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Tachometer or RPM Sensor

Old 01-04-2010, 02:57 AM
  #1  
ShoGinn
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Default Tachometer or RPM Sensor

So I am trying to make sure my power systems are running properly when bench testing.

I dont have an actual bench stand just planes with setups, I am trying to see if I am wasting money on knowing propeller RPM?

Reason I ask is with some of my ESC's (CC Phoenix) I can tune certain settings But just listening and feeling thrust isnt going to be a good test bed.

I have just been checking Watts/Amps and Temp and going with that.

Should I invest in an Optical Tachometer (like back in the nitro days)

Or Find some sort of pulse width sensor on one of the legs of the brushless motor?

Let me know what you guys think.

I know static testing isnt going to show everything but I want to make sure im doing all I can on the ground.
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Old 01-04-2010, 03:10 AM
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EagleTree has an adapter that connects to one or two of the ESC-to-motor leads and reads RPM's. Of course the EagleTree will read volts and amps without any extra components. But it won't measure thrust.
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Old 01-04-2010, 03:13 AM
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ShoGinn
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Ya Eagletree seems to be a good product, I have a Watt Meter already, I am just curious about either a stand alone optical RPM sensor ($30 or so) or to invest that $70 in an eagletree.

I know finding out how many poles your motor has etc can be a task.
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Old 01-04-2010, 04:55 AM
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Originally Posted by ShoGinn View Post
So I am trying to make sure my power systems are running properly when bench testing.

I dont have an actual bench stand just planes with setups, I am trying to see if I am wasting money on knowing propeller RPM?

Reason I ask is with some of my ESC's (CC Phoenix) I can tune certain settings But just listening and feeling thrust isnt going to be a good test bed.

I have just been checking Watts/Amps and Temp and going with that.

Should I invest in an Optical Tachometer (like back in the nitro days)

Or Find some sort of pulse width sensor on one of the legs of the brushless motor?

Let me know what you guys think.

I know static testing isnt going to show everything but I want to make sure im doing all I can on the ground.

Bottom line, RPM is interesting, but not the total answer. Depending on the model you are trying to fly, a motor that turns a 16 inch propeller at 6000 RPM could out perform a motor that turns a 14 inch propeller at 8000 RPM, even though they have the same watt input. It all depends on whether you've got a fairly slow low wing loaded model, or a fast high wing loaded model.

What works well in these decisions is a program such as www.motocalc.com that allows a prediction on how fast your model will climb at full power. If the prediction is around 2000 Feet per minute or more, that will be a very nice flyer.

But, a standard hand held tach is nice to use on your power system on its first flight. Write down the RPM you get, and compare that initial RPM to what you get after a number of flights. That's a nice indication of how your battery pack is standing up.

FYI, I fly A123 packs, and all three of my 6S2P 4600 Mah A123 packs have the same exact RPM performance AFTER a full second season of flying with them.

If you use one of those hand held tachs in your home, turn off all the lights, and aim a flashlight at your prop. Then aim your tach at the prop for an accurate RPM indication. Otherwise, you could be measuring the 3600 RPM generated by the lights in your home. Needless to say, be careful of that prop, even the little 6 inch props can hurt. (Sad experience!)

Last edited by kyleservicetech; 01-04-2010 at 05:16 AM.
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Old 01-04-2010, 05:10 AM
  #5  
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I've got one of these and it works pretty well. Cheap too.

http://www.hobbycity.com/hobbycity/s...idProduct=5609
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Old 01-04-2010, 05:56 AM
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If you intend to graph your results an Eagle Tree with rpm sensor is great... if you are just getting rpm for its own sake a Hobbico Digital Minitach goes for $16.95 at Tower these days. I have both, and both systems have their uses - the Hobbico is simple and straightforward and it is easy to read off the steady rpm figure and write it down... reading from the EagleTree's LCD display is not as easy since the rpm numbers continually fluctuate (by a very small amount) so you are "guessing". Of course the Eagle Trees graphing capability gives you everything later.
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Old 01-04-2010, 05:58 AM
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Originally Posted by ShoGinn View Post

I know finding out how many poles your motor has etc can be a task.
Count the magnets!
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Old 01-04-2010, 04:45 PM
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Originally Posted by kyleservicetech View Post
Bottom line, RPM is interesting, but not the total answer. Depending on the model you are trying to fly, a motor that turns a 16 inch propeller at 6000 RPM could out perform a motor that turns a 14 inch propeller at 8000 RPM, even though they have the same watt input. It all depends on whether you've got a fairly slow low wing loaded model, or a fast high wing loaded model.
I tend to disagree with this statement - not only can knowing your RPM be interesting, it can also be extremely useful. You can use it to determine the pitch speed of a setup, which is the theoretical maximum speed a prop/motor combination could pull an airplane to. You'll never actually reach the pitch speed, so it won't tell you exactly how the plane will perform, but it makes an excellent way to compare and select props.

It would allow you to make a very clear comparison between the 16-inch and 14-inch props in Kyle's example - let's call them a 16x8 and a 14x10. That means their pitch speeds would be 45 MPH and 75 MPH, respectively (RPM times pitch divided by 1056). We can immediately tell that despite their similar power, the 16-inch prop would be a good choice for 3D or a trainer while the 14-inch prop would be a better choice for a fast sport model.

I'll admit that MotoCalc can give you some useful information about how a motor setup will work, but it's no replacement for running your own tests - especially if you use motors for which not all data is available. When I'm doing my own testing, I'd like to know power, thrust, and pitch speed...but I usually don't bother with thrust, because I find it the most difficult to measure and the least useful overall - I once chose a power system based only on thrust and power, and the result was totally inappropriate for the airplane. If you make a few general rules for pitch speed in certain types of planes, and you follow the normal watts-per-pound power convention, you'll always end up with enough thrust, even without measuring or knowing what you have. I use something like this, although for larger, heavier planes, you'd likely need to adjust (increase) these estimates - I fly roughly 40" models with a 10-15 oz/sq. ft. wing loading, at 100-150 watts per pound:

Indoor: <20 MPH
Slow Fly (Slow Stick): 20-30 MPH
Park Flier, Trainer or 3D: 35-45 MPH
Sport/Aerobatic: 50-65 MPH
Fast/Racer: 70+ MPH

I use: http://www3.towerhobbies.com/cgi-bin...&I=LXPT31&P=ML
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Old 01-04-2010, 07:56 PM
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Originally Posted by aramid View Post
I tend to disagree with this statement - not only can knowing your RPM be interesting, it can also be extremely useful. You can use it to determine the pitch speed of a setup, which is the theoretical maximum speed a prop/motor combination could pull an airplane to. You'll never actually reach the pitch speed, so it won't tell you exactly how the plane will perform, but it makes an excellent way to compare and select props.

It would allow you to make a very clear comparison between the 16-inch and 14-inch props in Kyle's example - let's call them a 16x8 and a 14x10. That means their pitch speeds would be 45 MPH and 75 MPH, respectively (RPM times pitch divided by 1056). We can immediately tell that despite their similar power, the 16-inch prop would be a good choice for 3D or a trainer while the 14-inch prop would be a better choice for a fast sport model.

I'll admit that MotoCalc can give you some useful information about how a motor setup will work, but it's no replacement for running your own tests - especially if you use motors for which not all data is available. When I'm doing my own testing, I'd like to know power, thrust, and pitch speed...but I usually don't bother with thrust, because I find it the most difficult to measure and the least useful overall - I once chose a power system based only on thrust and power, and the result was totally inappropriate for the airplane. If you make a few general rules for pitch speed in certain types of planes, and you follow the normal watts-per-pound power convention, you'll always end up with enough thrust, even without measuring or knowing what you have. I use something like this, although for larger, heavier planes, you'd likely need to adjust (increase) these estimates - I fly roughly 40" models with a 10-15 oz/sq. ft. wing loading, at 100-150 watts per pound:

Indoor: <20 MPH
Slow Fly (Slow Stick): 20-30 MPH
Park Flier, Trainer or 3D: 35-45 MPH
Sport/Aerobatic: 50-65 MPH
Fast/Racer: 70+ MPH

I use: http://www3.towerhobbies.com/cgi-bin...&I=LXPT31&P=ML

Agreed:
As you indicate, the blade pitch and RPM are important, as they are directly related to how fast your model will fly, given enough power.

I'm using a Hacker A50-16S on 6S2P A123 cells, with a 16X12 APC-E prop on a Showtime 50 model. The motor turns that prop at about 5950 RPM, and powers that Showtime 50 model well. But, you could also put a 20X4 prop on that same Hacker A50-16S motor, and not have enough air velocity (Prop blast) through that prop to get the model off of the ground.

Also have a Hacker A50-12S on 6S2P A123 cells with a 14X10 APC-E prop on a scratch built 150% version of the bygone era Electrostreak. The motor turns that prop at about 7900 RPM.

Both models fly very well, and outperform the same model with a 70 class four stroke glow engine.

What I should have pointed out, is that the RPM is not as important as you might think, if you compare these electric motors to a two stroke glow engine. Most two stroke glow engines for models in the area of a 7-8 pound model will turn over at perhaps 11,000 or more. But you're talking about a prop on the order of 11 or 12 inches.

When I put together that Showtime 50 model with a 16X12 prop that only turned over 5950 RPM, I was worried that this model would be an embarassment at the field, and might not fly very well. (Wrong, wrong wrong!) Take off on grass is less than 30 feet.

It just seems that these larger diameter propellers perform better at a lower RPM than those glow engines with their smaller props that turn 50% faster, even with the same horsepower at the prop shaft.

And, even with motocalc or similar program, you absolutely must have a meter such as Astroflights whattmeter, or a Sears Roebuck Craftsman model # 82369 AD/DC clamp on ammeter. One big problem with these electric motors are, they are DUMB. They will happily turn over a propeller at twice the maximum wattage rating of your motor, untill everything burns up. And, you won't know unless you check your power system for how many amperes it pulls at full throttle.
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Old 01-04-2010, 09:37 PM
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Originally Posted by kyleservicetech View Post

If you use one of those hand held tachs in your home, turn off all the lights, and aim a flashlight at your prop. Then aim your tach at the prop for an accurate RPM indication. Otherwise, you could be measuring the 3600 RPM generated by the lights in your home.
You only need to turn of florescent -incandescent work fine with a tach. The incandescent element doesn't change rapidly enough with the A/C cycle to be read.

The Eagle Tree is worth it for the logging. They're not the only logger, but they may be the best for the money. With the logger, you can test in the air under various conditions and track it all over time. Not easy to see the extremes on the bench, especially under full load at speed.
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Old 01-04-2010, 10:38 PM
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Originally Posted by MaxAdventure View Post
You only need to turn of florescent -incandescent work fine with a tach. The incandescent element doesn't change rapidly enough with the A/C cycle to be read.

The Eagle Tree is worth it for the logging. They're not the only logger, but they may be the best for the money. With the logger, you can test in the air under various conditions and track it all over time. Not easy to see the extremes on the bench, especially under full load at speed.
Depends on the tach you have, one of my older ones picked up the ripple illumination from a 150 watt incandescant light bulb while trying to measure prop speed in my workshop.
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Old 01-05-2010, 12:01 AM
  #12  
ShoGinn
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Thanks for all the input, best thing about this hobby.. I've been doing it for over 20 years and still learn something everyday (doesnt help i took a 6 year break between nitro and electric)
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Old 01-05-2010, 12:31 AM
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Originally Posted by ShoGinn View Post
Thanks for all the input, best thing about this hobby.. I've been doing it for over 20 years and still learn something everyday (doesnt help i took a 6 year break between nitro and electric)

Yeah, I go back 50 years to the days of vacuum tubes and thyratrons, and transistor radios that cost a weeks wages. Now, its microprocessors, microcontrollers, digital circuits, high powered mosfets that make our ESC controls possible. Add to this those Lipo and A123 batteries. Those batteries, with the three phase ESC controls that power our three phase brushless motors, make our high powered electric models possible

And, with all of this stuff, include me with those guys that are still learning. Guess if you stop learning, you might be dead.
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Old 01-05-2010, 01:00 AM
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Originally Posted by MaxAdventure View Post
You only need to turn of florescent -incandescent work fine with a tach. The incandescent element doesn't change rapidly enough with the A/C cycle to be read.
I assure you my Hobbico reads 3600rpm on 2-blade and 2400rpm on 3-blade anywhere near an incandescent bulb... even reflection from a nearby wall is enough!

Here is my tach light - a MiniMagLite on a modified desk-lamp stand.
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Old 01-05-2010, 02:52 AM
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Originally Posted by Dr Kiwi View Post
I assure you my Hobbico reads 3600rpm on 2-blade and 2400rpm on 3-blade anywhere near an incandescent bulb... even reflection from a nearby wall is enough!

Here is my tach light - a MiniMagLite on a modified desk-lamp stand.
Same setup I use, but place my flashlight on any nearby box. FYI, if you want a really high powered flashlight, check out the two type "D" cell three watt LED type "Maglite" made in Ontario CA.

Not sure of other brands, but this is an interesting flashlight. I checked it on two cells, and again with just one cell, by jumping out one cell. The flashlight was just as bright on one cell as it was on two. The unit apparently uses a switching power supply inside, as when the battery voltage dropped by 1/2, the battery current nearly doubled.

Not cheap by any means, $34.00 at Sears Roebuck, but it is one of the brightest flashlites I've seen, and years ago I made one with a bunch of RC2400 nicads, and a snowmobile headlight.
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Old 01-05-2010, 06:35 AM
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Originally Posted by kyleservicetech View Post
Depends on the tach you have, one of my older ones picked up the ripple illumination from a 150 watt incandescant light bulb while trying to measure prop speed in my workshop.
Originally Posted by Dr Kiwi View Post
I assure you my Hobbico reads 3600rpm on 2-blade and 2400rpm on 3-blade anywhere near an incandescent bulb... even reflection from a nearby wall is enough!
Hmmm, my bad! I've never had that issue over several decades and several tachs. Maybe I've always had enough daylight through the windows.
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Old 01-05-2010, 06:35 PM
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Originally Posted by MaxAdventure View Post
Hmmm, my bad! I've never had that issue over several decades and several tachs. Maybe I've always had enough daylight through the windows.

Yeah, 99.9% of the time a light bulb will probably work just fine, but you've always got the guy in the back of the room that had problems using an optical tach with those bulbs.

I suspect that just where your incandescent bulb is located, compared to your prop is located might have an effect.
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Old 01-05-2010, 06:37 PM
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found a used tach on rcuniverse for $8 so i'll try that out and see if I can get it to work!
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Old 01-06-2010, 03:15 AM
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I just bought the GlowBee tach today for $28 cause I was in a hurry to measure my Telemaster 40 setup which I'm almost ready to fly (just need to cover it). I found the tach picked up the 3600 everywhere. I turned the lights off in the garage and used a flashlight behind my prop; works great.

I just measured RPM and current at full throttle on the recommended electric setup for this Telemaster 40 (3S LiPo, Rimfire 42X50 800KV, 13X8E prop). I calculated the motor voltage by dividing the tach RPM by motor KV (7300/800 = 9.1V). If this is fairly accurate, then measuring RPM on a brushless with known KV can give you the effective voltage it is seeing. I then calculated the power at 9.1 X 40A = 365 Watts. Anyone see anything wrong with this approach?

By the way, thanks for the encouragement about low speed large diameter props. Based on my calculations above, this combo will only draw 365 Watts (50 W/lb) but the motor is rated for 900 continuous. I am worried about even trying to take off with this low power, even though the setup is exactly as recommended by Hobby-Lobby. I don't want to increase voltage/RPM, just inject more power so I now see that I could go to an even larger prop; say 16 X 8 or 16 X 4. I want to get 50A (the motor max continuous limit) at full throttle. Using the 9.1 V, this is 450 W.
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Old 01-06-2010, 03:40 AM
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Originally Posted by alaursen View Post
I just bought the GlowBee tach today for $28 cause I was in a hurry to measure my Telemaster 40 setup which I'm almost ready to fly (just need to cover it). I found the tach picked up the 3600 everywhere. I turned the lights off in the garage and used a flashlight behind my prop; works great.

I just measured RPM and current at full throttle on the recommended electric setup for this Telemaster 40 (3S LiPo, Rimfire 42X50 800KV, 13X8E prop). I calculated the motor voltage by dividing the tach RPM by motor KV (7300/800 = 9.1V). If this is fairly accurate, then measuring RPM on a brushless with known KV can give you the effective voltage it is seeing. I then calculated the power at 9.1 X 40A = 365 Watts. Anyone see anything wrong with this approach?

By the way, thanks for the encouragement about low speed large diameter props. Based on my calculations above, this combo will only draw 365 Watts (50 W/lb) but the motor is rated for 900 continuous. I am worried about even trying to take off with this low power, even though the setup is exactly as recommended by Hobby-Lobby. I don't want to increase voltage/RPM, just inject more power so I now see that I could go to an even larger prop; say 16 X 8 or 16 X 4. I want to get 50A (the motor max continuous limit) at full throttle. Using the 9.1 V, this is 450 W.
You've got to watch the "prop blast" MPH from that prop. In theory, you could put a propeller with 24 inch diameter, and one inch pitch. It would pull roughly the same amount of watts from that motor, but it would have to be in a model that flys slower than five miles per hour :o That 16X4 prop would have a prop blast on the order of 26 MPH, so the models flying speed would have to be less than 26 MPH.

That Rimfire 42-50-800 motor weighs in at 7 ounces, so running it at 450 watts is about 450/7 or about 65 watts per ounce of motor weight. IMHO, that's fairly conservative.
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Old 01-06-2010, 03:41 AM
  #21  
ShoGinn
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9.1v seems real low for your motor to see.

With my 3s setups its roughly 11v on a fully charged battery according to the wattmeter.
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Old 01-06-2010, 03:43 AM
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Originally Posted by ShoGinn View Post
9.1v seems real low for your motor to see.

With my 3s setups its roughly 11v on a fully charged battery according to the wattmeter.
Good point!

My A123 cells run about 3 volts per cell at about 30 amps load, and those Lipos usually run about 20% higher than that.
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Old 01-06-2010, 04:03 AM
  #23  
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Thanks Kyleservicetech,

My battery voltage to the ESC IS 3S = 11.1 volts. However, the motor obviously does not see this due to drops in the ESC and wiring, which can be significant at 40 A. I have no means of directly measuring the 3 phase motor voltage yet the power must equal that voltage times the current which is easy to measure with my clamp on Ammeter. I figured I should be able to calculate the motor voltage from the RPM which I can now measure accurately with my tach.

Also, thanks for the advice on the airspeed. I heard that the Telemaster 40 can fly nicely at very low speeds so it may handle the larger prop just fine.
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Old 01-06-2010, 04:06 AM
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ShoGinn
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thats interesting alaursen I never thought about the actual voltage to the motor as a variable.
I just assumed that ESC voltage was the end all number
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Old 01-06-2010, 04:08 AM
  #25  
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Originally Posted by ShoGinn View Post
9.1v seems real low for your motor to see.

With my 3s setups its roughly 11v on a fully charged battery according to the wattmeter.
It may seem low, but if the manufacturer says the KV=800 and my measured RPM = 7300, doesn't that mean the motor must be "seeing" 9.1 volts? Maybe I don't understand KV correctly, however, one thing I know for sure is that battery voltage into the ESC is certainly not what the motor will get from the ESC output even at full throttle; right?

Oh I meant to ask; where is your wattmeter connected?
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