Old 02-23-2008, 05:30 AM
  #7  
AEAJR
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Join Date: Aug 2005
Location: NY, USA
Posts: 5,822
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WHO NEEDS A WATT METER?
A personal experience reveals their value.

I enjoy electric planes. They are quiet, convenient, can be fast or
slow and are fairly inexpensive to fly.

A few months back I picked up a Watts-up wattmeter.
http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXLMV0&P=ML

I thought it would be a good investment as I was doing more in the area of
mixing and matching motors, props, and the like. It is small and simple to
use so I put it in my field box. It wasn't long before it started to show
its value.

We were flying one afternoon when one of the club members felt he was not
getting
good performance from a new plane he had built. I put he wattmeter on the
plane and determined he was pulling about 9 amps. Turned out the pack he
was using really was not up to the load and the voltage was dropping off
excessively. As a result he was not getting the RPM out of the prop that he
expected. Problem discovered and cause identified in a few seconds. He
needed stronger battery packs.

A few weeks later we did the same thing with another plane. There was a
concern that the LiPo being used might be getting over worked. However the
Wattmeter showed that it was working well within its rated capacity. Flying
went on with confidence.

I recently purchased an Easy Glider Electric from another club member. He
had upgraded the motor from the stock speed 400 to a brushless, a 27 amp ESC
and was using 2 cell 2100 MAh LIPOs. I bought the whole package.

The plane flies very nicely on the 2 cell packs, but I had a 3 cell pack
that I thought I might add to the rotation and REALLY boost the power. The
ESC could handle 3 cell LiPo so I did not see a problem. I assumed the
system was probably running at about 18 amps which was within the rating of
this pack. Should be a good fit.

Fortunately before I tried it in the plane I put the watt meter on the
system. I was surprised to see that the system was running at 26 amps on
the 2 cell lipo packs. That was much higher than I had expected. It turned
out that the 2 cell packs were an excellent match for the motor and speed
control. The amp load was well within the specs of the 2 cell packs being
used and the plane flew very nicely on this combo.

If I had blindly put a 3 cell pack in there I would have pushed well past
the ESC's 27 amp rating and probably burned out the speed controller. Or,
in the case of my 3 cell pack, it would probably have pushed over 30 amps
into the system due to the higher voltage, but it was not rated for that
high of an amperage and would probably have had a short life working at that
level. I would have thought it was just a crummy battery pack but in fact I
would have been over working it.

Operating in the blind I would have ruined the ESC, or the pack, or both. A
very expensive mistake. Certainly more than the cost of the watt meter. It
had just paid for itself.

A few days ago I pulled out my old Electrajet to prepare to sell it. I had
purchased it almost 3 years ago, but had never really been happy with the
plane and my interests have turned more toward gliders and slow flyers
rather than a pusher jet. When I purchased it I also bought some cells and
made up some 8 cell packs. However it really didn't seem to have the zip I
thought it should. I just attributed it to the speed 400 motor and the
plane being too heavy.

I put the watt meter on the motor/battery combo. The motor sounded about as
I had recalled. When I checked the meter, low and behold, those 8 cell
packs were duds! They were 9.6V 8 cell 1000 MAh packs rated for 10C. At
rest, fresh off the charger they were reading 11 volts, but when I hooked
them up they were both dropping to 7 volts while delivering 9 amps. That is
way too much drop! The problem was not the plane or the weight of the plane
but the quality of the cells I had used.

I tried one of my 15C Lipo packs and that held voltage well, delivering 13
amps. The motor screamed! Now that was more like what I had expected.
Hummm, maybe I won't sell it after all. I just need to put better battery
packs in it.

I also tried a 1000 MAh 2 cell lithium pack that is rated at 10 C. The
voltage sagged to 6.6 volts almost immediately. The motor ran but I was
clearly over stressing the pack. This pack would have been ruined in very
few flights if I had used it to fly the plane regularly.

I share this story only to help you understand that, without a watt meter,
or the use of a multi meter with knowledge and skill, we are working in the
blind. We really don't know what is happening in our power systems.


WHO NEEDS A WATT METER?

While the watt meter is a nice to have, some people don't need one. If you
are buying RTF planes, or ARF or kit planes and are using the manufacturer's
supplied motor and battery packs, I would say you can be pretty confident
that all is well.

However, if you start mixing and matching motors, gear boxes, props,
controllers, battery packs and the like, you are really working in the blind
if you are not measuring the energy flow in the system. In my case, I
started making my own battery packs but I was not measuring their
performance. Now I know the true results.

There are a variety of watt meters out there. This one is easy to use and
fits nicely in my field box, but there are other good ones. If you are
going to upgrade your power systems or make up your own packs, you need a
watt meter. You can perform many of the same tests with a millimeter if you
know how to work with shunts and the like, but if you want a simple to use
tool that does exactly what you need it to do, this is hard to beat. It has
other uses too, so read the instructions, but for this use alone it paid for
itself pretty quickly.
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