Very interesting Setup from Mega Motors

[GriffinRU]

Originally Posted by squeeforever

Yup. And going down below that rating will decrease the light output as well as the amount of amps it uses, but this still doesn't have the extra variable like the motors (I don't think, or does it?). Please excuse me, I am kinda tired, and very confused :p.

Yup, motors are more complex, but that explains why current goes up with voltage. And then you can apply all the rest of the variables...

So, anybody used mega motors before?

Artur

[BrianG]

This thread seems a bit confusing. I agree with GD for the most part. Assuming the motor impedance is constant and the load is constant, an increase in voltage will increase current.

A motor is not a "constant power" device, so you can't say a motor is going to put out, say, 1800 watts no matter what voltage is applied. This is painfully obvious from the runtime achieved. If a motor always pulled 1800 watts, batteries would last a VERY short time. For a given voltage, it will draw the current it needs to do the job. The product of the voltage and the current creates the power. Finding amperage by dividing a constant wattage by the applied voltage is not correct, sorry.

However, motor RPM and inductance does have a factor in this though. Higher rpms can increase the back EMF, which is comparable to increasing the resistance, which decreases current. So, the "resistance" is not linear like in a pure resistor. So, an increase in voltage will increase rpms, which will increase the back EMF, which increases impedance somewhat, and decrease resistance, but not as much as the conversation here seems to imply.

Example, a 10v, a motor draws 10A. This is 100W. This equates to a 1 ohm "resistance".
However, at 20v, the same motor with the same load may draw "only" 18A. This is 360W. The impedance increased to 1.1ohms. A pure resistor would have stayed at 1 ohm, which would have developed 20A (and 400W). The rpms, and the increased back EMF they created, made the resistance go up a little. But it certainly did not go up enough to generate the same 100W.

[sleebus.jones]

Originally Posted by GriffinRU

I will make it simple...

If you get light bulb rated at 12V and apply 16V you won't get more light but it will draw more amps and won't last long. Why? Because of fixed resistance. But how much light it will produce will be based on how big is the light bulb. Now apply the same to the motor...

Artur

First problem is that you are wandering off into another area. I'm not talking about ohm's law (resistance) at all. Also, if you overvoltage a light, you will get more light out of it. If you shove more energy into a circuit, you are going to generate more heat. Heat is what makes light in an incandescent bulb. Make more heat, you make more light.

Take a look at this link here:

http://science.howstuffworks.com/question501.htm

Maybe that'll do a better job of explaining it than I have.

Sleeb

[GriffinRU]

Wow, even light bulb was hard...

BrianG and you are about back EMF...

Artur

[BrianG]

I just read the that HSW article and I didn't particularly like their lightbulb example. The way it was stated led the reader to believe a 100W lightbulb will always produce 100W no matter the applied voltage.

The incandescent lightbulb is also a bad example because the filament does not have a linear resistance either. Its resistance is close to 0 when no current is flowing (off) and the resistance increases as it heats up from the current flow. Incidentally, this is why a bulb amost always blows when you first turn it on - from the initial inrush of current while the filament resistance is lowest.

[GriffinRU]

Originally Posted by BrianG

I just read the that HSW article and I didn't particularly like their lightbulb example. The way it was stated led the reader to believe a 100W lightbulb will always produce 100W no matter the applied voltage.

The incandescent lightbulb is also a bad example because the filament does not have a linear resistance either. Its resistance is close to 0 when no current is flowing (off) and the resistance increases as it heats up from the current flow. Incidentally, this is why a bulb amost always blows when you first turn it on - from the initial inrush of current while the filament resistance is lowest.

100W of equivalent optical power, which will be based on temp of filament, which has its max at say 5000K then it melts...

Artur

P.S. howstuffworks -> internet is fantastic :)

[BrianG]

Originally Posted by GriffinRU

100W of equivalent optical power, which will be based on temp of filament, which has its max at say 5000K then it melts...

Artur

P.S. howstuffworks -> internet is fantastic :)

Yeahg, but they weren't really clear, that's the problem. People who don't know can be led to make some assumptions that aren't correct. Besides, light output isn't usually measured in watts...

For the most part, HSW really is good "stuff".

[squeeforever]

Originally Posted by BrianG

This thread seems a bit confusing.

I don't think your helping :p. Anywho, about the 1800 watts continuous. That is another variable that makes that equation irrevelent to this application. I was just stating that it is irrevelent even without that variable...

[BrianG]

Sorry. :dft001: :)

[crazyjr]

I'm not the best at brushless, but there are a lot of things going on at different cell counts:

As voltage goes up, resistence goes up and has the same effect as going up a wind (actually a wind may be drastic but you get the point)

As voltage goes up and temp goes up the magnet loses a little of its magnetism and kv goes up a little (opposite of the resistence in the winds)

Most of what i learned was from a semester of electronics and self experimentation with brushed slot cars, 1/24 scale 12 volt commercial setups.

One thing nobody ever mentions is bearings, cheap ones have a grease, that when the motor spins faster, can cause resistence. More expensive ones use a light oil and will spin faster with less resistence

As i said i'm no expert but there are a lot of things going on that i do understand and if i'm wrong about something, Please speak up as i want to understand as much as i can

[smhertzog]

Just a simple observation but the buggy in the link has a different motor. The wires are coming out of the back of the motor and it looks like there are only two wires going from the esc to the motor:032:
Another thing why would you bring the motor wires out of the motor on the shaft side right next to the pinion:032:
The heatsink is much more interesting than the oddball motor.

[Serum]

Cool!

This motor/mount is really awesome!!!

About the current draw;

Plain simple;

If a car is geared for 40 mph on 3S lipo, the current draw will be higher than that same car geared for 40 mph on 5S.

But.... It's rather uncommon to increase the number of cells to get more runtime... If you use more lipo's you obviously need more power. If you need more lipo's to get more speed, the current(A) will also increase.

The numbers they are talking about don't mean nothing to me. The current they take varies a lot. weight of the vehicle, maxximium amount of power the batteries can deliver before their voltage will drop, gearing, and the power they take under acceleration.

[coolhandcountry]

I think this was discussed one time before on voltage and amp draw.
With the same resistance. ( same motor on same gearing)
If you go from 3 cells to 6cells which is twice the voltage.
The amp draw will go up roughly double.
A light bulb with more voltage will burn brighter.
I know cause i have done this. It won't last long though.

[glassdoctor]

Originally Posted by sleebus.jones

Sorry, no it doesn't. As voltage goes up less amperage is needed to do the same amount of work. That is because of this relationship here:

Amps = Watts / Volts

Say for instance, we have a 1000w motor on a 6 cell pack:

139A = 1000w / 7.2v

Now, let's upgrade that to a 10 cell pack:

83A = 1000w / 12v

See? The amps went down, not up. If the amps stay the same when the voltage went up, that would mean the motor somehow make more power...which can't happen, because the motor is unchanged.

What this vendor is asserting with their numbers is that the same motor can do more work with more voltage. That makes no sense.

Sleebus

I'm sorry, but you are mistaken. Period. I don't know where to start. You have the whole concept backwards.

The formulas are correct but you are applying them in reverse.

Somebody here has to see this besides me....?

[glassdoctor]

[QUOTE=BrianG]This thread seems a bit confusing. I agree with GD for the most part.QUOTE]

That's because I'm right, BrianG :005:

Look at it this way.... run your car with a watt meter/data recorder in it... on whatever battery you normally run in it.

Now add one more cell to it and run it again. Amps (and watts) go up, not down.

That's just the facts, it's the way things work.

This is not meant to be confused with using higher voltage to be able to run lower gearing and/or lower kv motors to get greater efficiency. I think some are not focusing on the original question about the motor specs.

10v at 100A= 1000W
20v at 50A= 1000W
100v at 10A= 1000W

This is true, obviously.... but you can't apply it backwards to our cars and say that a given motor will put out a constant 1000W, therefore, if you run it on more volts, the amps will go down.

Again load (gearing) has a big effect on the amp draw, and it's possible that a setup geared for the same top speed may indeed draw less amps on 5s than on 4s with the same performance. I'm sure this is what everyone is thinking of if they disagree with what I have posted. But that's not the point in question.