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Old 12-27-2005, 07:50 PM   #1
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Default Home made Dyno does this work

Check out the attached link below.

For the guys who understand electronics will this work as a basic engine dyno?

http://uk.geocities.com/dave_pickett...ew_page_17.htm

Thanks for your help.
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Old 12-27-2005, 09:01 PM   #2
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PRO 2 RACER -- This is not a dyno...

I know what he is trying to do but in reality thast does not tell much about the motor unless you ubderstand what is going on.

He is reading the back EMF that the motor generates and that is a VERY good tool to tell motor performance, but not the way he is doing it.

I will refer you to this thread and post. Follow what we were doing and recommending in there and you might get a better undferstanding.

This is the post where it starts, just read from that point on.
http://www.rctech.net/forum/showpost...5&postcount=74
This is wher I started the posting, but read the entire thread, it is very educational.
http://www.rctech.net/forum/showthread.php?t=89485

The reasons why his motors always have a higher voltage in one direction v another is because of the motor timing, NOTHING ELSE. And there is NO NEED to run a motor in reverse to read the back EMF that it generates.

If after you read the CE DYNO thread you havve more questions, feel free to ask for help.

BTW, in that so called dyno examplre he is refering to power, yet I do not see a single POWER calculation, just back EMF readings.

In the thread that I refered you to, John Stranahan did an experiment using a known rotational source. This is a much more accurate way to test a motor.

If you follow the example I set out, and use the formulas I give, you should get a good understanding of motor performance. Like I said, I explained the problem and John took it a step forward by doing the test.

His only error in the experiment was that he did not introduce an efficiency factor. However, as a comparisn tool, it is VERY ACCURATE. Much more than the dyno that you are looking at. (I would not call that a dyno, nor the experiment I outlined).

I hope this will help you.

Isaac
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Old 12-27-2005, 09:05 PM   #3
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Sure -- within limits...

At least the test motor has a load on it (the slave motor)...

Without some form of load, any sort of "testing" is useless. BRP (Bud's Racing Products) sold a similar "dyno" for years and years...
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Old 12-27-2005, 09:12 PM   #4
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Quote:
Originally Posted by tfrahm
Sure -- within limits...

At least the test motor has a load on it (the slave motor)...

Without some form of load, any sort of "testing" is useless. BRP (Bud's Racing Products) sold a similar "dyno" for years and years...
I don't think so...

He is using a motor to drive the test motor. It is not being used as a load but as a source of ROTATIONAL SPEED to read the BACK EMF that the test motor generates.

If he would be doing this test at a KNOW speed, for example 1000 rpms, then you can use the motor equations to reverse engineer the motor.

What he is doing does not tell much. YOu are only comparing back emf and you have NO CONTROL settings as you do not know aty how many RPMs he is reading the voltages.

Isaac
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Old 12-27-2005, 09:45 PM   #5
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Thanks Issac,

So if I were to place an engine into a drill press with a fixed RPM and then measured the voltage it would be more accurate.

What I want to do (on the cheap) is to be able to compare engine to engine and a [articular engine before and after a re-build, ( to see if I got it right)

Am I right in assuming the higher voltage measured the better?
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Old 12-27-2005, 11:31 PM   #6
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Quote:
Originally Posted by BATT_MAN
I don't think so...

He is using a motor to drive the test motor. It is not being used as a load but as a source of ROTATIONAL SPEED to read the BACK EMF that the test motor generates.

If he would be doing this test at a KNOW speed, for example 1000 rpms, then you can use the motor equations to reverse engineer the motor.

What he is doing does not tell much. YOu are only comparing back emf and you have NO CONTROL settings as you do not know aty how many RPMs he is reading the voltages.

Isaac
Uh

I don't know. I read the little article, and it did not sound to me that he was measuring back EMF, infact I think back EMF would be EXTREMELY difficult to measure.

Back EMF, as I am sure you are aware...........is the voltage created in a winding while that winding is energized and thus moving through a magnetic field. In other words, as you power a motor and it begins to spin, some of the energy (electricity) that is being consumed by the motor is to merely counteract the back emf that is generated in the coil windings by the magnets. That current want to travel in the opposite direction as the current you are suppling to cause the armature to rotate. Thus it would take much more sophisticated equipment to measure back (reverse) emf.

It sounded pretty clear to me that he was using a slave motor as a Generator with the resistor and volt meter across it.(the slave motor) In fact its more like a slave Generator. The motors being tested had power applied to them, and they spun the generator slave, and he was testing the voltage produced by that generator slave.

Don't know if you have ever used a Turbo Dyno, but it works the exact same way, just with more sophisticated measuring equipment and constant voltage and variable load on the slave, etc....
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Old 12-28-2005, 10:40 AM   #7
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PitCrew -- I had not seen the diagram, I only saw the setup he had. Once I opened the word document I saw what he is doing and I stand corrected.

He is using the slave motor as a load, however not an accurate wat to do so.
He is also measuring the back EMF that the slave (load) motor generates to do his comparisons.


There are many variables here that make this dyno very innacurate, but it can be used as a comparison tool, thats about it.

Also, measuringback EMF is very easy. It is much better to do it with a scope or a data logger, that way you can get more samles/second, thus better accuracy.

By knowing the back EMF that a motor generates you can compute the Kt and Kv constants. From that you can derive just about everything needed to analyze a motor. The only true measure that you do not get is torque. Torque can be calculated from the Kt constant. TRUE torque can be measured (using different type of loads) or calculated by knowing the mass of a spining flywheel (like the Fantom dyno)

PRO 2 RACER -- If two motors have the same winding, same resistance, same brushes...you can get a very close comparison of the two of them by measuring the back EMF.


Thus two equal motors... The one with the highest Ke will develope more torque at the same current draw
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Old 12-28-2005, 07:14 PM   #8
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Default dyno(?)

Thanks guys ...... from down under.
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Old 12-29-2005, 01:58 AM   #9
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Quote:
Originally Posted by BATT_MAN
PitCrew -- I had not seen the diagram, I only saw the setup he had. Once I opened the word document I saw what he is doing and I stand corrected.

He is using the slave motor as a load, however not an accurate wat to do so.
He is also measuring the back EMF that the slave (load) motor generates to do his comparisons.


There are many variables here that make this dyno very innacurate, but it can be used as a comparison tool, thats about it.

Also, measuringback EMF is very easy. It is much better to do it with a scope or a data logger, that way you can get more samles/second, thus better accuracy.

By knowing the back EMF that a motor generates you can compute the Kt and Kv constants. From that you can derive just about everything needed to analyze a motor. The only true measure that you do not get is torque. Torque can be calculated from the Kt constant. TRUE torque can be measured (using different type of loads) or calculated by knowing the mass of a spining flywheel (like the Fantom dyno)

PRO 2 RACER -- If two motors have the same winding, same resistance, same brushes...you can get a very close comparison of the two of them by measuring the back EMF.


Thus two equal motors... The one with the highest Ke will develope more torque at the same current draw
Cool, good stuff.

I guess I am wondering about your definition of Back EMF. Not that you are wrong or anything, just that I am having a hard time fitting your description into my understanding of what back emf is.

As I understand, back emf is the "reverse electromotive force". This is electricity generated in a motor that is counter to the power going into the motor to cause it to rotate. This is why a motor with weaker magnets is more effecient, and will turn higher little or no load RPM (but produces less torque).
So, when you send power to a motor, like in your car, some of the energy being dissapated as heat is because it must overcome the back emf produced in the coils as they rotate through the magnetic field.

I guess in a sense, running a slave motor as a generator is in a sence creating back emf, and that is what is produced as voltage across that motor being run as a generator...

Can you enlighten me?
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