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Old 12-08-2005, 07:58 AM   #91
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John,
Thanks for posting the Robi info.

Batt Man-you mentioned most do not know what to look for with this dyno, I thought I knew but if I am missing a key indicator of how to tell a good motor then please let me know what to look for !!

Ok looking at the robi data above, I have always looked at the Left side numbers like max watts 128.0a, Average watts 81.2, Max RPM 26,462, Torque 96.9 etc.

But what are the values on the right side?
for instance RPM of 27,405 why is this higher than the max RPM on the left side of 26,462?

What does the torque figure represent? 175.7 Nmm
Does the Current and Resistence mean anything?
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Old 12-08-2005, 09:07 AM   #92
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Quote:
Originally Posted by erock1331
John,
Thanks for posting the Robi info.

Batt Man-you mentioned most do not know what to look for with this dyno, I thought I knew but if I am missing a key indicator of how to tell a good motor then please let me know what to look for !!

Ok looking at the robi data above, I have always looked at the Left side numbers like max watts 128.0a, Average watts 81.2, Max RPM 26,462, Torque 96.9 etc.

But what are the values on the right side?
for instance RPM of 27,405 why is this higher than the max RPM on the left side of 26,462?

What does the torque figure represent? 175.7 Nmm
Does the Current and Resistence mean anything?
erock1331

To start with, I used the WRONG WORD hmmm... BAD CHOICE from my side.

I should not make such generalizations as there are savy persons out there that are doing a great job and do know how to interpret the data.
If I insulted anyone, please accept my apology!

You asked a very good question about the numbers on the left side v right side and some of the motor values given. I will post an answer on that giving an explanation before the day is over.

I am very busy at the moment since we have been under a severe snow storm and extreme low temps in Denver, thus we are BEHIND in our work and shipments. As soon as I finish my responsabilities for the day I will post an answer.

Just a very quick reply... Resistance is very important. The resistance of a motor (armature + brushes) defines performance and I will elaborate on that when I post the answer. However, the only thing you can do to alter the resistance is by chaging/playing with the brushes.... Resistance, Back EMF... all those figures are important as they define a motor.

Here is a very quick example:

THE HIGHER THE RESISTANCE:
Lower stall torque (the Kt will have a different slope depending on resistence)
Lower speed (RPM)
Higher RPM/oz-in (this is the slope of the Speed - Torqur curve) = Rm/(Ke*Kt)
Ke is the back emf... do you star to see why these numbers are important? They all inter-relate ro each other and define the motors characteristics and performance.

Resistance also plays a role in the MECHANICAL TIME CONSTANT (seconds)

The winding (number of turns) is the RESISTANCE of the armature, then you need to add the resistance of the brushes to get the total motor resistance.

Thanks,

Isaac

Last edited by BATT_MAN; 12-08-2005 at 09:41 AM.
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Old 12-08-2005, 09:10 AM   #93
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Anyone got a Robi dyno or sale? Or do they make them anymore?

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Old 12-08-2005, 09:13 AM   #94
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EA
I think EddieO distributes/sells them in the US.

Otherwise you gotta call up Robi in Austria.
New they are about $800-850 range
Used Typically $400-500 range
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Old 12-08-2005, 09:17 AM   #95
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EA,

I will call you later about the Robi's

They do make them and they have a new disributor in the USA. I have no idead when product will be available. However, we (wife and I) have a trip planned for mid April to Europe and if we go, I will bring back with me a few dynos.

I think they are a bit more expensive because of the exchange rate. THe EURO is strong against the dollar... I think he sells them in Europe for around 1,200 euros...
ONE IS FOR YOU...DON'T EVEN ASK...

Isaac

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Old 12-08-2005, 10:21 AM   #96
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I have a dealer account with them, so I could resale them.....I never sold one though, as a price of a new one is like $900+ US....

I heard they were coming strong to the USA with a new distributor.....they take a long time to return emails too sometimes...


Later EddieO
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Old 12-10-2005, 03:52 PM   #97
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erock-long time, so I thought I would post my take on some of these measures. The higher RPM on the right is probably the estimated no load maximum RPM with the flywheel off. I don't think it is particularly useful, but if you run the motor pinion off on a power supply, at the voltage of the run, on the Integy type of instrument, you would get a value close to this. His estimate is probably made on mod motors, though. The torque value you quote is Maximum Torque. The only time we see this is right on the start of a race, so it is not a very useful tuning aid.

The Motor Resistance can be used by an motor winder to select the gauge of wire to use to match a good armature. They never tell us the gauge on mod motors (trade secret, I guess). Thats why all this "reverse engineering". If you trash the motor it is easy to unwind and measure the gauge. The armature resistance is easily calculated. I don't see much use to the tuner of stock motors for this number as what you want is more power. Since the resistance on the Robi includes the brushes this is a complicated factor to look at. The least resistance brushes don't give you the best power as they may develop too much friction. Most probably there is some optimum resistance in the brush. You might as well, then, try the brushes that are available and not pay much attention to resistance. Look at power as you try brushes and make sure in the race that they hold up for the 5 minutes and don't fade. I prefer they hold up for four heats. This is especially a problem in 19 turn class.

Motor Friciton tells you how quickly the motor spins down. If there is too much friction, a higher number, a bushing may be out of alignment. The magnet may be rubbing the arm. Some guys were melting the magnets loose at one time with motor temps a high 160 F. I even had some come loose at 140 F.


Induction is an important property that is not reported. It is a measure of how much magnetism you get per amp. In some races your motors induction has to be measured at the track to verify compliance with motor wind limits. So some tracks have the induction meter. Stock motors have a very high inductance due to lots of winds. Good corner control results along with poor top speed. Low wind single wrap mods have low induction. Great top ends, poor control in a corner; Poor part throtle response. The motor winders increase induction slightly by adding more wraps to a turn. Several wires are wrapped at once. This packs the wire tighter on the arm. You get more control in the corners at the expense of top speed. Trinity's flat wire probably has the best induction/turn of the motors available, but this limits top speed a little. This motor might be the result of an induction thread we had on their Web site.

For Stocks
A solid arm has good induction, good corner exit speed. A tri-rotor arm has poor induction and thus higher top speed, more suitable to ovals; poor corner exit speed. This is the only way we can control induction on a stock motor; buying a different arm-can, single, double, or trirotor.

Last edited by John Stranahan; 12-11-2005 at 06:44 AM.
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Old 12-10-2005, 05:08 PM   #98
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OK. I made my milling machine dyno. I tested a Trinity P2k Pro. The highest spindle speed is 2400 RPM. The motor produced .952 V. I had run in the motor until I got stable readings. Then I put in my secret combination of spring tensions (8.5 oz negative 8 oz positive This combination, which has to be measured, produced the best power, at 30 Amps 6.75 V, on Jorges dyno that he ever tested. Christian easily won the regionals with this combo. I heard racers mutter afterward directly to me, his motor was too fast he must have cheated. Not a chance.) and ran it in some more until I got a stable no load amp draw.

No Load amp draw 4.5 amp at 7.00 V. This value limits my precision.
Kv or RPM/volt = 2521 from the data on the milling machine.

I put a 10 ohm resistor in series with my Rivergate's adjustable voltage leads. I got 26.6 mV or .0266 V voltage drop at a .45 A current through the motor windings. Dividing gives me

Rm = .591 V/amp = .0591 ohm

Interestingly, this value matches exactly the resistance of a motor that I have in a Robitronic motor file measured by the dyno.

Next I calculated power with the formula that I posted above. I'll paste it here later. Here is the graph. Now the graph is a nice symmetrical power graph. The only trouble is that the power predicted is 60 or so Watts too high. Stock motors power peaks sooner (lower amps) in the graph on other dynos. About a 30 % error. The stall amp draw appears to be 116 amperes. That sounds high. I have measured 75 amps, 85 amps is probably a reasonable maximum looking at Robi Data

Io, Rm, Kv, Voltage
4.5, 0.0591, 2521, 7

Power is then = (Voltage -Current *motor resistance) (Current - No Load Current)

P=V-(I*Rm)(I-Io)

Anyway I have made the same test previously to try to test motor performance without spending the big bucks. I have to still remain skeptical. Power is best measured not calculated from constants. I have crawled over inspected and photograhed and studied the output of a dynojet full sized car dyno. They don't use current draw of the armature to measure power (there is no armature, gas engined cars are measured on a big heavy roller which contacts the tire) I don't know why the Fantom RC dyno that has the same basic design would, although I could be wrong.

I could have made mistakes in my calculation. The fact that I am in the ball park tends to say that I did not. Undoubtedly there are errors in the instruments and RPM but probably not 30%

Get a dyno or use the track dyno(time sheet) is what I have to suggest at this point. Eddie O's procedure near the maximum amp draw is about as good as any. My partial average power only distills this to a single number.

And I'll have to say I have learned a bit, which is why I participate. Give a little get a little back.
Attached Thumbnails
CE Dyno Question-theoretical-stock-motor.jpg  

Last edited by John Stranahan; 12-11-2005 at 11:47 AM.
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Old 12-11-2005, 11:35 AM   #99
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I knew that you guys were dying to see another graph (just a little humor), so here it is. This time I have estimated no load current of a P2K from the Robitronic Graph of Fan's P2K, 10 amps. This is different than on my motor. I have taken the Kv or RPM's volt from the right side of the Robitronic graph. I have taken the Rm or armature resistance from the right side of the Robitronic ouput, both by changing the units a bit. I have used these numbers to calculate a theoretical curve which is shown in the pink just below the power line from the motor I tested on the milling machine.

Now the power is more in line with what I would expect at 7 volts. Still high, though. We did use a real dyno for some of the data here. The amperage at peak power is at 60 amps. I believe this is in great error. I have tested lots of stock motors at 45 amps and 7.0 V. Unless you use really heavy springs (unavailable really heavy springs) the power is usually peaking at 40-45 amps for motors used in stock trucks on a high grip surface. The stall torque (or starting from a dead stop torque) is predicted to happen at 114 amps. I think this is too high. Anyway I remain skeptical still that this method can be useful for stock motors where a 1-2 percent difference is all that we look for.

Fan's P2K
Io, Rm, Kv, Voltage
10, 0.0645, 5243, 7
Attached Thumbnails
CE Dyno Question-robitronic-theoretical-power-7-v.jpg  

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Old 12-11-2005, 12:34 PM   #100
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Hi John,

I am sure all of us have learned and shared information that should be useful to most readers.

Now, regarding your calculations and errors that you mentioned:

It is common within the motor industry to accept tolerances of +/- 10% based on the constants. This is from motor manufacturers that implement ISO quality control standards. The r/c industry has VERY POOR QC regarding motor production. I have tested on accurate lab quality equipment three motors from the same manufacturer, same production day, and all test parameters the same...the results...WELL, the motors were way off. None of the motors were even close, the differences were as high as 20%

Testing a motor by inducing a known rotational speed as we spoke is only a good estimate, IT IS NOT A DYNO. To DYNO a motor properly we need to measure real torque and not calculate it.
However, your experiment was close enough. Taking into consideration that you are not using lab quality instruments I must say that you did a GOOD JOB.

Now, regarding the differnce in power output vs a dyno...You forgot to take the motor's efficiency into consideration (power losses). If you look at a motors efficiency rating at PEAK POWER, and you factor that adjustment, the power output will be much closer to that mesured on a real dyno.
Your explanations are accurate and you are passing along very good information with back up data to substantiate all your observations and findings. However, you must always factor out efficiencies, losses, etc. Only through repetitive tests and/or experience can you use the correct factors to get a close approximation.

The issue at hand, can you see how you can use the Ke[back EMF] (or Kv[Krpm/volt]) to start tuning a motor. Just look at the relationship between them and POWER...then torque comes into play, which is the final outcome. THe main numbers to look at a motor are the relationships between RPM and POWER as they relate to torque (or current)
Basicaly, you differentiate the values against torque (or current)

Well, I just wanted to get back to you with a prompt reply.

Later,


Isaac

Last edited by BATT_MAN; 12-15-2005 at 06:59 AM.
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Old 12-11-2005, 02:48 PM   #101
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Outstanding thread gentlemen, by far the best read to date on these forums


After reading this thread front to back, the question I need to ask is if I am to base my ce dyno on torque steps...how can I measure the amount of torque onboard in my car to get relevant #'s?

I would also like to ask what the recommended voltage setting is for a ce-45 dyno. I will be racing 6 cell dirt oval.

Some years ago while I was involved in pancar racing, ( I remember you Mr. Dunnigan )...when I had a ce dyno I focused on the 22 amp load readings. I have been away from this hobby now for some 6 years or so, alot has changed.


Thanks again for all your input and have a happy holiday!


Sheldon
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Old 12-11-2005, 03:49 PM   #102
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Sheldon-I have raced some 6 cell dirt oval. Pertinent torques and amps are very near the average amp draw as the motor does not slow very much. Run a full pack. Dicharge the leftover charge in the pack immediately on the Turbo 35 to calculate the average amp draw. More details on this if needed.

Another way is with a data logger that records the millivolt drop accross a shunt (heavy piece of wire or battery bar) that is connected in series with the battery. Radio shack used to have an inexpensive logging device that came with computer software, that I have seen used as much as 5 years ago. You run the car with the device on board and then download the data onto your computer at a convenient time. I don't know if they still carry it. Batt-man may have one in the works for RC use. I have never done this or felt the need as it was too much trouble. Now if a handy device were available for strictly RC use, I might give it a try.

I always used to use 7 volts on the turbodyno 45 as the batteries could usually maintain this average over the full race in stock. The track owner always used to use 6.75 Volts on a turbodyno 30. Not much difference in quality of the data.

Issaac-Thanks ,and thanks for your help.

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Old 12-11-2005, 04:07 PM   #103
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Thanks for the quick reply John. Details are good

I have been away from this hobby for some 6 years now...I am not even certain I remember how to properly calculate rollout the more details I can have the better off I am.

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Old 12-11-2005, 04:16 PM   #104
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damn; after the first long post i quit reading
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Old 12-11-2005, 04:47 PM   #105
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Sheldon-Here is the quick formula from John Wallace

Average Amp Draw =
(Milliampere-Hour x 3.6)/Race Time (in seconds)

You have to cycle a pack and get its capacity in milliampere-hour (mA-H). You do this on a Turbo 35 or similar device. Then run the same fully charged pack in the race. Discharge it, as immediately after as possible, on the T35 to get the mA-H remaining. Subtract the amount remaining from the packs capacity. This is the value you put into the formula above and the amount your car used in the race. Tell us what you get when you get a value.

On an oval this average amp draw and close by, is a good area to explore on the dyno.

on a Road course, the slower parts of the track, while the car is accelerating, will have more effect on lap times. Take your readings at the average and greater amp draws.

Last edited by John Stranahan; 12-12-2005 at 11:12 AM.
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