Is going from 7 to 8 cells (and higher discharge) enough to cook a brushed 400? - RC Groups
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Mar 28, 2005, 04:43 PM
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Peter Young's Avatar

Is going from 7 to 8 cells (and higher discharge) enough to cook a brushed 400?


I hope this doesn't look like a multiple post - this question has been raised in park flyers, but it's started to get more specific and I wondered if this forum might be better placed to answer my question.

I crashed my Sig Kadet EP42 on the weekend on it's first outing. The full dramatic story and pics are here:

The problem was that the motor burnt out, causing me to lose all control. My question is, did this happen because I used a battery pack with more cells than those recommended by the manufacturer (8 instead of 7)?

The comparison is as follows:

Motor: Sig 400 rebuildable brushed
Prop: 7x4 (I think)
Battery: Recommended - 7 cell 1100 nimh
Used - 8 cell GP1100 nimh

Others have tested the stock setup and found it drawing close to 20amps. Would taking it from 7 to 8 cells be enough to cook the motor?

I guess my question is - was there a one off fault with my motor, or is there a fundamental design flaw that Sig need to fix? If they have supplied a system that cannot handle high discharge 8 cell packs then there are going to be a lot of returns for this package.

Any thoughts?

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Mar 28, 2005, 05:15 PM
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vintage1's Avatar
I am entirely at a loss to know first of all who on earth recommended a 7x4 prop on what looks like a direct drive 400 can motor.

Usual prop is 5x5 5.5x4.5 or 6x3, and usual curent is 8-10A maximum.

20A on a can 400 is instant death. I am surprised it lasted even one flight.

You might get away with a 7.2v motor on that that what you used? That should draw 7-9A.

I think you have the wrong motor, or the wrong prop, or both!
Mar 28, 2005, 05:21 PM
Registered User
Looks to me as though you have over stressed the motor......

If it was designed to run on 7 cells with the stated prop and it draws 20 amps its definitley over propped.....and adding one more cell to it will hasten its demise.......
What you need to do is re motor the model....using either the 7 or 8 cell pack and prop it to a reasonable current.....Use a watt meter.. You should be able to achieve 70 watts on a speed 400 motor, but it depends on the voltage of the motor, there are variants.........
Mar 28, 2005, 05:22 PM
Registered User
Vintage - this is the motor / prop combination supplied (and installed) in the Kadet EP-42 ARF from SIG. And yes, it draws about 20A in stock form.
Mar 28, 2005, 05:28 PM
Registered User
What you are stating here then is:

on a 7 cell pack drawing 20 amps......the input power to the motor is,
We'll say 8.0 volts for a freshly charged pack onload with the stated prop.
8*20=160 watts......sorry I gotta disagree, no way would a 6.0 volt motor survive that sorta punishment.....but I stand to be proved wrong...

I do hope our thread starter resolves his problems......
Mar 28, 2005, 05:49 PM
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Peter Young's Avatar
gunfighter is right - the motor came installed on the plane, and the prop is the one supplied.

Sig describe the motor as:

"Super 400 motor (more powerful than standard 400-size motor)"

...and by the way, the prop is 7x5 (not 7x4 as I thought)!

The plane specs follow:

SPECS: Wingspan: 42" (1067mm)
Wing Area: 330 sq in (21.3 sq dm)
Flying Weight: 26.2 oz (743g)
Length: 34" (864mm)
Airfoil: Flat bottom, high wing placement
Mar 28, 2005, 05:59 PM
Registered User
A 7*5 prop makes it even worse!

I'd take a guess and say the motor is no std turn sp400 motor, but a special.

I'm surprised your esc survived all of this....unless its a high current job...>20 amps..
Mar 28, 2005, 06:17 PM
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Peter Young's Avatar
25amp rcline ESC (the plane comes supplied with a 30amp, but I wanted to leave this unopened so I could resell it easier). In most speed 400 applications the 20amp is plenty, although if I'd known what I know now I would probably have used the supplied 30amp Sig ESC.
Last edited by Peter Young; Mar 28, 2005 at 11:34 PM.
Mar 28, 2005, 06:36 PM
rpage53's Avatar

Not really an S400

The motor is a long can 400 or more like a 480. It should handle 20A but Sig includes a 30A ESC for head room. Going to an 8th cell wouldn't increase the current that much, but most of that current would just go to heating the already overloaded motor. The extreme case would be 20A*8V = 160W went to 25A*9V = 225W.

So to answer the question, the 8th cell might have caused the motor to burn out and stall and the high current draw overloaded the ESC which caused the BEC to shut down. Most can motors require a break in period first too to seat the brushes before full throttle operation.

Mar 28, 2005, 09:19 PM
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Peter Young's Avatar
Thanks - your scenario seems reasonable. I hadn't realised I was pushing this drive train to the edge by upping the cell count from 7 to 8. I normally fly a Komet with a 6V speed 400 and the same high discharge 8 cell battery pack. I do use a 5x5 prop, but have never had any problems.

My guess is that this is a lower turn (modified) motor to give it the power needed for this fairly big plane. I would have thought lower turn would mean smaller prop to compensate. So the combination of hotter motor and bigger prop must put this set-up right on the edge.

If this is right I think it's a pretty risky situation without having this clearly covered in the instructions (in particular they should warn against using an 8 cell pack). I bet I'm not the only one to try an 8 cell pack (especially if it's what you had on hand, rather than getting a 7 cell pack made up specially). Wouldn't most other 400 size planes be flying on 8 cell packs?

Anyone know the specs on this motor? There's nothing on the Sig web site that I can find.


Mar 28, 2005, 10:56 PM
Restful User
Jacques Flambeau's Avatar
That is a very curious motor setup. I'd be interested in knowing how/why they spec'd a 20A 160W 400 mptor.

Mar 28, 2005, 11:29 PM
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Peter Young's Avatar
not just specified, but actually supplied already installed. This plane is sold as a trainer for beginners, and the motor comes installed and covered by a cowl. Not many people buying trainers are going to suspect that this set-up is drawing heaps of current, and without a specific warning others are going to pop in an 8 cell pack (and maybe even a 3s lipo pack).

Maybe it's a bullet proof set-up that will fly happily all day on 8 cells (and maybe I was unlucky to get a dodgy motor). I guess that's my question - is the set-up risky to start with given the motor/prop combo, and will others who drop in an 8 cell high discharge pack have a similar smoking plane?

I should just say at this point that, not withstanding the motor problem, this plane has made me a big SIG convert. This was by far the best ARF I have ever had - easiest to build, best instructions, good hardware, nicely finished, looked great. I used to lean towards Great Planes and World Models for quality, but Sig have set the benchmark for me. That's why I am so disappointed - I miss my plane!
Mar 28, 2005, 11:51 PM
Registered User
If they sold it as is, then you should be able to send off to sig and get a replacement. Every manufacturer I have ever worked with has been very good about this even some in hong kong. The customer can expect reasonable operation of main line commercial products, and if they fail, the manufacturer should take care of it.
Mar 29, 2005, 12:21 AM
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Peter Young's Avatar
I'm hoping you mean "replacement plane" not "replacement motor". I actually don't want another very hot brushed 480 motor (next time I'll use a brushless). However a new fuselage would be nice. That's what I've asked them for.

I think the issue is a bit bigger though - is there a basic design flaw with this plane and motor combo? Do they need either a smaller prop or a warning in the instructions not to use an 8 cell battery?
Mar 29, 2005, 12:40 AM
Registered User
Maybe they just had a manufacturing flaw in your motor? And yes I was refering to the whole plane.

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