Every time I use my microwave oven the turntable spins the opposite direction it did the previous time. Why is this? I’ve seen it happen on every microwave I’ve used, do all models do it?
That happens in my current microwave oven too. I don’t know why, but I suppose it is a standard feature for turntables.
I don’t think all models do, I’m sure most of the newers ones do though.
As for the reverse spinning, there is probably some circuitry in it that reverses the polarity of the spinning motor everytime the door is opened. I guess it’s suppose to cook your meal more efficiently or something. Say you have to cook something for 04:00 min. You can stop it at 02:00 min, stir your food, and when you start it again, it will go the opposite way.
The turntable uses a shaded-pole synchronous motor. The direction it spins when it starts is essentially random.
I had a few minutes wait while cooking supper, so I tested how random it is:
Set the timer at two minutes
Started the over, and noted the spin direction
Opened and closed the door, and restarted 20 times
The spin sequence was: LLRLRLLLRLRLLRRLRRRR
It looks pretty random.
Desmostylus is right.
DC motors use brushes and a commutator to reverse the flow of current in the rotor as it turns to ensure that opposing fields continue to turn it.
Some types of AC motor use the inherent alternation of current in the mains supply to achieve a similar effect, but as a result of this, the direction of rotation is affected by the position in which the rotor is resting and the direction in which the alternating current happens to be flowing when the thing is powered up.