Author Topic: RC Snubber for Phase Regulator  (Read 1091 times)

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Offline RegiusTopic starter

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RC Snubber for Phase Regulator
« on: August 03, 2019, 12:32:33 pm »
Hi, I have a phase regulator and I'm using it for regulating the rpm of a fan - centrifugal blower. The motor is shaded pole induction motor (220VAC/28W). The regulator is working fine, but if the RC Snubber (R8+C8) is disconnected, the voltage and current are getting out of phase and false triggering / turn off of the triac occurs which is leading to very fast overheating and failure of the coil of the induction motor.
What type of capacitor should I use for minimizing the chance of its failure (I’m using this regulator for 8 hours a day).Or is there a way for adding additional circuit for protection if such failure occurs. I’m currently using Polypropylene capacitor rated for 250VAC/400VDC (Wima MKP 10 - https://www.wima.de/wp-content/uploads/media/e_WIMA_MKP_10.pdf). I’m not sure if a X2 rated capacitor will be suitable. For resistor I’m using 1W/Metal Film.

Also… the capacitor makes buzzing noise (the buzzing noise is slightly changing, when the firing angle is changing). Is it caused by piezoelectric effect or it is something else?
 

Offline MarkF

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Offline T3sl4co1l

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Re: RC Snubber for Phase Regulator
« Reply #2 on: August 03, 2019, 01:23:15 pm »
Try a larger capacitor, smaller resistor?

I'd probably go with a 400VAC+ capacitor not necessarily X2 rated, because the X2 rating allows self-healing (the cap momentarily short-circuits), and after enough self-healing the value decreases dramatically.  The voltage across a TRIAC can't be much over its rated voltage anyway, so there's no problem with transients.  Anyway, it's probably a small difference, and not one I would expect to show up until after years of operation.

And yeah, capacitors buzz, the mechanism is electrostriction.  Some do it more than others.  Want to say polypropylene does less than polyester?

Tim
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Offline RegiusTopic starter

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Re: RC Snubber for Phase Regulator
« Reply #3 on: August 04, 2019, 06:45:55 am »
Try a larger capacitor, smaller resistor?

I'd probably go with a 400VAC+ capacitor not necessarily X2 rated, because the X2 rating allows self-healing (the cap momentarily short-circuits), and after enough self-healing the value decreases dramatically.  The voltage across a TRIAC can't be much over its rated voltage anyway, so there's no problem with transients.  Anyway, it's probably a small difference, and not one I would expect to show up until after years of operation.

And yeah, capacitors buzz, the mechanism is electrostriction.  Some do it more than others.  Want to say polypropylene does less than polyester?

Tim

Thank you for the answer.
I will probably use the same type of capacitor but rated for higher voltages - WIMA MKP 10: 1000VDC/600VAC
There is an Application note from ST for "RC snubber circuit design for TRIACs" https://www.st.com/content/ccc/resource/technical/document/application_note/38/88/44/b8/2c/26/44/b8/CD00004096.pdf/files/CD00004096.pdf/jcr:content/translations/en.CD00004096.pdf
Their snubber calculation for a 26W pump is 10nF Cap and 620 Ohm Resistor.

I haven't tried polyester or other type of capacitors.
 


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