(Discussion)Most effient voltage??
 

Has anyone spent the time to figure out the most efficent voltage for a given motor??

Take the the most popular motor for example.
The BK 7 or 8 XL series.

I cant find the amp ratings or the KT (torque ratings). Please post a link, if you have one to the motor specs.

I am an electrican by trade and still have a hard time figuring out all the variables.

As voltage increase so does efficenty (spelling??). So we need to discover the best voltage for each given motor.

Being I have an 8xl I would like to figure out the best voltage.

For many motors the most efficient voltage should be when the motor is spinning to about 40,000RPM. That's like 5.5 Lipo cells or 16.5NiMh cells. Obviously you can't get these cells in halves, but that's about what they would be.

I am assuming you are reffering to no load. again 8xl as an example.

I am assuming the 1875 is the Volatge constant.
40,000/1875= 21.3 volts. Can you tell me what the amps rating is on these motors or the winding resistance??

5s lipos are 3.7*5= 18.5 nominal

6s lipos are 3.7*5=22.2 nominal

Just from the above caculation to get the BEST perfomace out of your motor you should be running 21.3 volts then gearing your motor to stay within the motor amp draw.

Now to clear a few things up. The voltage constant is the KV rating. The Motor should also have an AMP constant.
With that said as you lower your KV rating (voltage constant) you amp constant should go up letting to draw more amps thru the motor in turn creating more available watts. Sound about right??

So an 8XL is on the lower side of power as far as XL cans go but it has alot longer run time to get almost the same power.

To get a super fast motor you need a 12XL.
Voltage constant is 1251.
40000/1251= 31.9 volts. WOW.
This would make for a huge amp draw to get the same RPMS.
Anyone have apms on these motors??

Well if you look over at the lehner charts. The effency depends on the load against the motor as well. So it is really not about as much as volts alone. The load amount at certain voltage will draw a certain amps which in turn is the most efficent.

Perhaps you'll find this useful:
http://img.diytrade.com/cdimg/85838/...1082223033.jpg
I'm pretty sure the amp draw listed is where the motors are most efficient (they mean nothing for RC cars, though).

As the kv goes down, the resistance goes up, which means lower amp draw (indicated in the chart). As kv goes down, voltage should also go up. All of the XL motors are capable of producing the same power, but the 12XL would heat up less if it is spinning at the same RPM as an 8XL.

I am trying to get to the most efficent voltage. You guys are a big help.

Yes all the given loads are huge factors but to start with a no load situation and then gear it to the correct amps using a watt meter will give you the best perfomance out of you motor.

People are always asking whats the best for "xamount" of cells.

So I wanted to turn it around and use the most efficent motor for a given voltage.

I know I need to figure in amp draw in the equasion becuase of battery limitations but need to keep it simple at first.


So the most efficent voltage for an 8XL would be 21.3.
The max amps on it is 74 according to the charts. That would tell me I should should for a 5s lipo that is capable of an 74 amp burst. That should get me in the correct power factor.

After that I need to gear the motor to keep it under the temp limit.

I dont want to get into winding resistance vs voltage loss I dont think it is neccessary for cars.

The only thing to remember with resistance is... to buy the lowest resistance motor your batteries can handle to keep the heat down (for cars).

I want to run 4s lipos for a nominal voltage of 14.8v.

40000/14.8=2702 KV

So I would want a 6XL which would put me in the 95 amp range.
I would need a better controller the the 9920.
and I would need at least a 95 amp battery. So that is pretty much out. I dont want to upgrade my controller (yet) and dont want to pop for the batteries.

If I added 1 cell for 5s lipos I would be right in range for the 8XL.
3.7*5= 18.5 v
The motor should be able to handle roughly 74 amps.
9920 should handle the and 5s 8000 mah should cover any current I need.

Given me a grand total of 1300 watts. So the motors should be rated somewhere in the 1300 watt area.

I think I have it down enough to help a few people out. (I am getting asked all the time at the track).

I imagine most people with limited battery space are stuck using low voltages (unless they go Lithium), so they almost have to choose a low kV/high amp motor to compensate. It seems that trucks where space is not as much an issue should use a high wind, higher voltage, and then gear up.

I look at it this way; if I want 2 HP (which is around 730 watts), do I want high voltage and low current, or low voltage and high current? High current requires heavier wires, more heavy duty ESC, require beefier batteries, and more losses due to voltage drop and heat. Of course, your gearing options limit you somewhat as well; if you can't go above or below a certain gear ratio, then you are limited in choices for whatever top speed you are aiming for.

I'll tell you, there isn't much difference in real world speed between my 8XL and 10XL on 14 cells, but the truck using the 10xl is a bit lighter.

On a related note, does anyone know what the voltage limit is for these motors? I mean the voltage that it would take to arc through the thin varnish (insulation) on the winding. This is sometimes called a hi-pot test (high-potential). Not that we would run anywhere near that voltage, since it's probably ~200v+ but it would be nice to know how high we are able to go provided we stay at a safe kv rating.

I'm sorry?

Brian;
For sake of discussion.

1 hp = 745.699872 watts
Let me see if I am getting this correct.
To get the best perfomance out of the 2 motors you would need to run different voltages.

The voltage for the 8xl should be in the 21 volt range and geared for a max of 74 amps. For roughyl 1300 watts.

For the 10XL you could run roughly 26 volts. There by decreasing the amp's need to get the same watts. Some where in the 60 amp range.

So if you had room to get 26volts of batteries in your car/truck you could would run cooler.

A few other things that being an electrican that I want to interject. As you increase your voltage your amps will go down at the same watt level.
Also the more the voltage the more efficent things become.
Example a 120v 100 watt light uses .83 amps
a 277v 100w light uses .37 amps The more amps you have the more heat you create (current), however voltage does not create any heat.

So if you were able to add a few more cells to the 10xl you would get longer runs time and stay cooler also it would be less stress on your esc.

The main thing that determines the efficiency of a motor is the Io. That's amount of current the motor will draw at no load. Take an 8xl for example. Io is 3.6 amps. 21 volts @ no load means the motor is using 75.6 watts. This is known as iron loss. That's how many watts are automatically being wasted as heat, since the motor isn't doing any work at no load. The problem is, the 8xl will always be wasting an extra 75 watts of power due to friction in the bearings and inconsistancies in the magnetic field. However, the 10xl which has an Io of 2.3 amps will only waste 59.8 watts of power at 26 volts. So there you have a more efficient motor.

Now, when iron loss equals copper loss, (ohms x amps^2), you get the amp rate at which the motor runs the most efficiently. The fewer amps, the better.

Sorry, sorry, I [i]meant[/i[ to write "if I want 2 HP (which is around 730 watts per HP)". I let my thought get ahead of me and forgot to write the rest. :dft001:

And yes, I know 1 HP is a little more than 730W, I knew it was close and didn't want to convert it right then. A thousand apologies!