Very interesting Setup from Mega Motors - Page 2 - RC-Monster Forums

GriffinRU · 11.19.2006, 12:33 AM

Wow, even light bulb was hard...

BrianG and you are about back EMF...

Artur

BrianG · 11.19.2006, 12:51 AM

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.

squeeforever · 11.18.2006, 11:41 PM

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

BrianG · 12:17 AM

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.

squeeforever · 12:16 PM

Quote:

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 · 11.19.2006, 01:43 AM

Sorry. :dft001: :)

glassdoctor · 11.19.2006, 10:05 AM

[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.

sleebus.jones · 11.19.2006, 11:38 AM

Quote:

Originally Posted by glassdoctor

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.

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

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

GriffinRU · 01:24 PM

Quote:

Originally Posted by sleebus.jones

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

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

BrianG · 01:36 PM

Quote:

Originally Posted by sleebus.jones

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

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

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).

squeeforever · 11:53 PM

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.

GriffinRU · 11:59 PM

Quote:

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

crazyjr · 11.19.2006, 01:53 AM

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 · 11.19.2006, 02:55 AM

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 · 11.19.2006, 04:13 AM

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.