# Rotating motor direction

Discussion in 'Sensors and Actuators' started by Pharaday, Jan 18, 2016.

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Jan 18, 2016
Hello, I recently started to regret that we got all take hΩ tests in my circμit analysis class, so I joined this forum. I have taken a rotating motor out of a broken heater. The motor was used to rotate the heater left and right with an arm. It is a synchronous motor TYJ50-8A. I took the motor and connected it alone to the heater power cable (no transformer, it runs on 120 volts). I also installed a switch on the hot wire to turn it on and off. My question is this:

Every time I switch it on, the motor seems to randomly choose if it should turn clockwise or counter-clockwise. I have tested the time between activations, number of revolutions or partial revolutions, the position of the arm upon activation and nothing seems to predict which direction it will rotate.
What actually determines which way this motor rotates?

I refuse to believe that the motor just oh feels like rotating clockwise this time and ya know what we haven't done counter-clockwise in a while, let's do that.
Anyways, thanks a bunch, guys!

Last edited: Jan 18, 2016
2. ### Minder

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Apr 24, 2015
1ph synchronous motors can arbitrarily start in either direction, applications where it is important for the direction to be consistent such as wall clocks etc, there is a spring loaded clutch that fires it in the opposite direction if it attempts to start in the wrong direction.
M.

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Jan 18, 2016
Wait a minute, arbitrary? I need an explanation on how a machine can be arbitrary. I thought that there can be no randomness with machines or computers etc. You're saying the motor DID just oh decide to rotate a certain direction. I cannot accept this. no, there must be something that it's using to make this "decision". I'm not trying to be rude, but please explain how a motor can be arbitrary.

4. ### Minder

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Apr 24, 2015
Take a common 1ph induction motor, if the start (direction) winding is disconnected, the rotor will remain stationary at switch on due to the magnetic field oscillating across 180°.
If the shaft is given a sharp spin at turn on, the motor will run up to speed as normal, Whichever way it is spun.
The induction motor will never run up to synchronism however.
Your synchronous motor is very similar except the nature of the rotor which has a P.M. field enabling the synchronism.
But due to the fact it does not have the direction winding which an induction motor has, the direction will depend entirely on where the rotor lies WRT to the stator winding.
So some method (there are a couple) has to be provided to give it a certain direction at switch on. The same way the common 1ph induction motor has to have.
In some old wall clocks, the 'kicker' would fail and it was possible to see a clock running backwards.
M.

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Jan 18, 2016
Upon further consideration and some soul searching, and finding no "decision chip" on the motor, I thought about the power supply from the outlet. It occurs to me that it might be acting on the half wave sign (+\-) of the voltage. So it turns clockwise if the half wave is negative and counter-clockwise if the half wave is positive, or vice versa. Can anyone confirm this?

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Jan 18, 2016
And now I see your second post, minder. so you say it IS the position at activation that determines the direction? I tested that but saw no connection, unless the rotor position change can be extremely small. I pretty much eyeballed it. Either way, thank you. The concept of the motor itself being arbitrary was messing with my mind.

7. ### AnalogKid

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Jun 10, 2015
1. The motor rotor stops at an arbitrary angle with respect to the stator.
2. The motor stops with an arbitrary magnetic polarity on the rotor because power was removed at an arbitrary phase angle.
3. Power is reapplied at an arbitrary instantaneous positive or negative voltage/current/magnetic polarity.

Arbitrary cubed.

The turn-on inrush current into a power transformer primary can vary greatly for essentially the same reasons.

ak

8. ### hevans1944Hop - AC8NS

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Jun 21, 2012
You could easily rig up a circuit to start and stop the motor and count the number of times it rotates clockwise and number of times it rotates counter-clockwise when re-started. If the two counts are not substantially the same over a period of days or months or maybe years of operation, then the initial direction is not truly random.

Even if it does turn out to be "random" there are probably no practical applications for this "randomness" unless it somehow promotes more even cooking via microwave turntable rotation. I thought my microwave oven turntable reversing was a "feature" instead of a matter of chance. I have not noticed if it sometimes continues in the same direction...

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Jan 18, 2016
Thanks, Analog, I'm glad to see I was right about the sign of the voltage wave. but let's not be confused about where to place the "arbitrariness" or you'll mess with my mind again. The randomness comes from our arbitrary decision on when to deactivate the motor. As we all know, the motor could never be arbitrary, or based on random choice or personal whim, rather than any reason or system, as defined by Webster's big book o' words. It's "decision" is ultimately based on the polarity. but let me not drone on about semantics. You guys have been so helpful! Thanks a bunch! In case of curiosity, I'm using the motor to open and close an exterior window shutter from inside with a push button, so it would be nice if the motor alternated directions in a predictable way but it works ok this way.

Last edited: Jan 18, 2016

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Jan 18, 2016
Thank you, Hevans, I see that the randomness idea was bothering you too.

11. ### hevans1944Hop - AC8NS

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Jun 21, 2012
I became interested in the concept of randomness many years ago while trying to learn about quantum mechanics and the Heisenberg Uncertainty Principle. Albert Einstein didn't believe in randomness, commenting that "God doesn't play dice." Sure God plays dice... and loads 'em too! How else to explain "miracles" performed by God unless He manipulates randomness?

As for mere humans creating any sort of pure randomness... ain't gonna happen! Maybe we can't put our fingers on exact cause producing a given effect (at least at the macro level), but that doesn't mean pure randomness is responsible. Get down to the quantum level and the "rules" do appear to change. One of the "best" random number generators starts out with a "white noise" electrical signal. Such a generator would have an equal distribution of energy from DC to infinity, so such things don't exist. But if you limit the bandwidth to, say, DC to 1 GHz you can put together a pretty darned good "random number" generator. Even a DC to 1 MHz "white noise" generator will produce pretty good "random" numbers. As far as practical randomness goes, there is a little town in the Nevada desert that is self-supporting on practical randomness and the belief that something called "luck" has a long-term effect on that.

Hop

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Jan 18, 2016
I agree, Hevans. Some "random", unguided explosion happened and the particles landed just right so now we get those little tree seed\leaves shaped like an airfoil so they twirl slowly to the ground like a helicopter? Did you guys "randomly" choose what to use as a brain? Gentlemen, that was planned.

13. ### 73's de Edd

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Aug 21, 2015
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Sir Pharaday . . . . . . .

On that type of AC synchronous motor that you are working with , and its indeterminate start up direction, look at the center duplicated pic to the right and note the motor rotors 16 separated iron pole pieces and companion alum-i-nin- ni -yum- yum divider bars are being aligned PARALLEL to the plane of the motor shaft. Referencing with the BLUE line markup.
Over on the motor stator winding there are 16 pole pieces, one of which is referenced by the YELLOW line markup.
When the motor comes to stop, it will be randomly leaving the stator and rotor iron pole pieces being a little bit CLOSER to one another in that resting position. On power up the initial power surge will favor that closeness and that is the direction that the motor will initially get its magnetic kick to start up and rapidly build up its running speed in THAT direction .

Looking at the initial main picture at the left, note that the rotor there is designed with a twist /slope made to the construction of the iron pole pieces and the aluminum bars in rereference to the motor shaft axis..
Additionally the positioning of the rotor inside of the unit is such that the rotor is not exactly centered . . .front to back . . . on the
stators pole pieces. That dissolution of the 50/50 positional probability, along with the magnetic slope of the rotors pole pieces, assures that the initial rotational start of the rotor is made in one select direction.
If the motors directional need was for its running in the opposite direction, the iron pole pieces and the aluminum bars in reference to the motor shaft axis, will be sloping in the opposite direction. See that rough depiction . . . . at far right inset.

Sooooooooo . . . . in applications where the start and running direction of an synchronous, motor is critical . . . that's how its done on . . .appliances, fans and I particularly remember one case . . . . .of a phonograph motor.

( Oh "beaver construction", I was certainly one little " Angels antithesis" in my early teen age years . . . . .the pranked victim kept trying to put the needle on the records lead in groove . . .over and over and over and over . . . . . ..

Thasssit . . . . .

Technobilia:

73's de Edd

.

Last edited: Jan 18, 2016
14. ### duke37

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Jan 9, 2011
I have mains driven synchronous clock which has evil intentions. If the power goes off, it almost always starts in reverse so I leave it to run for a few minutes before trying again. I have to get the washing machine out to get at the switch, I think it is laughing at me.

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Jun 10, 2015
He?

16. ### Minder

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Apr 24, 2015
For starters the Induction motor in #13 does not have rotor poles until the current is induced (Induction motor!) in the shorted turns created either by aluminum or copper bars, without these the motor will not rotate and resemble a large choke or inductance to the applied AC.
The skewed lamination's in the rotor is for smoothness of operation only.
This method is used in high quality DC servo motors to prevent cogging at low rpm.
The randomness of the start direction of a synchronous motor in the OP type is normal and if it suddenly develops a random start direction it usually indicates the means of initial start direction has become compromised.
A God has nothing to do with it!.
M.

Last edited: Jan 18, 2016
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