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Soft-start for AC motor with an Attiny85?

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Chriss:
Hi!
A friend of mine asked me to make a controller for him to soft-start an AC motor which is around 900w.
He give me some parameter what I should achieve with an uC.
I should power a Triac with a 0-100Hz pulse which would practically rise in time from 0sec to 5sec. When 5sec is reached the signal should be 100Hz.

This is not a problem. It looks like a sweeper.

But I'm not familiar ( have no practice) in AC motors, my friend has, my problem with this setup is:
I can make this sweeper but the motor will probably not run even after reached the 100Hz after 5 sec cos I'm not sure how long should I keep full open the triac to put enough energy into the coil to start to spin the motor.

Am I righ ?

Somehow I should know how long should be the on state of the triac I suppose, it is not enough to generate just a signal of 10-20-30..Hz.
Somehow I feel there in his logic is something wrong.

Can somebody help me make me mor clear the logic what kind of signal should I create to make this think work?

Thanks for any advice and help.

rooppoorali:
Pardon me because I myself never worked on such a high-level project. Here are some contents that you may find helpful. At least you may get some ideas.

http://www.jetir.org/papers/JETIRBF06002.pdf

https://www.researchgate.net/publication/4297167_Implementation_and_Analysis_of_Microcontroller_Based_Soft_Starters_for_Three_Phase_Induction_Motors

Ian.M:
Singular "... a Triac ..." implies it could be a single phase motor.  There are several types of single phase induction motors, differentiated by whether the auxiliary/starting winding* is permanently powered, and by whether or not it has a series capacitor to provide the phase shift, with some switching in a starting capacitor in parallel to provide additional auxiliary winding current and thus torque during start and run-up.  The remaining type, shaded pole induction motors is very unlikely to be used at this power level due to their low efficiency.

If it has a switched starting winding or start capacitor, using any sort of electronic or time delay switching, that isn't likely to 'play well' with a TRIAC soft start circuit, so you'd need an extra TRIAC to manage the starting winding switching.   If it uses centrifugal switching, that should work OK, and your circuit will not need to take over the switching function.

Note that for induction motors with a switched starting winding, (rather than a permanently connected auxiliary winding), it typically has a fairly low duty cycle limit, and can burn out if the motor is restarted too frequently or if the start time is excessively extended, e.g. by shaft load that's marginal for the starting torque of the motor.  For those with a switched capacitor, (whether in parallel with the auxiliary winding's run capacitor or on its own simply in series with the starting winding), it is typically a non-polarized electrolytic and also has a low duty cycle limit as its can size is almost invariable insufficient to dissipate the heat from the I2R losses due to the start winding current flowing through its ESR.  If you let it absorb too much energy, its electrolyte boils and the capacitor vents, sometimes explosively - game over!

If the motor isn't protected by an overload relay, then your soft start circuit needs to perform that duty as well, which it may need sensors for, e.g. for motor speed, winding currents, and stator winding and/or start capacitor temperature.

* The second winding is generally known as the auxiliary winding if its powered while the motor is running and the starting winding if its only powered during start and run-up.  Some reversible motors use identical   main and auxiliary windings connected at one end by a run capacitor, and the other commoned together, the direction being determined by which side of the run capacitor you apply  power to.

pcprogrammer:
Controlling a triac with a variable frequency from 0 to 100Hz over time sounds odd to me since the triac switching off depends on the 50 or 60Hz mains frequency.

So you would need some synchronization with the mains frequency to make any sense of the control signal?

Just my thoughts, no expert on the subject.

Chriss:
I was also come acros the thinking about the control signal frequency and the main frequency problem.
However, I still trying to understand what kind of device could this be, and I comes to an idea, it could be probably likely more a 900W dimmer then a real soft - start device.

Could it be more an 900W AC dimmer?

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