Using SSR to switch transformer

[Arlet Ottens]

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.

I tried some experiments using phase control on the SSR to slowly turn
off the transformer over 50 cycles (1 second), followed by a test using
a few whole cycles starting at a zero crossing.

I noticed there was still some increased current surge (2 times normal
on the first half cycle), depending on the polarity of the first half
cycle of power. Apparently, the demagnetizing wasn't complete. However,
it is a lot better than starting with the core fully magnetized in a
unfavorable direction. Results may vary with transformer type.

You may also be able to put the core in a consistent magnetized state
after a test, and always start at opposite phase.

 

[[email protected]]

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?

Yes, don't waste your time on it.

 

[[email protected]]

I tried some experiments using phase control on the SSR to slowly turn
off the transformer over 50 cycles (1 second), followed by a test using
a few whole cycles starting at a zero crossing.

I noticed there was still some increased current surge (2 times normal
on the first half cycle), depending on the polarity of the first half
cycle of power. Apparently, the demagnetizing �wasn't complete.

Transformer cores do not hold any significant magnetic flux when there
is no current in the windings.


However,

it is a lot better than starting with the core fully magnetized in a
unfavorable direction.

Can't ever happen whatever you try.


Results may vary with transformer type.

You may also be able to put the core in a consistent magnetized state
after a test,

Nope, can't be done.


and always start at opposite phase.- Hide quoted text -

 

[Arlet Ottens]

Transformer cores do not hold any significant magnetic flux when there
is no current in the windings.

Mine does.

First picture shows current through 3 half cycles, starting and ending
with positive cycle.

http://c-scape.nl/transformer/first.png

Second picture shows same waveform, after a minute of inactivity. Notice
increased current in first half cycle due to core saturation. The effect
is also audible in the transformer.

http://c-scape.nl/transformer/second.png

This happens every time if the waveform starts with same polarity as the
previous one ended, even with considerable time inbetween. I also ran it
starting and ending with negative cycle, and the traces are the same
(but inverted).

Alternating positive and negative bursts shows no extra current.

The experiment has a 230V->27V toroidal transformer controlled by
SSR+MCU, and a small series resistor to limit the current. Secondary
side is permanently attached to 1 Ohm load.

 

[[email protected]]

Mine does.

First picture shows current through 3 half cycles, starting and ending
with positive cycle.

http://c-scape.nl/transformer/first.png

Second picture shows same waveform, after a minute of inactivity. Notice
increased current in first half cycle due to core saturation. The effect
is also audible in the transformer.

http://c-scape.nl/transformer/second.png

This happens every time if the waveform starts with same polarity as the
previous one ended, even with considerable time inbetween. I also ran it
starting and ending with negative cycle, and the traces are the same
(but inverted).

Alternating positive and negative bursts shows no extra current.

Without the voltage waveform as well as the current your trace is of
no value. I expect your not triggering quite how you think you are.

 

[Arlet Ottens]

Without the voltage waveform as well as the current your trace is of
no value. I expect your not triggering quite how you think you are.

I've added a voltage trace (measured across the primary) to the second
waveform:

http://c-scape.nl/transformer/second+v.png

During the current spike, the voltage drops due to current limiting
resistor in series with primary.

 

[[email protected]]

I've added a voltage trace (measured across the primary) to the second
waveform:

http://c-scape.nl/transformer/second+v.png

During the current spike, the voltage drops due to current limiting
resistor in series with primary.- Hide quoted text -

- Show quoted text -

You seem to be triggering at the zero crossing point and not on the
voltage peak. How big is your current limiting resistor? How are you
measureing the current waveform?

 

[Arlet Ottens]

You seem to be triggering at the zero crossing point and not on the
voltage peak. How big is your current limiting resistor? How are you
measureing the current waveform?

Yes, for this test I wanted to demonstrate worst case saturation effect,
so I'm triggering on the zero crossing point.

If I trigger the SSR on voltage peak, I get similar results, but
somewhat lower peak current, as expected.

http://c-scape.nl/transformer/second-peak.png

The series resistor is 33 Ohms. Current trace represents voltage across
this resistor. I'm using 3 voltage probes (one side of resistor, between
resistor and transformer, and other side of transformer), and tracing
the differences between them. Initially, I was using a 0.1 Ohm/10W
resistor, but it died with a flash and a bang during a current surge.
So I decided to pick something bigger.

 

[[email protected]]

Yes, for this test I wanted to demonstrate worst case saturation effect,
so I'm triggering on the zero crossing point.

If I trigger the SSR on voltage peak, I get similar results, but
somewhat lower peak current, as expected.

http://c-scape.nl/transformer/second-peak.png

The series resistor is 33 Ohms. Current trace represents voltage across
this resistor. I'm using 3 voltage probes (one side of resistor, between
resistor and transformer, and other side of transformer), and tracing
the differences between them.  Initially, I was using a 0.1 Ohm/10W
resistor, but it died with a flash and a bang during a current surge.
So I decided to pick something bigger.- Hide quoted text -

- Show quoted text -

33 ohms will seriously alter your results. What do you mean by
"tracing the differences"? does your scope have 3 channels? Or are you
not measureing them all at the same time. You should get almost no
surge if you fire at the top of the voltage waveform, if you are then
you are doing something wrong.

 

[Arlet Ottens]

33 ohms will seriously alter your results. What do you mean by
"tracing the differences"? does your scope have 3 channels? Or are you
not measureing them all at the same time. You should get almost no
surge if you fire at the top of the voltage waveform, if you are then
you are doing something wrong.

I sure that the 33 Ohm resistor alters the results, but the fact that
there is no current surge when using bursts of alternating polarity does
imply that the core is still magnetized to some degree. Also, with an
even number of half cycles, there are no current surges with repeated
bursts.

Without the series resistor, the effect is similar. Unfortunately, I
don't have a real current probe, so I can't measure the current without
the resistor.

Here are the voltage waveforms across the transformer primary, switching
3 half cycles at peak voltage _without_ series resistor. First burst,
followed by exactly the same burst a minute later.

http://c-scape.nl/transformer/first-no-R.png
http://c-scape.nl/transformer/second-no-R.png

You can see the voltage drop across transformer as current increases.
The lights in my room also briefly dim during second burst.

The scope has 4 channels, so everything is measured at the same time.

 

[[email protected]]

I sure that the 33 Ohm resistor alters the results, but the fact that
there is no current surge when using bursts of alternating polarity does
imply that the core is still magnetized to some degree. Also, with an
even number of half cycles, there are no current surges with repeated
bursts.

Without the series resistor, the effect is similar. Unfortunately, I
don't have a real current probe, so I can't measure the current without
the resistor.

Here are the voltage waveforms across the transformer primary, switching
3 half cycles at peak voltage _without_ series resistor. First burst,
followed by exactly the same burst a minute later.

http://c-scape.nl/transformer/first-no-R.pnghttp://c-scape.nl/transformer/second-no-R.png

You can see the voltage drop across transformer as current increases.
The lights in my room also briefly dim during second burst.

The scope has 4 channels, so everything is measured at the same time.- Hide quoted text -

- Show quoted text -

You only fire the first wave at the voltage peak not the rest, they
should be at the zero crossing point so the voltage is has no breaks.
When I was making tin can seam resistance welders I had exactly the
same set up as you, problems only occured when the transforme didn't
recieve alternating half cycles.

 

[Jan Panteltje]

Without the series resistor, the effect is similar. Unfortunately, I
don't have a real current probe, so I can't measure the current without
the resistor.

Here are the voltage waveforms across the transformer primary, switching
3 half cycles at peak voltage _without_ series resistor. First burst,
followed by exactly the same burst a minute later.

http://c-scape.nl/transformer/first-no-R.png
http://c-scape.nl/transformer/second-no-R.png

You can see the voltage drop across transformer as current increases.
The lights in my room also briefly dim during second burst.

Question: did you have the 1 Ohm load connected in teh experiments?
You transformer ratio is 230 / 27 = 8.51, so the 1 Ohm
is seen as the square of that in the primary, so as 72 Ohm.
I would expect the magnetic filed to collaps sooner with 1 Ohm load.
33 Ohm series with 72 Ohm load seems not a pleasant ratio.
Better use a lower shunt if the load is connected?

 

[Paul E. Schoen]

I sure that the 33 Ohm resistor alters the results, but the fact that
there is no current surge when using bursts of alternating polarity does
imply that the core is still magnetized to some degree. Also, with an
even number of half cycles, there are no current surges with repeated
bursts.

Without the series resistor, the effect is similar. Unfortunately, I
don't have a real current probe, so I can't measure the current without
the resistor.

Here are the voltage waveforms across the transformer primary, switching
3 half cycles at peak voltage _without_ series resistor. First burst,
followed by exactly the same burst a minute later.

http://c-scape.nl/transformer/first-no-R.png
http://c-scape.nl/transformer/second-no-R.png

You can see the voltage drop across transformer as current increases. The
lights in my room also briefly dim during second burst.

The scope has 4 channels, so everything is measured at the same time.

This effect is caused by a net DC component from an unequal number of
positive and negative excursions. In the test sets I design, we set up a
burst of 5 cycles for instantaneous trip testing of circuit breakers. While
the current is increasing, we make sure there are actually five full
cycles, and each subsequent pulse is clean and without a high current
surge. But if the timing is set so that there are 4.5 or 5.5 cycles, we get
large inrush currents.

This is, essentially, magnetic core memory, similar to that used in ancient
computers for data storage.

If the voltage waveform were reduced gradually, the net DC component would
be lower, until the magnetization becomes insignificant.

We tried reducing inrush current using large thermistors, but they would
eventually heat up and lose their effect, and we could not wait for them to
cool between pulses. Careful control of the number of pulses helped a lot,
but when the breaker trips, there are very often odd numbers of half-cycles
and the next test would have a large surge.

One way we found to minimize it was to apply a short burst of about 1/2 the
voltage, with the proper even number of half-cycles, and then the next
full-voltage pulses were OK.

Paul

 

[Arlet Ottens]

Question: did you have the 1 Ohm load connected in teh experiments?
You transformer ratio is 230 / 27 = 8.51, so the 1 Ohm
is seen as the square of that in the primary, so as 72 Ohm.
I would expect the magnetic filed to collaps sooner with 1 Ohm load.
33 Ohm series with 72 Ohm load seems not a pleasant ratio.
Better use a lower shunt if the load is connected?

Yes, the 1 Ohm load is always present.

The 33 Ohm series resistor was just for safe testing, and to prevent
more things from blowing up. I didn't have anything smaller. During
normal use, and with correct SSR trigger, it's not used.

 

[Arlet Ottens]

This effect is caused by a net DC component from an unequal number of
positive and negative excursions. In the test sets I design, we set up a
burst of 5 cycles for instantaneous trip testing of circuit breakers. While
the current is increasing, we make sure there are actually five full
cycles, and each subsequent pulse is clean and without a high current
surge. But if the timing is set so that there are 4.5 or 5.5 cycles, we get
large inrush currents.

This is, essentially, magnetic core memory, similar to that used in ancient
computers for data storage.

Yes. In my case, I can keep an eye on the cycle count, and make sure
there are always an integral number of cycles during normal operation.
The only case where this isn't possible is when the power is suddenly
removed, for instance when the machine is unplugged while in use, or a
circuit breaker trips. This is expected to be very rare.

To cope with those cases, I use a soft start at power up. See:
http://c-scape.nl/transformer/soft-start.png

The waveforms I referenced were created on purpose to test the effects
of bad timing, and to demonstrate the permanent magnetization of the
core. They don't reflect typical timing of the finished product.

 

[Jan Panteltje]

Yes, the 1 Ohm load is always present.

The 33 Ohm series resistor was just for safe testing, and to prevent
more things from blowing up. I didn't have anything smaller. During
normal use, and with correct SSR trigger, it's not used.

Right, so evaluating 33 + 72 = 105 Ohm.
Total power at 230 V is 230 x 230 / 105 = 503 W.
From that 33/105 goes into your resistor, so 158 W.
Did you use an electric heater as resistor?
Because 230 x 230 / 33 = 1.6 kW ;-)
Just curious to the setup...

 

[Arlet Ottens]

Right, so evaluating 33 + 72 = 105 Ohm.
Total power at 230 V is 230 x 230 / 105 = 503 W.
From that 33/105 goes into your resistor, so 158 W.
Did you use an electric heater as resistor?
Because 230 x 230 / 33 = 1.6 kW ;-)
Just curious to the setup...

Yes, it's an old 1.5 kW electric hotplate that I use for stuff like this.

 

[Paul E. Schoen]

Yes, it's an old 1.5 kW electric hotplate that I use for stuff like this.

One or more incandescent light bulbs is very useful for performing tests
where high currents might occur. There is about a 15:1 ratio of resistance
from cold to hot, and you get a very visual indication when something isn't
right.

I use an old car headlight for 12 VDC stuff.

Paul