Efficiency: 2 vs 1 motors, high voltage vs stronger motors

Efficiency: 2 vs 1 motors, high voltage vs stronger motors

Post by LearningCyc » Thu, 04 Jan 2001 16:51:40



OK, these are for you pros, electrical engineers, or any other experts out
there.

People are starting to modify their E-maxx systems wired running either one or
two motors and using aftermarket ESCs such as super rooster.

Assuming that each setup is to hit a particular speed  (Lets arbitrarily peg it
at 30mph) & torque ability using the exact same gearing, which of these sets up
would be the most efficient?  (Meaning using the least number of watts for a
given speed/acceleration)

One milder motor with higher voltage (i.e. 8-10 cells)
One hotter motor with lower voltage(i.e. 6 cells)
Two milder motor with higher voltage (i.e. 8-10 cells)
Two hotter motor with lower voltage (i.e. 6 cells)

Or are efficiencies about the same across the board and vary with the quality
of the electronics instead?

Thanks for any expert discussions.

MT

 
 
 

Efficiency: 2 vs 1 motors, high voltage vs stronger motors

Post by Tom » Thu, 04 Jan 2001 19:30:13



Quote:
> OK, these are for you pros, electrical engineers, or any other experts out
> there.

> People are starting to modify their E-maxx systems wired running either one or
> two motors and using aftermarket ESCs such as super rooster.

> Assuming that each setup is to hit a particular speed  (Lets arbitrarily peg it
> at 30mph) & torque ability using the exact same gearing, which of these sets up
> would be the most efficient?  (Meaning using the least number of watts for a
> given speed/acceleration)

Power is equal to the voltage times the current.  Voltage is equal to the
current times the resistance.  Therefore, power is equaled to the square
of the current times the resistance.  This means current is very
significant.

To put this into easy terms which you can understand, power companies
transmit electrical power at thousands and even millions of volts in order
to achieve power transmission efficiency.  It is then put through
transformers (each of which has its own efficiency losses) to step down
the voltage to something that is remotely safe.  Thus, it is worth it to
loose power through transformers in order to transmit electrical power at
high voltages.

In the R/C context, this means that higher voltages are more efficient
than lower voltages, since, for the same amount of power, the current is
less.  Since R/C cars deal with relatively high amounts of power for the
voltages used, the efficiency differences are somewhat significant.

Quote:
> One milder motor with higher voltage (i.e. 8-10 cells)
> One hotter motor with lower voltage(i.e. 6 cells)
> Two milder motor with higher voltage (i.e. 8-10 cells)
> Two hotter motor with lower voltage (i.e. 6 cells)

The higher voltage systems will be more efficient, however you may not get
sufficient performance with only one motor.  Wiring both motors in series
and both batteries in series would give the higest efficiency, but you may
not get enough power out of the motors.  The common motors used in R/C
cars aren't terribly happy at high voltages; they have a hard enough time
coping with 6-cell touring car use.

Quote:
> Or are efficiencies about the same across the board and vary with the quality
> of the electronics instead?

What you end up doing will end up being dictated by the equipment you
choose.  What's wrong with the original motors?

___
TTTTT   OO   M   M The sixth sick shiek's sixth sheep's sick.   |~~~|

  T    O  O  M M M So if it is in it or if it is on it, it is     *  
  T     OO   M   M        as it is, be it in it or on it.        `-'

 
 
 

Efficiency: 2 vs 1 motors, high voltage vs stronger motors

Post by Olev Pih » Thu, 04 Jan 2001 21:00:21


Quote:

> Assuming that each setup is to hit a particular speed  (Lets arbitrarily peg it
> at 30mph) & torque ability using the exact same gearing, which of these sets up
> would be the most efficient?  (Meaning using the least number of watts for a
> given speed/acceleration)

> One milder motor with higher voltage (i.e. 8-10 cells)
> One hotter motor with lower voltage(i.e. 6 cells)
> Two milder motor with higher voltage (i.e. 8-10 cells)
> Two hotter motor with lower voltage (i.e. 6 cells)

- Motors with more turns (i.e. "milder") have higher efficiency than
motors with less turns.
- Voltage increase gives more current (given the same motors, which are
the resistance). More total power, but the power comes from somewhere-
the batteries.

You have to figure out what you want to do. If you want 30 mph (or
whatever) you have to find out what is the required motor rpm for the
speed and find a motor that is the most efficient at this particular rpm
and load, which is generated by the rolling resistance and air drag of
the vehicle. This is impossible to calculate (at least for me;) knowing
nothing else.

IMO max efficiency would be with two mild motors. Hotter motors have
worse efficiency to start with and one mild motor would have to work
twice as hard, so it would probably be somewhere in the powerband where
the efficiency is worse. Number of cells is not important, it only
counts for total available power, not efficiency at certain speed.
If you want max power, the story is different. Hot motors with proper
gearing and higher number of cells are the deal. Don't cry about runtime
then though...

BR,
-olev-

 
 
 

Efficiency: 2 vs 1 motors, high voltage vs stronger motors

Post by the villai » Thu, 04 Jan 2001 23:43:01


Quote:
>One milder motor with higher voltage (i.e. 8-10 cells)
>One hotter motor with lower voltage(i.e. 6 cells)
>Two milder motor with higher voltage (i.e. 8-10 cells)
>Two hotter motor with lower voltage (i.e. 6 cells)

Ideally you want a motor which gives good low range torque (high number of
turns) and a motor which gives you a high speed (low number of turns). From
pratting around for years with two motors, i would recommend you go for a 19
turn and a 10 turn. You'll have to experiment wit hte gearing because I
don't have the specified machine.

I would also recommend you wire the motors in parallel and not series, for a
number of reasons namely, power is proportionaly to current sqaured, you
want to cause as little power loss int he wires as possible thus you should
wire in parallel.

as for the voltage, higher is better.

tv

 
 
 

Efficiency: 2 vs 1 motors, high voltage vs stronger motors

Post by Olev Pih » Sat, 06 Jan 2001 23:04:18


Quote:

> Ideally you want a motor which gives good low range torque (high number of
> turns) and a motor which gives you a high speed (low number of turns). From
> pratting around for years with two motors, i would recommend you go for a 19
> turn and a 10 turn. You'll have to experiment wit hte gearing because I
> don't have the specified machine.

Now this is something I certainly would not recommend:) Using Dual
motors they should both be the same, otherwise they would be fighting
each other instead of moving the car.

Quote:
> I would also recommend you wire the motors in parallel and not series, for a
> number of reasons namely, power is proportionaly to current sqaured, you
> want to cause as little power loss int he wires as possible thus you should
> wire in parallel.

The suggestion is correct, the reasoning not. The reason you should wire
the motors in parallel is that this way they both always get the same
amount of voltage, so they are always able to give out max power, no
matter what the other motor does. I try to explain:
Imagine dual motor monster with motors wired in series, crawling over a
rock, which gets stuck below front end, so that both (or at least one)
front wheels are in the air. This makes the load on front motor quite
small, which causes only small current flow through the front motor
(less load=bigger internal resistance=less current). If the motors are
wired in series, that means both motors have the same current passing
through (since thay are the same current circuit). The load on the rear
motor is much higher, internal resistance smaller, leaving only small
amount of voltage to the rear motor, which in turn means no power. Can
we say stuck?
With parallel wired motors this won't happen. Both motors always have
the same voltage, the front motor will just spin a lot faster due to
less load, but the rear motor is still able to move the thing.

Quote:
> as for the voltage, higher is better.

Mostly yes, especially when added weight is not a big issue.

BR,
-olev-

 
 
 

Efficiency: 2 vs 1 motors, high voltage vs stronger motors

Post by Tom » Sun, 07 Jan 2001 02:24:07


(*SNIP!*)

Quote:
> > I would also recommend you wire the motors in parallel and not series, for a
> > number of reasons namely, power is proportionaly to current sqaured, you
> > want to cause as little power loss int he wires as possible thus you should
> > wire in parallel.
> The suggestion is correct, the reasoning not. The reason you should wire
> the motors in parallel is that this way they both always get the same
> amount of voltage, so they are always able to give out max power, no
> matter what the other motor does. I try to explain:
> Imagine dual motor monster with motors wired in series, crawling over a
> rock, which gets stuck below front end, so that both (or at least one)
> front wheels are in the air.

(*SNIP!*)

Very good example, with one little itty bitty problem.  The vehicle in
question is an E-Max, which has both motors driving the same spur gear.
Therefore, this scenario doesn't apply, so, for efficiencie's sake, both
batteries and both motors should be wired in series.  If it were a
Clod-style truck (with independant transmissions front and rear), then I'd
suggest two motors and two batteries all wired in parallel.

___
TTTTT   OO   M   M The sixth sick shiek's sixth sheep's sick.   |~~~|

  T    O  O  M M M So if it is in it or if it is on it, it is     *  
  T     OO   M   M        as it is, be it in it or on it.        `-'

 
 
 

Efficiency: 2 vs 1 motors, high voltage vs stronger motors

Post by Remy Alexander Levin » Sun, 07 Jan 2001 03:45:01




Quote:


> (*SNIP!*)

>>> I would also recommend you wire the motors in parallel and not series, for a
>>> number of reasons namely, power is proportionaly to current sqaured, you
>>> want to cause as little power loss int he wires as possible thus you should
>>> wire in parallel.
>> The suggestion is correct, the reasoning not. The reason you should wire
>> the motors in parallel is that this way they both always get the same
>> amount of voltage, so they are always able to give out max power, no
>> matter what the other motor does. I try to explain:
>> Imagine dual motor monster with motors wired in series, crawling over a
>> rock, which gets stuck below front end, so that both (or at least one)
>> front wheels are in the air.

> (*SNIP!*)

> Very good example, with one little itty bitty problem.  The vehicle in
> question is an E-Max, which has both motors driving the same spur gear.
> Therefore, this scenario doesn't apply, so, for efficiencie's sake, both
> batteries and both motors should be wired in series.  If it were a
> Clod-style truck (with independant transmissions front and rear), then I'd
> suggest two motors and two batteries all wired in parallel.

> ___
> TTTTT   OO   M   M The sixth sick shiek's sixth sheep's sick.   |~~~|

> T    O  O  M M M So if it is in it or if it is on it, it is     *
> T     OO   M   M        as it is, be it in it or on it.        `-'

Of course the whole issue is moot if the esc or motors only will survive
being wired one way. Series is more efficient but slower for the motors (not
the batteries). How you wire the batteries depends on whether you want more
power or longer runs, in series essentially makes a single battery made up
of however many cells you put in the car, so a pair of 6cell packs make one
12 cell pack. In parallel you get double the run time because you split the
load between the packs, but each acts as a separate battery so you don't get
the doubled voltage. I am not really familiar with how the E-Maxx is set up
but I am fairly sure it's designed to run with at least the batteries in
series (causing the regular overheating problems which are mostly due to
cheaply built packs and the bare minimum esc Traxxas includes). I'm not sure
on the motors.

Remy

 
 
 

Efficiency: 2 vs 1 motors, high voltage vs stronger motors

Post by Eric Bryan » Sun, 07 Jan 2001 12:52:40


Quote:

>Very good example, with one little itty bitty problem.  The vehicle in
>question is an E-Max, which has both motors driving the same spur gear.
>Therefore, this scenario doesn't apply, so, for efficiencie's sake, both
>batteries and both motors should be wired in series.  If it were a
>Clod-style truck (with independant transmissions front and rear), then I'd
>suggest two motors and two batteries all wired in parallel.

You get two problems with running batteries in series.

The first is the issue of matching the packs to each other.  If the packs
aren't matched, the first one to dump will get a reverse charge from the
second.  That will shorten pack life.

The second is exceeding the voltage limit of the ESC.  I'm not sure if this
is the problem with the faulty E-Maxx ESCs, but it's possible (there's a lot
of things that could be wrong with that part).

Running the two motors and packs in series does cut the current in half, so
you'll reduce your IR losses in the wiring and ESC.  That'll certainly make
the ESC happier, since it'll need to dissipate less heat.  Assuming that the
ESC can handle a 14-cell pack, I'd run the series option.

Excuse my ignorance, but doesn't the E-Maxx run the packs in series and the
motors in parallel, so that each motor operates at 14.4 V?  I don't have one
in front of me right now.

Eric Bryant          Grand Haven, MI

http://www.novagate.com/~bryante

 
 
 

Efficiency: 2 vs 1 motors, high voltage vs stronger motors

Post by Tom » Sun, 07 Jan 2001 13:25:31



(*SNIP!*)

Quote:

> You get two problems with running batteries in series.

> The first is the issue of matching the packs to each other.  If the packs
> aren't matched, the first one to dump will get a reverse charge from the
> second.  That will shorten pack life.

This is very true.  This is why owners of E-Maxes are instructed to
purchase matched packs (that is matched to one another).  However, even
un-matched batteries work fine, so two unmatched packs should work fine
together too; just don't mix capacities.

Contrary to popular belief, you can run mixed capacities in parallel with
no ill effects, but that's for another discussion.

Quote:
> The second is exceeding the voltage limit of the ESC.  I'm not sure if this
> is the problem with the faulty E-Maxx ESCs, but it's possible (there's a lot
> of things that could be wrong with that part).

Hopefully this is obvious, since that was part of a previous discussion.

Quote:
> Running the two motors and packs in series does cut the current in half, so
> you'll reduce your IR losses in the wiring and ESC.  That'll certainly make
> the ESC happier, since it'll need to dissipate less heat.  Assuming that the
> ESC can handle a 14-cell pack, I'd run the series option.

That's the whole point for my suggestion.

Quote:

> Excuse my ignorance, but doesn't the E-Maxx run the packs in series and the
> motors in parallel, so that each motor operates at 14.4 V?  I don't have one
> in front of me right now.

Yes it does, however it uses 5-50 sized motors.  The more common (in R/C
use) 5-40 size isn't happy with that much juice, hence running them in
series.  Apparently the 5-50 motors that come with the E-Max are pathetic,
and even though they are larger than 5-40's, they are both less efficient
and produce less power.

___
TTTTT   OO   M   M The sixth sick shiek's sixth sheep's sick.   |~~~|

  T    O  O  M M M So if it is in it or if it is on it, it is     *  
  T     OO   M   M        as it is, be it in it or on it.        `-'

 
 
 

Efficiency: 2 vs 1 motors, high voltage vs stronger motors

Post by Olev Pih » Tue, 09 Jan 2001 21:33:33


Tom's correct, (as he always seems to be, for some reason;) I completely
forgot about the E-maxx using different approach than all the dual-motor
vehicles I've seen this far.

BR,
-olev-

Quote:

> Very good example, with one little itty bitty problem.  The vehicle in
> question is an E-Max, which has both motors driving the same spur gear.
> Therefore, this scenario doesn't apply, so, for efficiencie's sake, both
> batteries and both motors should be wired in series.  If it were a
> Clod-style truck (with independant transmissions front and rear), then I'd
> suggest two motors and two batteries all wired in parallel.