☠ Heat Damage to Motors (Magnets) ☠

DrkAngel

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Everyone has heard about "burning up a motor", typically this involves enough heat to "cook" the insulation on the windings or various shorts or arcing due to abrasion, vibration or excessive voltage etc.

The lesser talked about heat damage seems to be how temperature affects permanent magnets!
While "burning up" a motor might involve 300-400ºF +, magnet damage occurs at much lower temperatures!
Exemplified by a simple test, a motors permanent magnets "strength" deteriorates quickly at temperature above 176ºF and seems to totally disappear at 212ºF !
Prolonged and/or higher temperature can make deterioration permanent!

Magnet Experiments: What Happens When a Magnet is Heated

This entry was posted on June 10, 2014 by Apex Magnets.

Magnets can be found in many everyday items and technologies such as cars, phones and computers. It is because of permanent magnets' ability to create their own magnetic field that they are useful in various products and situations. However, they are not impervious. Magnet strength can be affected by certain environmental changes like temperature. The effect of temperature on neodymium magnets is one of the most interesting phenomenons to observe and evaluate. In this magnet experiment, we specifically explore how magnets react when exposed to extreme heat.

Safety Caution: Because this experiment involves potentially dangerous high temperatures and magnets, it is not intended for children and should not be conducted without the proper safety wear.
Supplies

Thermometer (212°F or 100°C)
Plastic tongs
2 Bar neodymium magnets
Safety goggles and gloves
Water
Stove
Pan
Plastic bowl
100 ferrous paper clips

Steps
Part 1: Room Temperature Test


Do a room temperature magnet test first. Pour the paper clips into a plastic bowl.
Submerse one of the neodymium bar magnets into the bowl of clips and remove it, recording the number collected.
Remove the paper clips from the magnet and set it aside. Replace the paper clips in the bowl.

Part 2: Hot Magnet Test

Hot water and metals can cause serious burns, so it is important to take necessary safety measures. Put on safety goggles and gloves.
Heat about ¾ cup water in a small pan until it reaches somewhere between 185°-212°F or 85°-100°C. At boiling point the water should be close to or within this temperature range. Use your thermometer to check that the degree is appropriate.
Using the plastic tongs, gently place the neodymium magnet in the water. Be extremely careful to avoid splashing the hot water.
Let the magnet heat in the water for about 15 minutes.
Carefully remove the magnet from the water with the plastic tongs and place it in the bowl of paper clips.
Observe and record how many paper clips are collected.
Wait until the magnet is fully cooled before attempting to handle it without tongs.

Results

The heated magnet will not pick up the paper clips, or it will pick up very few depending on the temperature and time it was heated. When heated above 176° Fahrenheit (80° Celsius), magnets will quickly lose their magnetic properties. The magnet will become permanently demagnetized if exposed to these temperatures for a certain length of time or heated at a significantly higher temperature (Curie temperature).

Heat demagnetization is also dependent on what types of materials make up a magnet. Some types of magnets such as Samarium-cobalt (SmCo) have higher heat resistance. There are also other types of Neodymium-iron-boron (NdFeB) magnets that are not as susceptible to heat induced flux degradation.
The Science Behind It

Magnets are made up of atoms. In normal environmental conditions, these atoms align between the poles and foster magnetism. When exposed to hotter conditions, the particles within the magnet are moving at an increasingly faster and sporadic rate. This jumbling confuses and misaligns the particles, causing the magnetism to be lost.

This indicates that motors are damaged by much less drastic "abuse" than previously advertised.
While the damage from temperature is not as drastic, it seems to be cumulative and progressive.

Each damaging incident will deteriorate magnets to some varying degree.

The insidious effect is that as the magnets lose their magnetism, achieving the same power will require more amps which will in turn produce more heat which will in turn cause more damage! Causing progressive and accelerating damage!
Battery range and performance will decrease ... when the motor might actually be responsible!
The motor becomes "worn out" ... going out with a whimper rather than a bang!

What to do?
Learn from your mistakes, on your next motors?
 
This is very old news, not sure why it is coming up now. It doesn't seem to occur very often, but it is possible. Burning windings is much more common than demagnetizing.

Insulation and Magnets have temperature specifications and ratings. Some are much higher than others. The quality and cost of the motor will affect the choices of the components they use in the construction.

The temperature at the windings (which are the source of the heat due to motor current squared times wire resistance) is much higher than out at the magnets.

If the magnets are damaged it reduces their field, which changes the KV of the motor, increasing the RPM per Volt and lowering the Torque per Amp. So it increases speed and reduces torque. Depending on the load this might be good, or it might be bad. If you measure the unloaded speed you can see the change, it will increase.
 
From what I remember from my old contest electric glider flying days, Bob Boucher of Astroflight purposely use Samarium Cobalt magnets (and I believe still does) for his electric motors. He knew that even thought they had a little less magnetic force, they could withstand heat far better than Neodyme magnets.
We used to fly in Eurostyle contests with the motor pulling 50 amps for about 30 seconds at a time, and those motors survived. They were far too hot to touch when tested at these amps.
Just sayin'.
otherDoc
 
I know, thermal transfer to magnets in hub motors is a minor concern ... large heatsink (motor casing and spokes) and open ventilation provide excellent cooling.

Most of my builds use sealed brush motors, so thermal deterioration in magnets is much more of a concern, for me.

With multiple motors @ 10,000 miles each, I have noticed a marked decline in performance over their lifespan.
Most noticeably after I loaned an eBike to someone who "wants to test it on some hills" ... big mistake.

With these motors I presumed that performance decrease was due to brush-communicator wear/alignment.
But the cumulative thermal abuse now seems the reasonable culprit.
 
Alan B said:
The temperature at the windings (which are the source of the heat due to motor current squared times wire resistance) is much higher than out at the magnets.

I wonder how the use of ferrofluid to bridge the stator/magnet gap changes this. Surely they would now be closer to equilibrium. Maybe ferrofluid can even shorten the life of a motor in certain instances.
 
Alan B said:
Some setups still using brushed motors, amazing.
2008 Snow Beast 24V 450w pumped to ~700w @ 37V and geared for 20mph, studded tires ~6000 miles on motor (50% Winter miles)
2008 eZip Trailz @ 11,000miles+ 24V 450w pumped to 25.9V 485w and geared for 20mph cruising
2009 eZip Trailz LS geared for 14mph, 25.9V 485w 12,000 miles +++ (rebuilt for large person)
Some newer too ...

Picked up supply of 24V 350w brush gear reduction motors for trike projects etc. ~$40 each ...
 
I have a supply of Neodymium magnet 2x8mm disks

Be interesting to test:
Match up several identical magnets, confirm with paper clip pick-up count.
Test for temperature deterioration in 212ºF boiling water for various times.
unheated as baseline-control
1 minute
4 minutes
15 minutes
1 hour
4 hours
etc.
magnets removed and cooled before testing >> paper clip pick-up count.

212ºF might be higher than necessary to deteriorate magnetism but is the only really simple-cheap way to maintain exact temperature.
 
Have you compared the efficiency loss of brushed motors vs Brushless? I see differences of about 10% in efficiency quoted in various places, but nothing precise. Over the lifetime of a motor the loss in efficiency accumulates to a huge total energy loss cost (if the motor is used much). This far exceeds the cost savings of the original motor, so the net savings is most likely a loss in reality.
 
DrkAngel said:
212ºF might be higher than necessary to deteriorate magnetism but is the only really simple-cheap way to maintain exact voltage.
Voltage?
 
amberwolf said:
DrkAngel said:
212ºF might be higher than necessary to deteriorate magnetism but is the only really simple-cheap way to maintain exact [strike]voltage[/strike] temperature.
Voltage?
Sorry, meant temperature ... must have been subconsciously developing a method of enhancing button magnets as battery nipples via electroplating! ... ?
 
For quite a few years, the general ES rule-of-thumb has been to never allow the hottest part of the motor to get up to 200F / 93C

Of course, if your motor is getting that hot, I personally believe you are wasting battery watts generating heat, because your motor is too small (or the volts are too low) for the job you are giving it. For a good system design, 140F / 60C is a much more reasonable continuous condition. I seem to remember that Astro motors all use Samarium magnets for their heat resistance...

Cheap Halls failing
Cheap winding enamel failing (causing a short)
Cheap solder failing on a joint that was already a weak connection from the factory
Cheap neo magnets slowly loosing their power
 
.
...
Test #1
DrkAngel said:
I have a supply of Neodymium magnet 2x8mm disks

Be interesting to test:
Match up several identical magnets, confirm with paper clip pick-up count.
Test for temperature deterioration in 212ºF boiling water for various times.
unheated as baseline-control
1 minute
4 minutes
15 minutes
1 hour:
4 hours
etc.
magnets removed and cooled before testing >> paper clip pick-up count.

212ºF might be higher than necessary to deteriorate magnetism but is the only really simple-cheap way to maintain exact temperature.
Test #2
Paper clip count method of magnet strength testing opens up alternate tests possibilities.
Magnet strength at various temperature
I will test comparative magnet strength at various temperatures:
110ºF
90ºF
70ºF -- base line
50ºF
30ºF
10ºF
etc
Battery is typically blamed for poor cold weather performance. I would like to determine if motor (magnet) temperature is an additional factor.
 
Please. Back on (rare) earth guys. There is no need to play with paper clips and boiling water. The demagnetisation curve of all magnets are well documented. Each type and grade of magnets are spec with maximum operating temperature and curie temperature.
 
DrkAngel said:
Test #2
Paper clip count method of magnet strength testing opens up alternate tests possibilities.
Magnet strength at various temperature
I will test comparative magnet strength at various temperatures:
110ºF
90ºF
70ºF - 19 paper clips-- base line
50ºF
30ºF - 19 paper clips
10ºF
etc
Battery is typically blamed for poor cold weather performance. I would like to determine if motor (magnet) temperature is an additional factor.
Cold shows no appreciable magnet strength loss ... 70ºF - 30ºF both pick-up 19 paper clips (2x8mm Neodymium disk).
 
DrkAngel said:
Cold shows no appreciable magnet strength loss ... 70ºF - 30ºF both pick-up 19 paper clips (2x8mm Neodymium disk).

See " If you start at room temperature (20°C) and get colder, the strength of the magnet increases by a small amount. Below about -125°C, the strength then drops much more quickly."(near bottom of page)

The whole article is worth reading. The two points I found very interesting are:

- The lost of magnetism with heat depends on the shape of the magnet.
- Below about 1000C, the loss of magnetism can be restored.


Also, a perhaps more accurate DIY test of magnetic strength is to use a book and see through how many pages the magnet can manipulate a paper clip or small nut.
 
Because of the many types and grades of permanent magnets, they can only be identified using a gauss meter. However, magnets are manufactured under patent. Some are punched with type and rating, and motor manufacturers usually spec the magnets that they are using.

Max operating temperature is the point when a magnet start to be damaged by heat.

Curie temperature is the point when a magnet is irreversibly demagnetized. Of course, like any ferrous metal, it can be magnetized again but this state of magnetization will be temporary.
 
There's a wide variation in magnet quality. Use cheap crap moderate efficiency motors and you get what you paid for, even if the price wasn't low. ie Follow the crowd and that's what you get, whether it's pushing current too high with heat growing by the square of current, or buying the same generic parts motors the crowd buys. To add insult to injury, go ahead and put your hubbie in big diameter bicycle wheels too.

In the meantime the fat guy in the jungle has ebikes with far greater power and performance seeing no motor degradation in over 8 years, and now that I've switched to automotive grade batteries I don't see battery degradation of significance either...All while spending less than anyone for high performance.
 
You're right that shape of the magnet is a huge factor.

The other huge factor is being against some non-saturated back iron. This is because well placed back iron relieves a huge amount of the stresses trying to relax the magnets field.

Just the no-load speed is enough to see if you're partial de-maged.

It happens occasionally but it's very rare, and I've personally not done it outside of tiny hobby motors.



Buk___ said:
DrkAngel said:
Cold shows no appreciable magnet strength loss ... 70ºF - 30ºF both pick-up 19 paper clips (2x8mm Neodymium disk).

See " If you start at room temperature (20°C) and get colder, the strength of the magnet increases by a small amount. Below about -125°C, the strength then drops much more quickly."(near bottom of page)

The whole article is worth reading. The two points I found very interesting are:

- The lost of magnetism with heat depends on the shape of the magnet.
- Below about 1000C, the loss of magnetism can be restored.


Also, a perhaps more accurate DIY test of magnetic strength is to use a book and see through how many pages the magnet can manipulate a paper clip or small nut.
 
Better link to referanced article - Measuring Magnetism

A very simple method for measuring magnet strength (field strength) is difficulty in turning motor!
Of course, you will need an identical comparison motor or recorded stats.
EG. Hub motor - rolling distance from specific speed on level surface till stop. average of opposing directions, (all factors identical - weight, wind, tire pressure etc.)

With my eZip gear reduction motors, simple test is difficulty in pushing bike backwards.
(Something I have noticed, in the past, but never quite directly equated to motor magnet field strength)
Defeats freewheel function and the ~8:1 rpm amplification demonstrates field strength nicely.
Identical motors with diminished magnet strength will turn (push) more easily!

Tested method on multiple eZips and confirmed vs suspected "worn out" motors!
 
When built with higher grades of permanent magnets, a motor will never suffer any demagnetization, simply because the magnets max operating temp is higher than the copper winding heat tolerance. So you will fry the motor before any damage can occur to magnets.
 
This was a very important parameter for us in the aerospace industry. We often baked the magnets at various temperatures for various times and developed a matrix of their specific demagnetization characteristics so that we could keep our systems performance up to par for decades in space.

Outside of the aerospace environment I have dealt with this in my personal hardware. The first few times it was baffling, as I thought Honda or Tecumseh surely could not have designed these motors this poorly, but they did! Saw this phenomena on the permanent magnet starter motors for large lawn equipment, and the electronic gear shift motor on a Honda Foreman ATV.

As said upthread, we often choose the higher curie temperature magnet over higher strength... interesting our choices always cost more! :lol:
 
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