Author Topic: Engineering question, safe rpm?  (Read 4358 times)

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TDC

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Engineering question, safe rpm?
« on: March 03, 2015, 09:32:14 PM »
I'm getting concerned about the safety of my magnet rotors. While unlikely, the thought of a big ceramic magnet hitting a child on the head is horrible.  I would be grateful if an ME or anyone else can provide some insight.
 
The rotor should run up to something like 1800 rpm with the chain drive I'm building.  The rotor is 300mm diameter with 50mm square magnets that weigh .349 kg.  There is a  31mm x 1.24mm stainless steel band around the perimeter and epoxy fills the void between magnets and the band.  Using an online calculator, I came up with 160 kg or 354 lbs. force.  Here's a spec sheet for the epoxy and a picture of my rotor.    http://www.devcon.com/prodfiles/pdfs/fam_tds_108.pdf http://www.fieldlines.com/index.php/topic,148618.0.html       







midwoud1

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Re: Engineering question, safe rpm?
« Reply #1 on: March 04, 2015, 06:29:02 AM »
 Windgenerator designs with ferrite magnets :

  Chris Olsen : http://www.anotherpower.com/board/index.php?topic=79.120

  Hugh Piggott : http://rurerg.net/2013/10/02/august-2013-ferrite-magnet-generator-for-a-2-4m-rotor-swt-for-battery-charging/

  Rgds . F .       midwoud1

midwoud1

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Re: Engineering question, safe rpm?
« Reply #2 on: March 04, 2015, 06:36:41 AM »

                         http://scoraigwind.co.uk/category/ferrite-magnets/
         

lifer

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Re: Engineering question, safe rpm?
« Reply #3 on: March 04, 2015, 08:35:33 AM »
A safer solution is to make a cage/case around the alternator, to "collect" any presumable flying objects.

SparWeb

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Re: Engineering question, safe rpm?
« Reply #4 on: March 05, 2015, 01:48:46 AM »
Without looking up the formulas, your numbers look close enough.
Epoxy is designed to work in shear:  Two surfaces sliding past each other, but restrained from motion by the bond between them.
Ignoring the steel band for a moment...

(50mm) * (50mm) = 2500 square mm = 3.88 sq. inches, contact area per magnet

If each magnet is trying to fly away with a force of 354 pounds, and the contact surface is 3.88 sq. inches, then the shear stress is 354/3.88 = 92 pounds per square inch

Most epoxies I know are good for thousands of psi.  When wet, that drops to many hundreds of psi, but still safe for you.

With the steel band, no problem at all.

Just bear in mind that the typical failure mode of an axial-flux alternator isn't "just" the magnets flying off - it's overheating, which leads to stator warping, which leads to rubbing of the rotor on the stator.  That's when the magnets are most likely to fly off.

Hey, you asked for an engineer's opinion!
 ;)
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
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MattM

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Re: Engineering question, safe rpm?
« Reply #5 on: March 05, 2015, 05:50:39 AM »
If only they made sheets of teflon to wrap the stator and magnetic disks.

You could then be in contact surface to surface with little fear of failure.  Plus the teflon is made for far higher temperatures than the magnets and it's acts like a great incompressible lubricant.

Mary B

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Re: Engineering question, safe rpm?
« Reply #6 on: March 05, 2015, 08:54:06 PM »
Telfon sheet is readily available but you don't want to know the price!

dnix71

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Re: Engineering question, safe rpm?
« Reply #7 on: March 05, 2015, 09:23:36 PM »
Nylon rubbing blocks have better wear ability than teflon. If you expect failure from warping, then have nylon blocks that stand up above the magnets and add squealers to the stator.

On automobile disk brakes there is a piece of metal near the base of the wear pad. When the pad gets low, it makes a nasty scraping noise against the rotor so you will know it's time to change the pads.

MattM

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Re: Engineering question, safe rpm?
« Reply #8 on: March 05, 2015, 11:21:08 PM »
I wonder how one nylon stacks against another.  They make patio furniture guides up to 1-1/2" internal diameter.  Probably just the right size for some magnets.

I know Teflon skid pads used under machinery are damn near indestructible and come in a variety of thicknesses.  5mm thick pads can handle several tons on them without a hint of complaint.  The trick is to place the pads between smooth surfaces.  Two Teflon pads together act as very low friction yet dry journal bearings.

Does nylon have this trait?

SparWeb

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Re: Engineering question, safe rpm?
« Reply #9 on: March 06, 2015, 01:22:13 AM »
I would prefer to make a stator that resists overheating, and leave enough air gap that if it does warp, it's less likely to touch the rotors.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
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TDC

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Re: Engineering question, safe rpm?
« Reply #10 on: March 06, 2015, 01:41:57 PM »
Without looking up the formulas, your numbers look close enough.

Most epoxies I know are good for thousands of psi.  When wet, that drops to many hundreds of psi, but still safe for you.

Thanks SW. I wanted to be sure I hadn't made a gross miscalculation or overlooked something. I've seen the destruction centrifugal force can cause, including a former coworker's x-rays, after he got out of the hospital.

I must confess, I don't understand your comment about the strength dropping when wet.  Water affects epoxy strength?

SparWeb

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Re: Engineering question, safe rpm?
« Reply #11 on: March 06, 2015, 06:11:19 PM »
Yes, humidity and heat both degrade the strength of epoxy.
The way axial-flux alternators are typically made, two different aspects the epoxy strength is relied upon.

One is the bond strength, that adheres the parts together.  This is especially vulnerable to moisture.  The epoxy bonding magnets to the steel rotor must not be contaminated, and the outer surface protected with paint.  Epoxy allows moisture to enter when exposed to the air.

The other is material strength - the strength of the block of epoxy itself is reduced by heat.  Some people still cast the stator windings in epoxy, and the rotors often are cast in epoxy too.  The thickened epoxy "cake" will soften when it gets hot, and this is what leads to the warping.  The heat is never evenly applied.  Most people don't use plain epoxy for stators any more, and this is why.

It might not be obvious, but when the alternator is running, the rotors do heat up.  Wind airflow helps cool them somewhat.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
www.sparweb.ca