That's because it's quite common for simple appliances to use a universal motor. https://en.wikipedia.org/wiki/Universal_motor https://en.wikipedia.org/wiki/Un

That's because it's quite common for simple appliances to use a universal motor. https://en.wikipedia.org/wiki/Universal_motor https://en.wikipedia.org/wiki/Universal_motor It basically doesn't make a difference what polarity you feed it, if it's positive going into the rotor then it's positive going into the stator as well. When the rotor changes polarity, so does the stator.