Electric motor with 1/rev torsional output 1

[goldcountry]

I've never studied motors. Is it possible to get a steady rpm, with a variable 1/rev torsional output??

Want to build a bench test setup for testing a mechanical device.

Help !

Steady rpm in the neighborhood of 1500 + or - say 20% or so (not critical). One per rev on the order of 1/3rd hp.

 

[waross]

How steady? How variable?

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

 

[BrianPetersen]

Explain what you mean by "1/rev torsional output".

Do you mean "torque inversely proportional to RPM"? (But you are calling for a "steady" RPM in which case both the RPM and the torque are going to be constant?)

Explain what you mean. Or perhaps, explain what it is that you are trying to do.

If push comes to shove, a modern servo drive can generate any motion profile you want that is within the capacity of the motor to do, or generate pretty much any torque relationship that is within the capacity of the motor. Servo drives can have a "positioning mode" in which they will apply as much or as little torque as it takes to follow a specified motion profile, or a "torque mode" in which they apply a torque that you specify regardless of the speed that the motor wants to go in order to apply it - provided, of course, that you remain within the capability of the system to do it.

 

[goldcountry]

That sound promising. Let's see if I can explain a little better: I want to spin a disc at nominal speed, say 1800 rpm. Low drag disc, low hp reqd. Then I want to input to the disc a one per rev torsional oscillation, of which I can control the amplitude. Would desire that the rms of the torque oscillation could go as high as one third HP.

 

[BrianPetersen]

Ohh, I see. A servo motor with a decent motion controller will do what you are looking for.

 

[goldcountry]

Many thanks! Will see if I can find such a critter.

 

[mikekilroy]

Absolutely you are on the right track considering a servo system!

But be very careful to define the cycle, load inertia, etc., VERY well; there is a LOT of variation in servo response capabilities!

For instance, we work daily with AC servo drives in the size you are talking about, capable of 800hz velocity loop bandwidth; not many, if any, other AC servo drive is capable of this (yet). Most work well up to about 100hz area. You are asking for a maintained constant speed with some undefined (as yet) sine/triangle/step? torque disturbance 1/rev @ 1800rpm, 30 times/sec, or a full cycle every 33 msec or less. And you want to be able to match that torque disturbance closely, so assume you are willing to accept only 10 'corrections' during that cycle, you are asking for a velocity loop response of 300hz or more.... You will require a VERY good servo system to accomplish this. Now if you were running only 180 rpm, any old servo would likely do.

We had an application similar to this where we were to simulate cam shaft loading for an engine test stand - again running at these type speeds. So a torsional torque was put into the servo each rev, and I must say, it was very nearly undo-able without overheating the servo motor! What appeared on the surface as a simple application like this had a lot of bandwidth requirement!

In addition, you do not say what your system inertia is. Since T=jw/t, where Torque required to move an inertia of j over a speed change of w rad/sec in time t, understanding total inertia is critical to determine your response need. In your application inertia may be your friend - or your enemy, depending exactly on what you are doing.

I would recommend you pass your specifics by some of us who help design such systems; besides, it is a free service a lot of us provide!

http://www.KilroyWasHere<dot>com

 

[zeusfaber]

How picky are you about the profile of your torque oscillation?

At 1/rev, you ought at least to consider the possibility of mechanical torque modification (a suggestion borne of 20 minutes spent truing the front wheel on the Lad's bicycle to stop it scuffing the brake blocks).

A.

 

[goldcountry]

You're right !! That's the way I started (being a mechanical type), but thought maybe I could eliminate a couple components if there was some gee-whiz electrical magic.

The cost appears to be cheaper with nuts and bolts than than with electrons flowing from - to + on the battery. (Just kidding!)

 

[mikekilroy]

Really cheaper mechanically? Please share cost comparison. thanks

http://www.KilroyWasHere<dot>com

 

[mikekilroy]

So is this idea of a servo dead?

http://www.KilroyWasHere<dot>com

 

[goldcountry]

Many thanks for your help. I'm still doing arithmetic. (Retired--no longer have access to a machine shop and neat electronic test equipment.)

Cost mechanically: Guesstimate about $300-400.

Been doing calculations using a steady rpm of 60 rev/sec, driving low inertia. On top of this #'s indicate an 'inertia load' of about 10-28 ft-lbs peak in a sign wave at one/rev (60 Hz). Or 20-56 ft-lbs peak to peak.

This help any?

 

[mikekilroy]

to drive at 60rpm takes some type of motor; did you include cost for that motor and its drive in your $ 3-400 total? The servo way would be to let the motor providing the 60rpm ALSO provide the modulated torque signal to the load... no extra mechanical parts?

http://www.KilroyWasHere<dot>com

 

[goldcountry]

Still doing arithmetic---made a grade school error---will have to back way off on one per rev torque and rpm. Nickel dime motors are around $100.

 

[mikekilroy]

ha! me too on math error; I said 60rpm instead of 60rps

http://www.KilroyWasHere<dot>com