Ideal Electric motor

[michael1965]

Hi,

I want to spin a disk (dimensions = diameter 1000mm, thickness 5mm, weight (inc spindle) 1.5kg) at a continuous 500rpm in the most energy efficiant way possible. What would be the best electric motor, the best configuration (below the disk or above), and what would be the approximate electrical usage of a motor performing such a task?

Thanks in advance for your observations.

 

[waross]

The most energy efficient way is to spin it in a vacuum to avoid windage losses.

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

 

[michael1965]

Hi,

I may well investigate the benefits of a vacuum in the future, but I'll be running the motor in normal pressure initially. What I really need to know is what would be the ideal electric motor given the job and the need for energy efficency.

 

[waross]

Once the motor is up to speed, the power required to keep spinning will be the sum of the losses. Mostly wind drag and a little bearing friction. You probably want a timing belt drive to drop from the motor speed to 500 RPM. The most economical motors are often 1750 RPM.
The belt drive may be the greatest source of losses.
Posting a link to this thread in the Gear and pulley engineering forum may get some help calculating the losses in a belt and pulley drive.
PS, post a link, don't double post.

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

 

[LionelHutz]

Given the little detail, I'd have to think something like a permanent magnet servo motor and controller would be best suited to that application.

The most efficient might be to make the disk act double duty as the motor too.

 

[ijl]

michael,
Can you tell a little more about your project. What kind of power is available (DC, AC, 1 phase 3 phases)? What kind of bearings, can the bearings still be selected? Does the disk spin freely, no loading? And (I am just curious), why "best electric motor"?

Without knowing anything more about your project, I would like to suggest a brushless outrunner motor (DC), with a low KV-coefficient, as used in model RC-aeroplanes.

 

[michael1965]

Hi,

Thanks for your replies, it's refreshing to find a forum where people actually respond to a post.

For an unknown reason I visualise the motor at the bottom, so with that in mind, the disk is 1000mm diameter, and 5mm thick, it will weigh a total of 1.5kg, including the spindle. The disk will rotate in normal pressure and temperature. There will be no loading other than the weight of the disk, wind resistance, and the friction caused by the top end of the spindle. The top end of the spindle will be pointed and will spin within a metal 'cup', such that only the pointed end will be touching the cup. The power source will be uk mains.

The reason I ask is that the power input will have an effect on the results of the test, and, not knowing anything about electric motors and their efficiencies, I decided that, rather than take a punt on a random motor, I would ask people who are experts in the field.

I'm humble in my lack of knowledge, amongst those whose undertanding is beyond me.

 

[Skogsgurra]

What is the purpose of the test? Are you going to drop things on that disk? Or have it make some work elseway? How important is the speed accuracy? All these things and a few more may influence the choice of motor. But, as of now, I would use a PM synchronouos motor and a small VFD.

Gunnar Englund

http://www.gke.org

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

 

[michael1965]

Nothing's going to be dropped on it, thrown at it, or fired at it. The disk will not mechanically interact with any other device - it's just going to spin. The rpm won't go below 100 or above 600, so an ability to easily change speed would be an advantage.

 

[mikekilroy]

michael, ALL motors have efficiency curves showing how they perform at different speeds and loads. Perhaps you should google eficiency and look at your choices in general terms. pretty much your choices seem ac induction or ac synchronous. both can have efficiencies in the 90+% area - AT PROPER SPEED/LOAD POINT(S) EG: a motor outputing rated torque at 0rpm has an efficiency of power in/power out =0%.... it goes up from there and then at some point turns back down as internal losses grow faster than the output power...

You said something startling in last post: The top end of the spindle will be pointed and will spin within a metal 'cup', such that only the pointed end will be touching the cup.

Up until this comment, your torque requirement seemed like it would be a ROM guess of 10#-in max say. IF the outer edge of the 1m disk is in contact with another metal surface all bets are off - the friction of THAT would be HUGE at that diameter and force you into a very large motor (10?20?hp). Remember that T=F*D, distance is 1m, force is coeff of friction of the two materials touching * force pushing them together....

 

[michael1965]

Sorry, I should have been clearer on my description, by spindle I mean a fixed axle or 'pin', one end of which will be attached to the motor, the end of which will be pointed and rest in a cup, and the disk fixed in the middle. There'll be no friction from any sources other than the motor, the pointed end of the pin in the metal cup (minimum contact) and wind resistance.

 

[OperaHouse]

How big is this disk you want to spin. I dismantled some old removible hard disk drives from the 60's. Remember when DEC was a powerhouse. That bearing assembly and belt drive would be ideal.

 

[LionelHutz]

If you desire easily variable speed then a permanent magnet motor and controller sounds the best. You'll have to check that the motor is OK standing on it's end and supporting this plate. Many are meant to mount horizonally only.

 

[imcjoek]

You don't really mention a budget or robustness requirement IE: What kind of environment will it operate in, for how long, and what are the consequences of failure?

You can be looking at anything from a $2000USD servo motor/controller or $10.99 Mabuchi with a $19.99 hobby-grade speed controller.

Without having a clue what you are doing with this thing:
I would look at a brushless hobby grade motor and controller. You could also drive your disc from it's OD with a rubber wheel. That takes care of speed reduction and let's you support the disc with proper bearings. This assumes your wheel runout is minimal though.

 

[michael1965]

Hi,

It's going to operate in a regular environment, it doesn't have to last forever as it's just for experimental purposes, and the consequences of failure would be having to buy another motor. As long as it can rotate between 100 and 600 rpm for an acceptable period of time without burning itself out (bearing in mind its load).

The information I need is: which motor, and what would be the power usage across the various speeds (say 100, 200, 300, 400, 500, and 600rpm).

 

[Skogsgurra]

There is no way we can tell you what power you need.

But you can find out all by yourself. Mount the disk the way it is supposed to run. Speed it up to your design speed with a small drill + rubber wheel. Let it spin and observe the time it needs to decelarate to still-stand.

Knowing the inertia will then make it easy to calculate the windage and other friction. That in turn will give you the size of the motor you need. Add 50% to compensate for the lower than rated mean speed.

If you can live with a less scientific approach, just select a 100 W motor. It may be overkill, but it will do the job. And we don't have to deal with incomplete questions.

Gunnar Englund

http://www.gke.org

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

 

[michael1965]

Thank you.