Motor Torque

[HRC]

I am figuring the amount of torque electric motors produce at full load. Full load being the KV of the motor per volt being used. This is the formula I use.

T = HP x 5252(constant)
________
rpm

Any thoughts or imput would be helpful.
Thanks
Dave

[ED66677]

the torque is equal to output power divided by rpm
C=P/ω
where C is in Nm(Newtonmeter), P is in W (Watt) and ω is in Rad/s
at full load no matter the KV, it is the actual rpm (of course depending of the KV, the input voltage and rpm loss under load) that has to be considered... and the actual output power not input power (input power minus loss).

[HRC]

Thanks for your input Emmanuel. At a glance looks like both formula will be close to same results. Thanks again.
Dave

[ReddyWatts]

Torque calculations

I have attached a file that calculates motor torque. Try it out.

Emmanuel created it.

[HRC]

Thanks for the help guys!! This will give me more to work with.
Dave

[marker]

The torque of an electric motor can be calculated if you know,
Kv
Io (no load current in amps)
I in ( the running current in amps)

Formula
1355/ Kv X (I in - Io) inch ounce

So using the example of the Feigao 540 8 XL
Kv 2084
Io 3.6 amps
I in 83 amps

1355/2084 X (83-3.6) =51.625 in oz (.3645 Nm)

Mark

[ED66677]

your formula is even better than mine Marker because you remove the Io, good point, but the result we (you and I) give is the torque at the unloaded rpm or KV so basically when no torque is produced!!!
let's say under 12V the 8XL spins at 25008 rpm t(his is unloaded) as soon as you load the motor, rpm's drop, so at this point the KV is lower, the torque is then higher, your formula really give the minimum torque of any Feigao XL motor, mine gives a value slightly higher that does not include Io, I've got to change that!

[Fluid]

...the torque at the unloaded rpm or KV so basically when no torque is produced...

Guys, the Kv rpm is not an unloaded rpm! Different motor makers rate it at different amperage loads, but there is definately plenty of torque/power available at the raw Kv rpm. I have even confirmed this from datalogging which shows plenty of power at the calculated Kv based on the ACTUAL voltage delivered by the packs under load - not the "nominal" pack voltage. I have seen Kv calculated at 20 amps up to over 50 amps, that is why comparing the Kv of different makes of motors can be deceiving.




.

[marker]

ED66677
Torque is directly related to the amps drawn, so in the first part of the calculation
1355/Kv the result is Kt (torque constant) "inch oz" per amp.
1355/2084 =0.650 in oz per amp
So if the Io is 3.6 amps for every amp above this the torque will rise .650 in oz.
ie 33.6 amps the torque is (33.6 - 3.6) X.650 = 19.5 and at 63.6 amps the torque is double 39, because the amps have risen another 30 amps.
If you are using a data logger because you know the amps, rpm and volts at a particular point you can calculate the output power of the motor and efficiency .

Mark

[HRC]

This is some GREAT information guys, thanks.
Something else I have been wandering about.

Could one consider an ESC as a phase converter? Seems to me they do some of the same functions. When current passes through the ESC could it be AC (alternating current) at that point. Has anyone taken voltage readings between the ESC and the motor under load as opposed to battery to ESC? I have never checked.
Thanks
Dave