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

 

[Compositepro]

A brushless DC motor is really an AC motor (usually 3-phase or three separate coils). It requires external commutation of a DC input. If you rotate the shaft, the electrical output is an AC signal with the voltage and frequency changing with speed.

 

[crshears]

Can you not control the speed of the driving motor by altering the voltage supplied to it?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[itsmoked]

Yes that will work fine.
You will need a brushless motor drive that can regenerate, most do. You will need a variable electrical load to make it all work. You'd have your shaft running and increase the electrical load on the BLDC generator until you detect a drop in the rotating shaft speed. At that speed you would note the voltage at and current going into, the variable electrical load. VxI=W

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

What is turning the shaft?
How much power do you expect to see? Watts?, Watts x 10?, Watts x 100?, kilo-Watts?

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

 

[Skogsgurra]

I have developed a difficulty to understand English lately.

What is your question about? Do you want to control the speed of that 7 mm shaft? Or do you want to measure the speed of that shaft?

Or do you want to check the generator/motor?

Or is it about a dyno? Like the test benches you use for ICE:s and such?

Or is it about creating some kind of overunity thing?

Tell us - and you will get a good answer. There are many of us that do this for a living.

One tip: Do not use a break. Brakes are better.

Gunnar Englund

http://www.gke.org

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

 

[itsmoked]

This could lead to 'break' Gunnar.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[AdamJ2]

Thanks to everyone who replied and apologies for the vague question.

Essentially I have device that rotates which is driving the shaft. The shaft is coupled to a brushless DC motor in order to generate electricity from the rotation of the device. What I would like to do is to test this electrical output at different rotational speeds. say at 50rpm, 100rpm, 200rpm etc and I was wondering if there is anyway a DC motor can be used to control this rotational speed of the shaft. My electrical background is a bit lacking so I'm not sure if what I'm suggesting is even possible but I thought it wouldn't hurt to check.

It's not a very large system, I'm expecting about 50-100W.

Thanks!

 

[itsmoked]

Adam you're not making a lot of sense here as you say you have a spinning shaft spun by some mysterious mechanism but you want to control the speed by some other thing? And, on the other hand you want to extract power. Normally a generator of any kind is a shaft spun by something coupled to a generator of some sort. The generator does its thing extracting while the prime mover, in your case, the mysterious mechanism, spins the shaft at a controlled speed adding more torque as needed to keep the speed the same. The generator is not used to control the prime mover speed.

If you have no way to control the prime mover then the only way is to load the shaft by extracting power via the generator. This is bas acwards. It can be done however. You must force a load on the generator that is there always to always control the shaft speed. You would need a controlled load bank of resistors to always assure the availability of having a controllable load. If you have a use for the generated electricity you would essentially modulate this by reducing the wastage of the load bank. A reversed situation from normal generation setups.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

Basically a really small regenerative dynomometer.

 

[cswilson]

It sounds to me that the OP is saying that in the actual application he will not have control over the speed of the shaft that will be driving his brushless motor as a generator.

However, to test/understand this generator action, he wants to drive the shaft with a DC (brushed?) motor that is under his control. Certainly this would be possible with a variable DC supply into this DC motor that is driving the shaft.

Curt Wilson
Omron Delta Tau

 

[waross]

Many DC motors and DC generators are the same machine.
With a given DC machine there is only a few Volts or a few RPM between a motor and a generator.
Use your brushless DC motor with a regenerative controller.
As you dial the speed down, the motor will load up the shaft and slow it down.
Measure the regenerated voltage and current at each speed of interest.
Caution: The motor must have more capacity than the unnamed device driving the shaft.

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

 

[waross]

If the controller uses a braking resistor and does not regenerate into the power source you will have to measure the voltage and current to the braking resistor.

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

 

[stevenal]

I believe the OP is describing a test motor-generator setup where both devices are brushless DC motors.

 

[electricpete]

Essentially I have device that rotates which is driving the shaft.

Adam you're not making a lot of sense here as you say you have a spinning shaft spun by some mysterious mechanism...

I'm stuck the same place as itsmoked
op - Please tell us what is the prime mover (the device that rotates which is driving the shaft).
If we don't know that, we cannot provide useful input on how to control the speed.

edit - cwilson's interpretation sounds plausible. Is that what's going on, op?

=====================================
(2B)+(2B)' ?

 

[AdamJ2]

Many thanks for everyone that took the time to reply to my question.

I believe cswilson has explained in a better way than I could, I essentially have no control over the speed of rotating shaft and what I wanted to know is if there's anyway a DC motor (which I am planning to use as a generator) can be altered in a way that makes it possible to control the rotation speed of the shaft.

I will attach a rough sketch to show what I mean, hopefully it will slightly clear things up.

 

[Skogsgurra]

Yes, please! Do that. This enigma is too much for me - and I think for a few more of us.

If you say that you have no means to control the speed of that rotating shaft, fine. Then you have no means of controlling the speed. Unless you overload it. Which isn't a good way of doing it. At least not if the generator taps only a fraction of the available output. Which I think it is.

A simple test would be to apply a bake and see if you can reduce the speed that way. If you can not, then you can forget about reducing the speed with a motor/generator with 50 - 100 W capacity.

BTW. Why brushless? A DC machine with brushes would be a lot less complicated. All brushless DC motors are not able to regenerate. And getting energy out of it in island mode is, I think, a tricky undertaking. With a brushed motor, it is no problem whatsoever.

Gunnar Englund

http://www.gke.org

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

 

[AdamJ2]

thanks again skogsgurra for looking into this problem.

Admittedly not the best sketch but it roughly shows what I'm working with.

so you see I have rotating device, similar to cylinder, which the shaft is connected to, then the shaft itself is coupled to a DC motor.
so again the issue is that I have no control over the speed of the shaft (although I predict it will be under 200rpm) and I was hoping that there might be a way to slightly control or reduce the rpm of the shaft through the motor. If all fails I'll simply use a mechanical way of reducing the rpm but I was hoping for a more elegant way of controlling the speed, if that was even possible.

There's no particular reason for using brushless, it just worked well with a similar project in the past so I thought why not use it again, but I appreciate your suggestion and I'll definitely look into using a brushed as it might be better.

 

[crshears]

The shaft is coupled to a brushless DC motor in order to generate electricity.

[Arghghgh! sound of hair being torn out] Motors DRIVE loads; they have an ELECTRICAL INPUT and a MECHANICAL OUTPUT. Is the device at the bottom driving the device at the top? If it is, you can alter the speed at which you rotate the shaft by using any number of electrical devices to alter voltage supplied to the MOTOR at the bottom.

Or is the copper colored box at the top a prime mover of some sort causing rotation of the shaft, which turns the DC machine at the bottom? If yes, and you wish to use the lower DC machine as a GENERATOR with a MECHANICAL INPUT and an ELECTRICAL OUTPUT, you can [provided you use the correct type of DC machine] place electrical LOAD on the machine to slow down the shaft speed.

What are the respective horsepower / kilowatt ratings of the prime mover and generator? Any change in speed will be inversely proportional to the rating of the driving machine over the rating of the driven machine.

I'm going to take a brake now...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Sparweb]

Do you mean that you only want to use the brushless DC motor as a tachomoeter?
BLDC motors are electrically and mechanically similar to synchronous AC generators, so in principle, you can get a frequency signal out of the BLDC motor coupled to the other motor's shaft, and it will be proportional to the shaft speed.

If you say "No" to that, then my next guess is that you want the two motors to fight each other until some target speed is reached. I don't think you can get far doing that.

You are confusing everyone because there are easier ways to regulate the speed of a motor. Lots of them.

If you want exact speed regulation, use a stepper motor.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF