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12 Volt power Supply

please forgive the supidity of this question but I am a mechanical
engineer and have very linited knowledge of electronics.

I have tried to biuld a 12 volt power supply for testing
motor/gearbox/pump units and the problem is that it keep blowing up

I have only a very simple circuit of a Transformer to drop the voltatge
down from 240 then feed this through a bridge rectifier ( International
Rectifier Part No. GBPC3502A ) and the connect the output via an
ammeter to the motor. once it is running it is fine, but somtimes blows
the refitier when it is turned on ( switching the AC input to the
transformer ) do I need more components in the circuit, if so what ?

The motors will draw up to 20 amps, when running the rectifier does get
hot but has a heat sink and fan from an old PC connected. but it
allways blows when you turn it on, not when it is running

All assistance greatfully received

Phil
 
please forgive the supidity of this question but I am a mechanical
engineer and have very linited knowledge of electronics.

I have tried to biuld a 12 volt power supply for testing
motor/gearbox/pump units and the problem is that it keep blowing up

I have only a very simple circuit of a Transformer to drop the voltatge
down from 240 then feed this through a bridge rectifier ( International
Rectifier Part No. GBPC3502A ) and the connect the output via an
ammeter to the motor. once it is running it is fine, but somtimes blows
the refitier when it is turned on ( switching the AC input to the
transformer ) do I need more components in the circuit, if so what ?

The motors will draw up to 20 amps, when running the rectifier does get
hot but has a heat sink and fan from an old PC connected. but it
allways blows when you turn it on, not when it is running

All assistance greatfully received

Phil

Phil

It must be the 'surge' current thats blowing the rectifier.
If the motor draws 20Amps with a moderate load, then I bet it draws
80Amps at switch on (Very high load during the first few hundred
miliseconds)
Your fan and heatsink wont help with the initial surge.

You'll need a more suitable bridge rectifier.

Steve Balstone
 
Phil

It must be the 'surge' current thats blowing the rectifier.
If the motor draws 20Amps with a moderate load, then I bet it draws
80Amps at switch on (Very high load during the first few hundred
miliseconds)
Your fan and heatsink wont help with the initial surge.

You'll need a more suitable bridge rectifier.

Steve Balstone

Cheers for that , Sounds like that is the problem, is there any way to
limit the intial current draw, a coil or somthing like that ?

Phil
 
Cheers for that , Sounds like that is the problem, is there any way to
limit the intial current draw, a coil or somthing like that ?

Phil

Mmmmmmmm

To keep it really simple, you could build a crude 'soft start' circuit.
Get a 0.5 ohm very high wattage (50W).
Connect this in series with the transformer output and the rectifier ac
input.
Now put a high current (20A) switch ACROSS the resistor.
To start.....
Turn switch off, turn on mains, motor will start slowly then throw on
the new switch after a few seconds
Putting these on the AC side NOT the DC side and that will save the
switch contacts from weilding themselves together!!

Steve Balstone
 
Cheers sounds like a nice easy way of getting it going, will put it on
a rely and push button to short out the resistor, so cannot be started
with the resistor shorted, and put a thermal switch on the relay so if
button not pressed then unit shuts down before getting too hot

Cheers again

Phil
 
J

John Popelish

Jan 1, 1970
0
Cheers sounds like a nice easy way of getting it going, will put it on
a rely and push button to short out the resistor, so cannot be started
with the resistor shorted, and put a thermal switch on the relay so if
button not pressed then unit shuts down before getting too hot

If you move that relay to the primary side, you will not need such
large contacts. The resistor will need to be about 200 ohms, in that
case. If left on, it could produce almost 300 watts of heat. But
this reduces the contact current to about 1 amp.
 
J

John Popelish

Jan 1, 1970
0
Jamie said:
switches on the primary side of the transformer can be
dirty and thus generate some high voltage spikes on the
secondary side when switched.
i guess the question here is, does this happen if you
simply turn on the supply with no motor load on it?
also, motors can be very inductive and having no capacitor
in the circuit can generate some nice HV energy if you are
getting arcs in the switching process.
i would try 2 things., a small non polarized cap on the
switch contacts and one on the secondary side ! something like
a .1 ceramic with the proper handling voltage. and for the
output of the bridge a larger cap to help suppress the pulse.

we had the same problem at work with a simple DC motor on a
Varistat with a bridge rectifier. , some times just turning on
the unit if the motor was on a light or heavy start, it would
short the bridge.
you may also want to look at getting some TVS diodes
(transient voltage suppression), don't use MOV's because
they only work x number of times before they short.
diodes/rectifiers have 0.6 volt on the average cutoff
of no current flow, this lack of load can allow for the xformer
to generate some nice HV with noisy switches and out of phase switching.
because the pulse width is generally short and wiring in
your application after the bridge produces some nice inductive
reactance compind with the motor, HV could simply be able to jump the
diodes voltage limits
and short for that moment.
the only other thing i can think of is that you have a heavy starting
load on your motor? if the motor starts quickly then the rectifier should
handle that.
i would at least try to use a 20 amp fast blow fuse on the output. if
you blow that on start up, then you have way too much starting torque in
which case you should have a bigger unit or current limited supply.
but try the caps first. i think that will cure your problem.

The bridge rectifier shorts out any motor inductive current, so that
is no problem. I don't that the transformer will generate anything
damaging with the motor connected across the rectifier. The main
power switch will have no inrush spike (caused by transformer core
saturation from the last power off half cycle being in the same
direction as the first one on power up) if there will be a 200 ohm
resistor in series for some time period. The contact that shorts that
out always has 200 ohms across it to act as a spark suppressor. I see
no problems if the primary side contacts are rated for 5 amps or so.
 
J

John Popelish

Jan 1, 1970
0
Jamie said:
say what you want, that was a cure we did to solve a long nasty
problem that was given to us after to many times of that part of the
equipment being down. used a set of HV probes on the scope to monitor
this problem and it was exactly that. out of phase/noisy contacts on the
primary side randomly generating HV pulses combined with motor the
bridge didn't like. the other guys try to keep a note on that equipment
to instruct the operators to make sure the variac was down to zero
before starting ! , but like any one else that works in a production
type job they don't give a crap about machinery.
when testing with just the motor on line with no caps, i was getting
aprox 2000 V or more pulse generated when testing across the bridged.
that is the AC-DC leg of the bridge., remove the motor load and i only
saw a aprox 1/4 of that which the bridge seem to handle ok.
put the .1 mica caps on the switch, secondary side and one on the +&-
output along with a small DC cap, switching noise gone, no HV over that
which was suppose to be there. and that unit has been running with the
same bridge for at least 3 or more years now.

if you read a lot of the instruction manuals for drives or sensitive
electronics, they will tell you to not! switch the primary side of a
transformer feed because of this problem. switching the secondary side
is much better to avoid this damaging noise.
higher end electronics have suppression components on the line to
remove that noise but a lot of the low end simple mini drives do not
go to far when protecting it self from this kind of damage in which
case you should be using a set of contacts to feed the drive at its
shortest point.
many supplies also use series inductors/chokes with a cap to remove
this problem.

I would rather see an X capacitor of a microfarad or so across the
primary.
 
J

Jamie

Jan 1, 1970
0
please forgive the supidity of this question but I am a mechanical
engineer and have very linited knowledge of electronics.

I have tried to biuld a 12 volt power supply for testing
motor/gearbox/pump units and the problem is that it keep blowing up

I have only a very simple circuit of a Transformer to drop the voltatge
down from 240 then feed this through a bridge rectifier ( International
Rectifier Part No. GBPC3502A ) and the connect the output via an
ammeter to the motor. once it is running it is fine, but somtimes blows
the refitier when it is turned on ( switching the AC input to the
transformer ) do I need more components in the circuit, if so what ?

The motors will draw up to 20 amps, when running the rectifier does get
hot but has a heat sink and fan from an old PC connected. but it
allways blows when you turn it on, not when it is running

All assistance greatfully received

Phil
switches on the primary side of the transformer can be
dirty and thus generate some high voltage spikes on the
secondary side when switched.
i guess the question here is, does this happen if you
simply turn on the supply with no motor load on it?
also, motors can be very inductive and having no capacitor
in the circuit can generate some nice HV energy if you are
getting arcs in the switching process.
i would try 2 things., a small non polarized cap on the
switch contacts and one on the secondary side ! something like
a .1 ceramic with the proper handling voltage. and for the
output of the bridge a larger cap to help suppress the pulse.

we had the same problem at work with a simple DC motor on a
Varistat with a bridge rectifier. , some times just turning on
the unit if the motor was on a light or heavy start, it would
short the bridge.
you may also want to look at getting some TVS diodes
(transient voltage suppression), don't use MOV's because
they only work x number of times before they short.
diodes/rectifiers have 0.6 volt on the average cutoff
of no current flow, this lack of load can allow for the xformer
to generate some nice HV with noisy switches and out of phase switching.
because the pulse width is generally short and wiring in
your application after the bridge produces some nice inductive
reactance compind with the motor, HV could simply be able to jump the
diodes voltage limits
and short for that moment.
the only other thing i can think of is that you have a heavy starting
load on your motor? if the motor starts quickly then the rectifier should
handle that.
i would at least try to use a 20 amp fast blow fuse on the output. if
you blow that on start up, then you have way too much starting torque
in which case you should have a bigger unit or current limited supply.
but try the caps first. i think that will cure your problem.
 
J

Jamie

Jan 1, 1970
0
John said:
The bridge rectifier shorts out any motor inductive current, so that is
no problem. I don't that the transformer will generate anything
damaging with the motor connected across the rectifier. The main power
switch will have no inrush spike (caused by transformer core saturation
from the last power off half cycle being in the same direction as the
first one on power up) if there will be a 200 ohm resistor in series for
some time period. The contact that shorts that out always has 200 ohms
across it to act as a spark suppressor. I see no problems if the
primary side contacts are rated for 5 amps or so.

say what you want, that was a cure we did to solve a long nasty
problem that was given to us after to many times of that part of the
equipment being down. used a set of HV probes on the scope to monitor
this problem and it was exactly that. out of phase/noisy contacts on the
primary side randomly generating HV pulses combined with motor the
bridge didn't like. the other guys try to keep a note on that equipment
to instruct the operators to make sure the variac was down to zero
before starting ! , but like any one else that works in a production
type job they don't give a crap about machinery.
when testing with just the motor on line with no caps, i was getting
aprox 2000 V or more pulse generated when testing across the bridged.
that is the AC-DC leg of the bridge., remove the motor load and i only
saw a aprox 1/4 of that which the bridge seem to handle ok.
put the .1 mica caps on the switch, secondary side and one on the +&-
output along with a small DC cap, switching noise gone, no HV over that
which was suppose to be there. and that unit has been running with the
same bridge for at least 3 or more years now.

if you read a lot of the instruction manuals for drives or sensitive
electronics, they will tell you to not! switch the primary side of a
transformer feed because of this problem. switching the secondary side
is much better to avoid this damaging noise.
higher end electronics have suppression components on the line to
remove that noise but a lot of the low end simple mini drives do not
go to far when protecting it self from this kind of damage in which
case you should be using a set of contacts to feed the drive at its
shortest point.
many supplies also use series inductors/chokes with a cap to remove
this problem.
 
E

ehsjr

Jan 1, 1970
0
Cheers for that , Sounds like that is the problem, is there any way to
limit the intial current draw, a coil or somthing like that ?

Phil

Yes there is, as discussed. However, it may well
be that the limited current will prevent your motor
from starting. Then it's cook the motor, cook the
circuit time.

Ed
 
A

Alan B

Jan 1, 1970
0
If you move that relay to the primary side, you will not need such
large contacts. The resistor will need to be about 200 ohms, in that
case. If left on, it could produce almost 300 watts of heat. But
this reduces the contact current to about 1 amp.

This brings to mind an interesting story. I used to write and maintain
software for automatic testing of electronic circuit cards. One particular
card had a "soft start" circuit similar to this, where a pair of resistors
were in the path at initialization, then shorted by relay action some
milliseconds later. The original test software author neglected to program
the shorting relays after turn-on. Cards were being sent to software as
defective, with comments such as "smoked resistors," "resistors glowed red"
and so on.

The resistors were hardy wire-wound types, so showed no signs of damage
when the circuit was cold. The cards routinely were returned to the system
with no defect noted. This started a circular process that took a bit of
time to figure out!
 
A

Alan B

Jan 1, 1970
0
please forgive the supidity of this question but I am a mechanical
engineer and have very linited knowledge of electronics.

I have tried to biuld a 12 volt power supply for testing
motor/gearbox/pump units and the problem is that it keep blowing up

I have only a very simple circuit of a Transformer to drop the voltatge
down from 240 then feed this through a bridge rectifier ( International
Rectifier Part No. GBPC3502A ) and the connect the output via an
ammeter to the motor. once it is running it is fine, but somtimes blows
the refitier when it is turned on ( switching the AC input to the
transformer ) do I need more components in the circuit, if so what ?

The motors will draw up to 20 amps, when running the rectifier does get
hot but has a heat sink and fan from an old PC connected. but it
allways blows when you turn it on, not when it is running

I agree with what's been posted so far, that you could either beef up your
bridge diodes, or create a "soft start" circuit at the primary. But I also
notice that you haven't indicated that you have any filter capacitors as
part of your design. I wonder if some smoothing at the DC output might be
a good idea?
 
A

Alan B

Jan 1, 1970
0
Yes there is, as discussed. However, it may well
be that the limited current will prevent your motor
from starting. Then it's cook the motor, cook the
circuit time.

Yeah, this could depend on how much torque the motor is required to have at
startup. If there was a way to limit the torque load until the motor is
turning, i.e. a clutch on the load, this might both allow a soft-start
circuit to work, and at the same time eliminate the need for it.
 
J

John Popelish

Jan 1, 1970
0
Alan B wrote:

(snip)
...But I also
notice that you haven't indicated that you have any filter capacitors as
part of your design. I wonder if some smoothing at the DC output might be
a good idea?

He has an inductive filter. The motor inductance averages the effect
of the voltage wave.
 
A

Alan B

Jan 1, 1970
0
Alan B wrote:

(snip)

He has an inductive filter. The motor inductance averages the effect
of the voltage wave.

OK, something to study up on. Motors have always been spooky to me.
 
E

ehsjr

Jan 1, 1970
0
please forgive the supidity of this question but I am a mechanical
engineer and have very linited knowledge of electronics.

I have tried to biuld a 12 volt power supply for testing
motor/gearbox/pump units and the problem is that it keep blowing up

I have only a very simple circuit of a Transformer to drop the voltatge
down from 240 then feed this through a bridge rectifier ( International
Rectifier Part No. GBPC3502A ) and the connect the output via an
ammeter to the motor. once it is running it is fine, but somtimes blows
the refitier when it is turned on ( switching the AC input to the
transformer ) do I need more components in the circuit, if so what ?

The motors will draw up to 20 amps, when running the rectifier does get
hot but has a heat sink and fan from an old PC connected. but it
allways blows when you turn it on, not when it is running

All assistance greatfully received

Phil


When you first start a motor, it draws a *huge*
amount of current with respect to the current it
draws once it is up to speed. At start up, you
want to be able to deliver that relatively hugh
current - and soft start does the opposite. The
motor needs that current to get started properly.
Restricting it with a soft start may cook the
both the motor and the powersupply.

Build or buy a supply that can deliver the needed
start current. Maybe an auto battery charger -
some of them have a very high current spring return
"start" position on the charge selector switch. You
could wire in a relay that transfers for a second or
two to provide that "blast" of current. I say maybe,
because I don't know how those chargers provide that
starting current, so further investigation would
be needed.

Even better would be to use an auto battery which can
supply hundreds of amps for the brief startup period.
There's no maybe in that solution.

Ed
 
J

jasen

Jan 1, 1970
0
Alan B wrote:

(snip)

He has an inductive filter. The motor inductance averages the effect
of the voltage wave.

it seems to me that doing that would soak up some of those voltage spikes
that the experts have been blaming for the rectifier failure. it probably
doesn't need capacitors of a size to fill the valleys of the rectified AC,
just something to soak up the spikes.

the experts were saying to put the capacitirs on the primary... I don't see
why they couldn't go after the rectifier...

Bye.
Jasen
 
ehsjr said:
When you first start a motor, it draws a *huge*
amount of current with respect to the current it
draws once it is up to speed. At start up, you
want to be able to deliver that relatively hugh
current - and soft start does the opposite. The
motor needs that current to get started properly.
(snip)

The motor torque is roughly proportional to the current, so limiting
the current to no more than full speed current just slows the initial
acceleration, unless there is some break away static friction or other
starting torque peak added to the inertial one.

The O.P. said nothing about being in a hurry to get the motors moving.
 
E

ehsjr

Jan 1, 1970
0
ehsjr wrote:



(snip)

The motor torque is roughly proportional to the current, so limiting
the current to no more than full speed current just slows the initial
acceleration, unless there is some break away static friction or other
starting torque peak added to the inertial one.

You need some minimum torque to get the motor going,
at all. If there is insufficient torque, it just
sits there cooking. Full speed current is a small
percentage of starting current, and isn't even
a valid figure. An unloaded motor vs a loaded
motor comes to mind. The valid figure is FLA -
full load amps. Compare FLA with LRA. LRA (locked
rotor amps) is the startup current - the current the
motor draws when it is stationary - and can be huge
with respect to the FLA (full load amps) - as much
as 6 times FLA. And who knows how much greater it
will be vs full speed amps, which is meaningless.

If the motor sits there drawing full speed current
and not moving, there's no airflow, so not only is
it drawing LRA, it is not being cooled by air movement
its spinning would cause via the attached fan, or
even just the armature if there is no fan incorporated
with the motor.

The posts recommending soft start - and not pointing
out the above - are playing fast and loose with the
OP's motor. Soft start, *properly* applied, is a
good thing. It may well be that something posted
might work. But the warning that it could cause the
motor to cook stands. The issue is not about getting
the motor up to speed quickly, it is about avoiding
the situation where the motor doesn't start at all.

Ed
 
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