Another Ebay story.

[David Strelitz]

We have a piece of domestic equipment that incorporates a small 24 volt DC fan. These fans are about 2"x2"
Stay with me on the back story...

These fans are made by Sanly in China. Purchased from Dimplex UK Spares, they are about £35.00. On eBay from China about £10.00 for the identical Sanly fan.
I had to replace a fan. I had a spare on a unit I am getting ready to sell.
The spare worked. Before Christmas a replacement arrived from China. Finally got round to doing the swap two weeks ago. New fan appeared to be DOA.
I convinced myself I had thrown the good one away and ordered another from China. It arrived and again, when I went to plug it in, it
appeared to be DOA. I then discovered that the wiring to the non reversible plug was reversed.
I did a quick "patch" to reverse the polarity and the fan worked. Both fans were badged Sanly 24vDC and both were wired reversed.

Contacted both suppliers.
Both asked for a VIDEO of the fan NOT WORKING...ffs!
After additional correspondence and me sending a photo of the reversed wiring, one supplier did an immediate refund but the other offered a full refund if I give them a 5star rating once the refund was received. Yeah right!!!

Neither suppliers answered my question as to whether the fans were genuine Sanly.
I am also baffled as to why the polarity on a DC motor would be a game changer, a limitation on my electrical knowledge.

[Brian Fretwell]

I only think there could be a diode somewhere to prevent the fan operating in the wrong direction when installed. Just to make it idiot proof.

[David Strelitz]

Thanks for the diode heads up Brian.
Just been reading about diodes in DC power circuits and that makes sense.
The unit is a Dimplex Optimyst fire. The fan is controlled from the PCB so I assume that must have a diode on it make it "idiot proof"...and stop reversed wired fans from ...fanning.😁

[Steve Klare]

It's also possible the motor has a catch diode connected between its terminals so that when a switch that's controlling it opens up, the diode forward biases and the energy stored in the motor's inductance has someplace else to go than arcing trough the just opened switch or transistor. Without this diode, the switch takes a beating and will fail much sooner.

It's just the nature of an inductor that it produces a huge burst of voltage when you try to stop the current instantaneously.

It's actually useful: this is what puts the spark in our spark plugs!