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Measuring Brushless Motor Resistance (Two Digital Meter Method)

Old 10-12-2009, 06:55 PM
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kyleservicetech
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Default Measuring Brushless Motor Resistance (Two Digital Meter Method)

What is involved in measuring the winding resistance of our brushless motors?? You don't need an elaborate setup. All you need is a two or three cell Lipo, or 12 volt battery, a power resistor on the order of five ohms, at 25 watts, and a pair of cheap digital volt meters. (One meter could be a Astroflight Wattmeter or similar) Heck, even a 12 Volt automotive tail light, or several tail lights in parallel will work, anything that draws an ampere or three. Applying higher currents will allow somewhat more accurate resistance measurements, at least up to the maximum current rating of the motor, or the 10 ampere limit of the DC Digital volt/ohm meter. More expensive meters such as the Fluke 87 meter allows measuring the voltage down to the 0.1 millivolt level.

Connect the battery such that current flows through the resistor, and the motor winding under test. (The motor and resistor are in series with each other.) Put one meter in SERIES with the resistor and motor, on the meters 10 Ampere DC range. (No you can't put the meter in series to measure the current, than take it out to measure the voltage. The meter itself on its 10 Ampere range has some resistance, and removing it will change the current through the motor under test.)

Measure the voltage ACROSS the motor windings with the first digital volt meter. Measure the current THROUGH the motor with the second digital meter. (Or Astroflight Wattmeter)

Now, the motors DC resistance will equal the voltage measured across the motor, divided by the current through the motor or R=E/I. The attached JPG photo shows it all.

If you measure 0.072 volts across the motor, and 2.5 Amps through the motor, the motor resistance is R=E/I or 0.071/2.5 which is 0.0288 ohms. Be certain to measure the motor voltage directly across the motor leads, and not across your clip wires to the motor. Measuring on the clip wires will add the clip's resistance to your total motor resistance.

(DO NOT CONNECT THE BATTERY DIRECTLY ACROSS THE MOTOR, THAT WILL BURN IT UP! )

Once you have this set up, it takes more time to read this instruction than to actually do the resistance measurement.

In the USA, you can buy a meter such as the Cen-Tech model 92020 for about $3.98 in American dollars. A much better meter is the Etek 10709 meter, I found one at Walmart for some $18.00. If you need more accuracy, more expensive meters can be used, such as the Fluke 87V, but mine cost some $350 USA dollars. It is accurate to 0.05% though. And a pair of Fluke 87V meters will give you far more accuracy than you need.

Look for a digital multimeter with a 2.00 AND a 200 Millivolt range. Even the lowly Harbor Freight $3.98 meter will work just fine. So if you measure 70 MILLIVOLTS and 2.5 Amperes, the resistance of your winding is 70mv/2.5 Amps, or 28 MilliOhms, (0.028 Ohms).

Just a note, even my $350 Fluke meter can not measure the winding resistance of a typical brushless motor. The resistance values are so low, that measuring them requires a "MicroOhmmeter, a device that can easily cost thousands of dollars. We had many of them at work, before I retired.
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Old 10-12-2009, 08:01 PM
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One, hopefully minor, aspect of these motor measurements I'd like to point out is that, when making measurements like this, best practice is to do what's called a four-wire measurement. A four-wire resistance measurement involves four separate connections to the element being tested. Two separate connections are made to each side of the element (motor winding lead in our case) being tested. The reason for this is that, otherwise the resistance of the connections on each side of the element are in series with the element you're testing. If however, you apply the high measurement excitation current ( a few amps in our typical case ) to the ends of the wires, and make your voltmeter connections on the wires slightly closer to the motor, then you will be measuring ONLY the motor resistance and any resistance in the contact is not reflected in the result. You'd be surprised how much resistance ( a few milli-ohms is common ) there is in a seemingly firm mechanical connection between two bare metal objects like a wire and an alligator clip. All decent quality lab DMM's with the capability to measure below 10 ohms provide four connections, two for providing the current, and two more for sensing the voltage drop.

When dealing with small resistances, the tendency to consider all points interconnected by wires and contacts as being equivalent is NOT valid. Every part of the conduction path has some finite resistance! Remember that you're only trying to measure the resistance of a piece of copper wire typically a few feet long!

kyleservicetech's drawing correctly reflects this important point. If the voltmeter had been attached above the top motor contact and/or below the bottom contact, the considerably less accurate results would be obtained.
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Old 10-12-2009, 08:07 PM
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Originally Posted by MustangMan View Post
One, hopefully minor, aspect of these motor measurements I'd like to point out is that, when making measurements like this, best practice is to do what's called a four-wire measurement. A four-wire resistance measurement involves four separate connections to the element being tested. Two separate connections are made to each side of the element (motor winding lead in our case) being tested. The reason for this is that, otherwise the resistance of the connections on each side of the element are in series with the element you're testing. If however, you apply the high measurement excitation current ( a few amps in our typical case ) to the ends of the wires, and make your voltmeter connections on the wires slightly closer to the motor, then you will be measuring ONLY the motor resistance and any resistance in the contact is not reflected in the result. You'd be surprised how much resistance ( a few milli-ohms is common ) there is in a seemingly firm mechanical connection between two bare metal objects like a wire and an alligator clip. All decent quality lab DMM's with the capability to measure below 10 ohms provide four connections, two for providing the current, and two more for sensing the voltage drop.

When dealing with small resistances, the tendency to consider all points interconnected by wires and contacts as being equivalent is NOT valid. Every part of the conduction path has some finite resistance! Remember that you're only trying to measure the resistance of a piece of copper wire typically a few feet long!
Agreed, that is how those MicroOhmeters are used. And, that is why you can't connect the "current source" to the motor with clip wires, and then measure the voltage drop across the clip wires, rather than the motor wires directly.

If you connect the "Current source" consisting of the battery and resistor to the "ends" of the motor wires, then measure the motor voltage just inboard from the "ends" of the motor wires, that would be in effect a four wire resistance test setup.

Yeah, I'm old enough to have used the Wheatstone Bridges that were common decades ago.
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Old 10-12-2009, 08:17 PM
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Originally Posted by kyleservicetech View Post
Yeah, I'm old enough to have used the Wheatstone Bridges that were common decades ago.
DANG! You must be nearly as old as ME!
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Old 10-12-2009, 08:36 PM
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Originally Posted by MustangMan View Post
DANG! You must be nearly as old as ME!
Yep, will be 68 this month

Forgot to mention, at work in the Service department we measured the contact resistance of our high voltage circuit breakers rated at 800 Amps, 38,000 volts, 16000 Amperes interrupting.

Those ceramic vacuum bottle circuit breaker contacts had 3/4 inch diameter solid copper bus bars for connections, typical contact resistance was on the order of about 1/2 Milliohm or less, depending on which breaker we were measuring. Those 800 Amp contacts were on the order of about 50-100 micro-ohms. We also tested them at hypot at about 80,000 AC volts. We had to test behind a lead shield, since at that voltage, they emit X-rays.

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Old 10-13-2009, 01:09 AM
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Originally Posted by kyleservicetech View Post
Yep, will be 68 this month

Forgot to mention, at work in the Service department we measured the contact resistance of our high voltage circuit breakers rated at 800 Amps, 38,000 volts, 16000 Amperes interrupting.

Those ceramic vacuum bottle circuit breaker contacts had 3/4 inch diameter solid copper bus bars for connections, typical contact resistance was on the order of about 1/2 Milliohm or less, depending on which breaker we were measuring. Those 800 Amp contacts were on the order of about 50-100 micro-ohms. We also tested them at hypot at about 80,000 AC volts. We had to test behind a lead shield, since at that voltage, they emit X-rays.
Sounds like fun stuff.

I'm surprised the contact resistance was that high. At 800 Amps that would mean a voltage drop of 40 to 80 mV. resulting in 32 to 64 watts of heat being generating in the contact!

I hadn't considered the X-ray emission problem. Certainly makes sense though.
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Old 10-13-2009, 01:16 AM
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Originally Posted by MustangMan View Post
Sounds like fun stuff.

I'm surprised the contact resistance was that high. At 800 Amps that would mean a voltage drop of 40 to 80 mV. resulting in 32 to 64 watts of heat being generating in the contact! Those interrupters are not small, they weigh in at some 10 pounds each. The whole three phase circuitbreaker is some 400 pounds.

I hadn't considered the X-ray emission problem. Certainly makes sense though.
That 60 some watts of heat is what limits the maximum continuous current those interrupters can handle. And thats for temperatures between minus 40 to plus 140 degrees F.

But, 800 Amperes times 38,000 volts equals 30 million watts or 30,000 Kilowatts, so 60 watts really is not much.

As to what can happen when the circuit breaker does not have enough fault current going through it to trip it out, take a look at this truck fire!

http://www.youtube.com/watch?v=wu8VZwNBgag

I taught circuit breaker control troubleshooting, testing and repair for many years for the company I worked for, and have heard many many stories about what happens when things go wrong. We once had a 1400 pound circuit breaker fail internally, and its backup breaker failed to clear the fault. The whole 1400 pound breaker was melted down to a puddle of metal.

FYI, try running a search in youtube for electrical powerline fires or substation fires. There are a LOT of them.

http://www.youtube.com/watch?v=OY72athcwvA&feature=related

http://www.youtube.com/watch?v=NQQMK...eature=related

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Old 10-13-2009, 01:38 AM
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Well, we've managed to drag this thread WAY OFF TOPIC! Interesting stuff though.
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Old 10-13-2009, 01:51 AM
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Originally Posted by MustangMan View Post
Well, we've managed to drag this thread WAY OFF TOPIC! Interesting stuff though.
You're right :o :o :o

Sorry about that.
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Old 10-13-2009, 06:39 AM
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Nice work guys!

A variation on this is usefull to check the resistance of your power connectors. Use the volt meter to measure the voltage drop across any connector in the loop while a watt meter is connected to see how well they are doing. You might be surprised at the variations you see. May be time to change a few connectors for new ones
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Old 10-13-2009, 04:53 PM
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Originally Posted by Larry3215 View Post
Nice work guys!

A variation on this is usefull to check the resistance of your power connectors. Use the volt meter to measure the voltage drop across any connector in the loop while a watt meter is connected to see how well they are doing. You might be surprised at the variations you see. May be time to change a few connectors for new ones
Yep, been there, done that with my Fluke 87V meter. For this you will need a digital multimeter with a millivolt range such as the Eteck meter mentioned in this thread. Or the Fluke meter with its added 10 times more sensitivity with its 659.9 millivolt range.

One reason all my models have the Anderson Powerpole connectors with the heaviest contacts available. Those Anderson units are made by AMP. www.digikey.com sells them but at the same price as the larger Hobby internet markets.
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Old 02-03-2010, 10:53 PM
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I have obtained several motors from Hobbycity. The motors seem to be well made but I think some of the specifications given are suspect. I measured the resistance between two of the three leads of one and found 0.023 ohms. The spec says it is 0.087 ohms. Quite a difference. This is critical for use with motocalc.
When a manufacturer gives a motor internal resistance is that the measured resistance or is it the resistance of one of the coils?
Is there a test to determine if the motor is delta or Y connected without disassembling the motor?
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Old 02-03-2010, 11:37 PM
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Originally Posted by dirtybird View Post
I measured the resistance between two of the three leads of one and found 0.023 ohms. The spec says it is 0.087 ohms. Quite a difference.
When I measure my own motor constants, it is usual for me to see numbers slightly different from those that are published... however, the difference you saw in resistance is quite large...

What was your method of determining the resistance of the coil?...

-Jim...

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Old 02-03-2010, 11:43 PM
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Hobby City pulls its specks (motor, esc, what ever) out of thin air far more often than not.

In many cases they simply copy the specs from a name brand motor they are trying to compete with.

They dont do any testing at all and I doubt their suppliers do either.

I have seen and read about many cases where you might order two "identical" motors and get two very different performance levels. Kv, Io, Rm very different. They are not even consistent within a single motor line.

You cant expect quality or honesty from them. The only reason to buy from Hobby City is the price. I figure your lucky if it works at all
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Old 02-04-2010, 01:08 AM
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Originally Posted by dirtybird View Post
I have obtained several motors from Hobbycity. The motors seem to be well made but I think some of the specifications given are suspect. I measured the resistance between two of the three leads of one and found 0.023 ohms. The spec says it is 0.087 ohms. Quite a difference. This is critical for use with motocalc.
When a manufacturer gives a motor internal resistance is that the measured resistance or is it the resistance of one of the coils?
Is there a test to determine if the motor is delta or Y connected without disassembling the motor?

I've used that two multimeter resistance test on my various Hacker A30, A40 and A50 motors, and have found that my measurements are quite close to those quoted by Hacker.

The only way to determine if the motor is "Y" or Delta, is to disassemble the motor and look for the "Center of the "Y" connections. It really doesn't make any difference, unless you change the motor from one to the other by taking the motor apart and re-connecting it. Do that, and you will find a very substantial difference in performance when changing from "Y" to Delta, or from Delta to "Y". (I've done this with those "Go Brushless" motors.)

Last edited by kyleservicetech; 02-04-2010 at 02:23 AM.
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Old 02-04-2010, 02:12 AM
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Originally Posted by dirtybird View Post
I have obtained several motors from Hobbycity. The motors seem to be well made but I think some of the specifications given are suspect. I measured the resistance between two of the three leads of one and found 0.023 ohms. The spec says it is 0.087 ohms. Quite a difference. This is critical for use with motocalc.
When a manufacturer gives a motor internal resistance is that the measured resistance or is it the resistance of one of the coils?
Is there a test to determine if the motor is delta or Y connected without disassembling the motor?
Very often the specification numbers, especially electrical parameters, on HobbyKing motor offerings are quite far off. Sometimes they look like they copied most of the specs from another motor and just forgot to change the values, or they got them mixed up. I'd be surprised if the folks typing the details on the web site have much understanding of this stuff. They are definitely NOT to be trusted. I have not had the experience of buying two "identical" motors and having them be very different, though I would not be surprised if it happened occasionally. I can easily envision the factory folks getting them mixed up before they're labeled, or the folks doing the winding making mistakes. I don't expect "bleeding edge" performance from their inexpensive motor products but I have found them to be pretty good for most of my low-budget applications.
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Old 02-04-2010, 04:49 AM
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Originally Posted by jjw View Post
When I measure my own motor constants, it is usual for me to see numbers slightly different from those that are published... however, the difference you saw in resistance is quite large...

What was your method of determining the resistance of the coil?...

-Jim...
I used the two DVM method outlined above. I am using the Sears clamp on ammeter for both the amp reading and the voltage reading. This meter reads DC amps and has an autoranging VM. Its quite accurate.
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Old 02-04-2010, 04:56 AM
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Originally Posted by dirtybird View Post
I used the two DVM method outlined above. I am using the Sears clamp on ammeter for both the amp reading and the voltage reading. This meter reads DC amps and has an autoranging VM. Its quite accurate.
FYI, that Sears clamp on ammeter is the Craftsman #82369, which has a 0-40 Amp, and a 0-400 Amp current range, for Either AC or DC. Most clamp on Ammeters are AC (Alternating Current) only, and will not work on our DC battery circuits. :o I checked my Craftsman meter against my $360 FLuke 87V meter, that Craftsman meter is within about 2%, even on its DC clamp on range for current.

Dirtybird has a really good idea with the Sears #82369 ammeter.
This clamp on ammeter has a real advantage, in that you can use the meter to quickly measure the current flowing through the motor winding by "clamping" the meter around one of the wires leading to the motor. Then, switch the meter to DC Volts, and measure the voltage drop. The motor winding resistance is then Resistance equals Voltage divided by Current.

If you only have one multimter, and use its Ammeter range in series with the motor, and remove that ammeter for the same meter for the voltage check, the test current will increase due to lower circuit resistance with the ammeter removed from the circuit. Removing that ammeter will affect accuracy of the resistance calculation.

I agree, that #82369 meter is one of the most useful meters I've got, and I've a whole collection of them.

Last edited by kyleservicetech; 02-04-2010 at 05:12 AM.
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Old 02-04-2010, 05:33 AM
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I have Hacker, Scorpion, Turnegy, and Eflite motors.
The Hacker equivalent motor to the Turnegy has an internal resistance, according to its spec, of 0.016. That means it will be more efficient than the Turnegy. But I find the Turnegy works just fine for me and it costs just $20. The Hacker costs $69.
BTW I have been told the Hacker is now being made in China.
I would like to be able to plug the Turnegy specs into Motocalc and get some meaningful results. I guess I just will have to run tests on each one I have.
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Old 02-04-2010, 04:32 PM
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Originally Posted by dirtybird View Post
I used the two DVM method outlined above. I am using the Sears clamp on ammeter for both the amp reading and the voltage reading. This meter reads DC amps and has an autoranging VM. Its quite accurate.
Well, my guess is your numbers are correct... Looks like you're going to have to measure all your own constants for the HobbyCity motors...

Lack of reliable motor constant data (from the manufacturers) has always been one of my pet peeves... but its not a big deal really, I measure and static test virtually every motor I buy anyway...

My bench set-ups...




-Jim.
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Old 02-04-2010, 05:08 PM
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Thats a nice setup Jim!

Its a good idea to static test ANY new setup even if its all name brand/hi quality stuff. Its even a good idea to re-check if you only change one component.

Motor kV's for example can vary as much as 5%-10% even with name brand motors. Far more with the cheep stuff.

ESC settings and resistance from one brand to the next can change power readings by another 10%-20% or more in some cases.

Ive seen as much as 5%-10% variations between "identical" APC props and much larger variations on others.

Then of course there are the batteries. New gen, hi quality packs can add as much as another 30%+ to power output compared to older, cheaper, hi Ir packs.

Depending how all those variables add up, you could have very very different results from two similar setups.

Thats also why the prediction programs like MotoCalc are good as a general guestimate at best.
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Old 02-04-2010, 05:09 PM
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Originally Posted by dirtybird View Post
I have Hacker, Scorpion, Turnegy, and Eflite motors.

BTW I have been told the Hacker is now being made in China.
Good grief, and one wonders why we have no jobs in the USA, and many other countries.

I just purchased a new Hacker A50-12S motor. The package says "Designed in Germany". Then it says, "Made in China". At any rate, its performance is identical to my other Hacker A50-12S motor.

Last edited by kyleservicetech; 05-09-2010 at 03:15 AM.
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Old 02-04-2010, 05:12 PM
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Originally Posted by Larry3215 View Post
Thats a nice setup Jim!

Its a good idea to static test ANY new setup even if its all name brand/hi quality stuff. Its even a good idea to re-check if you only change one component.

Motor kV's for example can vary as much as 5%-10% even with name brand motors. Far more with the cheep stuff.

ESC settings and resistance from one brand to the next can change power readings by another 10%-20% or more in some cases.

Ive seen as much as 5%-10% variations between "identical" APC props and much larger variations on others.

Then of course there are the batteries. New gen, hi quality packs can add as much as another 30%+ to power output compared to older, cheaper, hi Ir packs.

Depending how all those variables add up, you could have very very different results from two similar setups.

Thats also why the prediction programs like MotoCalc are good as a general guestimate at best.
Danged good point.

Last year, I changed a Hacker A50-10L motor battery source from 5S2P A123 cells to 6S2P A123 cells. With the same prop, the 5S2P was under powered, with 6S2P, I was exceeding the peak current rating of that motor.

What might appear to be a minor change on these electric powered models can result in drastic changes in horsepower output on those motors.
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Old 02-04-2010, 05:35 PM
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Originally Posted by kyleservicetech View Post
Danged good point.

Last year, I changed a Hacker A50-10L motor battery source from 5S2P A123 cells to 6S2P A123 cells. With the same prop, the 5S2P was under powered, with 6S2P, I was exceeding the peak current rating of that motor.

What might appear to be a minor change on these electric powered models can result in drastic changes in horsepower output on those motors.
Our electric power systems are VERY voltage sensitive due to the nature of the operation of propellers. Since the power necessary to turn a propeller rises as the QUBE of the RPM, and a motor tries to increase its RPM linearly with voltage, raising the voltage by a factor of 1.2 (from 5 cells to 6 cells) results in a (1.2)**3 or 1.73 factor increase in power! Since the voltage is only increased by 20%, the current has to rise 44% to account for the power increase.

In the absense of a MotoCalc simualation of the effect of increase voltage I'd probably go to a significantly smaller propeller if I were increasing the number of cells in my battery pack, make some measurements and move up carefully. As you noted, it's very ease to exceed your motor's limits with modest changes.
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Old 02-05-2010, 04:46 AM
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Originally Posted by kyleservicetech View Post
Good grief, and one wonders why we have no jobs in the USA, and many other countries.
Did you notice the Sears Craftsman #82369 meter is also made in China?
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