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# Calculation of inductors in Triacs

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#### deepakchikane

##### Full Member level 3
Dear All
pls look into my circuit attched to this.

I am facing problem with the Inrush sometimes & sometime not.

The problem is observed in Compressor (When it starts) also in the Motor where star to delta conversion take place it surges a huge amount of current for shorter Time periords.

This is uncertain behavior.
Will this problem will solve if i put inductor in series with the Triacs..??
Triac used:ACST12357-8FP
It(RMS)=12 AMP
F=50hz

Kindly help to sort the issue is coz of Triacs & if i put the inductor will it limit the current.
https://obrazki.elektroda.pl/2537186600_1442239978.jpg

Pls post idea of designing Inductor.

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Depending on class of motor, surge currents of 5x are normal, until back EMF from motor increases as speed approaches steady state to cancel. Motor may have some Remanence in iron core such that phase of stop current if opposite phase to next start will minimize surge from preventing momentary saturation of core. This is by chance.
If motor is shutoff by Triacs at zero-current crossing, then best simple start time is using ZCS trigger for triacs using any method.

Since motor is designed to handle this momentary heat load or may use starting Caps to improve PF for starting to improve torque, your Triac switches must also be capable of such surge currents.

Coil Rs determines the maximum torque and also maximum inrush currents. Motor torque and compressor Load profile with inertia determines the time of surge current. Underpowered motors will increase duration of surge time.

Is your problem overheating of triacs during startup or effects of surge current on Line voltage regulation and other devices?

deepakchikane

### deepakchikane

Points: 2
Current surges can occur when the motor stator saturates. Contactors and solid state relays are usually designed to handle the overcurrent.

You would need quite large inductors to limit the overcurrent. A better way could be to turn on the triac near the voltage maximum to avoid saturation.

Is your problem overheating of triacs during startup or effects of surge current on Line voltage regulation and other devices?

Hi Sunny

The Triac is failing at the Normal conditions.

It is failing when the High inrush current comes into the picture due to Compressor on off situations.

My triac is not able to sustain at normal temperature leave off the High Temperatures.

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You would need quite large inductors to limit the overcurrent. A better way could be to turn on the triac near the voltage maximum to avoid saturation.

Hi FvM

I am having an Transformer Ratio of 10:1 (NS:NP).
So it would be Around the 5 AMP for the 50 amp of secondary.
But devices would not handle the Compressor & motor inrush current.

Will be the inductor High for the 5 amp of primary current.
Thanks

Have you got sufficient heatsinking on your triacs? you probably need 40A + devices for reliability, usually 2 back to back SCR's are used to provide the necessary amount of silicon.... 1200V 50A each.

Also some triacs get hot when the gate is energised a certain way relative to the terminal volts - check the data sheet!

Yes.
i am driving it thru the Controller & Opto-isolator.

so it is isolated at 2.5kv insulation.

Y i need the very high capability if triacs (i.e:50 amp)

Can you insert phase control of Triacs ?
Current sensing?

More important, what is the total thermal resistance Rja?
This is a critical requirement with power loss in triac.

How much %voltage margin for Triac AC rating with startup BEMF 30%? 50%? more? I suggest 50% minimum.
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. A better way could be to turn on the triac near the voltage maximum to avoid saturation.

But which phase polarity near max?

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Sounds like you need to improve heat sink, and tandem up Triacs and Optodrivers for adequate ZCS pulse current .

But which phase polarity near max?
For a three-phase inductive load (without starpoint connection), you get optimal switching if two triacs are switched at the maximum of the respective interphase voltage and the third switch 1/4 period later.

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