Microwave transformer inrush current solution?

[Richard Rasker]

Hello,

I've been asked to look into a problem with an industrial installation with
3 kVA HV magnetron (microwave oven) transformers, where mains fuses keep
tripping due to the inrush current.

Normally, I'd look into NTC's or power resistors with bypass relays;
however, those solutions are useless in this case, because the magnetron
units are switched off and on very frequently (up to a dozen times a
minute). NTC's need at least a minute to cool down, and power resistors
(e.g. 5 ohms) would be dissipating a few dozen watts this way, even when
bypassed after a mere 100 ms.

So I wonder if it's possible to switch off just the magnetron's cathode
filament instead of the whole supply transformer. Obviously, the relay would
need to be a special type, with at least 6 kV insulation voltage between the
coil and the switching contacts, but for the contact rating itself a few
volts and amperes would suffice.

Does anyone have any experience switching magnetron elements this way?
If so, are there any problems to be expected? And where can I find relays
that can handle the required insulation voltage?

And oh, I also looked into a triac-based solution, but that has its own
snags: the need for zero-crossing current(!) switch-on timing, and bypass
relays to prevent several dozen watts of power dissipation due to the
triac's on-state voltage of up to 1.5 volts @ 16A -- yet this relay in turn
complicates switching off during zero crossing ...

Thanks in advance for any insights,

Best regards,

Richard Rasker

 

[Spehro Pefhany]

Hello,

I've been asked to look into a problem with an industrial installation with
3 kVA HV magnetron (microwave oven) transformers, where mains fuses keep
tripping due to the inrush current.

Normally, I'd look into NTC's or power resistors with bypass relays;
however, those solutions are useless in this case, because the magnetron
units are switched off and on very frequently (up to a dozen times a
minute). NTC's need at least a minute to cool down, and power resistors
(e.g. 5 ohms) would be dissipating a few dozen watts this way, even when
bypassed after a mere 100 ms.

So I wonder if it's possible to switch off just the magnetron's cathode
filament instead of the whole supply transformer. Obviously, the relay would
need to be a special type, with at least 6 kV insulation voltage between the
coil and the switching contacts, but for the contact rating itself a few
volts and amperes would suffice.

Does anyone have any experience switching magnetron elements this way?
If so, are there any problems to be expected? And where can I find relays
that can handle the required insulation voltage?

And oh, I also looked into a triac-based solution, but that has its own
snags: the need for zero-crossing current(!) switch-on timing, and bypass
relays to prevent several dozen watts of power dissipation due to the
triac's on-state voltage of up to 1.5 volts @ 16A -- yet this relay in turn
complicates switching off during zero crossing ...

Thanks in advance for any insights,

Best regards,

Richard Rasker

I presume that they're using a mechanical relay to switch the primary
of the transformer, and that simply switching to a slow(er)-blow fuse
or circuit breaker is not an option for some reason.

High inrush current is a function of the magnetization of the core
from the previous cycle and the point in the AC cycle when transformer
is energized.

You could use a commercial product such as these ones:-
http://www.fsm.ag/en/production-transformer-relay.php

 

[Richard Rasker]

Hello Vladimir,

That would dramatically shorten life of cathode. It will lose emission.

That is very interesting to know. Can I assume that this also happens when
switching the transformer including magnetron element off and on? Because
that's what these people are doing at the moment, and they may be interested
to hear that they can prolong the life of their magnetron elements.

If this only happens in my scenario (so turning the filament off and on with
HV present), then why is that?

....

Use SMPS with PWM regulation. That would allow for much smaller
transformer, too.

You mean switching just the HV off and on, instead of the whole magnetron
element, and keeping the cathode filament powered-up?

One problem is of course that they already have a complete set-up,
transformers and all, and don't want to replace most of the hardware. But
I'll certainly look for a HV switching solution.

Anyway, thank you for your advice already,

Best regards,

Richard Rasker

 

[Cydrome Leader]

Hello,

I've been asked to look into a problem with an industrial installation with
3 kVA HV magnetron (microwave oven) transformers, where mains fuses keep
tripping due to the inrush current.

Normally, I'd look into NTC's or power resistors with bypass relays;
however, those solutions are useless in this case, because the magnetron
units are switched off and on very frequently (up to a dozen times a
minute). NTC's need at least a minute to cool down, and power resistors
(e.g. 5 ohms) would be dissipating a few dozen watts this way, even when
bypassed after a mere 100 ms.

So I wonder if it's possible to switch off just the magnetron's cathode
filament instead of the whole supply transformer. Obviously, the relay would
need to be a special type, with at least 6 kV insulation voltage between the
coil and the switching contacts, but for the contact rating itself a few
volts and amperes would suffice.

yes, they make/made HV reed relays for this. Old microwaves had these to
switch the HV side from the transformer on and off for the defrost cycle.
I don't recall if the AC or side or the doubled up DC was switched though.
The transformer and heater coils were powered up the entire time, so you
never got crazy inrush current problems like you're facing.

 

[Bill Martin]

Vladimir, in normal magnetron / microwave ovens the heater winding is on the same transformer core
as the the HV winding, so switched at the same time.
Magnetrons normally last many years (> 10).

I've wondered about how that can work...maybe not so big a problem at
kilowatt power levels. I can assure you that it is very bad at megawatt
power levels! Magnetron life can be shortened to approximately zero in a
hurry if HV is applied when ther cathode is not at full temperature.

 

[Richard Rasker]

....


I presume that they're using a mechanical relay to switch the primary
of the transformer, and that simply switching to a slow(er)-blow fuse
or circuit breaker is not an option for some reason.

They're already on the slowest circuit breakers feasible, and 99 out of a
100 times things switch on without tripping. But this still means that thigs
go wrong every 15 minutes or so.

High inrush current is a function of the magnetization of the core
from the previous cycle and the point in the AC cycle when transformer
is energized.

I know. Remanence combined with same direction field from new inrush current
= saturated core = no more induction, just DC resistance (0.5 ohms). Hence
the triac solution, with switch-on at mains voltage zero-crossing, and
switch-off at current zero-crossing (plus perhaps a small extra timing
factor to allow for magnetic hysteresis). Now if only those triacs didn't
have this >1 volt voltage drop ...

You could use a commercial product such as these ones:-
http://www.fsm.ag/en/production-transformer-relay.php

Thanks, I'll contact these people for some pricing information (although
experience tells me that these things won't be cheap).

Best regards,

Richard Rasker

 

[Richard Rasker]

In home/office use, 10 years translates to some 2000 hours of operation
and 20000 on/off cycles. Not much.

These things will be running 8 hours daily, with an estimated 500 up to 2500
on/off cycles. Indeed a bit of a waste if they needs replacing every two
weeks or so.

I'll look for switchmode (and switchable) HV generators with separate,
continuous filament power.

Thanks for all the help.

Best regards,

Richard Rasker

 

[[email protected]]

try a suitable sized incandesent light bulb across the on/off switch/relay.When the switch opens the filament will be cold so the current through the xformer will continue and will gradually reduce as the filament heats. In other words you demagnatize the core each time it shuts off so that it won't be able to saturate at the next turn on.

Mark

 

[Lasse Langwadt Christensen]

Hello,



I've been asked to look into a problem with an industrial installation with

3 kVA HV magnetron (microwave oven) transformers, where mains fuses keep

tripping due to the inrush current.



Normally, I'd look into NTC's or power resistors with bypass relays;

however, those solutions are useless in this case, because the magnetron

units are switched off and on very frequently (up to a dozen times a

minute). NTC's need at least a minute to cool down, and power resistors

(e.g. 5 ohms) would be dissipating a few dozen watts this way, even when

bypassed after a mere 100 ms.



So I wonder if it's possible to switch off just the magnetron's cathode

filament instead of the whole supply transformer. Obviously, the relay would

need to be a special type, with at least 6 kV insulation voltage between the

coil and the switching contacts, but for the contact rating itself a few

volts and amperes would suffice.



Does anyone have any experience switching magnetron elements this way?

If so, are there any problems to be expected? And where can I find relays

that can handle the required insulation voltage?



And oh, I also looked into a triac-based solution, but that has its own

snags: the need for zero-crossing current(!) switch-on timing, and bypass

relays to prevent several dozen watts of power dissipation due to the

triac's on-state voltage of up to 1.5 volts @ 16A -- yet this relay in turn

complicates switching off during zero crossing ...

Is the dissipation in a triac really that much of a problem?

lots of stuff run of SSRs like: http://www.ebay.com/itm/SSR-Solid-S...ndom-Switching-Turn-On-Mode-30A-/321114452036


-Lasse

 

[Nico Coesel]

Hello,

I've been asked to look into a problem with an industrial installation with
3 kVA HV magnetron (microwave oven) transformers, where mains fuses keep
tripping due to the inrush current.

Normally, I'd look into NTC's or power resistors with bypass relays;
however, those solutions are useless in this case, because the magnetron
units are switched off and on very frequently (up to a dozen times a
minute). NTC's need at least a minute to cool down, and power resistors
(e.g. 5 ohms) would be dissipating a few dozen watts this way, even when
bypassed after a mere 100 ms.

And oh, I also looked into a triac-based solution, but that has its own
snags: the need for zero-crossing current(!) switch-on timing, and bypass
relays to prevent several dozen watts of power dissipation due to the
triac's on-state voltage of up to 1.5 volts @ 16A -- yet this relay in turn
complicates switching off during zero crossing ...

How about timing the relay so it switches at the zero crossing? Switch
on when the voltage is zero and switch off when the current is zero.
With a constant voltage the time a relay needs to close the contacts
should be pretty constant. With a microcontroller and volt / current
sensing you could make it self-adjusting.

 

[Richard Rasker]

How about timing the relay so it switches at the zero crossing? Switch
on when the voltage is zero and switch off when the current is zero.
With a constant voltage the time a relay needs to close the contacts
should be pretty constant. With a microcontroller and volt / current
sensing you could make it self-adjusting.

Mechanical relays are too inaccurate for this kind of timing -- typically up
to 10 ms (1/2 mains period) activation and switch-off time, with quite a bit
of tolerance (20% easily), not to mention a bounce time of at least half the
activation time. And indeed this would require a controller with software,
current and voltage sensing, a low-voltage supply ...
For now. it's probably easiest and cheapest to follow Spehro Pefhany's
advice and simply buy a couple of commercial transformer relays, especially
designed for the job.
For the next generation of magnetron drivers, I'll try and find switchable
HV supplies with a constant cathode filament voltage.

But thanks for thinking along anyway

Best regards,

Richard Rasker

 

[Martin Riddle]

Hello,

I've been asked to look into a problem with an industrial installation with
3 kVA HV magnetron (microwave oven) transformers, where mains fuses keep
tripping due to the inrush current.

Normally, I'd look into NTC's or power resistors with bypass relays;
however, those solutions are useless in this case, because the magnetron
units are switched off and on very frequently (up to a dozen times a
minute). NTC's need at least a minute to cool down, and power resistors
(e.g. 5 ohms) would be dissipating a few dozen watts this way, even when
bypassed after a mere 100 ms.

So I wonder if it's possible to switch off just the magnetron's cathode
filament instead of the whole supply transformer. Obviously, the relay would
need to be a special type, with at least 6 kV insulation voltage between the
coil and the switching contacts, but for the contact rating itself a few
volts and amperes would suffice.

Does anyone have any experience switching magnetron elements this way?
If so, are there any problems to be expected? And where can I find relays
that can handle the required insulation voltage?

And oh, I also looked into a triac-based solution, but that has its own
snags: the need for zero-crossing current(!) switch-on timing, and bypass
relays to prevent several dozen watts of power dissipation due to the
triac's on-state voltage of up to 1.5 volts @ 16A -- yet this relay in turn
complicates switching off during zero crossing ...

Thanks in advance for any insights,

Best regards,

Richard Rasker

Does the magnitron have a torroid transformer? There are gapped
torroids just for this problem.

You could also try bleeding off the core to reset it during the OFF
time. This may be the easist to try. Once the core is reset, you dont
have to worry which AC cycle you start on.
Likey the core is not reset and starting again on the same AC cycle
saturates the transformer and pops the fuse.



Cheers

 

[Jamie]

Hello,

I've been asked to look into a problem with an industrial installation with
3 kVA HV magnetron (microwave oven) transformers, where mains fuses keep
tripping due to the inrush current.

Normally, I'd look into NTC's or power resistors with bypass relays;
however, those solutions are useless in this case, because the magnetron
units are switched off and on very frequently (up to a dozen times a
minute). NTC's need at least a minute to cool down, and power resistors
(e.g. 5 ohms) would be dissipating a few dozen watts this way, even when
bypassed after a mere 100 ms.

So I wonder if it's possible to switch off just the magnetron's cathode
filament instead of the whole supply transformer. Obviously, the relay would
need to be a special type, with at least 6 kV insulation voltage between the
coil and the switching contacts, but for the contact rating itself a few
volts and amperes would suffice.

Does anyone have any experience switching magnetron elements this way?
If so, are there any problems to be expected? And where can I find relays
that can handle the required insulation voltage?

And oh, I also looked into a triac-based solution, but that has its own
snags: the need for zero-crossing current(!) switch-on timing, and bypass
relays to prevent several dozen watts of power dissipation due to the
triac's on-state voltage of up to 1.5 volts @ 16A -- yet this relay in turn
complicates switching off during zero crossing ...

Thanks in advance for any insights,

Best regards,

Richard Rasker

We run into that problem many times with inrush currents due to poor
contactor closures for motors and transformers.

You may have noticed that you don't always get the big initial surge
but when you do, it is very noticeable. Poorly or cheap designed contact
closures can cause bounce and depending where you are on the curve, if
can cause strong mag current.

What works well is an electronic closure with mechanical closure to
follow. This not only helps with this problem but also extends the life
of the contacts.

http://homemadecircuitsandschematic...9/adding-soft-start-to-water-pump-motors.html

Jamie

 

[Phil Allison]

"Richard Rasker"

They're already on the slowest circuit breakers feasible, and 99 out of a
100 times things switch on without tripping. But this still means that
thigs
go wrong every 15 minutes or so.

Remanence combined with same direction field from new inrush current
= saturated core = no more induction, just DC resistance (0.5 ohms). Hence
the triac solution, with switch-on at mains voltage zero-crossing, and
switch-off at current zero-crossing (plus perhaps a small extra timing
factor to allow for magnetic hysteresis). Now if only those triacs didn't
have this >1 volt voltage drop ...

** Switching on at a zero crossing creates the LARGEST possible *magnetic*
inrush surges with transformers - this is so because the AC wave takes a
whole cycle to average to zero volts.

For minimum magnetic surge current, you switch on at a voltage peak - this
way the AC wave averages to zero in a mere half cycle.

To make a transformer that does not surge with random switch on points,
simply wind the primary for double the AC supply voltage - this has the
same effect as using the transformer at double the usual supply frequency.

NB: In cases where the transformer is loaded with rectifiers and large
capacitor bank, surge current due to charging the caps dominates if you
switch on at AC supply peaks. This is not the case with the OP's question.


.... Phil

 

[Phil Allison]

"Richard Rasker"

They're already on the slowest circuit breakers feasible, and 99 out of a
100 times things switch on without tripping. But this still means that
thigs
go wrong every 15 minutes or so.

** Are the 16 amp breakers a part of the microwave units OR just installed
in the AC supply to each oven ? If the latter, then simply use a larger
breaker, ie 20amp.

Remember - supply breakers are only there to protect cabling, not
appliances.

IME the instantaneous tripping current of magnetic/thermal breakers is about
10 times the nominal amp rating and it seems your oven need a bit more than
160 amps.

BTW:

Old fashion wire fuses have much higher tolerance to very short surges like
transformer magnetic inrush.



.... Phil

 

[[email protected]]

Hello,



I've been asked to look into a problem with an industrial installation with

3 kVA HV magnetron (microwave oven) transformers, where mains fuses keep

tripping due to the inrush current.

Since when does a fuse "trip"??? They either open or not, but in English the word, trip, implies it is resettable, so maybe you're talking about circuit breakers??? Who would know? And who would know of it's a plurality of "fuses" or a single fuse, wherein a singular fuse is indicative of a miswiredinstallation.

Normally, I'd look into NTC's or power resistors with bypass relays;

however, those solutions are useless in this case, because the magnetron

units are switched off and on very frequently (up to a dozen times a

minute). NTC's need at least a minute to cool down, and power resistors

(e.g. 5 ohms) would be dissipating a few dozen watts this way, even when

bypassed after a mere 100 ms.

Not practical for the usual duty cycle modulated installation.

So I wonder if it's possible to switch off just the magnetron's cathode

filament instead of the whole supply transformer. Obviously, the relay would

need to be a special type, with at least 6 kV insulation voltage between the

coil and the switching contacts, but for the contact rating itself a few

volts and amperes would suffice.

Absolute last thing anyone would want to do.

And oh, I also looked into a triac-based solution, but that has its own

snags: the need for zero-crossing current(!) switch-on timing, and bypass

relays to prevent several dozen watts of power dissipation due to the

triac's on-state voltage of up to 1.5 volts @ 16A -- yet this relay in turn

complicates switching off during zero crossing ...

Yep, TRIACS are hardly ever used for power control, it's amazing the component even exits it's so cumbersome to use.

 

[[email protected]]

I know. Remanence combined with same direction field from new inrush current

= saturated core = no more induction, just DC resistance (0.5 ohms).

Saturated transformer peak inrush current is winding resistance limited so it makes no difference whether it got there by being cycled through zero or peak reinforcing residual magnetization. The only difference is a cycle of exra integration time.
The installation power distribution and circuit protection are miswired and selected badly.

 

[josephkk]

Hello,

I've been asked to look into a problem with an industrial installation with
3 kVA HV magnetron (microwave oven) transformers, where mains fuses keep
tripping due to the inrush current.

Normally, I'd look into NTC's or power resistors with bypass relays;
however, those solutions are useless in this case, because the magnetron
units are switched off and on very frequently (up to a dozen times a
minute). NTC's need at least a minute to cool down, and power resistors
(e.g. 5 ohms) would be dissipating a few dozen watts this way, even when
bypassed after a mere 100 ms.

So I wonder if it's possible to switch off just the magnetron's cathode
filament instead of the whole supply transformer. Obviously, the relay would
need to be a special type, with at least 6 kV insulation voltage betweenthe
coil and the switching contacts, but for the contact rating itself a few
volts and amperes would suffice.

Does anyone have any experience switching magnetron elements this way?
If so, are there any problems to be expected? And where can I find relays
that can handle the required insulation voltage?

And oh, I also looked into a triac-based solution, but that has its own
snags: the need for zero-crossing current(!) switch-on timing, and bypass
relays to prevent several dozen watts of power dissipation due to the
triac's on-state voltage of up to 1.5 volts @ 16A -- yet this relay in turn
complicates switching off during zero crossing ...

Thanks in advance for any insights,

Best regards,

Richard Rasker

The correct answer is to quit screwing around and upgrade the wiring to
properly support the load. Failure to do so can have legal consequences,
particularly in case of a fire.

Just ask your local fire marshall.

?-)