High speed motor for sinusoidal shaker?

[aschen]

Hello all,
I am new here and look forward to contributing and learning. I am trying to design a "shaker" with a fixed sinusoidal discplacement (~.02" P-P) with a max freq of around 250 hz (15krpm). I was thinking of using a electric motor with an eccentric crankshaft and some sort of conecting rod to drive my test specimen mounted on linear bearings. I have calculated that my peak torque will be around 2.5 ftlbs, but the duty cycle will be very low. A have a few questions:

Is this a reasonable thing to do? I considered adapting a electrodynamic shaker for my application but I was shocked at how large of a unit I would need.

Any sudgestions on what type of motor or a supplier to investigate. My brief research points me to DC brushless, but it is hard to find appropriate motors that spin to 15k.

any input is appreciated.

Thanks,
Aaron

 

[GregLocock]

So that's about 1 hp. Look at the motor/gearbox in an angle grinder.

However, I think you'll find that it is a tricky thing to design, could you consider using an eccentric mass instead?



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[bklauba]

Another option would be a brushless DC motor (some folks might disagree with the nomenclature, but that is the most accepted terminology). To make these work, you do need to "time" the firing of the drive powering the motor, and these have some sensitivity to vibration, an issue that will need to be addressed.

BK

 

[aschen]

2.5 ft lbs at 15k rpm is around 7 hp not 1 I think. As I said my research has pointed me to brushless DC which I have used in model aircraft in the past. What I am not sure of is how to power such a motor. I have done some research online but I am a bit confused. Is there an easy off the shelf controller that can be used to power one of these from ac mains? This guy looks like it would work ok for my app:

http://www.astroflight.com/store/st...=products:af-3220&sid=0001B6cx627p1CMAdg8I8F3

It has 3 hookups and is "sensorless". I am having dificulty figuring out the industrial jargon to find the appropriate drive controller.

Thanks fof the responces thus far,
Aaron

 

[BobM3]

I think you'll have a tough time finding a brushless DC motor near 7 HP that can turn 15k RPM. Is the 2.5 ft lbs the maximum torque (link at 90 deg to crank)? If so, maybe a flywheel would allow you to get by with a smaller HP motor.

You might want to look at universal motors for the high speed. But I'm not sure they're available at 7 HP either.

 

[aschen]

I think the fly wheel is a godd thought as that figure is peak as you guessed I dont think my original link worked correctly but here is the motor I was thinking of:

http://www.astroflight.com/store/pdffiles/3220.pdf

Anybody know a convenient way to power this guy? 7 hp at 4 lbs is quite impressive, but I know these motors can have huge power densities.

 

[aschen]

I dont think my original motor link worked. Here is the one I was considering:

http://www.astroflight.com/store/pdffiles/3220.pdf

The torque I mentioned is peak so I think a flywheel is a good thought. The Idea of alot of inertia spinning at 15k scares and excites me at the same time!

Thanks,
Aaron

 

[BobM3]

Wow, that's quite the motor. Microchip has some good application notes on controlling brushless motors. Try this link:

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function

I'm not sure that link will work (it looks funny). If not go to Microchip.com and go to Design..app notes...motor control....

The guys in the "Electric Motors & Motor Controls Engineering" forum might be able to help you pick out a drive.

 

[sreid]

With a flywheel you may not need much HP as the energy of deceleration will be returned to the flywheel. Your mechanical design will be important as your peak accelerations will be 64 G.

Brushless motors for 15 KRPM should not be a problem for smaller HP.

 

[BobM3]

From just a mechanical perspective, a crank and link would not be an efficient means to drive an inertial load sinusoidally. The motor/crank/link developes its max torque (link and crank at nearly 90 degrees) when the table needs it least (mid-stroke, accel = 0 for sinusoidal motion). Also it you are driving the table vertically, then in theory (if everything were infinitely stiff and lined up) you wouldn't be able to get it started if the weight of the table had moved the crank/link to its "bottom dead center". I think a flywheel is a must.

 

[aschen]

Thanks guys. I think everybody is right about the flywheel. Now I have to do some calculations to size the wheel. I better make sure our safty guy is not around when I run this beast! This is turning into a fun project mostely because I am not sure if it will work or not....

 

[BobM3]

Oops, you can still start it at bottom dead center.