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kyleservicetech
10-12-2009, 07:55 PM
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! :censor:)

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

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.:D 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. The Cen-Tech only has a 2.00 volt range, and is much less accurate for our purposes. My Etek 10709 meter does have a 200 millivolt range. 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.

MustangMan
10-12-2009, 09:01 PM
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.

kyleservicetech
10-12-2009, 09:07 PM
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. :cool:

Yeah, I'm old enough to have used the Wheatstone Bridges that were common decades ago.:Q

MustangMan
10-12-2009, 09:17 PM
Yeah, I'm old enough to have used the Wheatstone Bridges that were common decades ago.:Q
DANG! You must be nearly as old as ME!:Q

kyleservicetech
10-12-2009, 09:36 PM
DANG! You must be nearly as old as ME!:Q

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.

MustangMan
10-13-2009, 02:09 AM
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.

kyleservicetech
10-13-2009, 02:16 AM
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. :D

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=OY72athcwvA&feature=related)

http://www.youtube.com/watch?v=NQQMK1Rvq0A&feature=related

MustangMan
10-13-2009, 02:38 AM
Well, we've managed to drag this thread WAY OFF TOPIC! Interesting stuff though.

kyleservicetech
10-13-2009, 02:51 AM
Well, we've managed to drag this thread WAY OFF TOPIC! Interesting stuff though.

You're right :o :o :o

Sorry about that.

Larry3215
10-13-2009, 07:39 AM
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 :)

kyleservicetech
10-13-2009, 05:53 PM
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. :D

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 (http://www.digikey.com) sells them but at the same price as the larger Hobby internet markets.

dirtybird
02-03-2010, 11:53 PM
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?

jjw
02-04-2010, 12:37 AM
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...

Larry3215
02-04-2010, 12:43 AM
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 :)

kyleservicetech
02-04-2010, 02:08 AM
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.)

MustangMan
02-04-2010, 03:12 AM
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.

dirtybird
02-04-2010, 05:49 AM
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.

kyleservicetech
02-04-2010, 05:56 AM
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. :cool:

dirtybird
02-04-2010, 06:33 AM
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.

jjw
02-04-2010, 05:32 PM
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...

http://img267.imageshack.us/img267/3397/pdr0644.jpg
http://img26.imageshack.us/img26/8692/pdr0657.jpg

-Jim.

Larry3215
02-04-2010, 06:08 PM
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.

kyleservicetech
02-04-2010, 06:09 PM
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.

kyleservicetech
02-04-2010, 06:12 PM
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.

MustangMan
02-04-2010, 06:35 PM
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!:eek: 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.

dirtybird
02-05-2010, 05:46 AM
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?

kyleservicetech
02-05-2010, 05:58 AM
Did you notice the Sears Craftsman #82369 meter is also made in China?

Would not be surprised, but I couldn't find a "Made in China" label or decal anywhere on my meter.

It's getting more and more difficult to find anything not made in China now days. I just found on the Internet that the trade deficit between the USA and China was some 200 billion dollars for last year. Lets see, a dollar is about 0.004 inches by 2.5 by 6 inches. So, 200 billion of them is about seven million cubic feet, or something like 2500 rail road cars full of dollar bills.

Kind of puts a different light on things.

jjw
02-05-2010, 02:22 PM
Did you notice the Sears Craftsman #82369 meter is also made in China?

Did anyone think it would have been made in the good old USA?... not likely. Virtually all consumer class electronic manufacturing (and even most "professional" level) has gone offshore. Its a simple matter of economics. US manufacturing priced itself out of what the majority of consumers are willing to pay quite a few years ago... you can pick which side you want to wag your finger at.

Been in the PC manufacturing biz for many years... where everything is made in Asia... spent a lot of time at the Asian manufacturing sites and am intimately familiar with labor rates, working conditions, tax advantages... the US and other 1st world economic countries simply could not compete. As an example... the PC you're using right now, that you probably spent a few hundred dollars for, would cost 5 to 10 times that amount if it and all its components were to be made domestically. If you had to pay 5 grand for the average PC, would you buy it?

Not trying to make a political point here... just stating fact. Just look here on this forum and watch how many people are now trying to get things as cheap as they can by buying stuff directly from China at places such as HobbyKing, OKHobby, RCTimer, etc., etc... no longer even willing to buy Asian made stuff sold at a domestic retailer...

As I've always said... as much as people complain about quality and customer service, they'll vote with their pocket book everytime...

-Jim.

wingnutt
02-05-2010, 05:09 PM
Jjw – I am curious as to the value of measuring the DC resistance of a motors coils. Rm I can understand, since that can be used to help predict the performance capabilities of a motor in use. Other than checking for shorts or to make sure you didn’t miss a turn in rewinding, I cannot see where the value is useful, but would like to know if I am missing something.
A motor coils DC resistance is not the same as Rm and shouldn’t be used for performance prediction. I recently built a milliohm tester, using Lucien’s LM317 current source plans. On my most recent rewind, the DC resistance (Rdc) is 0.047 Ohm and the calculated Rm (at operation voltage) is 0.113 Ohm. Other motors I tested had the same discrepancy between Rdc and Rm.

Please understand that I really am curious and not trying to be contradictory at all. I just don’t want to not miss out on something.

dirtybird
02-05-2010, 09:42 PM
Jjw Ė I am curious as to the value of measuring the DC resistance of a motors coils. Rm I can understand, since that can be used to help predict the performance capabilities of a motor in use. Other than checking for shorts or to make sure you didnít miss a turn in rewinding, I cannot see where the value is useful, but would like to know if I am missing something.
A motor coils DC resistance is not the same as Rm and shouldnít be used for performance prediction. I recently built a milliohm tester, using Lucienís LM317 current source plans. On my most recent rewind, the DC resistance (Rdc) is 0.047 Ohm and the calculated Rm (at operation voltage) is 0.113 Ohm. Other motors I tested had the same discrepancy between Rdc and Rm.

Please understand that I really am curious and not trying to be contradictory at all. I just donít want to not miss out on something.

Whats Rm?
What does Motocalc use for their calculations?

wingnutt
02-05-2010, 10:56 PM
Rm is the motors 'running' resistance or impedance. It is the combination of the coils resistance, reactance, inductance, etc. while being pulsed by the esc. Rm is what Motocalc and other performance prediction software use.

dirtybird
02-06-2010, 12:05 AM
How do you calculate Rm?
If you can't or don't want to answer this, can you refer me to a book or an article that will tell me ?
When I went thru EE college(60 years ago) they did not have such a device.
They had a 3 phase synchronous AC motor but this appears to be a different motor.
In the description of the motor characteristics Motocalc lists resistance, KV, and no load current. When I changed the resistance from what the HC spec stated to what I measured, the predicted performance characteristics changed significantly.
I am not trying to argue, I just want to get to the bottom of this so I can get accurate performance predictions of these cheap motors

MustangMan
02-06-2010, 01:19 AM
How do you calculate Rm?
If you can't or don't want to answer this, can you refer me to a book or an article that will tell me ?
When I went thru EE college(60 years ago) they did not have such a device.
They had a 3 phase synchronous AC motor but this appears to be a different motor.
In the description of the motor characteristics Motocalc lists resistance, KV, and no load current. When I changed the resistance from what the HC spec stated to what I measured, the predicted performance characteristics changed significantly.
I am not trying to argue, I just want to get to the bottom of this so I can get accurate performance predictions of these cheap motorsIn the simplest sense Rm is the DC resistance of the windings between any two phase leads. Because it is substantially less than 1 ohm in any of our model motors it's a little difficult to measure with a standard multi-meter. The trick to getting a good measurements is to push a significant DC current, like an amp or two, through the winding and measure the voltage drop with a meter capable of millivolt resolution. The current must be known accurately as well. Knowing both the current and the voltage drop you can compute the Rm with Ohm's law ( R = V / I ). Note that you must measure the voltage drop at the point where the motor wires exit the motor, or very close to it to get an accurate measurement. Otherwise you'll be including the resistance of your test harness wiring in the measurement, which you don't want to do. Commercial instruments designed for this purpose use "4-wire" measurements rather than 2 wires like a standard DMM uses to measure resistance in order to eliminate the resistance of the measurement leads from the problem.

jjw
02-06-2010, 02:14 AM
Jjw – I am curious as to the value of measuring the DC resistance of a motors coils. Rm I can understand, since that can be used to help predict the performance capabilities of a motor in use. Other than checking for shorts or to make sure you didn’t miss a turn in rewinding, I cannot see where the value is useful, but would like to know if I am missing something.
A motor coils DC resistance is not the same as Rm and shouldn’t be used for performance prediction. I recently built a milliohm tester, using Lucien’s LM317 current source plans. On my most recent rewind, the DC resistance (Rdc) is 0.047 Ohm and the calculated Rm (at operation voltage) is 0.113 Ohm. Other motors I tested had the same discrepancy between Rdc and Rm.

Please understand that I really am curious and not trying to be contradictory at all. I just don’t want to not miss out on something.

Hi... For the purposes of estimating DC motor performance, Rm (or terminal resistance) is the motor coil DC resistance... I typically measure this using a constant current source and feeding this current through the coil, measuring the voltage across the coil, then using ohms law to calculate resistance.

When you say you calculate Rm, I can only guess that you are running the motor at some known volltage, current and load conditions and then back calculating the coil resitance... all completely valid.

The fact that you find different numbers than a measured DC coil resistance is probably due to other losses seen in real world motors... magnetic (or iron) losses (eddy current and hysteresis) and mechanical losses (friction and windage)...

-Jim.

Larry3215
02-06-2010, 03:11 AM
There has been quite a bit of debate on the value or validity of using the measured dc resistance vrs the calculated resistance.

There are a number of very smart people who's opinions I value who disagree on this point :)

Its my understanding the using the calculated 'Rm' ends up including the esc resistance and wiring, connectors etc. That might be fine for over all predictions for a specific "system" but doesnt necessarily tell you about the motor itself.

Changing the esc or its settings or the rpm and/or the power range your operating in all will effect the calculated values.

So if your calculated value was done using brand x esc at timing Y, but you end p runing the motor on brand z esc at tming Q, then you predictions may be meaningless.

At the same time, taking only the motors dc resistance ignores the esc and connectors, timing settings etc etc. But that may give you a more honest look at the motor itself if thats what your trying to evaluate or compare.

Both have value I suppose depending on the situation and what your looking for.

kyleservicetech
02-06-2010, 03:50 AM
How do you calculate Rm?
If you can't or don't want to answer this, can you refer me to a book or an article that will tell me ?
When I went thru EE college(60 years ago) they did not have such a device.
They had a 3 phase synchronous AC motor but this appears to be a different motor.
In the description of the motor characteristics Motocalc lists resistance, KV, and no load current. When I changed the resistance from what the HC spec stated to what I measured, the predicted performance characteristics changed significantly.
I am not trying to argue, I just want to get to the bottom of this so I can get accurate performance predictions of these cheap motors

Something I've done over the past few years, is set up a particular motor/prop/esc/battery pack. Then run the motor and prop, checking its current/voltage/watts with a meter such as an Astroflight whattmeter.

Then, assuming the battery is putting out the voltage under load as advertised, I check the actual motor RPM against the motocalc predicted RPM. If they vary substantially, I just save the motor under a different name, then "tweak" the motocalc motor characteristic "KV" number so the motocalc RPM matches the actual motor RPM.

If the battery has a different voltage under load, you can also "tweak" the battery constants to match the real world.

That way, as you change props in motocalc, the predicted RPM's should be a little closer.

Not very scientific, but it gets you a little closer. :)

Just a note, I've found that sometimes motocalc has the WRONG motor constants, as compared to the motors web pages. This was noted on the Hacker motors several times, as well as Eflites.

MustangMan
02-06-2010, 04:06 AM
How do you calculate Rm?
If you can't or don't want to answer this, can you refer me to a book or an article that will tell me ?
When I went thru EE college(60 years ago) they did not have such a device.
They had a 3 phase synchronous AC motor but this appears to be a different motor.
In the description of the motor characteristics Motocalc lists resistance, KV, and no load current. When I changed the resistance from what the HC spec stated to what I measured, the predicted performance characteristics changed significantly.
I am not trying to argue, I just want to get to the bottom of this so I can get accurate performance predictions of these cheap motorsCommercial/Industrial 3-phase motors are indeed a bit different in detail from our little motors but operate using the same general principles. Smaller 3-phase motors use squirrel cage rotors just like the smaller 1-phase motors use. In these motors the "field" magnetization of the rotor is "induced" by the slip between the theoretical rotation of the stator fields and the mechanical rotation of the rotor at a slightly slower speed. That's why induction motors never achieve true "synchronous" speeds. Larger motors, which operate synchronously, have field windings on the rotor which creates the field magnetization. These must be fed DC current through clip rings whereas our motors have permanent field magnets.

dirtybird
02-06-2010, 04:49 AM
In the simplest sense Rm is the DC resistance of the windings between any two phase leads. Because it is substantially less than 1 ohm in any of our model motors it's a little difficult to measure with a standard multi-meter. The trick to getting a good measurements is to push a significant DC current, like an amp or two, through the winding and measure the voltage drop with a meter capable of millivolt resolution. The current must be known accurately as well. Knowing both the current and the voltage drop you can compute the Rm with Ohm's law ( R = V / I ). Note that you must measure the voltage drop at the point where the motor wires exit the motor, or very close to it to get an accurate measurement. Otherwise you'll be including the resistance of your test harness wiring in the measurement, which you don't want to do. Commercial instruments designed for this purpose use "4-wire" measurements rather than 2 wires like a standard DMM uses to measure resistance in order to eliminate the resistance of the measurement leads from the problem.

I have an adjustable lab power supply(Mastec 5020) that I can connect to two motor leads in series with a 10 ohm resistor and adjust to 1 amp within +/- 0.1V. I need the resistor or the PS thinks its output is shorted. I then verify the current with the clamp on meter. I then read the voltage at the two leads with a millivolt meter. The MV reading then converts directly to milliohms.
The Sears meter has a 200MV range. The Harbor Freight $3 meter also has a 200MV range. Both meters agree.
The resulting reading is not the coil resistance. The coil resistance is 3/2xRmeasured if the motor is delta connected. It is 1/2x Rmeasured if the motor is Y connected.
The measured resistance I get agrees with the MFG spec for the Scorpion and Eflight motors but not for the Turnigy.
The Turnigy runs with a lot of vibration in the no load test. I suspect its bearing wont last long.
The Eflight is very smooth running but it is an inrunner.

kyleservicetech
02-06-2010, 06:27 AM
Commercial/Industrial 3-phase motors are indeed a bit different in detail from our little motors but operate using the same general principles. Smaller 3-phase motors use squirrel cage rotors just like the smaller 1-phase motors use. In these motors the "field" magnetization of the rotor is "induced" by the slip between the theoretical rotation of the stator fields and the mechanical rotation of the rotor at a slightly slower speed. That's why induction motors never achieve true "synchronous" speeds. Larger motors, which operate synchronously, have field windings on the rotor which creates the field magnetization. These must be fed DC current through clip rings whereas our motors have permanent field magnets.

If you want a demonstration of that "slip" between the three phase rotating magnetic field, and the squirrel cage rotor, try to locate an especially powerful magnet, samarium cobalt or similar.

I've got some that are 3/8 by 1 1/2 by 3 inches (They pull 400 pounds on a one inch thick steel block) Take that magnet, and swipe it very rapidly near an aluminum sheet, at least 1/4 inch thick. (Be danged careful with magnets like this. I got a finger smashed between one of those magnets and a steel magnetic structure. That finger didn't stop bleeding for 8 hours.)

The magnetic drag caused by the magnet inducing short circuit currents into the aluminum sheet is very noticeable. In fact, I was able to measure the voltage generated across a 1/2 inch sheet I've used on my oscilloscope. The scope showed a few millivolts across that sheet, indicating the short circuit current generated in that aluminum sheet was something around 1000 amperes. :cool:

But, 1000 amperes and a few millivolts still is only a few watts. :o

kyleservicetech
02-06-2010, 06:58 AM
Been in the PC manufacturing biz for many years... where everything is made in Asia... spent a lot of time at the Asian manufacturing sites and am intimately familiar with labor rates, working conditions, tax advantages... the US and other 1st world economic countries simply could not compete. As an example... the PC you're using right now, that you probably spent a few hundred dollars for, would cost 5 to 10 times that amount if it and all its components were to be made domestically. If you had to pay 5 grand for the average PC, would you buy it?

As I've always said... as much as people complain about quality and customer service, they'll vote with their pocket book everytime...

-Jim.

Yeah, my first PC was purchased in the early 1980's for $1500. A lot of money then, still is. The processor ran at a blazing 4 megahertz, it had one 8 inch floppy drive, and NO HARD DRIVE. Memory was something like 16,000 bytes. And that was made in the USA.

What I had before that was a cobbled up computer system built up with some 60 integrated circuits, from scratch with some 1000 feet of wire wrapping. Got it to work, and used it for simple word processing.

Learned a lot from that project, one of which was to damned certainly NEVER do that again.

Gohmer
02-11-2010, 11:20 PM
Well, we've managed to drag this thread WAY OFF TOPIC! Interesting stuff though.

Depends on what you find interesting. Puts me to sleep

kyleservicetech
05-09-2010, 04:15 AM
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. :blah:

hillbillynamedpossum
05-10-2010, 03:41 AM
Doesnt a megaohmmeter measure "loaded" resistance?

(FYI..... I work on stationary generator sets 7-60 KW as well as HVAC stuff)

I use a Freed megger that you plug in, let the tubes warm up and then after about 5 min you can check the insulation resistance and the loaded winding resistance. This is real handy to find a rotor or stator that has a winding that is opening under a load. The point is would this work to find the true resistance?


Possum

kyleservicetech
05-10-2010, 05:53 AM
Doesnt a megaohmmeter measure "loaded" resistance?

(FYI..... I work on stationary generator sets 7-60 KW as well as HVAC stuff)

I use a Freed megger that you plug in, let the tubes warm up and then after about 5 min you can check the insulation resistance and the loaded winding resistance. This is real handy to find a rotor or stator that has a winding that is opening under a load. The point is would this work to find the true resistance?


Possum

I'm not certain which kind of meter you have. The Megohmmeters we used before I retired were of the high voltage kind. They would apply up to about 5000 Volts DC, while measuring the DC current through the insulation under test.

None of the Megohmeters we had in the shop were capable of checking winding resistance. For that, we used Micro-ohmmeters, that could measure resistance down to one micro-ohm. (If you gotta ask how much these cost, you can't afford it!)

Per industry standards, we tested out equipment under hypot conditions, where we would apply up to 80 KV (Depending on unit type) 60 Hertz for insulation and contact high voltage withstand capabilities. They also have to withstand BIL (Basic Insulation Level) tests where some 200 KV is applied to the circuit breakers for microsecond periods of time.

It was real strange to watch a vacuum interrupter be tested at hypot until the vacuum arced over the OUTSIDE of the interrupter, a distance of some 10 inches. That knowing the vacuum contacts were only separated by about 1/2 inch inside the bottle.

These tests are conducted behind lead shields. That's how you generate X-rays.

hillbillynamedpossum
05-12-2010, 05:34 AM
My Freed meggers test at around 500 VDC. Ive got two, an old one I have never even tried to use and a fairly new one. They are pretty cool, like a cross between a hi pot and the "fish shocker" hand crank meggars that im sure you knocked the p*ss out of new guys with.

Any way, I was just curious. Id even be willing to peddle the old one to the right person.

Possum