Understanding Watt Meters

[Mike Downs]

I have swapped an OS 40 ft with the following set up,and I am getting the following results which I don't understand

Motor Turnigy D 3548-4 1100Kv 900w

Prop 12x8. ESC 80 - UBEC LIpo 3s 11.1v On the wattmeter I have the following A 28 W272 V9.5. Is this OK .....Any help appreciated!

[Simon Chaddock]

Mike

9.5V from a 3s LiPo is not good..

What capacity LiPo are you using..

[minty morton]

Hi Mike. Something is wrong there. I use the same size motor and get close to 900w with 3s. The battery voltage is low and this could be the answer. Was the pack fully charged at the start of the test?
Hope this starts to get an answer.
Minty

[minty morton]

Good point Simon. Too small a pack could well explain the voltage drop.

[Frank Skilbeck]

It's like the Lipo isn't fully charged, 9.5v indicates that the ESC Low Voltage protection is kicking in and throttling back the motor to protect the battery.

[Geoff S]

Posted by minty morton on 23/09/2017 16:51:18:
Hi Mike. Something is wrong there. I use the same size motor and get close to 900w with 3s. The battery voltage is low and this could be the answer. Was the pack fully charged at the start of the test?
Hope this starts to get an answer.
Minty

Though it seems odd, the power rating of the motor isn't an indication of how much power a system generates (or tries to). It's an indication of how hard you can drive it without damage. The main generation of power (or torque) is the voltage and the load (the prop size) imposed.

However, as everyone has said the battery voltage is low which indicates either one that's exhausted and needs to be recharged or a faulty battery with a high internal resistance. 1100 rpm/volt is quite high for a 12x8 prop, too. The main use of a watt meter is to set up a system with the right size prop to get the current/power into the region specified by the motor parameters (ie max current, mostly).

Geoff

[Mike Downs]

Hi all, Thanks for the suggestions.The battery is on charge so I will try again tomorrow and get back to you all.Geoff says that 12x8 may be too big.......any ideas?

[john stones 1 - Moderator]

Some comments here in rate n review.

Edited By john stones 1 on 23/09/2017 20:53:49

[Denis Watkins]

Just from the IC point of view, with a .40 plain bearing possibly giving 900w

Props on a .40 would ideally be 10 x 6, maybe 11 x 6 for noise reduction, but if an FP then plain bearing.

Will be watching with interest how the prop size evolves

Edited By Denis Watkins on 23/09/2017 21:15:28

[Mike Downs]

Minty,

What size of prop are you swinging?

[Frank Skilbeck]

ECalc, calculates, 8900rpm, 650watts and 65 amps on a 3s lipo.

[Geoff S]

The maximum current for the 3548/4 is 50 amps and that would give a maximum power of around 600 watts on a 3S. I would limit the current to (say) 40 amps and around 500 watts. To get more power you would need to run it on either 4 or 5 cells keeping to the same maximum current draw.

When choosing a motor, the main parameters to consider are the kv (rpm/volt) and the maximum current (bearing in mind that maximum is often for quite short times). The quoted power rating is a secondary characteristic really only of value for guidance.

The ECalc figures quoted by Frank (particularly the current) show that there's no way to achieve 900 watts on 3S without exceeding the maximum current rating. How heavy is the model? The usual rule of thumb is 100 watts/pound for aerobatic flight but IIRC correctly my Ballerina is set up at about 80 watts/pound and has more than enough power.

Geoff

[minty morton]

Hi Mike , I have the 1100w version (same size different make) in a bullet with 10" prop on 4s and smaller KVA in an addiction X with 3s using i think a 13" prop.cant remember exactly the figures. I've got them at home if needed.

I think we need to sort the battery voltage problem out first.

But I'm a long long way off an expert.

Minty..

[Dave Hopkin]

To try and clarify what you are seeing, perhaps some basics may help - a "Watt Meter" doesnt actually measure Watts, it calculates Wattage based on the measured Amperage and the applied Voltage using a derivative of Ohms Law - Watt = Amps * Volts

With electric motors their published power figures are usually a multiplication of the max rated voltage and max current flow rather than any measured values - to get that power output we have to set up the right running conditions and we only have Voltage and Load to play with for a given motor - the ESC doesn't really enter into the equation as long as its capacity is high enough (yes settings can alter performance but in simple terms...)

A Lipo should on full charge be giving 4.2V per cell, so your 3S fully charged should be giving 12.6V clearly yours is not giving anywhere near that, so give it a full balance charge and check that all three cells are at (or near) 4.2V when charged - then repeat the measurment

If the Lipo is ok but you are still not generating enough power you have two options

a) Increase the Voltage and go to a 4S pack

b) Increase the load (which will increase current draw) with a bigger prop

[Bob Cotsford]

Posted by Mike Downs on 23/09/2017 20:34:08:

Hi all, Thanks for the suggestions.The battery is on charge so I will try again tomorrow and get back to you all.Geoff says that 12x8 may be too big.......any ideas?

What capacity and C rating is the battery? I'd guess that the 12*8 will be trying to draw around 60A which needs a good battery, either in capacity or C rating.

[Mike Downs]

Hi again,

Battery fully cycled 12.4 V. With 12x8APC E 46A...444W....9.7V, Battery Changed to 14.8V same prop 49A....

623A.....13V. Only this time motor and battery getting HOT.....The only things not changed are the motor and ESC !Should I try a smaller prop ?

[Geoff S]

I haven't run 40 size glow engines for a while but I seem to remember that the usual prop was a 10x6 so start with that. Flat out that should be turning at about 10k rpm. I'd try a 10x6. What is the weight of the model?

The voltage drop you're getting indicates that the 4S battery you're using is OK but not stellar ( 50 amps is dropping 2v so the battery (and esc) have a resistance of about 40 milliohms or 10 milliohms/cell which is OK considering the esc has some resistance too).

The 3S pack is dropping 2.5 roughly so the series resistance is 54 milliohms or 18 milliohms/cell which is quite high. So not in as good condition as the 4S I would say.

The resistance calculations are only approximations because they include the esc in the source resistance. A good LiPo cell has an effective series resistance (ESR) of less than 5 milliohms and I regard as scrap any LiPo that goes over 15 and 10 is marginal.

What do you mean by hot? My calibrated finger can just about bear touching 50 deg. Is it hotter than that? Bear in mind that prolonged running on the ground results in stuff getting hotter than it would in the air - particularly flat out. I flew my Ballerina a few days ago. That turns a 12x6 prop at around 9k rpm IIRC and has a 4S LiPo. The battery is just warm after an 8 minute aerobaticish flight.

Geoff

[Mike Downs]

More tinkering, and I am still confused!

PROP 12X8 APC E

Batt A W V

11.1 29 272 9.5

14.8 48 617 13 *

Changed prop to 13X4 ( According to the leaflet with the motor LiPo 3s-13.8 (which I haven't got) LiPo -5s 12X6

11.1 28 264 9.4

14.8 30 441 12.6

*According to Geoff's calibrated finger the motor is less than 50 degrees!

Another thing...The AMP results on the wattmeter keeps dropping?.....

[John Privett]

Again, Mike, what capacity and C rating are your batteries? Also, are they brand new?

In both cases you are getting battery voltage dropping to 3.2v per cell or lower - which is well below what I would expect from a fully-charged battery.

[Geoff S]

Mike, have a look at this set of prop tests.

Geoff

[Mike Downs]

Thanks Geoff, I will have to send for of these sizes !

[Dave Bran]

Mike, before you do ANY MORE PROP TESTS, would you please do what several here have sensibly asked for and been ignored, that is please inform:-

1. Capacity in mAh of the pack

2. "C" discharge rating for the pack, both numbers if the pack has a steady and a surge rating

3. It's age/approx number of cycles

4. Voltage for each cell after balance charge, together with total.

(stating make might help, too)

[Mike Downs]

john,

The battery is Kokam 4000 3 S . the battery is about 3 years old,but it hasn't done many cycles. The C rating is 30

the model is a flying wing (own build) and it flew well on a OS .4- FP.

The all up weight IC 6lb14oz

The all up weight Electric is 6 lb 10 oz

Working on 100W per pound this works out to 700w .

the motor leaflet states 911w using 11.1V battery with a 13X8 Prop

[Mike Downs]

john,

each cell after balance charge is 4.2V,giving 12.6 total !

[Martin Harris - Moderator]

About 3 years old? Could be 4 years maybe - plus a year or so on a model shop shelf? LiPo batteries start ageing the day they are made so I'd suggest that from the information you've supplied that the battery is no longer capable of supplying current at a sufficient rate.

Amps WILL drop off as voltage declines so it's quite normal to see the highest current (Watts are directly related) during the first application of power unless the cells are cold enough that a bit of use warms them up enough to increase their internal efficiency...something you might notice on a winter's day.