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Dual direction TRIAC 3phase motor control problem

MPYC

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Hi dears

I wanna control the direction of a 3 phase AC motor. there are two types of circuits for it.
Image 1.jpg

and
Image 2.jpg

the lines voltages are 400 volts and I choose BTA12-600CW.
  1. The 600 volts for BTA12 is enough or do I need a higher voltage TRIAC for the purpose?
  2. It used 5 TIRACs in image 1 and 4 TRIACs in image 2. is the TRIAC 1 necessary in the first image?
  3. I choose the second schematic for lower component reasons. But I have some heavy crush in the test circuit. the burnt TRIACs are No one and three (from top to end) or No two and four. It's not always happening but it happens anyway. why?
  4. I use a 100 ohms resistor and 100nf capacitor as a snubber network. Is it proper?
thank you for your suggestion.
 

betwixt

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The schematics work in the same way, one using a Triac and the other two back-to-back SCRs. The main difference is how you trigger them, both SCRs in the same phase must be conductive at the same time but the triggering circuits between phases must be isolated from each other. How do you trigger them in the second design?

Note that in the second schematic, one phase is wired straight through, in the first it is wired through a Triac. You can simplify the first circuit by connecting L1 directly to the motor.

Brian.
 

MPYC

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The schematics work in the same way, one using a Triac and the other two back-to-back SCRs. The main difference is how you trigger them, both SCRs in the same phase must be conductive at the same time but the triggering circuits between phases must be isolated from each other. How do you trigger them in the second design?

Note that in the second schematic, one phase is wired straight through, in the first it is wired through a Triac. You can simplify the first circuit by connecting L1 directly to the motor.

Brian.
TNX Brain.

So if I replace TRIAC 1 in schematic one with a simple wire any change does happen?
--- Updated ---

Something strange happend.
photo_2021-07-22_19-16-34.jpg


3 and 4 completely broken and 2 damaged too.

But I didn't connect the motor to the circuit and I didn't feed any signal to Optotriacs. I just linked 400V to L1, L2, and L3 to inputs.
what is my mistake?
 
Last edited:

betwixt

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In a three phase motor system, the direction of rotation is changed by swapping any two of the phase wires. All the wires are essentially equal so it doesn't matter which two are swapped.

The intention is that one phase (L1 for example) stays permanently connected and the path between L2 and L3 is controlled by operating either Triac 2 and 3 -OR- Triac 4 and 5. If you trigger 2 at the same time as 5 or you trigger 3 at the same time as 4 you create an almost short circuit across L2 and L3 and the Triacs will be damaged. This is probably what is happening.

For rotation in one direction trigger Triac 2 and 3 but do NOT trigger 4 and 5.
To reverse the direction, trigger Triac 4 and 5 but do NOT trigger 2 and 3.

If you are using the opto-couplers, the easiest way to drive them is by wiring the ones connected to Triac 2 and 3 so they operate from a single signal. Do the same for the opto-couplers on Triacs 4 and 5. As long as you don't drive them both at the same time it should work fine.

Note that you will almost certainly have to bond the Triacs to a suitable heat sink when operating a motor.

Brian.
 

KlausST

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Hi,

Please understand that the photo of post#3 gives no information about how the damage can happen.
You need to show all the connections ... and additional informations. Maybe scope pictures of the optocoupler input signals. Maybe a complete schematic, maybe a hand drawn timing diagram....

Klaus
 

Easy peasy

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The SCR's ? triacs will die pretty quickly due to temp rise the way you have glues them to plastic - read data sheets about heat sinking ...
 

MPYC

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new circuit 2.png

I just simplified the circuit to this new circuit.
please attention, I didn't feed any voltage to pins 1 & 2 of MOC3063.
and there is no load for the circuit so it expected any heat if proper operation.
It seems TRIACs fire automatically.
 

Easy peasy

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shorting 2 phases , per the diagram above not such a good idea

if the triac is 1v5, 1A rms = 1.5 watts = bang with no heatsink ...
--- Updated ---

they get hotter for 3rd quadrant triggering too - look it up
 

KlausST

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Hi,

neither in photo of post#3 nor in drawing of post#7
* we can see which pin you used for what.
* we can see which triacs you switched ON
* we can see any timing of the control signal
* ..

You don´t give a single of the requested informations in post#5.
So my post#5 was a waste of time ???

Klaus
 

FvM

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The trigger circuit in post #7 looks o.k., except for the unusual 220 nF gate capacitor. Also a gate resistor will have little effect because triacs already provide a low input resistance.

Making a phase inverter circuit with triacs is however basically problematic, you must always be aware of possible mains surges causing self triggering of opto triac or main triac.

I see three-phase solid state relays suggested for motor reversing function, they are SCR based and have 1200 VDRM for 400-480V mains voltage. Usage of varistors for surge protection is recommended.
 

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