Wattmeters...for beginners - please keep it simple

[Tim Mackey]

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 Grigg]

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. Barry]

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 - Moderator]

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 Mackey]

Amen.

[A.A. Barry]

Would this be suitable

[Tim Mackey]

yep...its what I use.

[Steve Hargreaves - Moderator]

The original (& best!!)

 

Mind you they all do pretty much the same so buy the cheapest!!!

[TonyS]

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 Mackey]

Astro flight whattmeter, available from amongst others West London models.

[TonyS]

Thanks Timbo,

A Whattmeter ! - that explains the question mark   

[Tim Mackey]

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 - Moderator]

Wattmeters.........more wattmeters.....& even more wattmeters

 

[gary w]

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 Anderson]

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

[Dusty]

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 Bewes]

 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 

 

[leccyflyer]

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 Mackey]

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 7]

Apologies if its old news, but I found this "Watt Meter Simulator" ...

 

SIMULATOR 

[Steve Hargreaves - Moderator]

A great little tool Tony & very useful for illustrating what happens as voltage or whatever changes but I'm not sure how you would find/calculate some of the required values such as the load resistance or the battery internal resistance & if these are inaccurate they will have a huge impact on the results......

[Vecchio Austriaco]

sorry to put in my comment just now - did not see this thread before. 

My comment is nothing special - but let's stick to the right letters in the formulas to avoid confusion.

 

Tim writes

"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 letter for power is P not W - W stands for the unit watt.

So the formula is P=V x I

 

Cheers VA

 

[Malcolm Fisher]

Being a "Good Boy" and new to electric flight, I have read quite a few of these threads and bought a Neodym Wattmeter as suggested after a previous enquiry.

It's fine for showing the basic power, voltage and Amps and has helped me with my motors and props.

As well as these readings, the display changes and shows some readings in other values which I suppose are linked to battery perfomance. In my ignorance I haven't taken much notice of the readings apart from Watts, Volts and Amps as my main interest was making sure that I stayed within the limits of the various components and see what different props did.

Can anyone explain what these other readings are trying to tell me?

I would also like to know if the Wattmeter could be used to program an ESC when set up on the bench as a stand alone unit to test a set up without using radio. The particular ESC is programmable from the tranmitter by using throttle stick movements. Would using the control knob on the Wattmeter as the throttle make the meter act in the same way? Or something like a program card which I've heard of but only seen in illustrations?

Malcolm

[Tim Mackey]

It cant be used to "program" an ESC, but can "drive" one emulating a receivers throttle output curve - this eliminates the need to have radio at all for setting up purposes.

Most ESCs these days can be programmed with a separate card, but unfortunately they are not all the same, and you end up with loads of different cards to suit the various ESCs - they are a whole lot better than using the Tx sticks and beepings etc

I dont know the exact model you have, so cant say for sure what the other readings may be....most likely cumulative milliamps consumed.

[Cyclicscooby]

I've a degree in electronics, and a multitude of measuring devices, including 2 scopes (1 x storage), which until now have all been 'fit for purpose', but wiring up a multimeter to a plane and doing calcs in your head, is just an estimated guess..

 

I got a Bantam 'Two-Meter' at a show for £35 and its brill... Tells you everything you need to know at a glance... Beats a multimeter hands down for convenience..

 

One MAJOR issue I had was the first TWO pins on the balance plug are GND.. !! (my Flight-power balancer hasn't)

I didn't check, and ended up melting a balance wire to cell 1 as it was effectively shorted thu the balance pins.. GGrrr I don't know why its got 2 GND pins as i've never seen a lipo with a double GND, but i've had to bend pin 1 sideways and start at pin 2 so it doesn't happen again..

 

Luv

Chrisie.. xx