Power surge driving aircraft propellor.

[JeremyParticipant @jeremy15845]

When applying full throttle driving the airscrew of my Orlyonok A90 ekranoplan, the power comes in waves surging up and then down in cycles of around 3 seconds.

 

The set-up is a 9.6V NiMh battery driving a Park EFLM1010 with a 5.5″ x 2.5 prop via an Overlander 25A ESC. I have connected up Fischer 60 Amp wattmeter which shows the following:

 

2.6A surging to 7.7A

Voltage steady at 8.9V

23W surging to 51W

899uS

 

The motor spec is for continuous current 7A and max burst current 11A, so this not apparently being exceeded. The cut-off threshold for the ESC variable between 2.6V and 3.1V but is for Lipo cells. There is no option to turn it off for NiMh cells.

 

What is happening and what can I do about it? I would very much appreciate advice.

 

Jeremy

 

[JeremyParticipant @jeremy15845]

[ashley needhamParticipant @ashleyneedham69188]

Jeremy…hmmm…

 

Is the wattmeter connected to/ across the battery?

 

I think you would need to post on an aircraft forum for this sort of answer !

 

it is possible, and this is a wild guess, the surging may be because the loads on the motor are somewhat variable at less than full throttle and also the craft is stationary and so this will affect the throttle performance? I am wondering how sensitive aircraft ESc`s are. I think that aircraft mostly operate at full or 3/4 (say) throttle and there is not much need for anything less on an aircraft???

 

Is the ESc set up for Nimh cells? mine has that option…dont know about the cut off voltage though.

 

Ashley (sorry not much real help)

[JeremyParticipant @jeremy15845]

Ashley

 

The Pischler (not Fischer!) wattmeter is connected between the battery and the ESC. The ESC appears to be for Lipos with low voltage cut-off settings varaible according to the number of cells. Unlike the Seaking ESC used in my Bluebird, there is no option to disable it. I suspect that, somehow, the low voltage cut-off may be coming into play, although the voltage reading on the wattmeter is steady. The surging is occuring at full throttle. As you pointed out, there is little need for anything else. I will see what Overlander have to say.

 

It’s too cold to contemplate lake tests at the moment so I should have time to sort this out. I must admit to being temped by a Lipo to solve the weight problem at a stroke.

 

Thanks for your help.

 

Jeremy

[ashley needhamParticipant @ashleyneedham69188]

Is there no NiMh option in the settings??

 

There is only a weight problem if it does not perform as expected (after prop swapping)

 

repeat 100 times before going to bed.

 

Ashley

[JeremyParticipant @jeremy15845]

No NiMh option is shown. The low cut-off threshold is 2.6V and the number of Lipos can be autodetected or set manually. The cut-off mode can be set for reduce power or shut down. My problem is a reduction in power. I have e-mailed Overlander for advice.

 

Point 2 noted. I was only dreaming. I promise not to go for weight reduction nor to switch to Lipos until the full potential of the existing set-up has been explored – honest, guv!

 

Jeremy

[JeremyParticipant @jeremy15845]

Ashley

 

Replies on an aircraft forum point the finger at my battery:

 

The batteries look like consumer AA Nimh which probably can’t provide
the current required for this motor. A lipo replacement would likely
solve the problem.

I also would guess that your batteries are the problem. The power surging is likely the ESC entering into low voltage cutoff.

 

I recall you used a 9.6V NiCd battery whereas I have used an NiMh battery. A web site comparing the two states:

 

In
general, NiMH batteries cannot handle the high rate of charges or
discharges (typically over 1.5-2 amps) that NiCAD batteries can

 

It looks as though I will have to switch to NiCd or Lipo to solve the problem. The NiCd (see link below) would be a lot less expensive. Was this the type you used?

http://www.ebay.co.uk/itm/2x-Ni-cd-9-6v-1000mAh-Rechargeable-Battery-Pack-Tamiya-Connector-D235-/170775802442#vi-content

I would need to confirm that I can use the charger I already have for NiMh packs. A Lipo would save considerably on weight and would deliver more power.

 

Decisions…decisions…

 

Jeremy

[ashley needhamParticipant @ashleyneedham69188]

Nah, I have used a NiMh battery .. supplies up to 8 or 9 amps according to the spec (something you could look up for your make of cell) so well able to deliver the 7-8 A draw of the motor/prop setup.Proof of the jolly old pudding… my A90 has completed dozens of transpondic journeys…too many to recall, with no problem.

 

I recon the voltage cutoff thingy is detecting low voltage when there is a sudden current draw from the battery (non expert opinion there). If you are going to try anything, get a NiMh capable ESC. Another point..the Lipo`s come in 7.4 and 11.1v, so your Nimh battery may be throwing it, as its 9.6V…possibly the ESC thinks you have a badly discharged 11V pack? I didnt think that NiCd batts were being made still due to the poisonous nature of the things in landfill?

 

LiPo`s will require a dedicated charger and a little bag that stops the fire spreading when/if they catch fire, and they also blow up if they get wet (or am I just being alarmist)

ONLY KIDDING.

 

I found a 5×2 prop today by chance at Addlestone Models, would or may be just the thing to tame an 11V LiPo on the `plan…

 

Ashley (will not repeat advice as prev post)

 

 

 

 

Edited By ashley needham on 03/02/2012 18:21:11

[JeremyParticipant @jeremy15845]

Ashley

 

Now I know why it was so difficult to find NiCd batteries from the normal UK suppliers. No reply yet from Overlander – hopefully I will be able to switch off the low voltage cut-off even if the option is not shown in the programming options. I recall you used an Overlander ESC – can you let me know which one it was?

 

Re your final point, I still haven’t changed anything but progress across the pond could be interesting if I don’t resolve the surging issue. Of course, it might go very well across the ice which we are promised for this weekend…

[JeremyParticipant @jeremy15845]

Messsage sent itself before I had a chance to sign off….

 

Jeremy

[TelstarParticipant @telstar]

Hi Jeremy/Ashley
I think Ashley has the reason for the surging in his post 03/02.
If the ESc is on auto detect for Lipo cells it can very easily react as explaned in his (non expert)
opinion. You could try manuel select for two lipo cells (7.4v) and set min voltage at 3.1v per cell, giving a low voltage cutout of 6.2v. (I don’t know the ESc characteristics hence I’m talking generally) this may solve it.
I think if you connected a voltmeter across the battery (a ‘proper’ voltmeter with a pointer) you would see the battery voltage surging with the load, most digital meters have a long (reletivly) response time, and usually don’t show quick fluctuations in reading.
My estimate shows the battery’s internal resistance about 0.15ohms (NB estimate) thus if you called for 10A the battery voltage would fall to 8.5v perhaps low enough to trigger the Esc which would reduce the load (amps), the battery voltage would recover, and the cycle would repeat.

Nicads were indeed withdrawn from retail sale in UK several years ago. NiMh’s are considered practical replacement.

LiPo’s are not as critical as Ashley’s fears? Myself and a couple of others in the club have/are using lipo’s. If they were so dangerous would they be used in Mobile Phones, Cameras, etc. They are very suseptable to being run ‘flat’ ie if you discharge below the threshold voltage, that’s it, they will not recover. When being charged ‘normally’ they don’t get hot and burst into flames. If however you are Fastcharging they do get hot (just the same as NiCd’s used to) and are prone to bursting (hence igniting) just as NiCd used to explode (I’ve seen one or two NiCd go bang at electric speed events in the past).
When in the boat I take some precautions against water, but no more than I do with the NiMh packs, usually put the battery in a plastic bag, with an elastic band holding it closed round the wires and of course hope for a dry boat.

I’m looking forward to reading your findings
Cheers Tom

[ashley needhamParticipant @ashleyneedham69188]

I was thinking about making a set of temporary ski`s !!

 

My ESC is an Overlander “tornado gold” .

The parameters you can set using the transmitter to do the business are..

 

Brake on, Brake off, Nixx batt. LiPo batt, soft start. hard start, soft timing, hard timing, reset to factory settings.

 

The only thing I twiddled with was the Nixx setting for NiMh. There are so many brushless ESC out there I think its luck of the draw as to what you get in terms of capability and user frendliness. Certainly I just said to the man “better have an ESc for this shiny new brushless motor” and he said here you are squire take this one, should do you. An so I did, and it does !!

 

As an idea, try a 7.2v NiMh pack on the ESc, as this is nearer the 2 cell Lipo voltage..these ESC are self sensing on the battery front (by voltage) and so may assume you are then using a 7.4V Lipo, and not mess around?? worth a try… If it does work properly then, then you know what the trouble is !

 

Ashley

[ashley needhamParticipant @ashleyneedham69188]

I was thinking about making a set of temporary ski`s !!

 

My ESC is an Overlander “tornado gold” .

The parameters you can set using the transmitter to do the business are..

 

Brake on, Brake off, Nixx batt. LiPo batt, soft start. hard start, soft timing, hard timing, reset to factory settings.

 

The only thing I twiddled with was the Nixx setting for NiMh. There are so many brushless ESC out there I think its luck of the draw as to what you get in terms of capability and user frendliness. Certainly I just said to the man “better have an ESc for this shiny new brushless motor” and he said here you are squire take this one, should do you. An so I did, and it does !!

 

As an idea, try a 7.2v NiMh pack on the ESc, as this is nearer the 2 cell Lipo voltage..these ESC are self sensing on the battery front (by voltage) and so may assume you are then using a 7.4V Lipo, and not mess around?? worth a try… If it does work properly then, then you know what the trouble is !

 

Ashley

[JeremyParticipant @jeremy15845]

Tom

 

Problem solved as you suggested – thanks!. I selected 2 cells and the lowest voltage threshold, 2.6v – now the motor goes up to full speed and stays there. According to my Pischler meter, the battery at full throttle was supplying 9.4A with the voltage down to 6.7. I will report later how she goes on the water in my Ekranoplan thread.

Cheers

Jeremy

[ashley needhamParticipant @ashleyneedham69188]

Yo dude !! inspired guesswork that was.

 

Ashley

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