Maintenance of an old DC motor

Thread Starter

kalemaxon89

Joined Oct 12, 2022
259
I opened an old toy boat (battery-powered with an electric motor) that used to go round and round in the water.
It is about 20 years old, and I found it obviously in bad condition with rust and whatnot.

There were two 1.5V batteries which I removed, then connected the + and - of the electric motor to 3.3V coming from Arduino (I just need it to power ... I don't have a power supply at home). When connected, there are very small sparks in the + terminal and I can hear the noise of the electric motor that "should" move. Unfortunately it doesn't work, the little fan attached to the motor doesn't turn (you can't see it from the picture because it's behind).

I did the reverse, I turned with my hand the fan attached to the motor and with the multimeter I measured about 4.3mV.

WhatsApp Image 2024-04-14 at 10.13.00.jpeg

1) Does it not work because of the brushes inside the DC motor that have become encrusted?

2) Doesn't it work because of the low current the Arduino pin is supplying to it? (there were two 1.5V alkaline batteries in series ... I don't think they were providing that much more current than that pin so maybe that is not the problem)

3) What can I do at this point? I don't think that motor can be disassembled further so I would have to cut it in two to see inside the condition ... definitely better to buy another one and replace it, right? How do I know what specs the new one should have?

To repeat: the problem is that the DC motor axis does not move when powered with an Arduino pin (3.3V 20mA)
 

Ya’akov

Joined Jan 27, 2019
9,229
Even a toy motor like that one can draw great deal of current, more than amp, 20mA is not going to be enough to get that motor moving. It might keep it running, unloaded—but it surely won’t start it.

You need a different power supply, and you really, really don’t want to connect a messy inductive load like that motor directly to any pins on your Arduino—it could end in tears.
 

Jerry-Hat-Trick

Joined Aug 31, 2022
574
1. probably
2. Be careful not to destroy your Arduino! A PP3 9V battery is easy to connect briefly to the motor - it won’t destroy a 3V motor immediately and if the motor is effectively a short circuit the battery will get hot quickly so only try for a fraction of a second
3. It looks like a typical low voltage DC motor. Can you extract it from the boat without damaging the plastic? Lots of low cost 3V motors are for sale online, the length of the motor is not so important, the cross section needs to be the same or less because you can always pack it out, the shaft diameter ideally the same.
 

Ya’akov

Joined Jan 27, 2019
9,229
Cut your losses, look on eBay for a similarly shaped/sized 3v motor, they are quite common
3. It looks like a typical low voltage DC motor. Can you extract it from the boat without damaging the plastic? Lots of low cost 3V motors are for sale online, the length of the motor is not so important, the cross section needs to be the same or less because you can always pack it out, the shaft diameter ideally the same.
The motor might not need replacing. First, maybe it works just fine with sufficient current. If not, I would first see if it just needs cleaning and lubrication.

The front and rear bearings of the motor, which will almost certainly be sintered bronze bushings, may need a few drops (at most) of a light oil. The commutator of the rotor (the part that spins) might be corroded or simply tarnished. Very fine abrasive paper, a fiberglass brush, or even a green Scotch-Brite™ pad will fix that.

The motor is disassembled by carefully bending up the two tabs you will find symmetrically placed in the recesses on the back of the motor (in the green circle). You can use a small screwdriver to get under the end of the tab, and bend it enough that a fine pair of needle nosed pliers can finish the job.
1713086736353.png

Only bend the tab to be parallel to the motor. You are unlikely to break the tab off but the more it is bent the more the metal will suffer fatigue and eventually snap off. This is generally not an issue for the first time you take it apart if you are careful.

The back of the motor (black plastic) will pull out. Depending on the construction, you might damage the brushes that engage the commutator if you try to remove it without taking the rotor along. If that’s the case, you will have to remove the motor from the boat first, and remove the gear from the shaft so the rotor can be removed.

For that reason, you should probably do these things first. If you had more experience you‘d be able to try without the additional steps and do them if needed—but as a neophyte you are most likely to find out it is needed because you’ve damaged the brushes.

There are probably four contacts on the commutator which is at the rear of the rotor and will be brass. You can clean this but don’t use any thing that will make visible scratches in it because unlike carbon brushes the motor will use simple springy metal ones that don’t wear to conform to irregularities in the surface.

Clean by rotating, and around the commutator, not parallel to the rotor‘s axis. In a pinch, an old trick for cleaning is a pencil eraser, it can work well but be sure to remove all the rubbery particles that come off it from the motor parts.

A drop or two, not too much, of a light lubricant in the shaft at front and rear is a good idea. Not WD-40, which is not a lubricant*. Old school 3-in-1 oil or something more modern is fine, and very little is needed. In fact, it is probably best to tentatively reassemble the motor without bending the tabs back down and run if a bit to see if things are better. If so, take it apart again and wipe off any lubricant you can see.

The bronze bushings should be enough without additional lubricant, but they might have been gummy or dirty and running the motor with the lube in place can fix that. Reassemble again without bending the tabs, and test. If all is well, you can button it up—if not, you can repeat the lubrication and run it longer. The reason for bothering with this part is the tendency for lubricants to pick up dust and dirt leading to gummed up bearings.

*despite the fact that people constantly use WD-40 as a lubricant, the “WD” means “Water Displacement” and that’s what it is for. It is a very bad lubricant, and shouldn’t be used for one.
 

Thread Starter

kalemaxon89

Joined Oct 12, 2022
259
I should add that a AA cell can produce 500mA for a considerable period of time, and much more at peaks.
The 3.3v from the Arduino probably can't supply enough current to start the motor, esp if its caked up.

Cut your losses, look on eBay for a similarly shaped/sized 3v motor, they are quite common.
Regarding the current, you are absolutely right. In fact in any datasheet of 1.5V alkaline batteries the capacity comes to a few amps. I hadn't thought about it
 

Thread Starter

kalemaxon89

Joined Oct 12, 2022
259
The motor might not need replacing. First, maybe it works just fine with sufficient current. If not, I would first see if it just needs cleaning and lubrication.

The front and rear bearings of the motor, which will almost certainly be sintered bronze bushings, may need a few drops (at most) of a light oil. The commutator of the rotor (the part that spins) might be corroded or simply tarnished. Very fine abrasive paper, a fiberglass brush, or even a green Scotch-Brite™ pad will fix that.

The motor is disassembled by carefully bending up the two tabs you will find symmetrically placed in the recesses on the back of the motor (in the green circle). You can use a small screwdriver to get under the end of the tab, and bend it enough that a fine pair of needle nosed pliers can finish the job.

Only bend the tab to be parallel to the motor. You are unlikely to break the tab off but the more it is bent the more the metal will suffer fatigue and eventually snap off. This is generally not an issue for the first time you take it apart if you are careful.

The back of the motor (black plastic) will pull out. Depending on the construction, you might damage the brushes that engage the commutator if you try to remove it without taking the rotor along. If that’s the case, you will have to remove the motor from the boat first, and remove the gear from the shaft so the rotor can be removed.

For that reason, you should probably do these things first. If you had more experience you‘d be able to try without the additional steps and do them if needed—but as a neophyte you are most likely to find out it is needed because you’ve damaged the brushes.

There are probably four contacts on the commutator which is at the rear of the rotor and will be brass. You can clean this but don’t use any thing that will make visible scratches in it because unlike carbon brushes the motor will use simple springy metal ones that don’t wear to conform to irregularities in the surface.

Clean by rotating, and around the commutator, not parallel to the rotor‘s axis. In a pinch, an old trick for cleaning is a pencil eraser, it can work well but be sure to remove all the rubbery particles that come off it from the motor parts.

A drop or two, not too much, of a light lubricant in the shaft at front and rear is a good idea. Not WD-40, which is not a lubricant*. Old school 3-in-1 oil or something more modern is fine, and very little is needed. In fact, it is probably best to tentatively reassemble the motor without bending the tabs back down and run if a bit to see if things are better. If so, take it apart again and wipe off any lubricant you can see.

The bronze bushings should be enough without additional lubricant, but they might have been gummy or dirty and running the motor with the lube in place can fix that. Reassemble again without bending the tabs, and test. If all is well, you can button it up—if not, you can repeat the lubrication and run it longer. The reason for bothering with this part is the tendency for lubricants to pick up dust and dirt leading to gummed up bearings.

*despite the fact that people constantly use WD-40 as a lubricant, the “WD” means “Water Displacement” and that’s what it is for. It is a very bad lubricant, and shouldn’t be used for one.
Today at the university I will try to power it with a bench power supply by giving it 3V and a few amps. If that doesn't work then I will proceed by disassembling it.

I will keep you updated!
 

Ya’akov

Joined Jan 27, 2019
9,229
Today at the university I will try to power it with a bench power supply by giving it 3V and a few amps. If that doesn't work then I will proceed by disassembling it.

I will keep you updated!
Do be cautious—if the rotor is locked, you could burn up the windings. You can try spinning the motor to start it once you’ve applied the voltage, something I wouldn’t suggest if it was connected to an MCU pin.
 

Thread Starter

kalemaxon89

Joined Oct 12, 2022
259
Do be cautious—if the rotor is locked, you could burn up the windings. You can try spinning the motor to start it once you’ve applied the voltage, something I wouldn’t suggest if it was connected to an MCU pin.
The fact that it does not revolve indicates a locked rotor for some reason which has to be corrected first before applying power again.
I read the answer late.

With the bench power supply I set 3V and, one at a time, different current limits: 1A, 2A and finally 3A. The power supply provided a few watts of power, not more. It did not work for anyone. Moreover, this time I didn't even hear the little noise like stuck spatulas that with the uController pin could be heard.

I have no idea if the current I supplied is a lot, a little or too much because I have never dealt with electric motors.
Worst case scenario I broke it, but whatever.

Now I'm going to take it apart
 

Thread Starter

kalemaxon89

Joined Oct 12, 2022
259
The fact that it does not revolve indicates a locked rotor for some reason which has to be corrected first before applying power again.
Do be cautious—if the rotor is locked, you could burn up the windings. You can try spinning the motor to start it once you’ve applied the voltage, something I wouldn’t suggest if it was connected to an MCU pin.
WhatsApp Image 2024-04-15 at 18.43.14.jpeg

WhatsApp Image 2024-04-15 at 18.41.47 (2).jpeg

WhatsApp Image 2024-04-15 at 18.41.47 (1).jpeg

WhatsApp Image 2024-04-15 at 18.41.47.jpeg

At home I have silicone oil lubricant but you said it's not good, right?
 

Ya’akov

Joined Jan 27, 2019
9,229
At home I have silicone oil lubricant but you said it's not good, right?
Nice work. The brushes look a bit abused, but they are delicate so if you need to clean them do it very carefully.

There’s no problem with silicone based lubricants. Don’t get any on the commutator or brushes. If you do, clean it with isopropyl alcohol.

Did it spin with more current?
 

Thread Starter

kalemaxon89

Joined Oct 12, 2022
259
Nice work. The brushes look a bit abused, but they are delicate so if you need to clean them do it very carefully.

There’s no problem with silicone based lubricants. Don’t get any on the commutator or brushes. If you do, clean it with isopropyl alcohol.

Did it spin with more current?
What if the problem are the brushes? If I feed the motor with a certain voltage (when it is disassembled) .. with the multimeter I should be able to measure the same voltage at the ends of the brushes?
 

Ya’akov

Joined Jan 27, 2019
9,229
What if the problem are the brushes? If I feed the motor with a certain voltage (when it is disassembled) .. with the multimeter I should be able to measure the same voltage at the ends of the brushes?
I missed #11, sorry.

Much simpler is to use the resistance range on your meter. There should be practically no 0Ω from the terminal to the inner surface of the brush. You should also check the continuity of the winding on the rotor. There are two coils and each will be connected to opposite contacts of the commutator.

That is, pick a contact and call it 0°, the contact at 180° will be the other end of that winding, the one at 90° pairs with the one at 270°. Each pair should be connected together by the winding with some resistance, if not, one or both windings is open—this might actually be repairable. Even rewinding a small motor like that isn’t very hard if it comes to that.

As I said, though, the photo of the brushes makes me suspect they have been damaged. You may be able to remove the oxides and adjust them towards the rotor so they make better contact.
 

Thread Starter

kalemaxon89

Joined Oct 12, 2022
259
Much simpler is to use the resistance range on your meter. There should be practically no 0Ω from the terminal to the inner surface of the brush
Let's start in order by measuring if there is a shortcircuit in the brushes, using the multimeter.

1) Are you asking me to measure the resistance between the blue brush and the red brush? (look at this picture below)1713791499431.png
2) If the answer is yes, can you point me to where to point the two tips of the multimeter?
Between A and B? A and C? etc.
1713791632058.png
I don't understand the two brushes what they are, I see 3 brushes
 

Ya’akov

Joined Jan 27, 2019
9,229
A and B are common to each other. I want you to measure from A,B to where the red wire is connected, and from C to where the black wire connects. That is, to the terminals embedded in the plastic block.
 

KeithWalker

Joined Jul 10, 2017
3,135
There are only two split brushes. A and B are part of the same brush. They are bent out of shape. Before you re-assemble the motor, bend the metal brushes carefully until A and B are close together (like the two parts of C)and then adjust them to make A+B and C symmetrical with each other and nearly touching at the tips.
It is going to be difficult to get the back on the motor without bending the brushes out of shape again. The easiest way would be to push the shaft far enough out that the brushes can be aligned on the commutator, before moving the back and shaft into position together. To do that, you need access to the drive end of the motor shaft. What is connected to it? Does it have a gear on the shaft? Is there enough distance between it and the motor body that the shaft can be pushed in to give enough clearance at the commutator end to get the brushes re-aligned?
 
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Thread Starter

kalemaxon89

Joined Oct 12, 2022
259
A and B are common to each other. I want you to measure from A,B to where the red wire is connected, and from C to where the black wire connects. That is, to the terminals embedded in the plastic block.
I measured it.

In both cases (from A,B to the red wire and from C to the black wire) with the multimeter set to "200 scale" resistance, I measured a resistance that varies from 4ohm to 15ohm ... it fluctuated a lot because the hand shakes and the brushes are not fixed but bend at the slightest touch with the tip
 
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