DAK the science or reason for this....?

[Maroochydore]

A toy bubble machine. Powered by 3 x 1.2v batteries.

Toy stops working so before it is thrown (only a fiver) I thought I'd take a look.

Multi-metered through the connections, no issues so decided to open up the motor casing which was plastic and warm to touch.

Went to lift the motor - a diddly thing about 1 inch long - and burnt my thumb!

Put the multi-meter temperature probe on the metal motor casing and it read 55 degC!

So, would a 3.6V DC (approx) generate enough power to heat the motor - not under power - and why would it do so?

Any theories or answers greatly received for my knowledge bank.

[GrahamPlatt]

So, would a 3.6V DC (approx) generate enough power to heat the motor - not under power - and why would it do so?

Planly, it did. Why - insulation on windings probably phucked.

[quelquod]

The motor probably stalled from some mechanical reason and took excessive current.

[servodude]

what should happen in a simple DC motor is:
- the current from the battery generates a magentic field in the motor coil
- this pushes against magnets on the spindle to turn it
- the spindle moves and wipes brushes on to different contacts which causes the the magnetic field to change position pushing the magnets further round (the magic of commutation)
- this repeats and the motor spins up to speed

as it spins it generates "back EMF" that acts against the applied voltage reducing the current in the coil
and it hits steady state speed when the load torque, current, voltage and back EMF balance

If the spindle does not rotate there's no back EMF generated and the winding acts like a heater (it's just a coil with a constant current going through it)

Eventually that will melt the insulation on the winding wire, the coil will short to itself, the impedance seen by the supply will drop, the current goes up further still and things get even hotter
- you can normally smell this

If the spindle can rotate with your fingers but doesn't with the power applied it sounds like your magnets have come away

[csearle]

Any theories or answers greatly received for my knowledge bank.

The electrical energy from the battery was converted to heat (I agree with quelquod). Once the heat is being generated it flows to cooler places. If that heat flow is impeded the temperature of the motor rises to reach an equilibrium, in your case 55°C.

C.

[Urbandreamer]

So, would a 3.6V DC (approx) generate enough power to heat the motor - not under power - and why would it do so?

Any theories or answers greatly received for my knowledge bank.

Voltage is NOT power.

So, do you know how much current your battery could supply?

You provide the voltage of 3.6V, so I'm going to assume a 18650 lithium battery. Which could supply 3.5A, producing 12W at that voltage.
Many soldering irons, intended to melt a metal alloy, are 15W.
Such devices also don't cool down instantly, once power is removed.

Others have explained things like insulation brake down and seized motors. I should also point out that a moving motor can force air through it to cool it.

Ps this sort of thing is why decades ago manufacturers told people not to fit rechargeable batteries. The products had not been tested to fail in a safe manner in the event of a short circuit with the new battery technologies. These days they have been designed to pass those tests.

[gryffron]

Also, Power creates heat. The ultimate temperature it reaches depends on the area available to dissipate the heat.

So a very tiny thing, like a lamp filament, could glow white hot with 12w. A huge thing wouldn't even feel warm. A little motor, obviously, could get to 55deg.

Gryff

[Mike4]

You provide the voltage of 3.6V, so I'm going to assume a 18650 lithium battery.

From the OP:

"Powered by 3 x 1.2v batteries."

[Mike4]

You provide the voltage of 3.6V, so I'm going to assume a 18650 lithium battery.

From the OP:

"Powered by 3 x 1.2v batteries."

And answering my own post I'm not sure I've ever encountered a 1.2v battery. Probably 1.5v cells, very discharged.

[servodude]

From the OP:

"Powered by 3 x 1.2v batteries."

And answering my own post I'm not sure I've ever encountered a 1.2v battery. Probably 1.5v cells, very discharged.

1.2V is the nominal standard for NiMH AA rechargeables
1.5V alkaline actually get there very quickly
Both bottom out nearer 0.9V but the NiMH get there in less of a slope

[Maroochydore]

Many thanks to all of you, especially servodude. I'm somewhat wiser now.

[Mike4]

And answering my own post I'm not sure I've ever encountered a 1.2v battery. Probably 1.5v cells, very discharged.

1.2V is the nominal standard for NiMH AA rechargeables
1.5V alkaline actually get there very quickly
Both bottom out nearer 0.9V but the NiMH get there in less of a slope

Ah yes I'd completely forgotten about those! My head has been in LiFePO4 for years now and then UD mentioned the 18650 cell...