BEWARE of "Too Much of a Good Thing!"

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BEWARE of "Too Much of a Good Thing!"

New postby retiredVTT on Tue Oct 27, 2009 5:12 pm

A very few years ago...like maybe six..we had lipo cells capable of a 8C discharge rate. A 1000mah pack was capable of a 8 x 1000ma = 8 amp discharge into a motor without harming itself.
Oh, we did manage to raise that discharge figure in our ever quest for speed and you know what we got...a nice warm pack that was so proud of itself for delivering those extra watts of power that it "swelled right up with pride", but lo and behold it had damaged itself beyond repair and was relinquished to the garbage can..!

So, fast forward six years and we are using cells that now have a normal discharge rate of 20C or higher and a burst rate (good for 15 secs only) of 40C. So now that same little 1000mah cell can safely deliver not 8 amps but 20 amps to our motor, so we must be sure that our motor and our esc are rated for those 20 amps.

But now all of a sudden, we have cells rated at 40C normal discharge...! And we also have components that are failing for reasons that might not be obvious at first.

So it has become very necessary that a Wattmeter or similar instrument be used when setting up a power system.

I think we are all guilty of wanting to get the most from a system. Recently I burnt out two motors when operating them from a 4S pack. But they did not fail immediately. (one was rated only for 3S but the other was rated for 4S)

The synario goes something like this, you try the motor on an older 4S pack and you see great results for a few flights, then you see an ad for the latest greatest high C rated pack...wow gotta have one of those!
You get it, slap it on, and hey, these guys have really made a great pack and you enjoy the flight (if your lucky you might get a few flights). But then all of a sudden you realize that the motor is burnt out!

So..what gives..? (continued below)
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Re: BEWARE of "Too Much of a Good Thing!"

New postby retiredVTT on Tue Oct 27, 2009 5:54 pm

Well, it's the old story, we hotrodded the motor (with a higher octane fuel, got increased rpm from the old Chevy, and promptly blew out the transmission..!

What is happening with the newest lipos...(and will continue to happen as we approach 60C, 80C, whatever) is that the internal resistance of the lipo cell is being lowered, with the latest materials and manufacturing processes; so that less of a voltage drop is happening within the cell, and more of the cell voltage is felt across the motor, which in turn causes more amps to flow through the motor. (basic Ohm's law..)

As an example...if we have a lipo pack that when delivering amps to the motor, it lowers it's output voltage by three volts, and then we replace it with a newer pack which only drops it's voltage by one volt, we are effectively applying two additional volts to the motor. If the current delivered to the motor from the first pack was equal to the rated motor current...the current delivered from the second pack will be more than the rated motor current, and since power in Watts is a factor of Voltage x Amps...you could actally see a 25% rise in power from the motor when run on the newer pack. If either the motor or esc cannot handle the additional power, one or both could fail.

Bottom line is to use those Wattmeters again when using new packs...and be ready to upgrade some or all of the components to avoid problems.

To expound (another nice word) on this further, keep in mind that any good designer of power systems will build in a safety factor of say 25%. If you feel that a 600 Watt motor is what is going to propell your model properly, then use a motor rated at 800 Watts. If a 45 amp current draw will be the norm, your Esc should be rated at 60 amps. You are building in a safety cushion, and the system will likely survive the next level of higher C packs that come along.


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Re: BEWARE of "Too Much of a Good Thing!"

New postby steve on Tue Oct 27, 2009 7:23 pm

Good post Bill.....

The watt meter is an invaluable tool....You only need to burn out 1 or 2 motors/ESC's to see the value in them...

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Re: BEWARE of "Too Much of a Good Thing!"

New postby retiredVTT on Wed Oct 28, 2009 1:23 am

Thanks Steve, as I commented above, we are definitely seeing an increase in component failures at the field from those of us that tend to push the power or speed envelope.
Performance that just mere months or a few years ago was unheard of from electric powered models is now almost common place, and moving forward so rapidly that if you don't keep up with it, you start letting that magic blue or white smoke out of components. It's in all the components we buy, it's up to us to keep it in there..!

Another whole topic of continuing discussion is the "screeching sounds" we hear now and then eminating from the electric motors we use. There are several reasons and causes for this and I don't pretend to know all of them but I keep reading..!

First off is the case of starting up our motors. If we just open the throttle a click at a time, the motor can reach a point where it pulsates and emits noises but does not rotate....try this on an electric drill, same thing..! So we should literally bump the throttle to create motion, then lower the left stick to get to an acceptable low idle speed. The Esc has no idea what position the motor is in UNTIL the motor actually moves...sort of a chicken and egg synario.

The second case of "screeching motor" we come upon is the one when we slam the throttle open quickly...sometimes it sounds like the ole fingernails on the chalkboard deal. Horrible..!

Generally this is a "timing" problem...in that the esc is providing pulses to the motor windings faster than the motor can handle them or at best it's providing them at the wrong time....and the solution is to change the timing of the esc by reprogramming it.

But lately I'm reading of another condition that can result in the same noise, and it goes something like this. Your motoring along and slam the throttle open, a very large current flows into the motor and saturates the magnetic core to the point that no additional motor strength is created, a terrible racket is heard, (I believe I read that at this instance a very large spike of current enters the motor), and one of the suggested cures for all of this is to reduce the size of the propeller.
The theory here is that the propeller was large enough that the motor was on the verge of being magnetically saturated and the sudden additional in rush of current from those new high C packs simply put the motor over the edge.

So the quick thing to try in this case would be a different size prop. Next I would try reprogramming the esc or changing the esc as some esc's will work better with certain motors, and finally I would go to another motor...probably a larger one to see if it could handle the high current pack.

(..more on this subject two posts below..)

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Re: BEWARE of "Too Much of a Good Thing!"

New postby retiredVTT on Wed Oct 28, 2009 1:46 am

One more "too much of a good thing" is a condition I witnessed last weekend.

We had the HobbyKing Funjet hotrodded with a big ole fat Turnigy motor drawing fuel from an equally fat 3200mah pack and spinning a 6/5 prop and while on cruise control it was behaving fine..but when given it's marching orders, it responded with the nastiest collection of sounds and jerky movements you could imagine. What to do..? Do you suppose this itty bitty prop was turning so fast that it displaced every molecule of air around it..? (I believe they call it cavitation..)
Humm...pretty wild theory, but let's try a bigger prop. So we went up to a 7/5 prop, and hello..! No noise, just smooth and fast and groovy flight...and big smiles from the face of Chris as he criss-crossed the field.
So now what..? Yup, let's try a 7/6 prop since the motor was still cool after the 7/5 flight.
Ah yes...she like's 7/6 even more!

So there was a case of having so much motor and pack that the weak link was the small prop.... An easy fix!

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Re: BEWARE of "Too Much of a Good Thing!"

New postby retiredVTT on Fri Oct 30, 2009 11:21 pm

..here is more info on "screeching motors" from Lucien Miller of Scorpion Products:

*jackosmeister,*

"I have had my test motors loose sync with the ESC when pushing them to extreme limits of voltage and RPM on small props. Increasing the timing angle helps in most cases, but when you have the magnetic field switching polarity in a motor that fast, you can begin to saturate the core, and once that happens, the ESC cannot get it's proper feedback signals, and it loses sync with the motor and you end up with the equivilent of a short circuit, and that is where you get the massive current surge. When you have a 12-pole stator with 14 magnets, you can only turn them so fast before you start running into this problem.

All motors have a maximum operating RPM due to the limitations of how fast you can expand and contract the magnetic fields within the core of the motor. Once you hit the saturation point, it cannot spin any faster. The only option is to use a Higher Kv motor or one with fewer magnets, so it rotates more during each phase cycle, and reduces the number of times the ESC has to change phases to make the motor rotate."

Lucien


....although not stated here in so many words, what I'm reading into this "core saturation" problem is that the high C (40C) lipo cells are contributing to this situation. Some Esc's are not able to keep up with the extra power going into the motor and they fall out of sync..)

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