Using SSR to switch transformer

[Arlet Ottens]

Hello,

I'm considering using an SSR to switch the primary side of a
transformer. The transformer is a toroid, 230V -> 27V. The secondary
side is fixed to a 1 Ohm nichrome wire heating element.

I've read some articles that say switching on at zero crossings causes a
high inrush current, and that it's best to switch on at peak voltage.

If I follow this advice, are there any other issues I need to look out
for ?

What kind of (surge) current capabilities does the SSR need to have for
reliable operation ? (DC resistance of primary side is about 1 Ohm)

My original idea was to switch the secondary side using a pair of SCRs,
but at that current it would require a fairly decent heat sink. If I can
switch the primary side, the dissipation would be reduced considerably.

 

[stp]

Hello,

I'm considering using an SSR to switch the primary side of a
transformer. The transformer is a toroid, 230V -> 27V. The secondary
side is fixed to a 1 Ohm nichrome wire heating element.

I've read some articles that say switching on at zero crossings causes a
high inrush current, and that it's best to switch on at peak voltage.

I think - just opposite. With fast SSR you will have gradual increase
current with voltage
on the transformer primary - do the calculations and simulation.
dI=E*dt/L
The secondary will follow.
Zero voltage crossing switching is popular in heating, lightning etc.

 

[Spehro Pefhany]

Hello,

I'm considering using an SSR to switch the primary side of a
transformer. The transformer is a toroid, 230V -> 27V. The secondary
side is fixed to a 1 Ohm nichrome wire heating element.

I've read some articles that say switching on at zero crossings causes a
high inrush current, and that it's best to switch on at peak voltage.

Probably correct- the core will tend to saturate most under those
conditions.

If I follow this advice, are there any other issues I need to look out
for ?

I guess you could phase control it on.

What kind of (surge) current capabilities does the SSR need to have for
reliable operation ? (DC resistance of primary side is about 1 Ohm)

Worst case 230V/1 ohm = 230A if you don't put a thermistor in series.
Probably translates to a 50A or higher 'puck' SSR.

My original idea was to switch the secondary side using a pair of SCRs,
but at that current it would require a fairly decent heat sink. If I can
switch the primary side, the dissipation would be reduced considerably.

Best regards,
Spehro Pefhany

 

[[email protected]]

Hello,

I'm considering using an SSR to switch the primary side of a
transformer. The transformer is a toroid, 230V -> 27V. The secondary
side is fixed to a 1 Ohm nichrome wire heating element.

With a resistive load, you will not have an inrush current with zero
voltage crossing. It will be easier to find a relay that will handle
the 3.17A primary current at 230V than to find one that will handle
the secondary current of 27A at 27V.

 

[Phil Allison]

I'm considering using an SSR to switch the primary side of a
transformer. The transformer is a toroid, 230V -> 27V. The secondary
side is fixed to a 1 Ohm nichrome wire heating element.

I've read some articles that say switching on at zero crossings causes a
high inrush current, and that it's best to switch on at peak voltage.

I think - just opposite.


** YOU do not think at all.

Learn a thing or two about transformes beforee making as ASS of yourself
here again.



.......... Phil

 

[Phil Allison]

Arlet Ottens

I'm considering using an SSR to switch the primary side of a
transformer. The transformer is a toroid, 230V -> 27V. The secondary
side is fixed to a 1 Ohm nichrome wire heating element.

With a resistive load, you will not have an inrush current with zero
voltage crossing.

** That is ABSOLUTE BOLLOCKS !

YOU need to learn a thing or two about transformers too - pal.




........ Phil

 

[Phil Allison]

"Arlet Ottens"

I'm considering using an SSR to switch the primary side of a transformer.
The transformer is a toroid, 230V -> 27V. The secondary side is fixed to a
1 Ohm nichrome wire heating element.

I've read some articles that say switching on at zero crossings causes a
high inrush current, and that it's best to switch on at peak voltage.

** Correct.

If I follow this advice, are there any other issues I need to look out for
?

** Designing a drive cct for the SSR that will NEVER accidentally switch
near zero volts is not that easy.

What kind of (surge) current capabilities does the SSR need to have for
reliable operation ? (DC resistance of primary side is about 1 Ohm)

** Needs to be safe with a 300 amp, one cycle surge.

Better get a 40 amp one.



......... Phil

 

[Terry Given]

"Arlet Ottens"




** Correct.




** Designing a drive cct for the SSR that will NEVER accidentally switch
near zero volts is not that easy.






** Needs to be safe with a 300 amp, one cycle surge.

Better get a 40 amp one.

........ Phil

Phil is spot on.

The magic number you need to look for is usually called Ifsm - the
amount of single-cycle surge current it can cope with. Phils
recommendation ought to be about right, but check the SSR datasheet to
be sure. or post the part you think is right, and we can check for you.

Cheers
Terry

 

[Mook Johnson]

"Arlet Ottens"


** Correct.


** Designing a drive cct for the SSR that will NEVER accidentally switch
near zero volts is not that easy.




** Needs to be safe with a 300 amp, one cycle surge.

Better get a 40 amp one.



........ Phil

I'll bite, why does switching on the voltage zero cross cause inrush
current in a transformer?

My thinking is from the old NASA incident with tape wound transformers
without a gap. Sometimes when the device was turned off the core was high
up on the BH loop. The next time if the startup voltage pushed it even
higher into the same direction, the core would saturate for 1 cycle. In
their case it blew the fuses randomly from the high inrush current. They
solved the problem with a gap so the remnant flux density returned to a low
value any time the device was powered off. (from Magnetics Inc literature)

If you switched at the zero cross for a low frequency signal the remnant
flux density should be close to zero and therefore have high permeability
for the first startup cycle. (assuming you switch on at zero cross as
well.)

Where am I off on this? Is it because B laggs voltage due to B being a
volt*second product?

 

[Phil Allison]

"Mook Johnson"
"Phil Allison"

I'll bite, why does switching on the voltage zero cross cause inrush
current in a transformer?

** Cos it maximises the low frequency transient included in the initial few
cycles of the wave train.

For zero crossing, it take one cycle for the average value to equal zero.

For peak crossing, this is met in one a half cycle.

This low frequency transient drives the core into saturation during the
initial half cycle - much more so if it is extended.

If the core is already magnetised in an unfavourable direction at switch
on - the surge duration so much the worse.




....... Phil

 

[[email protected]]

Hello,

I'm considering using an SSR to switch the primary side of a
transformer. The transformer is a toroid, 230V -> 27V. The secondary
side is fixed to a 1 Ohm nichrome wire heating element.

I've read some articles that say switching on at zero crossings causes a
high inrush current, and that it's best to switch on at peak voltage.

If I follow this advice, are there any other issues I need to look out
for ?

What kind of (surge) current capabilities does the SSR need to have for
reliable operation ? (DC resistance of primary side is about 1 Ohm)

My original idea was to switch the secondary side using a pair of SCRs,
but at that current it would require a fairly decent heat sink. If I can
switch the primary side, the dissipation would be reduced considerably.

It is nearly impossible to answer unless you mention what you think
is the secundary current.

Main point with semiconductor switching in the primary is the fact
that the slightest
imbalance will cause a DC component, so DC current to flow.
Also may well need snubbers etc.
Consider a normal relay
If you want proportional control, secondary is what I personally would
prefer,
a resistive load is simple, could be SCR's or a bridge rectifier
followed by some MOSFET,
perhaps with PWM.
It will need a housing anyways, metal housing makes good heatsink.

 

[Phil Allison]

It is nearly impossible to answer unless you mention what you think
is the secundary current.

** Can't you divide 27 by 1 - fuckwit ???

Main point with semiconductor switching in the primary is the fact
that the slightest
imbalance will cause a DC component, so DC current to flow.

** Drivel.

Also may well need snubbers etc.

** Yawn.....

Consider a normal relay

** But the OP has his heart set on using a SSR.

If you want proportional control,

** But the OP don't.

secondary is what I personally would prefer,

** Totally INSANE CRAP !!

At 27 amps rms !!!!!!



........ Phil

 

[Jan Panteltje]

There once was a man from Erlangen..
http://en.wikipedia.org/wiki/Georg_Simon_Ohm

My error, I only grabbed 1 Ohm (that man) primary.
Too much speed reading

At 27 A I think secondary SRC would be no problem, neather a bridge + MOSFET.

 

[Spehro Pefhany]

On Tue, 18 Mar 2008 04:43:44 -0700 (PDT), the renowned

It is nearly impossible to answer unless you mention what you think
is the secundary current.

There once was a man from Erlangen..
http://en.wikipedia.org/wiki/Georg_Simon_Ohm

Main point with semiconductor switching in the primary is the fact
that the slightest
imbalance will cause a DC component, so DC current to flow.
Also may well need snubbers etc.
Consider a normal relay
If you want proportional control, secondary is what I personally would
prefer,
a resistive load is simple, could be SCR's or a bridge rectifier
followed by some MOSFET,
perhaps with PWM.
It will need a housing anyways, metal housing makes good heatsink.

Best regards,
Spehro Pefhany

 

[Fred Bloggs]

I'm considering using an SSR to switch the primary side of a
transformer. The transformer is a toroid, 230V -> 27V. The secondary
side is fixed to a 1 Ohm nichrome wire heating element.

That 1R is reflected into the primary circuit as 1/(27/230)^2=72R making
the maximum possible inrush current due to the secondary loading
1.41*230/72=4.5 Amps peak. This 4.5 Amps then adds in quadrature to the
transformer magnetizing current, both currents passing through the
series ohmic resistance of the primary winding.

I've read some articles that say switching on at zero crossings causes a
high inrush current, and that it's best to switch on at peak voltage.

The big unknown is the residual magnetization of the core at turn on,
but the worst case is usually zero flux because the initial permeablity,
and therefore the inductance of the core, is smallest under that condition.

If I follow this advice, are there any other issues I need to look out
for ?

What kind of (surge) current capabilities does the SSR need to have for
reliable operation ? (DC resistance of primary side is about 1 Ohm)

My original idea was to switch the secondary side using a pair of SCRs,
but at that current it would require a fairly decent heat sink. If I can
switch the primary side, the dissipation would be reduced considerably.

As you can see from the calculations, the 1R in the secondary
contributes little to the inrush compared to the core initialization or
possible supersaturation effects, both of which may last for many line
cycles before reaching steady state. There are lots of rules of thumb on
overrating the SSR, bit if you /have/ to get it right the first time,
then you need a soft start module:
http://www.crydom.com//userResources/productFamilies/54/crydom_sst.pdf

 

[Arlet Ottens]

It is nearly impossible to answer unless you mention what you think
is the secundary current.

Main point with semiconductor switching in the primary is the fact
that the slightest
imbalance will cause a DC component, so DC current to flow.
Also may well need snubbers etc.
Consider a normal relay
If you want proportional control, secondary is what I personally would
prefer,
a resistive load is simple, could be SCR's or a bridge rectifier
followed by some MOSFET,
perhaps with PWM.
It will need a housing anyways, metal housing makes good heatsink.

After reading the various bits of advice, I decided to order a big SSR
(50A continuous) to build a prototype.

I'm using a small MCU to produce the control signal for the SSR, with a
simple zero crossing detector for synchronization. The current in the
primary coil is measured by a current probe on the scope.

After some experimenting, I now have:

- Initial turn-on done using a soft start mechanism, as suggested by
Fred Bloggs, but instead of using a secondary module, the MCU is doing this.

- When the core is fully magnetized, the MCU keeps track of the
direction of the magnetic field.

- Further proportional control is done by switching at zero crossings,
but only when core is magnetized in favorable direction.

Even without a snubber, switching at voltage peaks only produces small
transients (*much* less than switching a 100W light bulb). Inrush
current during soft start phase is only slightly higher than normal.
Once the core is magnetized, and the MCU switches with alternating
polarity, currents are the same as steady-state currents.

Significant DC currents are easily avoided, and small ones that I added
for testing don't seem to have much effect on the performance.

The SSR has no heat sink and only gets slightly warm. Much better than
switching on secondary side which produces more than 25W of heat. The
soft start mechanism is a nice bonus.

 

[Jan Panteltje]

After reading the various bits of advice, I decided to order a big SSR
(50A continuous) to build a prototype.

Some solid state relais already have a zero crossing chip.
Does yours?

Beep

 

[Arlet Ottens]

Some solid state relais already have a zero crossing chip.
Does yours?

No, I picked one with random turn on (Crouzet 84 134 320), otherwise
there's no way to control the inrush currents.

The ones with zero crossing logic seem to be more prevalent, but there
are still plenty of choices for random turn on types.

I picked this big one for my prototype because it had a better chance of
surviving any timing errors. For production, I'll look for something
smaller and cheaper.

 

[Paul E. Schoen]

No, I picked one with random turn on (Crouzet 84 134 320), otherwise
there's no way to control the inrush currents.

The ones with zero crossing logic seem to be more prevalent, but there
are still plenty of choices for random turn on types.

I picked this big one for my prototype because it had a better chance of
surviving any timing errors. For production, I'll look for something
smaller and cheaper.

Your solution seems to be ideal. For my application, I cannot use a soft
start, because I must apply a full current waveform to a circuit breaker to
simulate a fault, and measure its response time accurately. The best I can
do is time the initial application of the test voltage to coincide
approximately to the expected zero crossing of current, based on the
impedance of the source and the load, which are both mostly inductive.

Also, in my case, the load itself opens while the test set has voltage
applied, and it may open at any point on the waveform. Thus arcing and
inductive spikes are inevitable, and the transformer may have some remanant
magnetism after the breaker trips, which will then cause a high inrush on
the next test.

But I have considered adding a demagnetizing sequence after the trip is
detected. I think a series of phase modulated pulses with gradually reduced
duty cycle might reduce the magnetization of the transformer to a minimal
level for the next test.

Any thoughts on that?

Thanks,

Paul

 

[Jan Panteltje]

No, I picked one with random turn on (Crouzet 84 134 320), otherwise
there's no way to control the inrush currents.

The ones with zero crossing logic seem to be more prevalent, but there
are still plenty of choices for random turn on types.

I picked this big one for my prototype because it had a better chance of
surviving any timing errors. For production, I'll look for something
smaller and cheaper.

OK.
I wonder if I had to do it myself if I would not simply have used a normal relay.
As the load is Ohms, I would expect little problems, you will
see a resistor transformed at the primary.
My microwave switches a thousand times about a kW with a very small relay...
Right now while it is doing the French fries.