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[DPRG] Motor Calculations

Subject: [DPRG] Motor Calculations
From: Rick Bickle rbickle at intconsys.com
Date: Wed Aug 29 10:10:47 CDT 2007


Thanks for the info. The bearings that I'm using are tapered roller
bearings. In order to reduce cost, I am using heavy duty trailer stub
axles with hubs. 

I think I have misstated the speed. The speed of rotation should be 30
RPM. I must have been thinking in terms of seconds.

The system will rarely ever make a full revolution. It will do a lot of
starting and stopping though.


-----Original Message-----
From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org] On
Behalf Of Chuck McManis
Sent: Wednesday, August 29, 2007 12:44 AM
To: Rick Bickle; DPRGlist at dprg.org
Subject: Re: [DPRG] Motor Calculations

Hi Rick,

More questions than answers but if you get a bit more data the answer
would fall out of the calculations so to speak.

>1. Will need to rotate a weight of about 600 lbs. evenly distributed 
>with a  maximum distance from center of 2 feet.

So 600 lbs is roughly 272 Kg of mass,

    First you need to know what sort of bearing this weight is on,
    it will contribute potentially significant friction into the

    Next you will need to know at what speed you will be rotating
    this mass. Given its 272 Kg it is going to be carrying a *lot*
    of angular momentum.

    Finally you will want to know what other loads (besides bearing
    friction and wind resistance) are going to be applied to the

The reason these other bits of information are important is that if the
bearing is very low friction then you can start the thing spinning with
a pager motor, if its on an axle with a steel on steel oiled bearing you
may find you need a bit more starting torque.

>2. The motor will need to rotate a weight of about 800 lbs. evenly 
>distributed with a maximum distance from center of 4 feet.

Same questions.

>The motors should be strong enough to get up to a maximum speed of 2 
>rpm within 1 second.

Ok, this gives us almost enough information (actually it is if we assume
a frictionless bearing and no other resistances). The formula for
angular momentum is Iw (Moment of Inertia * angular velocity), if you
calculate the angular momentum of your 600lb disk, its approximately
1/2MR^2 (M = Mass, R = Radius) so keeping everything in SI units for
your first disk, 272 Kg * your angular velocity is 720 degrees / minute
or 12 degrees/sec and your radius is .61 meters.
So about 608 degree*Kg*m^2/sec. That's about 608 joules or 608
watt-seconds. Since a 1 hp motor is about 750 watts, that means for your
600lb disk you can use a 1HP motor to get it spinning at 2RPM (ignoring
bearing friction and other factors), you'll probably want something
better than that.

For your 800lb disk with the 4' radius your looking at more than 3200
wattseconds so something bigger than a 4.4HP motor.

So in the first case I'd start with a 1.5HP motor (unless you know the
bearing is high friction) and in the second case a 5HP motor.


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