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Wattmeters...for beginners - please keep it simple

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Tim Mackey23/08/2009 12:34:33
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Unless you have been living under a stone, you will almost certainly have heard of a "Wattmeter". This invaluable, and inexpensive piece of measuring equipment is the best way of ensuring that your electric powertrain is correctly matched.  All components within an electric powertrain are designed to operate within specific  parameters, and we need to ensure that our system is going to produce the power we are looking for.
The "Wattmeter" ( I will call it a meter from now on ) is used for bench testing your setup before you commit themodel to flight. Heres how it works. You will have seen in the main intro section how the motor and battery and ESC all connect up. The meter is temporarily inserted between the battery and the speed conrollers main cables and will provide you with a readout of exactly how much current your "rig" is consuming, how many watts of power the system is providing, and also how many volts the battery is maintaining when under full load. Normally the rig is secured safely on the bench or whatever, and the throttle is advanced ( ensure you stand behind the prop / fan and that if the worst happens and the prop flies off, it will not damage anything ). Let the motor run for around 30 seconds or so, and make a note of the figures mentioned above. They will likely fluctauate alittle during the test, but you will get the trend. One of the main things this will show for you is the power - in watts -  that the set up is producing.
Watts are derived from multipying the volts of the battery ( under load ) by the Amps consumed. W =I x V ( I is the symbol for current, or amps ).
The more volts you have the higher the watts, the more Amps you have the higher the watts. Assuming that the battery you select is not likely to be changed for a higher voltage version, then altering the propeller is the biggest single factor in altering the current that the motor will consume.

Lets say your motor is designed for a 6 X 4 prop, on the chosen battery, but you want to try and get a litlte more thrust, or climbing performance from your setup. Using the meter, you could now fit a slightly larger diameter prop, and monitor the current being drawn. Now you can see the amps that the motor is pulling, and therefore ensure you do not "prop up" to the point where the maximum figures allowable are exceeded. These maximum figures are also applicbale to your battery, and your speed controller, and indeed exceeeding any or all these could prove very expensive, or even dangerous. The modest cost of a suitable meter will be re-couped the very first time you use it and discover that your power figures are beyond the maximums allowed. Various models and features are available these days, including a less versatile "clamp style" ammeter which as the name suggests, simply clamps around the poer leads, so negating the need to actually "break into" the cabling and fit extra connecters etc. However, these will not show all the data that an "inline" meter will, and also be aware that many of the cheaper clamp meters will only measure A/C ( alternating current )  - we need to measure D/C ( direct  current)
Heres a whattmeter
 
And heres the clamp meter....

 

Edited By Timbo - Moderator on 23/08/2009 16:36:47

Stephen Grigg24/08/2009 05:24:57
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I was surprised when I first used a watt meter how much the watts change with differenrt sized props and buy getting the best match how much more efficient the motor the becomes.So much to learn so little time to do it,Thankyou god for Timbo
A.A. Barry28/08/2009 13:54:49
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Timbo, I have taken your advise, ...wattmeter......which is the most suitable, "plug in" or the "clamp" ones???????
 Being an Automotive tech. is the accuracy  with the clamp one finite enough for model application??, the ones I have used have a rather large "clamping circle" to get the measurement???
Good stuf this
A.A Barry  

Edited By Timbo - Moderator on 29/08/2009 16:17:05

Steve Hargreaves - Moderator28/08/2009 14:06:09
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For me the beauty of a wattmeter is that it shows current & voltage under load (very important!!!) as well as watts....a clamp meter will only (in my experience) show current so you need to measure the voltage & do some maths to get the watts (volts x current). Measuring the voltage entails another meter & all the connection hassle that entails......!!
 
Given that wattmeters are so cheap (£25 or so & it could save you that in its first usage if it shows you are puling 40A through a 30A ESC!!!) I just don't think its worth being without one or looking for alternatives.....
 
Buy a wattmeter & be happy......
Tim Mackey28/08/2009 14:41:35
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Amen.
A.A. Barry29/08/2009 15:27:37
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Astro Flight Super Whatt-MeterWould this be suitable
Tim Mackey29/08/2009 16:16:22
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yep...its what I use.
Steve Hargreaves - Moderator31/08/2009 17:12:40
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The original (& best!!)
 
Mind you they all do pretty much the same so buy the cheapest!!!
TonyS31/08/2009 17:27:41
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Hi,
I read this some time ago and then decided to go and buy one. Trouble is I can't seem to find any from searches of Wattmeter on the various online shops I normally use. I was excited to see Barry's post "Would this be suitable"  hoping to get the braqnd etc - sadly all I can see is a blue square with a ? in the middle followed by the text. (not sure what's happening there.... 
Any recommends...?
Thanks 
Tim Mackey31/08/2009 17:56:09
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Astro flight whattmeter, available from amongst others West London models.
TonyS31/08/2009 18:19:22
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Thanks Timbo,
A Whattmeter ! - that explains the question mark   
Tim Mackey31/08/2009 20:17:52
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Indeed... Catches a few out does that one  - I even had a member accuse me once of being unable to spell Watt-meter - shock horror - I threw him out ( not really )
Steve Hargreaves - Moderator01/09/2009 08:59:04
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gary w08/09/2009 22:50:57
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Ye that one works well but i use the multi-functional one by etronix's its a lipo balancer/watt-meter and voltage checker at a good price £35 from BRC Hobbies.
It does everything at a low price? 
Ed Anderson04/04/2010 15:57:21
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I ahve used the Watts Up  wattmeter with great success.  
 
You will have to solder on the conenctors that match what you use for your battery and ESC, but after that it is very easy to use and small enough that you can mount it in the plane if you want to take some peak readings during the filght.
 

Edited By Ed Anderson on 04/04/2010 15:58:58

Dusty04/04/2010 16:17:08
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I use the Watts Up too Ed! I like the way it displays the peak readings so you can write them down for referance etc after testing various combos. I notice they have come down price considerably the last year or so... Without a Watt meter one would be lost?
Eddie Bewes09/06/2010 12:07:39
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 Hi, Having read all the the different comments about all the watt meters I am at a loss as to which one to choose from,I have looked at Aurorra and their meter is £56-05,
BRC Hobbies meter is £27-00,modelmaniacsonline.co.uk, multiplex M X 8120 meter is
£32-00, RC Electronics "watt up" meter & power analyzer is £52.35. I am a total beginner to model aircraft and I chose to go down the electric route as I am not very fond of IC power and all the mess that goes with it, It is not the price that I am concerned about but which one is best for me as when I look at them, they all seem very hard to understand as I am not very electrical minded 
 
leccyflyer09/06/2010 12:21:30
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My Astro Wattsmeter, that served me well for many years is now mostly unused in favour of a Graupner clamp meter. The big thing with the clamp meter is that it is non-invasive, easy and simple to use, which encourages it's use. The Wattsmeter maybe gets 5% of the use, and only then when I specifically want to test how the batteries are holding up under load.
 
There's also the small matter of inserting what I measured on mine as 13 inches of extra wiring between the battery and the ESC, which could potentially cause problems. Others' Wattsmeters wouldn't have as much wire, since I retained the original Astro plugs and fitted short extensions to my usual connectors.
 
Nevertheless I still find the clamp meter easier to use, as do those who borrow it. True you don't get any reading other than the most critical amps drawn, which is what preserves your electric bits from letting out the magic smoke. If I were only able to have one meter it would be the clampmeter.
Tim Mackey09/06/2010 12:47:04
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Posted by Eddie Bewes on 09/06/2010 12:07:39:

 Hi, Having read all the the different comments about all the watt meters I am at a loss as to which one to choose from,I have looked at Aurorra and their meter is £56-05,
BRC Hobbies meter is £27-00,modelmaniacsonline.co.uk, multiplex M X 8120 meter is
£32-00, RC Electronics "watt up" meter & power analyzer is £52.35. I am a total beginner to model aircraft and I chose to go down the electric route as I am not very fond of IC power and all the mess that goes with it, It is not the price that I am concerned about but which one is best for me as when I look at them, they all seem very hard to understand as I am not very electrical minded 
 
 
They will all do pretty much the same thing Eddie. Its like most things, cars, washing machines, toasters etc - sometimes you get what you pay for, other times you pay for the name, and some people like a badge.  I like VFM.
Tony Smith 709/01/2011 16:55:33
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Apologies if its old news, but I found this "Watt Meter Simulator" ...
 

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