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Triac Question

D

Dan

Jan 1, 1970
0
I have an conventional AC motor control circuit that uses an MOC3012
driving a BTA06 sensitive gate triac. The design has been working well
for several years, however it has a random problem where the motor
"bumps" or rotates slightly and randomly upon initial power-up. The
motor drives a gearbox whose output shaft must not move unless
commanded to do so. I've made sure the MOC3012 is not being turned on
during power-up. I've also tried loading the triac's gate to gnd, A1,
A2, etc. with a variety of resistors with no improvement. I swapped
the MOC3012 for an MOC3043 zero-crossing driver and the problem
improved considerably, but not completely. As you might have guessed,
the power-up 'bumping' of the motor occurs more frequently if there is
a noisy load on the same circuit as my controller. It really
misbehaves with flourescent lights and soldering irons. My customer
will no longer accept any movement of the gearbox shaft, however
slight.

Any suggestions?

TIA,
Dan
 
P

Phil Allison

Jan 1, 1970
0
"Dan"
I have an conventional AC motor control circuit that uses an MOC3012
driving a BTA06 sensitive gate triac.


** Which version - there are lots.

The design has been working well
for several years, however it has a random problem where the motor
"bumps" or rotates slightly and randomly upon initial power-up. The
motor drives a gearbox whose output shaft must not move unless
commanded to do so. I've made sure the MOC3012 is not being turned on
during power-up. I've also tried loading the triac's gate to gnd, A1,
A2, etc. with a variety of resistors with no improvement. I swapped
the MOC3012 for an MOC3043 zero-crossing driver and the problem
improved considerably, but not completely. As you might have guessed,
the power-up 'bumping' of the motor occurs more frequently if there is
a noisy load on the same circuit as my controller. It really
misbehaves with flourescent lights and soldering irons. My customer
will no longer accept any movement of the gearbox shaft, however
slight.

Any suggestions?


** Read the data sheet for your triac.

http://www.datasheetcatalog.com/datasheets_pdf/B/T/A/0/BTA06-600SW.shtml

Add a good snubber, use a "snubberless " version.



......... Phil
 
D

Dan

Jan 1, 1970
0
Thanks for the quick replies and good advice guys.

I put in a 100 ohm/0.1uF snubber and so far so good. The glitch is
random, but seems to have ceased. Spehro, I think you nailed this one!
I did consider the snubber, but discounted the idea thinking the
problem was gate related rather than across the anodes of the triac.

Thanks again for your help.
Regards,
Dan
 
S

Spehro Pefhany

Jan 1, 1970
0
I have an conventional AC motor control circuit that uses an MOC3012
driving a sensitive gate triac. The design has been working well
for several years, however it has a random problem where the motor
"bumps" or rotates slightly and randomly upon initial power-up. The
motor drives a gearbox whose output shaft must not move unless
commanded to do so. I've made sure the MOC3012 is not being turned on
during power-up. I've also tried loading the triac's gate to gnd, A1,
A2, etc. with a variety of resistors with no improvement. I swapped
the MOC3012 for an MOC3043 zero-crossing driver and the problem
improved considerably, but not completely. As you might have guessed,
the power-up 'bumping' of the motor occurs more frequently if there is
a noisy load on the same circuit as my controller. It really
misbehaves with flourescent lights and soldering irons. My customer
will no longer accept any movement of the gearbox shaft, however
slight.

Any suggestions?

TIA,
Dan

Swap the MOC3012 for something like a MOC3082, add a snubber if that
doesn't work. And make really sure that your drive circuit isn't
triggering the optoisolator.


Best regards,
Spehro Pefhany
 
J

john jardine

Jan 1, 1970
0
Dan said:
I have an conventional AC motor control circuit that uses an MOC3012
driving a BTA06 sensitive gate triac. The design has been working well
for several years, however it has a random problem where the motor
"bumps" or rotates slightly and randomly upon initial power-up. The
motor drives a gearbox whose output shaft must not move unless
commanded to do so. I've made sure the MOC3012 is not being turned on
during power-up. I've also tried loading the triac's gate to gnd, A1,
A2, etc. with a variety of resistors with no improvement. I swapped
the MOC3012 for an MOC3043 zero-crossing driver and the problem
improved considerably, but not completely. As you might have guessed,
the power-up 'bumping' of the motor occurs more frequently if there is
a noisy load on the same circuit as my controller. It really
misbehaves with flourescent lights and soldering irons. My customer
will no longer accept any movement of the gearbox shaft, however
slight.

Any suggestions?

TIA,
Dan

Sounds like you've got a single half cycle switch through. Most likely time
for it to happen, is when the supply is switched on at the peak of the
incoming voltage waveform.
In the UK this can amount to a +/- 340V step edge turning up at the triac
main terminal over the space of a few uS. The triac's dv/dt rating is
massively exceeded and it switches on by itself for the rest of that single
half cycle. Hence the twitching of a motor or lamp etc. This problem is
completely separate from the normal action of the gate terminal and trigger
pulse.

As suggested, you need a snubber across the triac. Something like 0.1uf or
0.22uF in series with maybe a 100 ohm resistor. Best to just buy the snubber
item as a standard part. 1nF may be capable of doing the job but better
safe than sorry.
Idea is that the capacitor instantly passes the nasty voltage edge across to
the other main terminal of the triac. Essentially both sides of the triac
are lifted by the voltage edge at the same time, hence it cannot crowbar due
to excessive dv/dt across it.
For the same reason you may also need another snubber across the opto triac.

One other technique is available which is to "prewarn" the triac of the
incoming waveform, so it never sees a switch-on transient. This method
simply bypasses the contactor/relay with a high value resistor.
 
J

Jan Panteltje

Jan 1, 1970
0
I have an conventional AC motor control circuit that uses an MOC3012
driving a BTA06 sensitive gate triac. The design has been working well
for several years, however it has a random problem where the motor
"bumps" or rotates slightly and randomly upon initial power-up. The
motor drives a gearbox whose output shaft must not move unless
commanded to do so. I've made sure the MOC3012 is not being turned on
during power-up. I've also tried loading the triac's gate to gnd, A1,
A2, etc. with a variety of resistors with no improvement. I swapped
the MOC3012 for an MOC3043 zero-crossing driver and the problem
improved considerably, but not completely. As you might have guessed,
the power-up 'bumping' of the motor occurs more frequently if there is
a noisy load on the same circuit as my controller. It really
misbehaves with flourescent lights and soldering irons. My customer
will no longer accept any movement of the gearbox shaft, however
slight.

Any suggestions?

TIA,
Dan

Fist thing you need to figure out is:
1) Is the big triac firing by itself or
2) Is the controller sending pulsens when it should not.

In case (1) make sure you also have overvoltage proctection across the triac
(VDR perhaps), proper RC snubbers, the works.
In case 2 you will have to scope for supply noise, pickup by wires, etc,
use RF (metal) shielding perhaps.
 
I have an conventional AC motor control circuit that uses an MOC3012
driving a BTA06 sensitive gate triac. The design has been working well
for several years, however it has a random problem where the motor
"bumps" or rotates slightly and randomly upon initial power-up. The
motor drives a gearbox whose output shaft must not move unless
commanded to do so. I've made sure the MOC3012 is not being turned on
during power-up. I've also tried loading the triac's gate to gnd, A1,
A2, etc. with a variety of resistors with no improvement. I swapped
the MOC3012 for an MOC3043 zero-crossing driver and the problem
improved considerably, but not completely. As you might have guessed,
the power-up 'bumping' of the motor occurs more frequently if there is
a noisy load on the same circuit as my controller. It really
misbehaves with flourescent lights and soldering irons. My customer
will no longer accept any movement of the gearbox shaft, however
slight.

Any suggestions?

TIA,
Dan

A triac is not the best device for the application, the current and
voltage waveforms are out of phase so when the triac turns off at zero
current there is an instantanious voltage across it which can fire it
again. Hence the need for a big snubber network. There are "special"
triacs made by Telcon that are supposed to address this problem. A
pair of thyristors is the best way. A zero crossing MOC shouldnt be
correct for the same reason. Your problem seems to be EMI. You should
have a filter/surge limiter on the AC supply, a MOV accross the triac
and a triac that has a voltage rating of at least twice the MOV
voltage.
 
P

Phil Allison

Jan 1, 1970
0
** Groper FUCKWIT Alert !!!

A triac is not the best device for the application,

** It is perfectly OK.


the current and
voltage waveforms are out of phase

** Probably not by very much.


so when the triac turns off at zero
current there is an instantanious voltage across it which can fire it
again. Hence the need for a big snubber network.


** No serious issue.

There are "special"
triacs made by Telcon that are supposed to address this problem.


** Yeah - called snubberless.

Stale news here - fuckhead.


A pair of thyristors is the best way.


** Not true at all.

A zero crossing MOC shouldnt be
correct for the same reason.


** Zero crossing is OK - less volts at switch off.

Your problem seems to be EMI.


** No it is not - you fucking tenth witted prick.

The OP's main problem occurs at switch on.


You should
have a filter/surge limiter on the AC supply,


** Go SFA to do with the fast voltage rise ( dv/dt ) at switch on.

a MOV accross the triac
and a triac that has a voltage rating of at least twice the MOV
voltage.


** No need with an AC motor load and a simple snubber on the triac.




....... Phil
 
S

Spehro Pefhany

Jan 1, 1970
0
Thanks for the quick replies and good advice guys.

I put in a 100 ohm/0.1uF snubber and so far so good. The glitch is
random, but seems to have ceased.
Spehro, I think you nailed this one!
I did consider the snubber, but discounted the idea thinking the
problem was gate related rather than across the anodes of the triac.

It's far more likely to be related to the optoisolator than the triac.

Both the breakdown voltage rating and the dv/dt rating of the
optotriac in the optoisolator are far less than the WORST version of
the BTA06. It's rated for only 10v/usec (less at higher temperatures
than 25'C- down to less than 2V/usec at 100'C) and 250V Vdrm, whereas
the MO3083 is rated for 800V Vdrm and guaranteed not to turn on with
dv/dt of 600v/usec (1500v/usec typical, both at 25C).
Thanks again for your help.
Regards,
Dan


Best regards,
Spehro Pefhany
 
D

Dan

Jan 1, 1970
0
The production boards are using MOC3012 drivers which are spec'd at
12V/uS typ @ 25C. Because of UL approval, swapping the '3012 for
another driver such as the '3083 as you suggest is a less desirable
option. Even though they are both UL approved components, the cost to
update the UL paperwork for the part change is around $1,500. I
know... it sounds crazy to me too.

My BT06-400C datasheet shows a dVdt of 200V/uS. I have not tried a
snubber on the driver's output yet. I did however short the opto's
input to guarantee that the driver is not getting turned on
accidentally via the micro.
 
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