Naked Science Forum
Non Life Sciences => Technology => Topic started by: chris on 24/05/2013 23:23:52
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Why does the turntable in the microwave sometimes turn to the right and sometimes rotate to the left? If cooking something and it's rotating, say, right, and you stop the microwave, then re-start it, often it reverses and starts turning left instead. Why?
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It's not a design feature (say to even-out wear) it's an inherent property of the type of motor used to drive the turntable ...
... A single-phase (or two-phase derived from single phase) stator winding is possible, but in this case the direction of rotation is not defined and the machine may start in either direction unless prevented from doing so by the starting arrangements.
http://en.wikipedia.org/wiki/Synchronous_motor#Operation
I believe the rotation direction depends on the position of the rotor when the (AC) power is applied.
[ some say the rotation is dependent of the phase of the AC when the power is applied ]
This may just be me, but it seems the position it comes to rest when the power is cut causes it to rotate in the opposite direction when started again, so the rotation direction usually alternates every time it is started up
If rotation direction is only dependent on the phase of the AC then it should be random like flipping a coin. But if it "remembers" the direction it was spinning in previously, and rotates in the opposite direction, then rotor position when power is applied defines the direction of rotation, not AC phase.
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Most of the time they come to rest with one pole or the other nearest the magnet pole, attracted to it. When power is reapplied this then is repulsed, and as the gear set will have less friction in the reverse direction ( the gears will have some slack play, when running this makes it noisy but allows it to run freely, and when stopped it causes a bias) and will tend to turn into that direction with less friction. This then gives some momentum to the rotor and it carries on in the same direction.