DC Motor Thyristor Failures??

Guest
Does anyone know why Thyristors used in a DC motor speed control
application tend to fail? Or what would be the reasons to look at
when they fail in an otherwise normally operating system?

I'm talking large DC motors (100+ HP) using a 600VDC main and armature
current in the hundreds of Amps. The application is an electric
transit bus. We have an abnormally high rate of Thyristor failures,
usually in the shorted condition. There is nothing wrong with the
control circuit design. The system will be running fine for a long
time and then we'll have a failure. There is no specific time or
mileage that they tend to fail at, but it's fleetwide and presents a
major operating headache.

Thanks.
 
On Fri, 09 Apr 2004 16:05:16 -0700, the renowned none@none.123 wrote:

Does anyone know why Thyristors used in a DC motor speed control
application tend to fail? Or what would be the reasons to look at
when they fail in an otherwise normally operating system?
Click to expand...
Too much voltage, too much current, inadequate drive circuit, too much
junction temperature due to poor installation or design, too much
thermal cycling.

I'm talking large DC motors (100+ HP) using a 600VDC main and armature
current in the hundreds of Amps. The application is an electric
transit bus. We have an abnormally high rate of Thyristor failures,
usually in the shorted condition. There is nothing wrong with the
control circuit design. The system will be running fine for a long
time and then we'll have a failure. There is no specific time or
mileage that they tend to fail at, but it's fleetwide and presents a
major operating headache.
Click to expand...
What does the manufacturer of the speed controls say? Are they
electric trolley buses with power coming from overhead collectors? Are
there any correlations with weather (heat, thunderstorms) or other
identifiable factors?

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
They shouldn't. This looks like a transient problem where spikes reach a
level that is too much to bear. What kind of motors are they? Could there
be anything that causes spikes? Brushes wearing out maybe? Any correlation
with vehicle or motor age/mileage?

Regards, Joerg.

none@none.123 wrote:

Does anyone know why Thyristors used in a DC motor speed control
application tend to fail? Or what would be the reasons to look at
when they fail in an otherwise normally operating system?

I'm talking large DC motors (100+ HP) using a 600VDC main and armature
current in the hundreds of Amps. The application is an electric
transit bus. We have an abnormally high rate of Thyristor failures,
usually in the shorted condition. There is nothing wrong with the
control circuit design. The system will be running fine for a long
time and then we'll have a failure. There is no specific time or
mileage that they tend to fail at, but it's fleetwide and presents a
major operating headache.

Thanks.
Click to expand...
 
<none@none.123> wrote in message
news:toae705m5dusapornivovkn0e8hmo80ti0@4ax.com...
|
| Does anyone know why Thyristors used in a DC motor speed control
| application tend to fail? Or what would be the reasons to look at
| when they fail in an otherwise normally operating system?
|
| I'm talking large DC motors (100+ HP) using a 600VDC main and armature
| current in the hundreds of Amps. The application is an electric
| transit bus. We have an abnormally high rate of Thyristor failures,
| usually in the shorted condition. There is nothing wrong with the
| control circuit design. The system will be running fine for a long
| time and then we'll have a failure. There is no specific time or
| mileage that they tend to fail at, but it's fleetwide and presents a
| major operating headache.
|
| Thanks.

Get some more data about your system and what was happening when the
things failed. Easier said than done, it's broke. Why? It's broke. What
were you doing when it broke. It's broke. Arghhhhh.

Best guess, for me.... is.

The driver is braking and the motor is in regen. If it relies on the
supply to dump current back and that supply goes open, like the
cantalever or whatever connects the bus to the bus (grin) bounces or
something, then you get an overvoltage on the thyristors and they go
bang.

Solution is transient overvoltage protection on the DC bus, something
meaty.

DNA
 
On a sunny day (Sat, 10 Apr 2004 13:02:02 +0100) it happened "Genome"
<Genome@nothere.com> wrote in <0mRdc.75$872.26@newsfe1-win>:

none@none.123> wrote in message
news:toae705m5dusapornivovkn0e8hmo80ti0@4ax.com...
|
| Does anyone know why Thyristors used in a DC motor speed control
| application tend to fail? Or what would be the reasons to look at
| when they fail in an otherwise normally operating system?
|
| I'm talking large DC motors (100+ HP) using a 600VDC main and armature
| current in the hundreds of Amps. The application is an electric
| transit bus. We have an abnormally high rate of Thyristor failures,
| usually in the shorted condition. There is nothing wrong with the
| control circuit design. The system will be running fine for a long
| time and then we'll have a failure. There is no specific time or
| mileage that they tend to fail at, but it's fleetwide and presents a
| major operating headache.
|
| Thanks.
Having worked with high power thyristor, but 38 years ago, in THAT
Click to expand...
time (as John Woodgate a while ago mentioned), so called 'hot spots'
were a failure reason.
At these very high currents you need REPETITIVE trigger pulses from the
driver, during the priod the SCR has to be on.
If not, the thing may physically only 'partly' switch on (and I mean
with partly: 'only a small area of the silicon', and that will take the
whole current.
A so called 'hot spot' occurs, and the thing shorts (say melts).
So the driver MAY seem OK, but you will really need a strong enough
trigger of a several kHz pulse train in say a 50 or 60 Hz period, or any
on period.
KEEP triggering that thing, with as much as is safely allowed.
JP
 
I read in sci.electronics.design that Jan Panteltje
<pNaonStpealmtje@yahoo.com> wrote (in <c592mt$2pk2$1@news.wplus.net>)
about 'DC Motor Thyristor Failures??', on Sat, 10 Apr 2004:

KEEP triggering that thing, with as much as is safely allowed.
Click to expand...
Especially if you are using it with inductive loads (as you are) and
even more if you are using it in the 'third quadrant' mode, i.e. MT2
negative with respect to MT1 but with positive gate current.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
On Sat, 10 Apr 2004 17:23:27 +0100, the renowned John Woodgate
<jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that Jan Panteltje
pNaonStpealmtje@yahoo.com> wrote (in <c592mt$2pk2$1@news.wplus.net>)
about 'DC Motor Thyristor Failures??', on Sat, 10 Apr 2004:

KEEP triggering that thing, with as much as is safely allowed.

Especially if you are using it with inductive loads (as you are) and
even more if you are using it in the 'third quadrant' mode, i.e. MT2
negative with respect to MT1 but with positive gate current.
Click to expand...
They won't be triacs at that current/voltage level. SCRs are about the
only (thyristor) game in town.

The rise time of the gate current and magnitude is also critical to
avoiding hot-spots.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
I read in sci.electronics.design that SP <spffSNIP@interlogDOTyou.knowwh
at> wrote (in <dffg70hakm50m1tfrad90bhjf2gu9kkt1a@4ax.com>) about 'DC
Motor Thyristor Failures??', on Sat, 10 Apr 2004:
They won't be triacs at that current/voltage level. SCRs are about the
only (thyristor) game in town.
Click to expand...
Quite right. Wool-gathering rules!
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
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