Professor Motor brake upgrade

[SlotsNZ]

It isn’t even  “electronics” Greg. It is a basic electrical principle, and the same principle that makes a motor rotate when you apply voltage to it - simply the flip side of the same coin, and it is something that people normally learn their first month choosing motors etc in  club racing.

I find it hard to conceive that a guy who debates the finer parts of alternative digital systems electronics across the forums is sitting here pleading ignorance on the basic principles of motor braking. 

And it is not about “not upsetting those who are good with electronics”, it is simply about not writing wrong information that would create confusion for new spotters. 

 

[MrFlippant]

I can't explain why I understand some things and not others. I'll note that while I understand WHAT digital systems do, I don't understand the specifics of the magic behind them. Your reaction is what I meant by "upsetting" people.

DC power makes a motor go. Reversing the polarity makes it go the other way. Inserting resistance into the power reduces the amount of power going to the motor, and therefore the speed of the motor. Removing the power makes it stop going. Shorting that power makes the motor stop hard. Inserting a resistance into the short makes the motor stop less hard. 

Is that all correct? Would you say that I understand it? 

I still can't explain why those things are the way they are. I can kind of understand how a resistor, which does what it says, and resists the flow of energy, will therefore reduce the amount of energy (note I'm not saying volts or amps here, 'cus I honestly don't know which is being resisted... both? Volts? I'm guessing), will make the motor go less quickly. That logic follows for me. But why does resisting the flow of energy out of a motor, that has become a generator by virtue of it still spinning after power is removed, make it stop less hard? That's backward to me... which I guess stands to reason, since it's now backward from how the motor GETS power, but it just doesn't click for me.

If I could explain WHY it doesn't click for me, then I probably wouldn't have to, because then it would click for me. 

I'm sorry if this frustrates you. I'm glad that you understand not only how these things work, but also why. 

I also did not intend for this to continue going so far off topic. I was merely responding to a previous post about how it's hard to understand for some people, including myself. 

This is why you won't ever see me "debating the finer points" of the actual electronics behind any system. I don't repair chips, or design chips, or replace resistors in things. I can solder wire to motors and braids. That's pretty much the extent of my prowess on HOW these things DO what they do. Does that mean I'm not allowed to understand, or debate, WHAT they do from a practical, USER point of view? I should think not. 

[Sports Racer]

Put the car on the track, lift the rear tyres so they don't touch and rev it, then lift the car out of the slot. Note how long it takes for the wheels to stop turning.

Put the car back, wheels up again. Rev it but instead of taking the car out of the slot, put your finger on both rails (as you back off the throttle so there's no power going to the track).

Try it with a wet finger and you'll see the motor stops quicker.

Now stick your tongue on both rails as you back off the throttle and see how quick the motor stops.

When you pick yourself up off the floor you'll notice that (amongst other things) even though there was no power going to the track your tongue felt like it was stuck to the southern end of a northbound Jumbo jet with the afterburners turned on. That's because a turning electric motor generates electricity which was transferred to your tongue. What your tongue did is create a short circuit, sucking out what little electricity was being generated by the motor. The better the material to create the short the faster the electric motor stops when the throttle is backed right off. 

Actually there was no need to use your tongue but you're over 2000 miles away and don't know where I live. 

Cheers

Paul

[MrFlippant]

I couldn't find a LOL button, so I guess I'll use emojis...

I appreciate the attempt to explain how it works.

I understand WHAT happens. Gaining a true understanding of why is not so easy. Not being permitted to explain it the way that my brain wants me to just means I'll never try again.

Now that everyone has focused their attention on my inability to describe it with appropriate scientific terminology, the others in my predicament can sit back and learn from my mistake. 

[MrFlippant]

I think I'm slowly getting it. This video was VERY helpful, and also explains why some people might say that the power generated by the motor under braking is flowing back into itself. If this video does NOT apply, then I'd appreciate a similar video that DOES apply, if not also an explanation as to why this video is not appropriate.

(Note, I'm not sure about the shunt field stuff... does that apply to our use?)

 

Edited November 17, 2021 by MrFlippant

[Garry J]

Forget the shunt field, we have permanent magnets.

When you disconnect the power from a running motor it will continue to spin according to its momentum plus the momentum off whatever it was driving.

At this point the motor becomes a generator because it's being driven, not driving.

BUT THE TORQUE IS REVERSED WHEN THE MOTOR IS GENERATING, so if you connect it back on itself it is effectively attempting to drive itself in the opposite direction to whichever way it was running under power. Once the motor slows the generated voltage reduces until it eventually stops.

Adding a resistance to the circuit simply reduces the voltage the motor is feeding back on itself, and therefore reduces the braking effect.

To see your motor act as a generator, a common motor test is to free wheel a test motor using another motor to drive it and measure the test motor's output voltage. Good for relative comparisons only.

Edited December 8, 2021 by Garry J

[Shaynus]

Interesting info there Garry J, thank you.

I appreciate it is a relative thing, but what does that test voltage tell you? If a motor puts out a higher voltage than another, what difference does that make, stronger braking effect?

[Garry J]

I've never done it myself, but I guess if it puts out more voltage it's more efficient.

The idea is to set up a jig with a fixed motor to drive the test motors with a bit of tube between the shafts. Apply a fixed voltage to the drive motor and see what you get at the other end.

My usual way to test motors is to run them on a few volts and choose the one with the least vibration. A combination of the two might work.

 

[kalbfellp]

The best way to test is to do the opposite use the test motor to drive a “ fixed “ motor that can  be used as the generator, measure the voltage and Amps of the fixed motor to calculate the output. The fixed motor will need a resistive load on it. It is then a Dyno.

[Garry J]

Yes, you are right Phil.

I had it backwards, never having done it myself.