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static switch

I use three triacs as a switch for a three phase motor instead of
relay. But here I have a problem, when the overload occurs the triacs
gets damaged. Please give the way to prodect the triacs.
 
J

John Fields

Jan 1, 1970
0
I use three triacs as a switch for a three phase motor instead of
relay. But here I have a problem, when the overload occurs the triacs
gets damaged. Please give the way to prodect the triacs.
 
J

John

Jan 1, 1970
0
I use three triacs as a switch for a three phase motor instead of
relay. But here I have a problem, when the overload occurs the triacs
gets damaged. Please give the way to prodect the triacs.

A three-phase motor will draw 5 to 10 times the nameplate current upon
starting or with a locked rotor or a missing phase.

1. Choose a triac which can handle that much current.
2. Choose a breaker which will trip with that much current.*

*Motor may be equipped with an internal self-resetting thermal breaker
(Klixon, for example).

For more severe faults (phase-to-phase or phase-to-ground), you must protect
with sub-cycle fuses whose I^2 * T (eye-squared-t) rating is less than the
triac's I^2 * T rating.

Good luck.

John
 
H

Homer J Simpson

Jan 1, 1970
0
I use three triacs as a switch for a three phase motor instead of
relay. But here I have a problem, when the overload occurs the triacs
gets damaged. Please give the way to prodect the triacs.

What makes you think you have the skill level to do this?
 
B

Boris Mohar

Jan 1, 1970
0
I use three triacs as a switch for a three phase motor instead of
relay. But here I have a problem, when the overload occurs the triacs
gets damaged. Please give the way to prodect the triacs.

Why did you put "static switch" in the subject line? What does this imply?
 
R

Robert Baer

Jan 1, 1970
0
John said:
A three-phase motor will draw 5 to 10 times the nameplate current upon
starting or with a locked rotor or a missing phase.

1. Choose a triac which can handle that much current.
2. Choose a breaker which will trip with that much current.*

*Motor may be equipped with an internal self-resetting thermal breaker
(Klixon, for example).

For more severe faults (phase-to-phase or phase-to-ground), you must protect
with sub-cycle fuses whose I^2 * T (eye-squared-t) rating is less than the
triac's I^2 * T rating.

Good luck.

John
Yup; the trouble is that it seems nobody has been specifying I^2*T on
power semis for about 20 years.
Even if ignitrons were still being made, i bet that spec would be
dropped for them also.
And the engineers that were around then are retired or have departed...
The newbies do not have a clue when asked...
 
T

Terry Given

Jan 1, 1970
0
modern machines are high efficiency, and end up about 10x to 20x (10% to 5%)


a LOT less.
Yup; the trouble is that it seems nobody has been specifying I^2*T on
power semis for about 20 years.
Even if ignitrons were still being made, i bet that spec would be
dropped for them also.
And the engineers that were around then are retired or have departed...
The newbies do not have a clue when asked...

work it out from Ifsm.....

its spec'd for real power semis, just not toys ;)

Cheers
Terry
 
J

John

Jan 1, 1970
0
Terry Given said:
modern machines are high efficiency, and end up about 10x to 20x (10% to
5%)


It has been a few years since I worked in motor controls. I accede to your
more recent knowledge.

In any case, the OP should be able to get the locked-rotor rating from the
manufacturer.

a LOT less.


work it out from Ifsm.....

its spec'd for real power semis, just not toys ;)

That's a good one!
Cheers
Terry

I quickly looked up a triac on Mouser, the T1620W made by ST. The I2t is
given. I know this does not mean that the I2t is given for all (toy) triacs
and thyristors, but it seems likely that the info is available for most. I
designed motor control power stages from 1980 through 2000 and I never ran
across a thyristor data sheet which did not have I2t given. However, my work
was with Westinghouse, Westcode, and, later, Powerex thyristors. We never
used triacs.

OP: what triacs are you using? What are the motor nameplate ratings?

John
 
N

Noway2

Jan 1, 1970
0
I use three triacs as a switch for a three phase motor instead of
relay. But here I have a problem, when the overload occurs the triacs
gets damaged. Please give the way to prodect the triacs.
There are two ways that I have seen this problem addressed, when I
worked as an engineer at a company that made static switches. Both of
these answers have been mentioned in the replies to this thread: use
fuses, or oversize the semiconductors to handle whatever you are going
to throw at them.

You mentioned triacs in your post, but you may also want to consider
SCRs and IGBTs.
 
T

Terry Given

Jan 1, 1970
0
John said:
It has been a few years since I worked in motor controls. I accede to your
more recent knowledge.

In any case, the OP should be able to get the locked-rotor rating from the
manufacturer.





That's a good one!




I quickly looked up a triac on Mouser, the T1620W made by ST. The I2t is
given. I know this does not mean that the I2t is given for all (toy) triacs
and thyristors, but it seems likely that the info is available for most. I
designed motor control power stages from 1980 through 2000 and I never ran
across a thyristor data sheet which did not have I2t given. However, my work
was with Westinghouse, Westcode, and, later, Powerex thyristors. We never
used triacs.

I'm doing two projects with toy thyristors at the moment.

TIC106 - no. and a ratshit datasheet

BTA/B10 - yes.

BTW67/69 - yes.

I think the conclusion we can draw here is that lousy thyristor
datasheets dont include IIt.

However its a different story when we look at FETs, IGBTs & diodes. I
havent seen a toy diode, FET or IGBT spec IIt yet, but its pretty common
with the big stuff. interestingly enough, none of the Semikron miniSKiiP
datasheets have IIt.

but everyone has Ifsm, and there is a relationship between the two; an
IIt value based on Ifsm is lower than the actual fusing IIt, so it gives
a reasonable safety margin.

If only manufacturers would tell us the die dimensions!
OP: what triacs are you using? What are the motor nameplate ratings?

John

Cheers
Terry
 
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