Dual direction TRIAC 3phase motor control problem

Thread Starter

MPYC

Joined Apr 17, 2015
19
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.
 

Ian0

Joined Aug 7, 2020
3,181
Both circuits work the same. There's a triac in L1 (first diagram) where there isn't in L3 (second diagram) - you can leave it out.
If you switch off all the other triacs the motor will stop
Two back-to-back thyristors are the same as a triac.
A capacitor between gate and cathode of a thyristor is a really bad idea - it slows down the switch-on and is likely to make it fail.
I would suggest wiring the MOC3062 #2 and #3 in series and #4 and #5 in series so that they both switch on together, and adding some interlock so that you can't get all 4 on at the same time.
100Ω/100nF is a reasonable choice of values for a snubber.
 

LowQCab

Joined Nov 6, 2012
1,078
There needs to be extensive operational interlocks, including,
no changes allowed with a running Motor.

If You have a running 3-Phase-Motor, and You arbitrarily switch it to Reverse,
something is going to explode.
The Current will go through the roof, and the Magic-Blue-Smoke will be released.

And of course, if Forward and Reverse are turned on at the same time,
there will be instant destruction.

With a few more Triacs, You can implement Motor-Braking.
.
.
.
 

Thread Starter

MPYC

Joined Apr 17, 2015
19
Both circuits work the same. There's a triac in L1 (first diagram) where there isn't in L3 (second diagram) - you can leave it out.
If you switch off all the other triacs the motor will stop
Two back-to-back thyristors are the same as a triac.
A capacitor between gate and cathode of a thyristor is a really bad idea - it slows down the switch-on and is likely to make it fail.
I would suggest wiring the MOC3062 #2 and #3 in series and #4 and #5 in series so that they both switch on together, and adding some interlock so that you can't get all 4 on at the same time.
100Ω/100nF is a reasonable choice of values for a snubber.
Thank you lan0

is there any benefit to use SCR instead of TRIAC?
 

Ian0

Joined Aug 7, 2020
3,181
Thank you lan0

is there any benefit to use SCR instead of TRIAC?
Yes - they are more robust (but not if put capacitors between gate and cathode)
But not so robust that they would survive if you changed the motor direction whilst it was running! Like @LowQCab said.
There's a big improvement in reliability between MOSFETs/IGBTs and Triacs, and a further but smaller improvement between triacs and Thyristors.
 

MaxHeadRoom

Joined Jul 18, 2013
23,585
So essentially a VFD, these typically use much different commutation style and reproduce a variable frequency sine wave.
Also the typical VFD has automatic decel and accel when issued a reverse command.
The Chinese versions can be had for a little more than $100.00, DIY is a moot point at that price?
 

Ian0

Joined Aug 7, 2020
3,181
So essentially a VFD
Not quite that advanced, it looks to me like a three-speed drive - forwards, backwards and stopped!
I suppose phase-firing could reduce the speed by reducing the torque and increasing the slip on an induction motor.

(And I've only just got used to VFD no longer meaning Vacuum Fluorescent Display)
 

Thread Starter

MPYC

Joined Apr 17, 2015
19
There needs to be extensive operational interlocks, including,
no changes allowed with a running Motor.

If You have a running 3-Phase-Motor, and You arbitrarily switch it to Reverse,
something is going to explode.
The Current will go through the roof, and the Magic-Blue-Smoke will be released.

And of course, if Forward and Reverse are turned on at the same time,
there will be instant destruction.

With a few more Triacs, You can implement Motor-Braking.
.
.
.
thank you

is it necessary to complete the motor stop before changing direction? It seems just feed more current than nominal current, isn't it?

but something happens when I test the circuit. I make another circuit to avoid firing cross TRIACs(2 and 5 or 3 and 4) at the same time. I didn't connect the motor to the circuit and I didn't feed any signal to OptoTriacs but TRIACs 3 and 4 were completely exploded and 2 damaged too. what is the reason?
 

MaxHeadRoom

Joined Jul 18, 2013
23,585
Keep in mind an AC induction motor at the point of switch on, is essentially a transformer with shorted turn secondary's.
Reversing without detecting zero RPM aggravates the situation further.
 
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