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Do DC permanent magnet motors get old and weak?

 

Not specifically a Unimat question, but some people here seem to know a heck of a lot about brushed, DC motors. Do these permanent magnets get tired after a while? I scavenged one from a lawnmower labeled 24VDC/500 W/4300 rpm. When I hook it up directly to the 24V battery pack it only gets up to maybe 300 rpm. I replaced the brushes and made sure the bearings spun nicely and the battery is fully charged. Am I missing something, or is it junk?

Tim in Vermont
 

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Hi Tim,

I¡¯m not sure it applies to older DC motors but I do know that you can¡¯t take a stepper motor apart without damaging the magnetic field.? The structure around the armature serves as a ¡®keeper¡¯ for the magnet.? Remember the old horsehoe magnets also required a small bar across the end when the magnet was stored.

So it¡¯s quite possible on a newer lawnmower DC motor that the motor is designed to not be taken apart.

John

?

From: [email protected] [mailto:[email protected]] On Behalf Of vt_biketim
Sent: April 29, 2024 4:43 PM
To: [email protected]
Subject: [Unimat] Do DC permanent magnet motors get old and weak?

?

Not specifically a Unimat question, but some people here seem to know a heck of a lot about brushed, DC motors. Do these permanent magnets get tired after a while? I scavenged one from a lawnmower labeled 24VDC/500 W/4300 rpm. When I hook it up directly to the 24V battery pack it only gets up to maybe 300 rpm. I replaced the brushes and made sure the bearings spun nicely and the battery is fully charged. Am I missing something, or is it junk?

Tim in Vermont

 

DC motors wear out. Think of car starters and even back further to car generators. They had to be periodically taken apart and the armature turned down and new brushes installed. Most of the new motors are throw aways, can not be rebuilt or if you can get them apart, no parts available.

?Jeff

On Monday, April 29, 2024 at 07:59:20 PM EDT, John Dammeyer <johnd@...> wrote:


Hi Tim,

I¡¯m not sure it applies to older DC motors but I do know that you can¡¯t take a stepper motor apart without damaging the magnetic field.? The structure around the armature serves as a ¡®keeper¡¯ for the magnet.? Remember the old horsehoe magnets also required a small bar across the end when the magnet was stored.

So it¡¯s quite possible on a newer lawnmower DC motor that the motor is designed to not be taken apart.

John

?

From: [email protected] [mailto:[email protected]] On Behalf Of vt_biketim
Sent: April 29, 2024 4:43 PM
To: [email protected]
Subject: [Unimat] Do DC permanent magnet motors get old and weak?

?

Not specifically a Unimat question, but some people here seem to know a heck of a lot about brushed, DC motors. Do these permanent magnets get tired after a while? I scavenged one from a lawnmower labeled 24VDC/500 W/4300 rpm. When I hook it up directly to the 24V battery pack it only gets up to maybe 300 rpm. I replaced the brushes and made sure the bearings spun nicely and the battery is fully charged. Am I missing something, or is it junk?

Tim in Vermont

 

In the model railroading world, permanent magnet DC motors definitely have problems with magnets weakening.? The result is that the motor loses power runs hotter.

Some have put in new magnets which are often a stack of smaller magnets since it's hard to find a solid one-piece magnet the correct size.

This is a typical in the older open frame motors that look like this.:

Inline image

And this is one where a number of small magnets have replaced the one large one which probably got weak.

Inline image

Charles E. "Chuck" Kinzer



On Monday, April 29, 2024 at 04:43:05 PM PDT, vt_biketim <tchock59@...> wrote:


Not specifically a Unimat question, but some people here seem to know a heck of a lot about brushed, DC motors. Do these permanent magnets get tired after a while? I scavenged one from a lawnmower labeled 24VDC/500 W/4300 rpm. When I hook it up directly to the 24V battery pack it only gets up to maybe 300 rpm. I replaced the brushes and made sure the bearings spun nicely and the battery is fully charged. Am I missing something, or is it junk?

Tim in Vermont

 

Is your battery capable of providing 500 watts of power? Is your wiring capable of carrying nearly 21amps of current. That's what you're asking your power supply for.?

I have one or two supplies that can handle that much power, and a bunch that cannot. If the supply is rated for 24vdc, at 500 watts, it should have a bit over 20833ma or nearly 21 amps of current available.? If if it does, check your wiring and connections. Should be a minimum 12 AWG wire, per this calculator:? https://www.calctool.org/electronics/24v-wire-size

Bill in OKC?

William R. Meyers, MSgt, USAF(Ret.)

Aphorisms to live by:
Good judgement comes from experience. Experience comes from bad judgement.?
SEMPER GUMBY!
Improvise, Adapt, and Overcome.
Physics doesn't care about your schedule.
The only reason I know anything is because I've done it wrong enough times to START to know better.
Expect in one hand, expectorate in the other. See which one gets full first.



On Monday, April 29, 2024 at 06:43:04 PM CDT, vt_biketim <tchock59@...> wrote:


Not specifically a Unimat question, but some people here seem to know a heck of a lot about brushed, DC motors. Do these permanent magnets get tired after a while? I scavenged one from a lawnmower labeled 24VDC/500 W/4300 rpm. When I hook it up directly to the 24V battery pack it only gets up to maybe 300 rpm. I replaced the brushes and made sure the bearings spun nicely and the battery is fully charged. Am I missing something, or is it junk?

Tim in Vermont

 

Hi Tim if it has ceramic magnets they will loose their strength due to heat. If you find the motor runs hot in a short period of time they have possibly lost their magnetism. You might be able to find a magnito zapper to re energise them. I have one but I'm in Brisbane Australia.
 

Good points, Bill. The Neuton mower was a simple arrangement of battery pack, switch/relay board, and motor. After removing the pieces from the mower carcass I simply charged up the battery pack and touched the leads to the terminals. All original wiring and the original battery, just bypassed the switch and relay. I do think the reason for taking it out of service was a slow output speed.
 

Yes, and no, and yes, and so on.

Traditional magnets used in DC motors will lose their magnetism eventually. They can be re-magnetised, and sometimes you can get modern replacements. Modern rare earth magnets - neodymium etc - don't but if they get up to 150¡ã C they lose their magnetism and they don't recover when they cool down. Your lawnmower motor has probably lost its magnetism, and if you tried to get power out of it it would just get very hot. As noted if you wanted full power it would take a lot of current, but a couple of car batteries would be up to the job. With the motor idling it shouldn't draw more then an amp or so, and you should easily get full speed.?

At any rate I would say the motor has had it, and probably not worth attempting to fix it.
 

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Check for carbon packed between commutator? segments or shorted or open windings, you need a low reading ohm meter for an accurate check.? Perhaps run it on about 6 volts and load it down and check for dead or weak spots in the rotation.? Dead spots are usually open or shorted windings.

On 4/29/24 17:43, vt_biketim wrote:

Not specifically a Unimat question, but some people here seem to know a heck of a lot about brushed, DC motors. Do these permanent magnets get tired after a while? I scavenged one from a lawnmower labeled 24VDC/500 W/4300 rpm. When I hook it up directly to the 24V battery pack it only gets up to maybe 300 rpm. I replaced the brushes and made sure the bearings spun nicely and the battery is fully charged. Am I missing something, or is it junk?

Tim in Vermont

 

Another check is check for short circuit from the windings to the shaft of the armature. Put your ohm or mega meter from the commutator segments to the shaft there should be no reading.

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