The Right Batteries

[trillian]

Be careful of connecting a higher voltage to the motor than it's rated for. It's likely that the wire coils will overheat and possibly start to damage the internal insulating materials on the wire coils causing short circuits, especially if the power demand on the motor is at or close to it's maximum continuous power rating. In-runners have a tendancy to operate at higher temperatures than outrunners. That plus the metal jacket you're putting over it... it would be risky over powering it as well.

Looking at the specs on the 910KV motor it says it's OK up to 8s. Probably just cooling will be the challenge.

[Bob H]

I missed the bit about the different motor. Was commenting on the OP thinking it was still a max 5s rated motor being used

[trillian]

I missed the bit about the different motor. Was commenting on the OP thinking it was still a max 5s rated motor being used

It's still a valid point because I'm so used to motors that are run on 12s being run on 6s etc. I've never known how they determine voltage ranges except logically that if you take a high KV motor and run it on too high voltage it will spin at crazy speeds.

[Bob H]

Where P is power in Watts, I is current in Amps, V is voltage in Volts and R is resistance in Ohms...

We all know that P = V x I but V also equals I x R so we can substitute V in the first formula to produce P = I x I x R or P = I² x R. The resistance of the wire will be measurable and we know what kind of power output we'd be looking for so we can work out how much current we will need to overcome the resistance in order to generate that power by rearranging the formula to I = (P/R)^-2. Now that we know the ball-park current range we'll be using in addition to the resistance, we can work out how much heat (if I could remember the formula) will be generated - electrical resistance is the same function as friction between two surfaces: in this case the friction is caused by electrons passing across the atoms of the conducting wire. Same thing, different size scale.

All this, of course is basic electrical maths and takes no account of efficiency losses. But, if you know the resistances involved in the motor windings and the heat dissipation characteristics of the materials the motor is made from, you can calculate the maximum amount of current you can pass through the coils before you start having heat problems. Once you have that figure plus the power output you're looking for you can work out how much voltage you need to apply in order to get the current you need/want.

Far more complicated than that obviously (for instance we're actually pulsing the current through the coils so we're in effect working with AC currents, not DC which means we would need to work with electrical impedance rather than resistance) as you still have to take into account the motor turning and how much heat that is going to generate in addition to the heat caused electrically but it gives an idea of how it all fits into place.

[Grumpy]

Electric motors are inducive loads, not resistive. So simple V=I x r doesnt work for motors.

As an example of just how 'wrong' V=I x r is for a motor just take a typical heli motor, for example the motor I have in my Goblin 630, a Scorpion HKIII 4035 560kV..According to Scorpion spec the motor resistance is 0.014 Ohms. I'm running 12s which has a nominal voltage of 44.4V

So lets calculate what current (Amps) I would expect to see if V = I x r held true.

Transposing V=I x r for I

I = V/r

I = 44.4 / 0.014 = 3171A

So if V = I x r worked for motors my Scorpion would draw 3171 Amps

[Bob H]

Electric motors are inducive loads, not resistive. So simple V=I x r doesnt work for motors.

As an example of just how 'wrong' V=I x r is for a motor just take a typical heli motor, for example the motor I have in my Goblin 630, a Scorpion HKIII 4035 560kV..According to Scorpion spec the motor resistance is 0.014 Ohms. I'm running 12s which has a nominal voltage of 44.4V

So lets calculate what current (Amps) I would expect to see if V = I x r held true.

Transposing V=I x r for I

I = V/r

I = 44.4 / 0.014 = 3171A

So if V = I x r worked for motors my Scorpion would draw 3171 Amps

You didn't read my last paragraph did you Grumpy? Just a quick skim through the first few lines and a "he's talking b"££$%t" wasn't it? I know they're induction devices not resistive. I was trying to give Trillian a very basic starting point by keeping it simple as you'd have seen by reading the last paragraph.

[Grumpy]

Actually I did read it, I was just pointing out just how 'wrong' the results are if assuming simple 'V = I x r' applies to motors... Like a factor of 20 times out in the real world case i quoted.

[Granpappy]

If we are trying to work out how hot the motor gets then perhaps the following:-

Motor is rated at 2400 watts. If it's 80% efficient then 1920 watts goes towards driving the load, a bit goes towards other losses and a bit of heat is gained from friction. The rest goes towards generating heat.

So therefore 450 watts = 450 joules per sec would go towards generating heat. (In a vacuum)

The other side of the coin is:-

Motor surface area, ambient air temperature, humidity, air density, and total volume of air in cubic meters per sec passing over the total surface area. If you know all the figures for that, then you will know how hot it will get at max power.

[bigron]

Hi Bob H,

I have acquired a 910 kv engine and will be using that which is rated 2s-8s so should be ok, if you didn't notice I am using the stock Raptor fan and clutch aswell as the motoer has a built-in internal cooling fan. Anyhow I have all the bits now, programming card, 6v digi servos etc so when it is put together I will up load a video of the test flight.

Thanks to all of you above for your help and interest,

Nigel.

[bigron]

Hi trillian, I flew the chopper this arvo all went well, I used the small 6s 2700mah battery and flight time was to say the least dissappointing about 2.75 mins before the engine started to slow. I landed it and within 30 secs or so the SC started to beep twice warning of low battery. However it all went well. What I will do is change the timing setting from auto to 18 degrees after reading blogs that say setting auto makes the motor run hot, having said that the motor housing was quite warm but the actual motor and ESC were quite cool.

When I get better batteries I will take a video of it in action and post it here.

Nigel.

[trillian]

I'm glad to hear it has flown! Yeah if you can get some video that would be cool. I've been terribly lazy with my Robbe Milli conversion but I've come too far not to finish it. Seeing other conversions is always good motivation!