter read more than 10 volts I figured that was a good motor, and it went into a car. If the motor couldn't give me 10 volts it went into a drawer. I have a whole lot of disqualified motors in a drawer, waiting for Judgement Day or something.

I should mention that all of my Rattler cars got a second, critical test. They had to achieve lap times lower than 1.8 seconds on my banked oval track. If they couldn't do that the motor got stripped out and replaced with a better one.

And if you are curious the best lap times I ever achieved with a Rattler on that track was a bit south of 1.6 seconds. But those were miracle laps.

So all my Rattler cars saw track time before being packed and shipped. Even the cars that I sold to collectors and never came out of the display box have scuffed tires. None genuine without it.

[Barc 1]

Thanks Ed. A perfect explanation of what is happening.

[slothead]

Thanks for your original post. Good mental exercise to think logically about what is going on when something confounding happens. I believe there's a logical explanation for everything, but no reason to expect it has to be simple.

[Barc 1]

Thanks , I knew there had to be an explanation. The fact this motor had a capacitor across the terminals and the other ones didn’t, means all things were not equal. Both you and Ed identified this capacitor as the culprit and it makes perfect sense that this stored energy is adding to the voltage available, and when you combine this with PWM it all makes perfect sense.

[32lbking]

Hey thanks guys I knew Carrera digital used PWM, but didn't know exactly what that was. Now I think I do. Just enough to be dangerous... uh oh
Randy