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Which Watt Meter

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Andy C19/01/2019 19:15:13
158 forum posts

Up to now I have simply built electric planes with recommended set ups. However, I want to experiment a little so obviously need a watt meter. Are they all much of a muchness? Will a cheap one from Ebay or hobbyking cut the mustard?

Thanks

Geoff Sleath19/01/2019 19:50:27
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3434 forum posts
297 photos

I have two. The first I bought is an Astroflight one that cost £50 10/15 years ago, so quite expensive. The second is one from HK which also works as a LiPo checker/balancer. I almost never use the Astroflight one and the HK one is taken the field every time I go (mainly for its LiPo checker function). When I first got the HK meter I checked it against the Astroflight version and they matched as accurately as practical (we aren't looking for +/- 1%) so I'm happy with it.

So yes, get one from HK. They're very cheap now and have potential to save much more than their cost.

Geoff

Bruce Collinson19/01/2019 19:56:49
404 forum posts

Or support a good UK business and buy from 4Max.

Dave Hess20/01/2019 01:06:38
303 forum posts
18 photos

I've used most of the cheap Chinese wattmeters that you can get from Ebay from £8 to £13. They all work perfectly. The thee button type are a bit more expensive, but have more functions. For aeroplane motors watts aren't as important as amps. Wh/Ah are useful for checking battery capacity..

Don't forget that any watts or amps you measure are power in, not out, and the efficiency depends on the speed the motor spins. It can be anything from about 30% to 80%, and low current with low efficiency can damage the motor much quicker than high amps with high efficiency, e.g. 50 amps at 80% efficiency gives 10 amps burning power and 40 amps at 60% gives 16 amps burning power.

Edited By Dave Hess on 20/01/2019 01:15:06

Attilio Rausse20/01/2019 10:31:29
105 forum posts

Dave don't think your statement re efficiency makes any sense, its just too many amps that will kill the motor.🤣🤣

Tim Ballinger20/01/2019 12:06:03
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556 forum posts
229 photos

Personally I prefer to have a lipo voltage sensor integrated into every set up. Allows me to monitor overall voltage and individual cell voltage before after and during flight. I also invested in an in line current sensor which when used with the voltage sensor allows a watt read out as well. I use the radio’s TM to monitor and record the data as well as display real time if required during sytem testing and set up. If you leave it connected during flight and later download the log data you can enjoy endless fun correlating with your in flight manoeuvres.

Tim

gangster20/01/2019 12:07:34
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965 forum posts
17 photos

Attilio your post makes perfect sense to me and I have almost always used the maximum current for the motor to be the one parameter that I based my choice of set up on. ( yes I know voltage is important but I am solely a 3s or 4s man). What has confused me and indeed caused me to buy elswhere is that one of our most respected supplier of batteries sell a range of motors quotin power rpm and voltage but not max current nor can they definitively quote one

Geoff Sleath20/01/2019 12:15:06
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3434 forum posts
297 photos
Posted by gangster on 20/01/2019 12:07:34:

Attilio your post makes perfect sense to me and I have almost always used the maximum current for the motor to be the one parameter that I based my choice of set up on. ( yes I know voltage is important but I am solely a 3s or 4s man). What has confused me and indeed caused me to buy elswhere is that one of our most respected supplier of batteries sell a range of motors quotin power rpm and voltage but not max current nor can they definitively quote one

Couldn't agree more. The voltages used for model aircraft are relatively low and extremely unlikely to break down the insulation covering the wire in an electric motor. The current on the other hand, combined with the wire's resistance can do untold damage and is the most important limiting characteristic.

Geoff

Dave Hess20/01/2019 12:23:43
303 forum posts
18 photos
Posted by Attilio Rausse on 20/01/2019 10:31:29:

Dave don't think your statement re efficiency makes any sense, its just too many amps that will kill the motor.🤣🤣

It's basic physics - the law of conservation of energy. The energy you put in has to go somewhere. If you have an efficiency of 60%, it means that 60% of the energy going in is converted to motive power. The rest will be converted to heat and a very smal amount to sound. The amount of power is the rate of that conversion, so at 60% efficiency and 100W on the wattmeter, roughly 60w motive power and 40w heat. The ESC is not 100% efficient either, so a small proportion, something like 5% to 10% won't reach the motor an will be converted to heat in the ESC, which is why they can burn too if you put to much power through them.

The current that you measure on an ameter or wattmeter is not the current going through the motor, so it's beter to consider the power going through it. The current is split into three phases and pulsed at a high frequency by the ESC, so it becomes much more complicated to try and explain what happens in the motor. That's why I prefer to think about power and efficiency, though for some strange reason, the motor manufacturers and testers very rarely give us the full throttle speed vs efficiency tables or graphs, which are really useful to see whether the motor is likely to burn.

Sorry for a not simple answer. Unfortunately, the closer you look the harder it gets, though if you look at it from the principle of conservation of energy, it is simple.

gangster20/01/2019 12:40:16
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965 forum posts
17 photos

quite so Dave but surely way over and above what we need to know and indeed what we could even measure. If a manufacturer quotes a maximum current the best we can do is measure the dc current into the esc. Not the pulsed 3 phase into the motor. I have no doubt that the power, max current that we are quoted are what we can measure into the esc and do not take efficiency of motor or esc into consideration. With the normal motors we use together with a cheap meter as previously described we will not go far wrong provided we play safe and allow a bit of headroom

Bob Cotsford20/01/2019 13:06:18
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8032 forum posts
444 photos
Posted by Dave Hess on 20/01/2019 12:23:43

Sorry for a not simple answer. Unfortunately, the closer you look the harder it gets, though if you look at it from the principle of conservation of energy, it is simple.

and that's why I don't look too closely. Current rating - a little bit = reasonably safe limit. Kv to suit the prop size and cell count that will fit or vice-verse if I already have the motor. Wattmeter to check, Bob's your Uncle. I use E-calc to double check that it won't flame out for unfamiliar setups.

Tim, the telemetry is good to monitor the LiPo condition and capacity left in flight but it gets costly if you have a number of models. Come to that, depending on your budget you might find setting up telemetry expensive full stop. A wattmeter is a good place to start if you are new to electrics if not essential equipment.

Tim Ballinger20/01/2019 13:40:07
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556 forum posts
229 photos

True enough Bob. Costs me £11 for each model to which I add the voltage sensor. I reckon the peace of mind is worth it . Also true regarding the TM but having invested in FrSky I may as well make use of it.

Tim

PatMc20/01/2019 13:58:17
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4222 forum posts
521 photos
Posted by Dave Hess on 20/01/2019 12:23:43:

The current that you measure on an ameter or wattmeter is not the current going through the motor,

The current measured is the rms current through the motor windings.
OTOH the applied voltage measured is not the voltage across the motor windings when the motor is running.

The voltage across the windings = applied voltage – back emf

This voltage can be calculated as = measured current x resistance of windings

The heat generated in the motor = measured current x (resistance of windings)² Watts

All of which simply means that the greater the measured current the greater the heat generated in the motor.

Q.E.D. current is the limiting factor for our motors.

Wingman20/01/2019 14:13:08
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1105 forum posts
405 photos

What is the matter with you lot?? The OP asked for a recommendation for a WATTMETER and you're all waffling on and on about volts and amps - 9 out of 13 posts off topic - give it a rest!

Attilio Rausse20/01/2019 14:14:00
105 forum posts

If you allow the motor to draw more amps than it is designed to do, you will burn the motor out irrespective of the efficiency, Amps is what is measure by the wattmeter, I can see what your reasoning is but if you follow your reasoning then if the motor is drawing 100A and is 80% efficient you state that there will only be '20A burning power' but you are forgetting the 80A driving power, it's still drawing 100A.

Attilio Rausse20/01/2019 14:15:58
105 forum posts

Wingman - you are right, however equally important is that a newbie doesn't get his sums wrong.

Wingman20/01/2019 14:36:47
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1105 forum posts
405 photos

Yep Attillo the wattmeter saves you having to do sums.

gangster20/01/2019 14:45:41
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965 forum posts
17 photos

 

 

 

Wingman. I am not totally sure it is too far off topic. The guy wants a watt meter which in my opinion on is vital if you are to choose your own power train for a model. Some basic meaning of what you are measuring must help what does not help is waffling about advanced theory to put the everyday modeller off for life

So let’s keep it simple for the average sub kW motor in an average model

Max amps is the main decider . If you draw more than the maker says you deserve smoke . Ignoring efficiency back emf etc keeping the maths simple .decide the power you want for the model ,choose a motor of that power then divide that power by the voltage If that figure comes to more than the max current either look for a bigger motor or goes for a battery of more cells (within the voltage spec of the motor) Then look at prop size and follow the makers recommended range If that is way too small for the model in question you want a motor of lower kv ie 1000 not 1500 etc Do not however push the motor to max current and go for the next size up of esc doing that allows for the fact that we are simplifying the maths to basics and that the manufacturer is pushing his spec too hard finally downy run on the ground too long it will draw less in the air (back emf again)

 

Edited By gangster on 20/01/2019 14:50:42

Geoff Sleath20/01/2019 16:04:46
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3434 forum posts
297 photos
Posted by PatMc on 20/01/2019 13:58:17:
Posted by Dave Hess on 20/01/2019 12:23:43:

The current that you measure on an ameter or wattmeter is not the current going through the motor,

The current measured is the rms current through the motor windings.
OTOH the applied voltage measured is not the voltage across the motor windings when the motor is running.

The voltage across the windings = applied voltage – back emf

This voltage can be calculated as = measured current x resistance of windings

The heat generated in the motor = measured current x (resistance of windings)² Watts

All of which simply means that the greater the measured current the greater the heat generated in the motor.

Q.E.D. current is the limiting factor for our motors.

I agree with Pat

Geoff

Gary Manuel20/01/2019 17:37:18
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1953 forum posts
1514 photos

When choosing an electric setup, CURRENT and WATTAGE are both important. Exceed the maximum of either and you are asking for trouble.

A motor with a maximum current of 45A that runs nicely at (say) 40A on a 3 cell setup may overheat at 40A on a 4 cell setup if the maximum power rating is exceeded.

PS I use a HK Watt meter and can confirm that it works well.

Edited By Gary Manuel on 20/01/2019 17:41:21

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