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Old Sep 19, 2005, 10:24 AM
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Setup - Hi amps vs Hi volts

I saw some great modls at NEAT this weekend and was considering building something bigger this winter. I saw a Fliton Extra set up with a high voltage pack - something like 6s1p and as a 3 or 4s 2p. What are the pros and cons of producing power each way?
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Old Sep 19, 2005, 10:39 AM
Master of 1 point landing
Naperville, IL
Joined Jul 2005
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Quote:
Originally Posted by mexico
I saw some great modls at NEAT this weekend and was considering building something bigger this winter. I saw a Fliton Extra set up with a high voltage pack - something like 6s1p and as a 3 or 4s 2p. What are the pros and cons of producing power each way?
Higher voltage drops total amps for the same power out(watts = amps*volts). However you have to consider the motor wind as well. A 10 turn motor will draw more on 3S than 2S. If you increase the wind count to, say, 20 to spin a bigger prop, your 3S 20 wind will draw less amps than a 2S 10 wind for the same power output.

Dont just think that bumping up the number of cells in series will decrease the amps, it decreases amps for the SAME POWER OUTPUT (via prop and HEAT).

Hope that makes sense.
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Old Sep 19, 2005, 10:46 AM
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That makes sense and I understand that. But that is my question. Not how that calculation works but how do you choose which way to produce the desired wattage? Why choose hi voltage or why choose higher current? What are the pros and cons of each? More cells lower mah or less cells and higher mah
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Old Sep 19, 2005, 11:44 AM
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higher voltage power systems are more effecient and your power system will usually run cooler. Current is what produces the heat, so high volts and low amps are better. The only down side is you have to run more cells with high voltage, thus more money and more weight.
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Old Sep 19, 2005, 12:12 PM
Giz
Tony Rogers
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Bath, UK
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The choice of high current versus high voltage affects 3 areas: motor, ESc and battery pack.

Motors:
The main thing with motors is to try and run them as near to their maximum efficiency as you can. Different winds allow you to choose whether this at a high or low current. Losses in the motor are made up of copper losses and iron losses. Copper losses = I * I * R so losses rise rapidly with increasing current. Iron losses = Io * V where Io is the no load current so iron losses increase with voltage. It may seem like a good idea to run a high kV motor (lower number of turns) because the internal resistance will be lower causing lower copper losses, but invariably, the no load current will be higher causing higher iron losses. In fact it can be shown that all motors of the same type/size are just as efficient as each other but at different voltages/currents.

So, with motors, pick your prop and then choose wind and gearing to run most efficiently.

ESCs:
Up until fairly recently, it was fairly easy to get high current ESCs but harder to get high voltage ones so it was easier to go for higher current. Now there are more high voltage ESCs available. I don't know whether there are efficiency differences with high voltage or current with ESCs but they shouldn't give off a lot of heat anyway so shouldn't be too wasteful. One thing to remember though is that a battery elimination circuit is usually only available up to about 10-12 volts so if you go high voltage you are going to need either a separate switching BEC or receiver pack.

Battery Packs:
Losses in battery packs come from the internal resistance of the cells and is proportional to the current drawn. However, a pack designed for a high current set up will use larger cells or cells in parallel to supply the current and capacity at that current. Larger cells or cells in parallel will cause the pack resistance to be smaller. A pack deisgned for high voltage use (at the same weight) will have more, smaller cells in series which will cause the pack internal resistance to be higher.

Consider two 100 watt capable packs: one 3s2p and one 6s1p. Say the cell resistance is .020 ohms.

3s2p. Pack resistance = .030 ohms. Current = 10 amps. Losses = 10 * 10 * .03 = 3 watts.

6s1p. Pack resistance = .12 ohms. Current = 5 amps. Losses = 5 * 5 * .12 = 3 watts.

So again, there is little difference really. A low current setup will cause lower losses in the wiring and connectors but these should not be getting hot anyway so I think that the differences are negligable.

My Conclusion:
The differences between high voltage setups and high current setups is not all that great. Consider things like BEC/servo requirements and cost of ESCs. Choose your battery power requirement first based on power, duration and weight considerations. Consider what battery configurations are available such as cell capacities, current delivery capability and cell weights. Sometimes you can't get the cell size you want. I have an aeroplane that is designed to use 3s 4000 packs. I chose a motor for 4s but I could not find a 4s 3000 pack at the appropriate weight. I used 4s 4000 so my pack weighs 30% more than it should!

Choose the propellor that you want to use. This is usually the largest diameter that will fit the aeroplane with an appropriate pitch.

Compare motors with the available battery packs to find one that will turn your chosen propellor (geared if necessary) with one of the available packs to deliver the required power. Choose an ESC to suit.

One final thing that I would say is that the recent availability of high voltage systems does allow some existing motors to be used efficiently in situations that they were not designed for which can give added flexibility. I am using a Mega motor with a low kV, that was designed for sailplanes on 8-10 round cells, at about 24 volts in a Midi Fan! It will eventually use a 7s1p pack which will allow a very low pack weight (albeit with short duration). Other motors that would have provided that power required may have needed 3s3p or 4s2p meaning 9 or 8 cells.
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Old Sep 19, 2005, 12:20 PM
Master of 1 point landing
Naperville, IL
Joined Jul 2005
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Quote:
Originally Posted by mexico
That makes sense and I understand that. But that is my question. Not how that calculation works but how do you choose which way to produce the desired wattage? Why choose hi voltage or why choose higher current? What are the pros and cons of each? More cells lower mah or less cells and higher mah
High current = expensive ESC, High voltage = expensive battery. Trade offs on each one.
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Old Sep 19, 2005, 12:32 PM
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Toronto Canada
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Giz,

So can one conclude from your statement below that today's motors can take more voltage than their rated max? I just bought a MpJet motor (AC 28/20-7 Mk2 - MPJ 20201 seen at http://www.mpjet.com/motor/brushless...window_en.html) that I've been running with on 3 cell lipos with an 11x8 APC E prop and getting about 290 watts at about 29 amps.

However I wanted to get up to its max of 400+ watts but don't want to increase the amps. So I was thinking of going the route of increasing the volts to 4 cell lipo packs ... however it's only rated for 6-10 cells ... so am I stuck with having to prop up to increase the amps to get up to 400 watts ... or can I (based on the "impression" I get from your statement) get away with using the 4 cell pack even though it's rated for 12 volts max?

-----------------------------------------------------------------------
"One final thing that I would say is that the recent availability of high voltage systems does allow some existing motors to be used efficiently in situations that they were not designed for which can give added flexibility. I am using a Mega motor with a low kV, that was designed for sailplanes on 8-10 round cells, at about 24 volts in a Midi Fan! It will eventually use a 7s1p pack which will allow a very low pack weight (albeit with short duration). Other motors that would have provided that power required may have needed 3s3p or 4s2p meaning 9 or 8 cells."
-----------------------------------------------------------------------
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Old Sep 19, 2005, 01:08 PM
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USA, CA, San Francisco
Joined Aug 2004
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Quote:
Originally Posted by wollins
Giz,

So can one conclude from your statement below that today's motors can take more voltage than their rated max? I just bought a MpJet motor (AC 28/20-7 Mk2 - MPJ 20201 seen at http://www.mpjet.com/motor/brushless...window_en.html) that I've been running with on 3 cell lipos with an 11x8 APC E prop and getting about 290 watts at about 29 amps.

Unfortunately not all manufacturers rate their motors the same way. The motor Giz is talking about is a Mega, and they are notorious for take waaaaay more abuse than they are rated for and not complaining a bit. I have one that is rated somewhere around 200W, I believe. I put 450-500W through it on a regular basis, it doesn't overheat (in fact I'm more concerned about my batteries getting too hot and they *are* rated for that power), produces great power and is still going strong after a year of abuse. Unfortunately there's no way to know which manufacturer is being conservative in their ratings and which is exaggerating, other than these forums. Poke around, ask questions and look for other people running similar setups to yours and find out how hot their motors are getting, etc. When in doubt, though, ya gets what ya pays for...
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Old Sep 19, 2005, 02:07 PM
"RC ADDICT" in Toronto!
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Toronto Canada
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Quote:
Originally Posted by Neil Walker
Poke around, ask questions and look for other people running similar setups to yours and find out how hot their motors are getting, etc.
Will do ...
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Old Sep 19, 2005, 02:11 PM
Giz
Tony Rogers
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Bath, UK
Joined Sep 2003
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Quote:
Originally Posted by wollins
Giz,

So can one conclude from your statement below that today's motors can take more voltage than their rated max?
Yes, that's quite right, especially with a brushless.

Motors are limited by two things in the main. Temperature at which windings or magnets cook/get damaged and the maximum rpm that the motor can spin at. Maximum rpm is a physical limit where things start to fly apart. So, provided that you stay below the maximum rpm and it does not get too hot, you can increase the voltage. The maximum voltage that can be used on a motor without a prop is the maximum rpm divided by the kV (rpm per volt) of the motor. So a motor with a max rpm of 50,000 and a kV of 2000 can be used at 25 volts with no load.

Brushed motor have an additional limitation at a voltage where excessive sparking occurs at the brushes.

However, increasing voltage with the same prop will cause the motor to draw more current so if you increase the voltage, you may need to reduce the prop size.

Quote:
Originally Posted by wollins
Giz,

I just bought a MpJet motor (AC 28/20-7 Mk2 - MPJ 20201 seen at http://www.mpjet.com/motor/brushless...window_en.html) that I've been running with on 3 cell lipos with an 11x8 APC E prop and getting about 290 watts at about 29 amps.

However I wanted to get up to its max of 400+ watts but don't want to increase the amps. So I was thinking of going the route of increasing the volts to 4 cell lipo packs ... however it's only rated for 6-10 cells ... so am I stuck with having to prop up to increase the amps to get up to 400 watts ... or can I (based on the "impression" I get from your statement) get away with using the 4 cell pack even though it's rated for 12 volts max?
The maximum recommended speed of that motor is 12,000 rpm and the kV is 950. So maximum no-load volts is 12,000 / 950 = 12.6 volts. it will spin slower with a prop on. There is also a maximum rpm quoted at 14,000 rpm which would give a maximum voltage of 14.74 volts. A 4s pack will give maybe 13-14 volts under load which would make it a bit marginal, rpm wise, but you may well get away with it. If you have a 4s pack, you could always spin it up gradually with your intended prop whilst checking the rpm to make sure that it does not go above the maximum.
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Old Oct 06, 2005, 01:55 PM
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txfly's Avatar
frisco,tx
Joined Oct 2003
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i know this thread is old , but heres a question : dont high amps cause your battery to "sag" in voltage? is this a factor to consider??
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Old Oct 06, 2005, 09:02 PM
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East Anglia, UK
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Quote:
Originally Posted by txfly
i know this thread is old , but heres a question : dont high amps cause your battery to "sag" in voltage?
[
Yes

Quote:
is this a factor to consider??
Not really. You will have put in a fatter battery knowing that this will happen.

Whether you have 8 cells at 40 amps or 16 cells at 20 amps, if you want the same weight the 8 cells will be 'fatter' cells.

In the old days of SUB C only, motors were designed or selected to run at about 15-30A (that being a comforable level for a 1700 Nicad SUB C), and you got more power by upping cell count.

Thes LIPO days, you can parallel cells to your hearts content. There is still a TENDENCY to limit current to about 40A and go up in cell count to get the power, but its not inviolable. People flying with 8000mAh packs are quite happy to draw up to 70A out of them..

In general hi current low voltage or vice versa is of no great importance: Todays motors are wound and tuned to specific pack voltages. And a plethora of windings is available to match whatever you decide is 'right' for you, your wiring standards and your ESC..
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Old Oct 06, 2005, 11:09 PM
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Mesa, AZ
Joined Aug 2004
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so you're saying that if you have two batts, say 3S @ 1000mAh, and two motors, one 1000Kv and the other 2000Kv but all other specs identical, and you wire them up appropriately (3S2P for the 2000Kv motor and 6S1P for the 1000Kv motor) there's no real world difference in duration, power, etc? Assuming your motors each are producing 200W of power, the 2000Kv motor would be drawing 18A @ 11.1V (9A off each pack in parallel), and the 1000Kv motor would be drawing 9A off the combined series pack.

There's no advantage in going one way or the other?

A real world example would be the Neu 1107/3Y vs the 1107/6D.
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Old Oct 07, 2005, 05:35 AM
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East Anglia, UK
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Quote:
Originally Posted by bsoder
so you're saying that if you have two batts, say 3S @ 1000mAh, and two motors, one 1000Kv and the other 2000Kv but all other specs identical, and you wire them up appropriately (3S2P for the 2000Kv motor and 6S1P for the 1000Kv motor) there's no real world difference in duration, power, etc? Assuming your motors each are producing 200W of power, the 2000Kv motor would be drawing 18A @ 11.1V (9A off each pack in parallel), and the 1000Kv motor would be drawing 9A off the combined series pack.

There's no advantage in going one way or the other?

A real world example would be the Neu 1107/3Y vs the 1107/6D.
Pretty much yes.

You would find the low KV motor had twice the winds, half the idle current and 4 times the resistance, but, all other things being equal, the power losses come out the same.

There are some second order effects: The ESC is likely (but not guaranteed) to be a little better at high voltage low current...and the gauge of wire in the pack can be lighter at lower current. But if the wiring and ESC semiconductors are optimnised, even those issues show themselves to be pretty much neutral.

Its totally different when you look at a SINGLE motor. There you can find the absolutely best voltage for it, for best power at best efficiency. Too many or too few cells will result in less efficency.
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Old Oct 07, 2005, 06:10 PM
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Bundaberg, Qld, Australia
Joined Jun 2002
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I will be following this thread closely, very interesting stuff. I had not known what copper and iron losses meant before. How did the motor lose iron and copper? Now I understand that those are effeciency losses, correct?
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