[Stoves] Calculating the RPM

Crispin Pemberton-Pigott crispinpigott at gmail.com
Tue Feb 12 13:15:09 CST 2013


Dear Frank

 

The motor will not slow down under no load if you drop the voltage because
it is a synchronous motor. You would have to change the frequency of the AC
to change the speed.

 

That said, if it was lowered enough the motor couldn't 'keep up' to the 60
cycle synchronous rotation and would slip, slowing down as a result, however
that slippage is strongly dependent on the load which would be really
inconvenient. Being a square box the speed would actually change 4 times per
rotation as the load alternately came and went. 

 

I suspect that the motor is rated at 1725 RPM under full load not actually
'all the time'. Under no load it might be very close to 1800 (60 cycles per
second * 60 seconds per minute / 2 sets of windings gives 1800).

 

As it is geared down so much I suspect the motor will not be working hard at
all. Given that you want about a 30:1 reduction and there is a general rule
that you should not step down more than 7:1 at a time, I suggest you get 2
sets of 1-1/2" and 8" pulleys in series which will give you 28.4:1.

 

If your speed is 1725, the shaft will turn at 60.64 under load. That is
pretty close. It might go a tiny bit faster but the pulleys are not all that
accurate anyway so you would have to measure it to know. It is slow enough
you can count while watching.

 

With such an installation you should use Goodyear belts (the ones that are
fan belts but look like they have big teeth). If you drop the voltage it
will only have to slip a little (magnetically speaking) in order to drop one
or two RPM but it will fluctuate a little during the turn because the box is
square.

 

I have a book here for calculating how much power can be transmitted through
the belt contact angle on the small pulley based on their centre distances,
if it matters.  I don't think it will. Pick a centre distance that matches a
common belt.

 

Regards

Crispin

 

 

Dear Frank

 

1: Box shape and diameter are VERY important.  At "critical speed", for a
given combination of rotational speed and container diameter, centrifugal
force will hold the pellets stationary against the container wall, and there
will be zero tumbling and pellet wear. A machine running at say 95% critical
speed will create very little abrasion, but a machine running at say 60% of
critical Speed will create a great deal of abrasion effect, because of the
cascading action.

 

2: Do you want to turn the drum/container/box for a fixed time, OR a fixed
number of revolutions?

 

3: One could calculate the required HP, but the simplest and least costly
thing is to try the 1/3 HP motor, and if it does not overheat after say 4
consecutive tests, it is good for the job. If it does overheat, then you
have proven that you need a bigger motor.

 

Best wishes,

 

Kevin

----- Original Message ----- 

From: Frank Shields <mailto:frank at compostlab.com>  

To: 'Discussion of biomass cooking stoves'
<mailto:stoves at lists.bioenergylists.org>  

Sent: Tuesday, February 12, 2013 2:30 PM

Subject: [Stoves] Calculating the RPM

 

Greetings Stovers,

 

Building equipment for determining the hardness of pellets and biochar I
need something to turn a 9 kg box at a speed of 60 RPM +/-  1.

I have a 1/3rd HP motor that is rated at 1725 RPM

Fitted with a 2" pulley attached to the motor and going to an 8" pulley
gives me a shaft turning 431 RPM

Then another 2" pulley going to an 8" pulley on another shaft gives me 108
RPM

I plan to adjust the 108 down to 60 using a voltage regulator. 

 

 

Is this a good plan? 

And is there another factor I need to include for the weight of the box I am
turning?

 

Thanks

 

Frank

 

 

Frank Shields

Control Laboratories, Inc.

42 Hangar Way

Watsonville, CA  95076

(831) 724-5422 tel

(831) 724-3188 fax

www.controllabs.com

 

 


  _____  


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