Power Draw of Motors?

[vekenti]

Hello,

Kind of confused on how power draw works. I'm wondering is it based more on load or pure rpm of the motor?

My main concern is that since my trail truck (Redcat Gen 8) is geared pretty low for crawling, does that mean that it will eat through batteries when I'm just walking through the hiking trails? The motor would be spinning pretty fast just to keep it at those speeds, but the motor wouldn't be under too much stress.

Can't seem to find the answer to this so I figured I would ask here.

Thanks for the time,
Noah.

[crawlhog]

it would give you a longer run time, say you put in a 15turn motor which would make the truck faster and drain the battery faster. if you put in a 55 turn motor it would drain the Batt. much quicker.


If im wrong some one please correct me

[lonleycreeper]

A watt meter will answer all your questions.

That said my single speed Scx10II on 3s is pretty quick with stock gearing it does about 1mile of trai per 1000 mAh. My 2spd version geared much lower drains the battery about 2x faster. My big heavy 11lb, Scx10II axles gen 1 trans and frame is almost as efficient as the Honcho but not quite.

[MAC FAB]

The easiest way to run longer is get greater rated batteries. If you run 4000mah 20c batteries, for example, buy the larger, say 8000mah 100c, lipos and extend your drive time. RPM while driving on flat land has less load on the motor, less current draw or fuel consumption. Go climb a 45 degree hill keeping top speed burns more fuel. Consider pulling a 28 foot travel trailer with your F150 with EcoBoost. On the flats mileage will be ok at 65 mph and away you go. Now go pull a long steep hill at 65 mph, you will either overheat your truck or run out of gas. Larger capacity batteries give you the 100 gallon fuel tank and the shot of NOS. By only changing the battery the run time increases dramatically. Changing electronics is the best route, but $$$ spent there. I'll catch flack for this comment, but look at some inexpensive electronics, and kv down and volt up your battery. Just know by doing so the torque load has potentially increased on the driveline which may cause failures there. Plan B. Provided you can run a higher s count with the current electronics without cooking them, then gear down your existing setup and accomplish what you want, a long drive time. The cheapest way out in the end may be fleet of batteries instead of a bunch of other upgrades. You'll get 2 or 3 lesser mah and c value batteries than a big one. Back to square 1, many trips to the gas station. How long do you run now vs what are you looking for?

[Hardcoretam]

Originally Posted by crawlhog

it would give you a longer run time, say you put in a 15turn motor which would make the truck faster and drain the battery faster. if you put in a 55 turn motor it would drain the Batt. much quicker.
If im wrong some one please correct me

A 55 turn motor will not drain the battery quicker. Higher turn motors of the same design as the lower turn, draw less amps at the same voltage.

Originally Posted by MAC FAB

The easiest way to run longer is get greater rated batteries. If you run 4000mah 20c batteries, for example, buy the larger, say 8000mah 100c,

The C rating on a battery does zero to increase the run time.
You use it with the Mah as a multiplier to determine the max constant amps it can supply

A 8000Mah 10C battery will perform and last as long in a crawler as a 8000mah 100C as 10C 8000Mah can supply 80amps constant and crawlers don't draw anywhere near that constantly. (XL-5 stock ESC has an 18Aamp constant rating)

[OSRC]

MAH is the size of the bucket and the C rating is the size of the hose you use to drain it.

If the motor is spinning along, not stressed out much as your going down the trail, that's a pretty ideal scenario as long as everything is cool temp-wise. Gearing it taller and having the motor working harder at lower RPM might gain you some runtime, but it is much harder on the electrical components - especially the ESC.

I try to gear my trucks to be running about 75% of throttle at walking speed. Seems to be the sweet spot for me.

[Voodoobrew]

I am not an electrical engineer but this is my understanding. feel free to correct any of this because some of it is probably wrong. and it probably doesn't really answer the OP.
Lower turn motors have the potential for drawing more power. This does not mean that they do. both a lamborghini and a ford pinto can go 60 mph at 12 mpg but only the lambo can do 200 mph. the thing is what your asking is not an easy answer, its one of those your mileage may vary questions because there are so many factors involved. A high quality motor will probably more efficient and get you more run time.
The turn rating is determined is basically the number of wraps of wire around a rotor, or half of the magnetic force in your motor. but the resistance of the wire and the diameter of the wire varies between manufacturers. So the end results are not always the same. The more copper you can fit in there the bigger the magnet, the more resistance it has the more amperage it can draw. some of that amperage is power, some is heat. This is efficiency.

a few terms to understand.

Torque is measured as weight/distance. It the ability to get your car moving, more torque means you can increase your speed faster, so its better for short distance races like the quarter mile and jumping off the line. Also it helps the maintain a constant rpm when force is applied against the rotation like when a crawler tries to slowly climb up a wall.
Speed is measured in rpms and is pretty easy to understand. more speed makes your car reach higher speeds. This has not bearing on how long it takes for your car to reach that speed. With low torque you may have a motor that can go 200 mph but it might only get up to 50 mph in the quarter mile.
Power is basically torque * speed . You can have 2 motors with the same power output with vastly different outcomes.

Turns is the number of times the copper wire is wrapped around the stator. by doubling the amount of turn you can double the resistance therefore increasing power, but twice as much wire won't fit into a motor
Wire Gauge, is the size of the wire, and higher gagues means smaller wire. This has a perportional(? or inverse?) relationship to motor turns, higher turns means higher wire gauge (or smaller wire).
By decreasing the number of turns can increase the wire size and you get more speed, more heat, more electrical resistance (potential for power).
Generaly a Higher turn motor will have a lower resistance then a low turn motor. This means less power.

Voltage.... Increased Voltage=Increased power. I'm guessing here but I believe this is by increasing the rpm and not the torque. However you may double the voltage and double the gear reduction to double the torque but maintain the same speed.

The amount of copper in a motor has the largest effect on torque in an rc brushed motor.

Originally Posted by JohnRobHolmes

Motor torque is linear to length and the square of the diameter (assuming magnetic flux density is not reduced). Fir example, revolver snubnose has 10mm long stator and standard 540 is 20mm stator, standard is twice the power. Comparing a motor with 23mm rotor to 28mm rotor is about 1.5 times the potential torque, assuming rotor didn't displace our ability to cram magnets in.

so

Originally Posted by Voodoobrew

So the magnum would be (28*28)22=17,249 torque potential?
and the 550 ver would be (23*23)29=15,341 torque potential?
and the 540 ver would be (23*23)22=11,638 torque potential?
Meaning the magnum should have 12% more torque than the 550 and 32% more torque then the 540 wet magnet variants of the same motor?

Basically larger motors have more torque, also hand wound motor might pack in more copper then machine wound motors so also more torque.

So we know more turns is less power.
In the rc world More turns is also less speed.
assuming all motors in question have the same amount of copper in them.
torque should stay about the same between turns but the difference is in the speed and there for power.

If you want longer run times you want less power or higher efficiency.
You will get both of these from higher turn motors.

Personally I always run lower turn motors. all of the following motors are 2200kv or the same speed.
27T 3 slot 540 (I rarely use these)
21T 3 slot 550 (#1 in most torque)
13T 5 slot 540 (#1 in smoother startup)
10T 5 slot 550 (My prefered motor, #2 in torque, #2 in smoother startup)
This doesn't Include magnum motors from holmes which have more torque since they have a bigger stator then 550 motors but in a 540 size can.

Ways to Increase speed in order of preference; increase voltage, decrease turn, Decrease gear reduction (increase pinion teeth, decrease spur teeth)
Ways to Increase torque in order of preference; increase gear reduction (decrease pinion teeth, increase spur teeth), Increase motor size
Ways to increase runtime; consider brushless motors, Higher mAh on batteries, decrease either speed or torque.

the thing with runtime is you have to give something up,
do you want to give up more money? then go brushless or buy a higher quality motor.
do you want to give up speed and power? Then get a higher turn motor, or reduce voltage
Do you want less torque and power? then get a higher turn motor and larger pinion
Do you want your rig to weigh more: then get a bigger battery. or just buy extra batteries and keep them in your pocket.

Me, I just buy extra batteries.
I wouldn't want to give up the torque/power as this makes the throttle feel squishy, gutless and sluggish to me.
I am ok with less speed usually.
If i need the speed and efficiency I would go brushless.