using a DC motor to control rpm of shaft 1

[AdamJ2]

Hello everyone,

I am intending on buying a brushless DC motor and use it as a generator to get the electrical output from a 7mm rotating shaft by coupling it with the motor. I need to somehow test the power output at different rotational speeds of the shaft. Is there anyway or kind of brushless DC motor that can somehow control the rpm or do I have to use a mechanical break? Any advice would be highly appreciated.

Thanks

 

[Skogsgurra]

AJ2,

Lots of us have been trying to help. With very little success, so far.

I guess that the reason is a complete lack of knowledge (you) what words really mean in this context and that most engineers (we) in the drives field expect that the meaning of key words are understood by both parties involved.

To me, at least, you seem to be an honest guy that want to do "something" that you haven't been able to describe in a coherent or plausible way. In a post 16 May 19 06:46 I asked a few questions that I don't think there's an answer to and the reason could well be that you haven't got the terminology right. That leads to a "discussion" where Newspeak and Fake Facts mixed with wishes and misunderstandings leads nowhere.

That could be avoided if you tell us WHAT you is trying to do instead of telling us in a vague and incomprehensible way HOW you are going to do it.

I can understand your hesitation to reveal what you may think is a valuable secret - you may think that you have uncovered some secret way of making energy available at a low cost, or something similar - but, if that is the case, it is highly unlikely that you have done that. Lack of understanding is behind most failed inventions.

So, start over again and tell us, in the words you would use to describe to a fellow mechanical engineer, the following very simple facts:

1. What makes the vertical long shaft in your 20 May 19 19:55 drawing rotate?
2. At what speed does it rotate?
3. If speed varies - what are the upper and lower limits for the speed?
4. What torque is available from the shaft?
5. By controlling, do you mean "take command over" i.e. influence the speed? Or do you mean check/measure/log/record/whatever the speed? Or both (measuring AND controlling the speed)?
6. When you say (16 May 19 19:22) 50 - 100 W. Where did you get those numbers? From other measurements? Or a nameplate somewhere? Or is it a guess or, perhaps, what you wish that you can get?
7. What are you going to use that power for? This is not a totally irrelevant question. The answer may guide you or us to better understanding.
8. Does this contraption exist IRL? If so, make a photograph and show us. It is easy, click the camera icon and drop the picture there.

As I said, you seem to be an honest guy. But sometimes, honesty is not enough. Basic engineering knowledge and some respect for facts is also needed. Or you may end as the President of a mighty Western country.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[AdamJ2]

crshers,

just to clarify, the copper coloured box is the prime mover which rotating freely. Assume it's a turbine driven by water or air where I cannot essentially control the speed of that rotation. Now if I simply connect a shaft only to the copper box and nothing else at all it will rotate freely at 200rpm (for argument sake). I cannot do anything to change the source of the rotation. The purpose of the motor is to get the mechanical input from the shaft and get an electrical output.

sparweb,
thank you for your reply, your second guess is close to what I am looking for. I essentially just want a way to control the rotational speed, doesn't have be the motor itself, but I wanted to check if that's possible. clearly, it seems from all the replies that it is not the best option.

Skogsugraa,
I appreciate your interest in the problem and taking the time to inquire about it, I genuinely do.
to answer your some of your questions
1- the shaft is connected to a water turbine that rotates due to water stream at a roughly fixed speed, again for argument sake say it's 200rpm. I cannot control the source of the rotation directly and that's the main problem. sharing the whole design will probably add to the confusion.
2- around 200rpm, it will be approximately fixed at that speed.
3- The variation is very insignificant and you can assume it's zero, because I am doing it under controlled conditions.
4- that I have yet to test, but I could check on that and get back to you.
5- by control I mean take command of the speed. assume the shaft is not connected to a motor, in that case it will rotate indefinitely at 200rpm. What I would ideally like to do is for example change that rotational speed to 180rpm or 150rpm or so on. I do not have to do it very precisely, I can measure the rpm easily, controlling it is the challange. It doesn't have to be done through the motor, any other suggestions are highly appreciated.
6- 50W to 100W is a guess based on a similar design of similar size, properties and test conditions made elsewhere. I cannot be 100% sure that will be my output but it's a fair guess.
7- I am not planning to use the power for anything at this stage, I just wish to measure and record it to better understand the potential of the design. So I will likely connect the motor to a multimeter.
8- Unfortunately it does not exit at the moment.

please let me know if I can clarify on anything else.
Thanks.

 

[BrianPetersen]

So it is a totally conventional prime mover and generator (not motor!) system, just in small scale.

 

[AdamJ2]

Brian, yes exactly, I see now the source of the earlier confusion, I should have probably explained differently.

 

[BrianPetersen]

In a hydroelectric power plant, the AC generator is locked to grid frequency (normally 50 or 60 Hz depending on where you are in the world), the generator shaft and thus the turbine will be locked to a speed that depends on how many poles the generator has, and fiddling with the volume flow rate and angle of attack of the water flow and so forth varies the torque that the turbine produces at that fixed speed, and thus the active power fed to the grid by the generator.

If you have a DC generator, it won't be locked to an AC frequency and will thus not be locked to a fixed operating RPM but the general control strategy is the same.

 

[Skogsgurra]

Very good, AJ2!

But you were driving us crazy. Braking a shaft with a DC motor is very easy. Couple a big enough motor to the shaft - as you said - but you started talking about a brushless DC motor. Then get a set of incandescent lamps with a suitable voltage and connect them to the motor/generator output and see the speed drop as you connect more and more of them. You can also use a rheostat (a variable power resistor) for the same result. Or, if you have a separately excited motor, keep resistance constant and go from zero excitation current and up. That will increase voltage of the DC machine and hence the load on the shaft.

Or use a mechanical brake, which you know more about than I do.

A very commendable set of tests, I think. And you know what? You will probably learn from it. Inquiring minds do.

Sorry for being harsh. But there were reasons...

Brian, not all hydraulic generators are tied to the grid. Especially not little ones like the one discussed here.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

A used automotive alternator may fit the bill.
It should easily handle the power that you are expecting.
Remove the voltage regulator and control the field directly.
The automotive alternator will generate an AC potential but will internally rectify the AC to DC.
Connect a resistive load to the output terminal.
Start your prime mover and slowly increase the voltage applied to the field.
As the field voltage is increased and the field is strengthened, the output voltage will increase.
As the output voltage increases, the current through the load will increase.
Volts time Amps will give you the power in Watts that you are producing.
You may have to increase the speed of the alternator with pulleys and a belt.
As you increase the voltage applied to the field, the output will increase until you stall the prime mover, burn out the alternator or burn out the alternator field.
You will want a variable source of DC from zero to about 14 volts for the field.
If you can get this set up, come back and we will help you tweak it.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[AdamJ2]

Brian,
thanks for the suggestion, in my case I'm working with a stand alone system that will not be connected to the grid.

Skogrussa,

thank you, the exactly the kind of answer I was looking for. I think your suggestion is probably the most straightforward way to go about this. Hopefully it will work and I won't have to resort to the mechanical breaking system I have which I'm fairly certain will work, but it causes a lot of major inconveniences outside the scope of this post. Also, no worries, your previous comments are probably for the best

Bill,
many thanks for your detailed suggestion, I will have a go at skogrussa's idea and if that prove challenging I will certainly try to set up the automotive alternator. Don't be too surprised if you see me back here soon.



Good to see that we're getting somewhere no.

 

[waross]

Some general comments about DC machines.
The difference between a DC motor and a generator is only a few Volts or a few RPM. Let's call it a DC machine, machine for short.
It may have a shunt field or a permanent magnet field.
When the machine is turning and the field is energized it is acting as a generator and developing a voltage called the back Electro-Motive-Force in the case of a motor. The back EMF is very close to the applied voltage.
For the sake of example consider a small motor connected to a shaft that is driven by a much larger motor at a constant speed.
We will ignore the small current taken by the field.
Both motors are running at 1800 RPM.
The small motor or machine is drawing enough armature current to power windage losses.

Now increase the voltage supplied to the smaller motor. There will be a greater difference between the supplied voltage and the back EMF.
The current will increase as the DC machine tries to turn the larger motor faster.

Now decrease the voltage applied to the DC machine. the back EMF remains the same because the motor is being driven at a constant speed.
The armature current will decrease to zero and then increase in the opposite direction. The machine is now a generator.
Note: It may be necessary to use a load resistor isolated from the supply voltage by a diode.

It takes very little change in the applied voltage for the DC machine to go from motoring to generating.
It takes a greater change in applied voltage to go from full output as a motor to full output as a generator but still only a few percent of the applied voltage.

The actual amount of voltage change to go from zero output to full output either as a motor or as a generator is determined by the rated voltage and the resistance of the armature circuit. The resistance of the armature is typically very low. The lower, the better the efficiency of the machine.

Now if the applied voltage is held constant and the speed of the driving motor is varied we will see the machine go from motoring to generating with only a few RPM change in speed.

A shunt machine at constant speed will go from motoring to generating with only a small change in the field current.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[AdamJ2]

Bill,

Thanks a lot for your detailed explanation, it cleared a lot misconceptions that I had for some reason and I've already saved it for future reference.

I think it's time for a brush up on electrical engineering 101.

 

[itsmoked]

Bill; I've seen old car generators (as apposed to alternators) motoring. It was pretty surprising.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hi Keith. I ran into a couple of units that I am sure that you will like.
They were on garden tractors and looked like an automotive starter.
They were belted to the flywheel.
Inside, one field pole had a winding similar to a generator, fairly small wire, lots of turns.
The other field pole had a starter type winding. One layer of heavy, flat, conductors.
The armature winding was heavier than a normal generator but lighter than a starter.
When running, the voltage regulator controlled the light gauge field and the unit generated as a shunt generator.
When battery power was supplied to the other field winding, in series with the armature, the unit motored as a series motor and started the small engine.
Old car generators motor just fine, but when they do, there is often an interesting story behind it, grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Oh that's right Bill! I remember seeing those in my autoshop days.

Amazon Motor/Starter Delco

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Wow. Still available. I haven't seen one in "the iron" for about 30 years and it was old then.
Just as I remember them, including the reduction in diameter.

Bill
--------------------
"Why not the best?"
Jimmy Carter