Go Back  R/C Tech Forums > General Forums > Electric Off-Road
Power Calculation for Bat Voltage/KV of Motor? >

Power Calculation for Bat Voltage/KV of Motor?

Power Calculation for Bat Voltage/KV of Motor?

Reply

Old 10-21-2009, 09:21 AM
  #1  
Tech Adept
Thread Starter
 
Join Date: Sep 2009
Posts: 211
Default Power Calculation for Bat Voltage/KV of Motor?

I'm trying to calculate or show in mathematical terms the average run times in comparison to motors of varying sizes and voltage inputs. Example:

2200kv @ 14.8v (4s) = x ( 0% ) using this as the base to compare.
2100kv @ 14.8v (4s) = y ( -+ yp% )
1700kv @ 18.5v (5s) = z ( -+ zp% )

What x, y, z is could be average power draw @ half load, average runtime in minutes in straight line, I have no idea. Anyone chime in here as there must be some way of measuring in mathematical terms to show the differences in runtime you will receive between each motor/voltage setup

seg
segmentfault is offline  
Reply With Quote
Old 10-21-2009, 09:24 AM
  #2  
Tech Fanatic
iTrader: (2)
 
jmcvicker's Avatar
 
Join Date: Jan 2008
Location: SE Pennsylvania
Posts: 814
Trader Rating: 2 (100%+)
Default

Need to include the wattage of the motor itself. KV is not the complete picture. Some motors have higher wattage ratings than others - and also, higher KV can also mean more watts used.

Watts = volts * amps. However, a 1100w motor will draw more than a 1000w motor, possibly lowering runtime. It would also have more torque (power) and you could gear it higher which may mean longer runtimes if you aren't on the throttle as much. Maybe.

Runtime is "how much wattage is my motor needing over time".

If the motors are all the same wattage, then KV changes could help you figure out runtime.

The 5S 1700KV solution will typically always give better runtime than your 4S 2K+ setups - unless you use 5S 4000mAh vs. 4S 5000mAh.
jmcvicker is offline  
Reply With Quote
Old 10-21-2009, 09:32 AM
  #3  
Tech Adept
Thread Starter
 
Join Date: Sep 2009
Posts: 211
Default

Originally Posted by jmcvicker View Post
Need to include the wattage of the motor itself. KV is not the complete picture. Some motors have higher wattage ratings than others - and also, higher KV can also mean more watts used.

Watts = volts * amps. However, a 1100w motor will draw more than a 1000w motor, possibly lowering runtime. It would also have more torque (power) and you could gear it higher which may mean longer runtimes if you aren't on the throttle as much. Maybe.

Runtime is "how much wattage is my motor needing over time".

If the motors are all the same wattage, then KV changes could help you figure out runtime.

The 5S 1700KV solution will typically always give better runtime than your 4S 2K+ setups - unless you use 5S 4000mAh vs. 4S 5000mAh.
Yes, wattage would be required, however I don't believe many of these motor companies post wattage ratings, do they? It would have to be measured with a multimeter in series to the motor while subjecting it to load. Some of the motors I do not possess so it would be tough to gauge.
segmentfault is offline  
Reply With Quote
Old 10-21-2009, 09:45 AM
  #4  
Tech Fanatic
iTrader: (2)
 
jmcvicker's Avatar
 
Join Date: Jan 2008
Location: SE Pennsylvania
Posts: 814
Trader Rating: 2 (100%+)
Default

Neu put up their constant/peak wattages for the 1500 series, which helps show the diff between 1512 and 1515 which are basically Buggy vs. Truggy (or Castle 2650kv vs. 2200KV).

In the Tekin line of motors, you have Y and D winds - apparently, the Y winds are said to use less battery per KV than D winds. If you think about it - the wire winds and all the electromechanics going on are important to the efficiency of the motor and power delivery. Your battery never "touches" anything - everything goes through the magnetic field between the motor winds and the rotor and then to the wheels. So, if you are looking at the same brand of motor (ie. Tekin) then your best bet is to call the company and talk it out with them - maybe they can give you insight into the motor choice for best runtime (if that's the goal) or for most power for your track, etc. If only runtime, then get the biggest battery with smallest KV you can fit.
jmcvicker is offline  
Reply With Quote
Old 10-21-2009, 10:09 AM
  #5  
Tech Elite
iTrader: (6)
 
kufman's Avatar
 
Join Date: Feb 2004
Location: Elburn, IL
Posts: 3,461
Trader Rating: 6 (100%+)
Default

There are way too many factors involved here. Not only do you have the voltage, current and power numbers, you also have to consider driving style and track size. You also have controller parameters that will affect run time. The quality of the batteries will also play a part in it too. You could have two different 4000mAh batteries and get different runtime from them depending on their internal resistance. There are just too many different variables to consider.
kufman is offline  
Reply With Quote
Old 10-21-2009, 10:31 AM
  #6  
Tech Elite
iTrader: (19)
 
Join Date: Dec 2007
Location: Chicago, IL
Posts: 2,854
Trader Rating: 19 (100%+)
Default

Yeah, there is no real way to do this consistently. Too many uncontrollable variables. However, I think most will tell you that to complete a 15 minute main:

4s 5000 = 15 minute on a 1700-2050kv motor.
5s 4000 = 15 minute on a 1550-1700kv motor
6s 3300 = 15 minute on a 1350-1550kv motor

From what I have seen, 4s setups tend to burn 250-300mah a minute. 5s typically is 200-250 a minute and 6s 165-200 a minute.

Size of track, driving style, traction, batteries, and gearing call all play a part in run time.

Keep in mind that you also typically get more run time practicing versus racing a true main.
Edumakated is offline  
Reply With Quote
Old 10-21-2009, 10:52 AM
  #7  
Tech Adept
Thread Starter
 
Join Date: Sep 2009
Posts: 211
Default

Thank you for your all your replies, and it's basically pretty much what I expected. I'm going to try to calculate average power draw as half of the total wattage for the motor, then go from there. I'll post my results and see if we can come up with something to at least ballpark the differences between motors.

What happens when you put a 6s in a 2200kv motor as opposed to a 4s in a 2200kv motor? Obviously you will go quicker, but if you were to decrease the size of the pinion and motor timings and actually control the added acceleration and speed by practice, theoretically because of ohms law (w = v * a,) wouldn't you have longer runtimes?

6s, 22.2v * 5a (c value for a point of calculation) = 111 value
4s, 14.8v * 5a (c value for a point of calculation) = 74 value

thats a 50% increase in total power of the battery. With a 50% increase, and considering you can keep wheel spin and acceleration near the same with the 6s as with the 4s, shouldn't you also increase your run time by 50% with no other factors considered?

Obviously the weight of the battery is heavier, and there are a few other values to consider such as voltage drops across the wires, electronics, etc.

Anyone have experience with this situation?

seg
segmentfault is offline  
Reply With Quote
Old 10-21-2009, 11:02 AM
  #8  
Tech Elite
iTrader: (19)
 
Join Date: Dec 2007
Location: Chicago, IL
Posts: 2,854
Trader Rating: 19 (100%+)
Default

Originally Posted by segmentfault View Post
Thank you for your all your replies, and it's basically pretty much what I expected. I'm going to try to calculate average power draw as half of the total wattage for the motor, then go from there. I'll post my results and see if we can come up with something to at least ballpark the differences between motors.

What happens when you put a 6s in a 2200kv motor as opposed to a 4s in a 2200kv motor? Obviously you will go quicker, but if you were to decrease the size of the pinion and motor timings and actually control the added acceleration and speed by practice, theoretically because of ohms law (w = v * a,) wouldn't you have longer runtimes?

6s, 22.2v * 5a (c value for a point of calculation) = 111 value
4s, 14.8v * 5a (c value for a point of calculation) = 74 value

thats a 50% increase in total power of the battery. With a 50% increase, and considering you can keep wheel spin and acceleration near the same with the 6s as with the 4s, shouldn't you also increase your run time by 50% with no other factors considered?

Obviously the weight of the battery is heavier, and there are a few other values to consider such as voltage drops across the wires, electronics, etc.

Anyone have experience with this situation?

seg
No, you will probably get less run time if the pack energy is consistent (4s 5000 = 6s 3300). A 2200 motor on 6s is 48k RPMs which is going to generate a lot of heat and wasted energy. Even if you gear down to the speed equivalent to a 4s on 2200, the energy used to spin at 48k rpm is negates the benefits of the higher voltage.

Every motor has it optimal efficient rpm range even though most of the motors are good to 60,000 rpms. IMHO most of these setups run most efficient when the motor rpm is between 28-32k rpm. Once you start going above and below you generally start seeing higher amp draws even if you are using higher voltage. this is why mating the right kv motor with voltage is critical.
Edumakated is offline  
Reply With Quote
Old 10-21-2009, 02:36 PM
  #9  
Super Moderator
 
Join Date: Oct 2007
Location: Rochester Hills Michigan
Posts: 7,232
Default

Originally Posted by jmcvicker View Post
However, a 1100w motor will draw more than a 1000w motor, possibly lowering runtime. It would also have more torque (power) and you could gear it higher which may mean longer runtimes if you aren't on the throttle as much. Maybe.
You gotta be kidding, a higher powered motor, geared higher, will have a longer runtime???

Seems you keep making the same mistake, thinking a motor produces peak power regardless of the applied voltage. It is just not that simple. Not even as a trend or guideline.
Dave H is offline  
Reply With Quote
Old 10-21-2009, 03:01 PM
  #10  
Tech Master
 
RBMike's Avatar
 
Join Date: Jan 2007
Location: Santa Clara CA
Posts: 1,264
Default

There is some truth to that. I have a seen 5S 2200Kv CC motor in a buggy get great run times with something like 17/46 gearing & the throttle end point turned down to 70 or 80% (after calibrating the radio at 100%). Certin combos of way under utilized, way over powered set-ups can run higher run times.
RBMike is offline  
Reply With Quote
Old 10-21-2009, 03:05 PM
  #11  
Super Moderator
 
Join Date: Oct 2007
Location: Rochester Hills Michigan
Posts: 7,232
Default

By turning down the endpoint you are effectively changing the applied voltage. That is a different situation, further demonstrating my point that power changes with applied voltage. The comment that caught my attention did not mention changed voltage (it was mentioned later however, but with a lower power motor with the higher voltage, which does make sense).
Dave H is offline  
Reply With Quote
Old 10-21-2009, 03:23 PM
  #12  
Tech Master
iTrader: (18)
 
Join Date: Dec 2006
Location: Coral gables, Florida
Posts: 1,265
Trader Rating: 18 (100%+)
Default

I use 220-225 mah per minute with my losi 2100kv on 4 cell. With my old battery pack that has a higher internal resistance I use more mah per minute, if I remember correctly around 250 mah per minute.
infanterene is offline  
Reply With Quote
Old 10-30-2009, 09:03 AM
  #13  
Tech Regular
 
Join Date: Oct 2009
Location: not in a tree, CA
Posts: 455
Default

Originally Posted by RBMike View Post
There is some truth to that. I have a seen 5S 2200Kv CC motor in a buggy get great run times with something like 17/46 gearing & the throttle end point turned down to 70 or 80% (after calibrating the radio at 100%). Certin combos of way under utilized, way over powered set-ups can run higher run times.
How low can you turn the throttle end point down without heat stressing the batteries/esc too much?

from Castle's website:
It's not a good thing to dial down the top throttle endpoint, especially when you can re-gear, or just go down in cell count. The batts, the ESC and the motor will all run hotter than normal in a system with turned down endpoints to slow it down.
lusifur is offline  
Reply With Quote
Old 10-30-2009, 09:13 AM
  #14  
Tech Elite
 
UK.hardcore's Avatar
 
Join Date: Aug 2007
Location: Belgium.
Posts: 2,199
Default

why no wattage is given has an easy answer, it will be higher when you put more load on a system. till it blows up basicly.

also temperature has great effect on a system's performance, when the battery is cold it doesn't work as good versus a bit warmer but motor and controller will be more efficient when colder.

in offroad even the dirt has significant effects on the systems, when you have lots of resistance in soft dirt your car will need more power to drive true it.

List goes on, i could be typing till tommorow night.
UK.hardcore is offline  
Reply With Quote
Old 10-30-2009, 10:51 AM
  #15  
Tech Elite
iTrader: (154)
 
ta_man's Avatar
 
Join Date: Nov 2006
Location: New Jersey
Posts: 4,630
Trader Rating: 154 (100%+)
Default

Originally Posted by Dave H View Post
You gotta be kidding, a higher powered motor, geared higher, will have a longer runtime???

Seems you keep making the same mistake, thinking a motor produces peak power regardless of the applied voltage. It is just not that simple. Not even as a trend or guideline.
Yup! Higher powered, higher geared motor can have more runtime.

Very first tests I did with 1/8th scale buggies I had a Novak HV6.5 geared 9:46 compareed to a Neu 1512 2.5D geared 13:46. Ran each for exactly 10 minutes (stopping the clock when marshalling was required). Recharged the packs to determine mAHr usage and found the Nue, even though it was faster on the track, drew less current. Neu averaged 14 amps over the 10 minutes and the Novak 18 amps over the 10 minutes. Less average amp draw translates to more runtime.

Neu is rated at around 1200 watts, Novak around 600-700.

Of course, what I really believe is that the design differences between the two motors contributed more to the difference in amp usage than the fact that the Neu is a higher powered motor.

Last edited by ta_man; 10-30-2009 at 10:26 PM.
ta_man is offline  
Reply With Quote

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service