Power consumption on multi engined model

[Cliff Bastow]

I am fitting out my Avro York and thinking about a connector between the wing and fuselage to supply the esc and motors, and also for the batteries.

My question is what will be the total draw of 4 x 30amp esc? will it be 120amp or am I missing something?

Cheers Cliff.

[Bob Cotsford]

it will be the sum of whatever current the motors draw rather than the ESCs ratings, though that does give a maximum possible current if you don't leave any headroom when specifying them.

[Danny Fenton]

The power required for multi engines is a lot less than I expected. My Mossie for example, was specced for much higher but went fine at around 30A per side, a pair of 20 x 12 props, with low kV motors was more than adequate.

Cheers

Danny

[Cliff Bastow]

The power set up recommended uses 4 x 30 amp esc so I assume each motor will draw less than 30amp, lets say 25 amp max so what will be the total draw at the battery is basically what I am asking. Normally I use Deans connectors but I am a bit worried they wont take enough amperage.

Is it what each motor draws added together?

[Frank Skilbeck]

Posted by Cliff Bastow on 02/05/2020 21:02:56:

The power set up recommended uses 4 x 30 amp esc so I assume each motor will draw less than 30amp, lets say 25 amp max so what will be the total draw at the battery is basically what I am asking. Normally I use Deans connectors but I am a bit worried they wont take enough amperage.

Is it what each motor draws added together?

Yes, think of amps as fuel flow, so if you had one tank feeding 4 engines then the flow from the tank would be the total flow going to all engines.

[Dad_flyer]

You will also need plenty of capacity in the wiring. I have not made a multi, but I do want to and have been following several builds, including your magnificent York.

Many people separate four engines as inner and outer pairs on the Rx, but also on two batteries. That means only the double current on each battery, connector, loom instead of four times. You could of course split one battery to two looms.

The wires are much longer than usual as well, which gets heavy. Heavy guage solid 'magnet' wire is lighter and cheaper than flexible silicone. The recent thread here on a big depron Antanov shows that I think.

[Bob Cotsford]

I think you need to be looking at at least XT90 connectors or larger sizes of bullet connector with anti-spark resistors. As D_F suggests I think I'd be looking at twin batteries, even then Deans wouldn't be my connector of choice.

[Simon Chaddock]

With a multi you do have to think where you are going to mount the ESC's and the battery(s). The battery to ESC length has to be considered.

If the battery(s) are mounted in the fuselage and with the ESCs close by then each motor has its own wires so no heavy cables are required.

By far the most elegant and lightest wiring solution, if it is possible, is to mount individual battery and ESC combinations in or around each motor then only the ESC 'signal' wires are long.

With this sort of arrangement you do have to keep a close watch on the individual battery state to avoid one going down before the others although a situation no worse than with an IC engine multi.

[eflightray]

I'm just finishing my fourth four motor model, all have the batteries and ESCs in the fuselage, (don't forget to ensure suitable cooling where ever you mount them).

The motor are as already mentioned are, one channel and battery to inner pair, another channel and battery for the outer pair. Using two mixed receiver channels also allows differential throttle if fancied.

I prefer to do a 'set throttle range' on each ESC separately, it also allows me to do an individual check of amps drawn and prop rpm.

Having the 'power' inside the fuselage does mean multiple connections if using plug-in-wings, (which I prefer for easier transport), but it's something I accept and not a problem.

Ray.

[Robert Parker]

Hi Cliff,

I have use XT 60 or 90 connectors on all my multi-motor electric models but ensure your cables to the ESC's are large enough to carry the current.

Just as an aside, take look at the thread "lengthening wires", some debate and schools of thought.

Regards

Robert

[Martin Harris - Moderator]

Just thinking out loud - have you considered a higher cell count to reduce the current required?  The result could reduce both weight and cable management issues along with reducing losses due to voltage drops across the various connections at higher currents.

Edited By Martin Harris on 03/05/2020 15:50:27

[PatMc]

Posted by Martin Harris on 03/05/2020 15:43:38:

Just thinking out loud - have you considered a higher cell count to reduce the current required? The result could reduce both weight and cable management issues along with reducing losses due to voltage drops across the various connections at higher currents.

Edited By Martin Harris on 03/05/2020 15:50:27

I was thinking exactly the same & looking for motors of lower Kv to use the same size props. There aren't many motors of similar weight & size as the recommended ones in the Kv range that could take more than 4s using the props specified. However IMO these dimensionally identical 980 Kv motors should be a drop in fit & give near identical power & prop speed on 4s whilst dropping the current pulled by 33%.

OTOH a little online research would turn up several equally suitable motors at around half the price. Motors need to be around 60g, 28mm dia & 950 - 1000Kv.

Edited By PatMc on 03/05/2020 17:43:32

[PatMc]

Posted by Bob Cotsford on 03/05/2020 11:41:37:

I think you need to be looking at at least XT90 connectors or larger sizes of bullet connector with anti-spark resistors. As D_F suggests I think I'd be looking at twin batteries, even then Deans wouldn't be my connector of choice.

Bob, I think anti spark connectors are only necessary at 6S or greater, the magnitude of current handled isn't a factor that needs considering.

[Bob Cotsford]

You're probably right Pat, I'm in default 6S mode!

[PatMc]

Bob, if there were any circa 700kv motors of the required size size available 6s would be my choice for this model.

[Vince G32]

For what its worth this is my setup.

Each motor is 200w (14.8V) (13.5A) giving a grand total of 800w (14.8V) (54A) to power a 72" Tony Nijhuis Avro York.

I prefer to have short battery wires and long motor wires (there are advantages/disadvantages for both setups).

The 5.5A SBEC within the 40A ESC powers the receiver and 5 mini servos.

The 5A UBEC powers the retracts only.

The 4A BEC's withing the 20A ECS's are redundant.

[Chris Walby]

Vince, 

 Looks good, have you tried the motors attached to the esc's on a test rig? Not had any problems myself, but others have had issues motors running at different RPM?

[Vince G32]

I've bench tested the motors, they are all 200w, give or take a few watts. I've never had issues with motors running at different speeds.

 

Difficult to see how that could happen if all the motors are from the same manufacturer, the same KV and all four ESC's receive the same voltage from the LiPo's.

 

Should have said, the LiPo's are connected in parallel within the XT60 dual adapter. I've tried to standardise my Lipo's. I use one 14.8v 2200mAh in small models like the Marlin EDF. Then connect two in parallel to power my 40 size models and three in parallel to power 60 size models.

 

[Chris Walby]

IIRC its when some ESC's have not been "spanned" e.g. taught min and max settings/range of input from the RX that there is an issue, as I say I have not had that issue myself.

 

A few interesting things when using differential throttle control on the WooHoo, but that was intentional as I want it to spin. 

 

Again depends on your battery stock as to whether you parallel or series lipos to get the power output, technically higher voltage reduces your losses (I squared R), but higher voltage ESC and motors are more expensive so up to the individual. 

[Vince G32]

Talking about motors running at different speeds. I've only noticed that happen on an electric control line model. When you do a loop you put a little more load on the propeller, that in turn increases the current. Due to the internal resistance of the battery the voltage will drop a little. The ESC algorithm will reduce the RPM slightly, its very subtle.

 

[Vince G32]

You have a very good point about setting the max/min range of the ESC's (didn't know it was called spanning). So easy to forget to do that the first time you connect it all together.

 

Ohms law tells me higher voltage is better. But as you say small motors are normally designed to work on a lower a voltage (high KV). It's all a bit of a compromise. My motors are 1100KV, not ideal, I need to use very small props 7x4 to get the power down to 200w.

 

4max sell a 870kv 35/35 size motor that would be a better. But I already had the 1100KV motors, so its a done deal.

[Vince G32]

Just theorizing (too much time). We are both looking at this from the electronics point of view. If you were running a low voltage, high current system, with different long wire lengths and diameters the resistance of the wires may vary? Hence each ESC will detect a different voltage, the RPM may differ? but that would be impossible to detect. I think I'm feeling very subtle changes of frequency more to do with the vibrations coming down the control line wires to my hand?

 

The next two paragraphs are not my work, they are copy and pasted from another forum.

 

"If there is a significant distance between the battery and the motor, it is MUCH better for the controller to be as close as possible to the battery, with the three motor phase wires being the longer section. In an effort to reduce voltage ripple and voltage spikes that result from being forced to have longer battery wires, it is sometimes helpful to add capacitors to the red/black pos/neg right next to the controller.

This is because the motor can absorb quite a bit of voltage ripple and voltage spikes, but the controller is much more sensitive. The key metric is to use Capacitors with a low-ESR. A physically larger capacitor with high capacitance doesn't hurt, but it also doesn't help either. A capacitor has two leads, positive and negative. Simply connect the two leads to the two battery cables, in a place near the controller."

 

Back to me. If there was a dry joint somewhere? that would cause major issues.

[Chris Walby]

10 hours ago, Vince G32 said:

Just theorizing (too much time). We are both looking at this from the electronics point of view. If you were running a low voltage, high current system, with different long wire lengths and diameters the resistance of the wires may vary? Hence each ESC will detect a different voltage, the RPM may differ? but that would be impossible to detect. I think I'm feeling very subtle changes of frequency more to do with the vibrations coming down the control line wires to my hand?

 

The next two paragraphs are not my work, they are copy and pasted from another forum.

 

"If there is a significant distance between the battery and the motor, it is MUCH better for the controller to be as close as possible to the battery, with the three motor phase wires being the longer section. In an effort to reduce voltage ripple and voltage spikes that result from being forced to have longer battery wires, it is sometimes helpful to add capacitors to the red/black pos/neg right next to the controller.

This is because the motor can absorb quite a bit of voltage ripple and voltage spikes, but the controller is much more sensitive. The key metric is to use Capacitors with a low-ESR. A physically larger capacitor with high capacitance doesn't hurt, but it also doesn't help either. A capacitor has two leads, positive and negative. Simply connect the two leads to the two battery cables, in a place near the controller."

 

Back to me. If there was a dry joint somewhere? that would cause major issues.

IMHO I don't think very minor voltage variations will make a lot of difference to the RPM compared with incorrect spanning of the ESC's. Plus if the RPM increases then the load increase hence the voltage will reduce so I recon its probably all a bit self limiting unless the actual ESC setting is different.

 

Next two paragraphs:

All ESC's are not the same, but I spoke to George at 4Max about battery to ESC lead length and he said for his ESC's the problem was designed out. I have a TN DC3,  Mosquito and a Grumpy Tiger Cub (all with 4Max kit) where the ESC's are in the nacelles (better for cooling IMO) and no problems or added capacitors.

 

Dry joint....yep had one of those on a BH Mossie. Bought second hand and reportedly pre flown, but the C of G was miles away from where it should have been + a couple of major control issues would have meant it would have been a very short single flight. Anyway before the maiden on various checks it would kick one prop backwards a few degrees (like 30ish) on just one motor about 1 in 8 times, but the pick up ok all the way through the RPM range. So on with the maiden and apart some trimming all was going well so time for a couple of low fast passes. On the second pass there was a odd sound and one motor powered off and stopped, I chopped the other throttle. I then decided to keep it low (30 ft) and do a gentle bank to the right and see if I could get it back in the field. No real piloting skill as it dropped a wing and spun with it pointing vertically at the ground. I instinctively put a bit of up in and was think of whether the gear should go down of not and it met the ground with wings level!

 

 

The problem was found - a factory made motor lead bullet connector dry joint, hard to find as it was supported by the heat shrink sleeve until cut away, but under full load blew the remaining solder out of the bullet.

 

PS looks like two skid marks, but from memory there might have been three 😉

 

 

[MattyB]

21 hours ago, Chris Walby said:

Next two paragraphs:

All ESC's are not the same, but I spoke to George at 4Max about battery to ESC lead length and he said for his ESC's the problem was designed out.

 

This has been stated before on this forum, and whislt I know George is held in high regard here I'm not aware of any explanation having been presented of how this has been achieved. Given 4-Max ESCs do not look obviosuly different from others, don't cost much more or less than other (non-German) ESCs and the theory of how to protect from voltage spikes is well understood, I maintain a healthy scepticism of this claim until some explanation is presented. 

[PatMc]

1 hour ago, MattyB said:

 

This has been stated before on this forum, and whislt I know George is held in high regard here I'm not aware of any explanation having been presented of how this has been achieved. Given 4-Max ESCs do not look obviosuly different from others, don't cost much more or less than other (non-German) ESCs and the theory of how to protect from voltage spikes is well understood, I maintain a healthy scepticism of this claim until some explanation is presented. 

Ditto.