Very interesting Setup from Mega Motors
 

Any thoughts on this setup?

Artur

Well, it seems to defy the laws of physics at first glance. The amp draw goes up as the battery voltage goes up...that doesn't make sense. All things being equal, higher voltage should equal less amp draw.

Also, the back-calculated Kv rating varies with voltage, another weird thing:

1. 3S = 1306Kv
2. 4S = 1250Kv
3. 5S = 1281Kv
4. 12v = 1320Kv

Something's not right, or someone's being sloppy.

Sleebus

that would be cool, but it would take a lot of gearing or batteries to wake it up. I'd like to see that heatsink as a bolt-in for any XL, I'd definetly go brushless if it were possible

Actually this is not true at all... their numbers are correct.

Amp draw increases with voltage. Given the same motor, etc... more voltage always means more current.

The high voltage/low amp draw thing is due to using a different motor/gearing setup with the change in voltage.

This is how a system can operate on lower amp draw and still make the same power (watts). You use a much milder amp draw motor and just feed it more volts.

I don't know, 1300kv seems a bit low. You'd almost have to use 5s to get the RPMs out of it. I'm not really crazy about the looks either, although that is a minor consideration.

It looks like a xl motor with a weird heatsink. Should run cool though.

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

What happen to the motor when you apply more volts?
Hint -> Power=torque*RPM
Torque and Amps are twins...

Artur

How they fix motor inside? Is it a clamp style?
It helps in fixing motor in 2 planes, but still requires to align with differential, which is usual.

Artur

Alot of this is over my head, but for instance, the XL motors are all rated for around 1800 watts (I think), so if the dimensions, etc of the motor stays the same, but the wind/RPM's change, the watts is gonna be a constant. Example:

1800W / 22.2V = 81A

1800W / 14.4 = 125A

But what if the KV changed? The watts would be the same, but wouldn't the amperage change alot with the KV? Yes, it would. A higher wind XL motor (like a 9XL) on 18 cells will pull less amps than a 7XL on the same amount of cells, will it not?

Basically I see it like this. The formula you gave would work if the RPM's were a constant between each motor. That formula might work for other things without the extra variable...Now this may be wrong...But this is the way I understand it.

Squee, you've got it spot on. Motors are designed to do a specific amount of work. It's just like a horse, no matter how hard you whip it, once it's going as hard as it can, it's not going to work any harder. If you need more work done, you either need:

1. A different horse

or

2. More horses!

Yes, if the KV changes, you've changed "horses" and the draw will be different for the same work done. It's not as over your head as you think! :)

Artur: When you apply more volts, you get more RPM, up to the limit of the work the motor is able to do. It's the same analogy with the horse, you'll only get so much out of the motor, regardless of what you do.

Sleeb

Ok, so after reading this thread several times, it seems as though you and glassdoctor are both correct depending on the situation...

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

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

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.

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

Wow, even light bulb was hard...

BrianG and you are about back EMF...

Artur

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 :)

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".

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...

Sorry. :dft001: :)

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

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.

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.

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.

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....?

[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.

Yeah glass, that's what i am saying.............

Ah. So, this manufacturer is lying on their motor dataplate:

http://i94.photobucket.com/albums/l9...es/motor01.jpg

As much as has been argued otherwise, a motor is a constant power device, and the proof is right there in the picture. As voltage goes up, amp draw goes down. If it didn't, then if you ran this motor on 208v, it would magically become a 3.5 HP motor, which is not possible. When you run it on 208v, it draws less power. It's not able to do any more work (which means horsepower, which is equivalent to 746 watts) than it was on 110v, which is exactly why the amp draw goes down. The motor power is set by the design, not the voltage.

Either I'm wrong, or someone changed the laws of Thermodynamics while I was sleeping.

Sleebus

The motors we use in RC will never run at constant power.

On the previous page you mentioned after BrianG's that Ohm's Law has nothing to do with this. In fact, it does. The motor has a resistance that is constant for the most part. If you do not change the gearing when increasing voltage, then the resistance is constant. You cannot use constant power to determine the amp draw, as the only thing that would be close to determinging the MAX power is the battery, but the only possible way for you to be running at constant power is if you were running at full throttle the entire time and the speed/terrain/acceleration/battery voltage were also constant.

So, since power is not constant, the amp draw will increase as voltage increases (assuming a fixed load, IE same gearing). If you use higher voltage AND the load decreases (smaller pinion or larger spur) then the amp draw decreases.

I will explain later, below is more confusing then helping. SORRY

Quote:

Ther is a difference between this motor and motors we use in RC. This motor power determines by induction of the motor windings and once they saturated there is no more power, but you can burn it :) In our case we have constant magnet inside and we can drive coils until saturation (if there is a core) or until meltdown, obviously there is no need to get more magnetic power then constant magnet has...

Artur

P.S. It is very simple "how it works"
Cool link and more models on the side bar

Sleebus, read my post.

(http://www.rc-monster.com/forum/show...4&postcount=27)

And if you look closer to the picture of that motor, it has got TWO setting, one for high and one for low voltages.

Please stop being sarcastic sleebus, this is just a discussion, nothing changed while you where sleeping, that motor always had two settings.

Ah! Well, I must admit, I did not see any such setting. I'm squinting at it now, and I honestly can't see what you are talking about, but I do trust you that it's there. That would explain why the motor is acting so weird electrically.

I'm just trying to inject a bit of humor. I shoulda put a ;) at the end. Sorry that I came across as a jerk.

Sleeb

Sleebus, I can see where you might be coming from with the whole "work" thing, but there are assumptions made which are incorrect.

Motors are not constant power. The power listed on a motor tag will be at the nominal rated voltage. Actually, they usually list this power as "VA" which takes power factor (basically an efficiency factor) into consideration, but I won't go into that. If you double the voltage, you WILL increase current and power by a substantial amount. Maybe not exactly double because of the change in rpm (therefore beack EMF, ie: resistance), but close.

Most dual voltage (120v/240v) AC motors I've seen have alternate sets of windings/connections for the optional voltage; or if three phase, you can hook the windings in wye instead of delta or vice versa. And that's what the little diagram on the right looks like - a way to wire the windings for higher voltage. The motor windings would not like it if you tried to double the current going through them. So, the alternate wiring was designed to increase resistance enough so the power remains the same with higher voltage.

Besides, how could a passive device be constant power? The resistance would have to change. The only way for a device to be a constant power device is if it had active circuitry to read the voltage and be able to adjust its resistance to develop less current to generate equal power.

Ohms law is Ohms law, and it works - always. It gets a little more complicated than simple Ohm's law for AC motors because you have to take in drive frequency, rpm, and winding inductance (which changes the voltage to current phase angle).

Much better link then "howitworks"

So, nobody tried mega motors?

Artur

Look on the right side. It says low voltage then right below it has high voltage.

Yeah, on the right of that picture.