Author Topic: Add "soft start" to Isolation transformer  (Read 14694 times)

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Offline Ian.M

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Re: Add "soft start" to Isolation transformer
« Reply #50 on: September 08, 2016, 06:36:44 am »
You are right about the need for a high power resistor with an adequate surge rating, but the problem with the voltage sensing relay approach is that a toroidal transformer surge tends to be highly asymmetric, due to core saturation, therefore the relay coil will see effectively half wave rectified DC from the beginning of the surge.  A time delay relay is a much safer choice.
 

Online Zero999

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Re: Add "soft start" to Isolation transformer
« Reply #51 on: September 08, 2016, 08:53:48 am »
You are right about the need for a high power resistor with an adequate surge rating, but the problem with the voltage sensing relay approach is that a toroidal transformer surge tends to be highly asymmetric, due to core saturation, therefore the relay coil will see effectively half wave rectified DC from the beginning of the surge.  A time delay relay is a much safer choice.
I agree. Ideally you want a timer relay which can run off the mains but if you can't find one you can make your own from a 555 timer, run off a bridge rectifier + smoothing capacitor, connected to a lower voltage winding wound on top of the toroidal transformer.
 

Offline edpalmer42

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Re: Add "soft start" to Isolation transformer
« Reply #52 on: September 08, 2016, 04:27:03 pm »
First, the OP doesn't have a toroidal transformer, he has a regular transformer.  IanJ, who has the toroidal transformer, doesn't feel he needs a soft-start.  So the characteristics of a toroidal transformer, while interesting, aren't part of the original conversation..

Second, a timer is an acceptable solution for a device that presents a stable load to the AC network, but the OP has a general-purpose isolation transformer that will most likely have different loads - which may have starting surges of their own.  The timer has no knowledge of the load or the surge so the delay may be too short or too long.  If too short, there will still be a surge when the relay operates.  This might require a larger relay to prevent burning the contacts.  If the delay is too long, the series limiting resistor will have to be much larger to handle the surge current and power dissipation.  In my example of a 300W bulb protected by a 20 ohm 25W resistor, the resistor is instantaneously carrying a load of something like 10 KW due to the bulb's low initial resistance.  This is the second resistor I put in the circuit.  The first one couldn't take the surge.  Even though the total power dissipation is low, the surge current, which is far beyond the resistor's rating, caused the resistance wire to open.

So for variable load situations, a time-delay relay is not a good solution.  If transformer characteristics upset the relay's operation, then some form of voltage-sensing circuit could be used to provide more reliable operation of the relay.  Note that for anything other than hobbyist uses, a voltage-sensing circuit is required.  Pull-in voltage for relays is not well-defined.

Ed
 

Offline uncle_bob

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Re: Add "soft start" to Isolation transformer
« Reply #53 on: September 08, 2016, 04:42:18 pm »
Hi

The gotcha with all of the ARRL surge suppression stuff is that it is *not* targeted at the "isolation transformer problem". All of it is designed for another purpose: To take out the surge caused by charging the capacitors on a big DC supply. That is indeed a very real issue and you can calculate what needs to be done to take care of it. Capacitors slurp up a big dose of current when they are given a voltage step. That's the way they work. You *can* put a choke in series with the caps to suppress the current spike. That is an expensive thing to do. The choke does *not* immediately move a lot of current when presented with a voltage step, thus no problem.

Bob
 

Offline edpalmer42

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Re: Add "soft start" to Isolation transformer
« Reply #54 on: September 08, 2016, 06:45:54 pm »
Hi Bob,

Yes, the ARRL idea was originally used to suppress the surge of capacitors charging and that could be handled by a rather large choke.  But does it matter?  A surge is a surge.  The transformer surge will likely be shorter than the capacitor surge, but the relay can handle both.  It quite happily handles my incandescent lamp surge.

I like the idea that the resistor is in the circuit for as long as necessary, whatever that is, but no longer.  Yes, there may be some cut-and-try work required to get things working with your particular transformer and relay.  I've noticed that some relays don't spec pull-in voltage and different transformers would certainly have different surge characteristics.

Ed
 

Offline raptor1956

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Re: Add "soft start" to Isolation transformer
« Reply #55 on: September 08, 2016, 08:14:05 pm »
If the core saturation is well understood to last, at most, 6 cycles, then a time delay relay can be set some short period longer than that -- perhaps 0.3 seconds or there abouts.  I'm not sure I see the need for a higher power resistor given the small total ENERGY it needs to handle -- the bigger issue I'd think is current handling ability.  But, if you used a 20? resistor with a mains voltage of 120VAC the actual max current should be less than 8.5A no matter what the core is doing or, in the case of a PS with huge caps, what the caps want to do.  So, someone need to explain just how and why a larger power resistor is needed given the relatively low energy it need to handle.


Brian
 

Offline julian1

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Re: Add "soft start" to Isolation transformer
« Reply #56 on: September 08, 2016, 08:28:29 pm »
Quote
what level of peak inrush current should one expect from a toroidal transformer rated at,

Wikipedia doesn't give the reference but states,

Quote
Toroidal transformers, using less copper for the same power handling, can have up to 60 times inrush to running current. Worst case inrush happens when the primary winding is connected at an instant around the zero-crossing of the primary voltage,

I've seen similar multiples quoted elsewhere.

Offline raptor1956

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Re: Add "soft start" to Isolation transformer
« Reply #57 on: September 08, 2016, 09:14:17 pm »
Quote
what level of peak inrush current should one expect from a toroidal transformer rated at,

Wikipedia doesn't give the reference but states,

Quote
Toroidal transformers, using less copper for the same power handling, can have up to 60 times inrush to running current. Worst case inrush happens when the primary winding is connected at an instant around the zero-crossing of the primary voltage,

I've seen similar multiples quoted elsewhere.


Good info...

So 60X the 6.25A rating would be ... 375A -- that would be worst case but no CB in the 15A-20A range would handle that without tripping.  But, with a 20? current limiting resistor on the input of a 120VAC mains supply the peak current would be less than 8.5A no matter what the core was up to or the phase angle of the voltage.


Brian
 

Offline edpalmer42

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Re: Add "soft start" to Isolation transformer
« Reply #58 on: September 08, 2016, 09:36:47 pm »
If the core saturation is well understood to last, at most, 6 cycles, then a time delay relay can be set some short period longer than that -- perhaps 0.3 seconds or there abouts.  I'm not sure I see the need for a higher power resistor given the small total ENERGY it needs to handle -- the bigger issue I'd think is current handling ability.  But, if you used a 20? resistor with a mains voltage of 120VAC the actual max current should be less than 8.5A no matter what the core is doing or, in the case of a PS with huge caps, what the caps want to do.  So, someone need to explain just how and why a larger power resistor is needed given the relatively low energy it need to handle.


Brian

That was my thinking when I used the first resistor to protect my projector bulb.  I think it was still 20 ohms, but maybe 10 watts.  It worked for awhile, but then it opened like a fuse.  So I did the calculation.  120V, 20 ohms, 8.5 amps.  So?  Wait.  20 ohms, 10 watts means that full load current is ...  0.7 amps.  Oh.  Maybe it didn't like a current surge over 10 times its rating.  Oops!  Let's try a 25W resistor.  That's only about 8 times its rating.   ::)  Well, it works.   Whatever.   ;)

In case anyone is wondering, you'd need a 1500 watt resistor to get 8.5 amps through a 20 ohm value.  I could do it.  I've got a couple of electric stove elements in my junk box that would be perfect, electrically.  But I don't think I will.  :)

Ed
 

Offline FlyingHacker

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Re: Add "soft start" to Isolation transformer
« Reply #59 on: September 08, 2016, 09:41:52 pm »
I've got a couple of electric stove elements in my junk box that would be perfect, electrically.  But I don't think I will.  :)

Aw, why not? Then you would have a coffee warmer to go with it!  :-DD
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Offline raptor1956

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Re: Add "soft start" to Isolation transformer
« Reply #60 on: September 08, 2016, 09:57:27 pm »
If the core saturation is well understood to last, at most, 6 cycles, then a time delay relay can be set some short period longer than that -- perhaps 0.3 seconds or there abouts.  I'm not sure I see the need for a higher power resistor given the small total ENERGY it needs to handle -- the bigger issue I'd think is current handling ability.  But, if you used a 20? resistor with a mains voltage of 120VAC the actual max current should be less than 8.5A no matter what the core is doing or, in the case of a PS with huge caps, what the caps want to do.  So, someone need to explain just how and why a larger power resistor is needed given the relatively low energy it need to handle.


Brian

That was my thinking when I used the first resistor to protect my projector bulb.  I think it was still 20 ohms, but maybe 10 watts.  It worked for awhile, but then it opened like a fuse.  So I did the calculation.  120V, 20 ohms, 8.5 amps.  So?  Wait.  20 ohms, 10 watts means that full load current is ...  0.7 amps.  Oh.  Maybe it didn't like a current surge over 10 times its rating.  Oops!  Let's try a 25W resistor.  That's only about 8 times its rating.   ::)  Well, it works.   Whatever.   ;)

In case anyone is wondering, you'd need a 1500 watt resistor to get 8.5 amps through a 20 ohm value.  I could do it.  I've got a couple of electric stove elements in my junk box that would be perfect, electrically.  But I don't think I will.  :)

Ed


I've been playing around with some 30ga wirewrap wire which is rated at 338mOhm/m and a rated ampacity of 0.86A.  It can handle 10A for 10sec, 15A for 1 sec, and 86A for 1msec. 

But, it's hard to know how the power resistor is constructed so there may in fact be something in the construction of that type of resistor that has a brick wall limit of current that's in the single digits...


Brian
 

Offline edpalmer42

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Re: Add "soft start" to Isolation transformer
« Reply #61 on: September 08, 2016, 10:14:09 pm »
I've been playing around with some 30ga wirewrap wire which is rated at 338mOhm/m and a rated ampacity of 0.86A.  It can handle 10A for 10sec, 15A for 1 sec, and 86A for 1msec. 

But, it's hard to know how the power resistor is constructed so there may in fact be something in the construction of that type of resistor that has a brick wall limit of current that's in the single digits...

Or maybe there was a manufacturing flaw in that resistor.  Don't know.  It was one of those gold colored aluminum/ceramic types so there was no way to get into it.  I didn't even think to try, I just realized that I'd put too much current through it and moved on.

Ed
 

Offline Ian.M

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Re: Add "soft start" to Isolation transformer
« Reply #62 on: September 08, 2016, 11:51:59 pm »
But, it's hard to know how the power resistor is constructed so there may in fact be something in the construction of that type of resistor that has a brick wall limit of current that's in the single digits...

Or maybe there was a manufacturing flaw in that resistor.  Don't know.  It was one of those gold colored aluminum/ceramic types so there was no way to get into it.  I didn't even think to try, I just realized that I'd put too much current through it and moved on.

Meaningful brick wall current limits are rarely found in resistance elements. However brick wall energy limits are common.

Consider a wirewound or foil resistance element of some known resistance alloy.   The ultra short non-repetitive pulse power limit is directly related the thermal mass of the resistance element itself, and for a known alloy can be estimated from the energy that would be required to raise the resistance element to its melting point.
For longer pulses the thermal mass of the former or substrate becomes significant and once you get out to the whole second timescale, its the thermal mass of the whole resistor.

However stresses in the element may cause it to neck near its melting point causing hot-spotting on subsequent pulses, it may gradually oxidise, and thermal degradation of its insulation may produce byproducts that corrode it, so the repetitive limit will be considerably less.   

*SOME* resistor manufacturers have good datasheets with pulse power ratings, typically 5 to 10 times the continuous rating for whole second timescale pulses, and a few even have application notes: e.g. http://www.ttelectronicsresistors.com/pdf/application_notes/Pulse-Overload_AN.pdf

A mains rated heating element will certainly withstand the pulse power of a mains soft-start application, but it would be advisable to add a thermal fuse or cutout rated for an inductive load in case the shorting relay fails.

For bench use, you can also use NTC thermistors designed for anti-surge applications, but for reliability reasons, even though they may be rated for continuous operation, one should use a shorting relay, which also reduces the risk they will still be hot, and thus ineffective if the  supply is briefly interrupted.  One *ASS*U*ME*s the user will be smart enough not to repeatedly frobulate the upstream switch, as that can cause worst-case core saturation with the thermistor(s) already hot enough to be near minimum resistance.

Controlling the surge while staying within the resistor's pulse power limits is $EXPENSIVE$ and either requires a very high wattage resistor or a higher value resistor (to limit the peak power) than would initially be expected.  The purpose of a bench isolating transformer does not and should not include soft-starting its load, as the load should have its own soft start circuit if required.   As such, the use of a second relay to delay load switch-on till after the anti-surge resistor has been shorted and the transformer current has stabilised, is likely to be cost-effective at higher VA ratings, as a much smaller resistor can be used if it only has to handle the unloaded transformer surge. 
« Last Edit: September 09, 2016, 12:01:02 am by Ian.M »
 

Offline FlyingHacker

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Re: Add "soft start" to Isolation transformer
« Reply #63 on: September 09, 2016, 12:07:48 am »
Controlling the surge while staying within the resistor's pulse power limits is $EXPENSIVE$ and either requires a very high wattage resistor or a higher value resistor (to limit the peak power) than would initially be expected.  The purpose of a bench isolating transformer does not and should not include soft-starting its load, as the load should have its own soft start circuit if required.   As such, the use of a second relay to delay load switch-on till after the anti-surge resistor has been shorted and the transformer current has stabilised, is likely to be cost-effective at higher VA ratings, as a much smaller resistor can be used if it only has to handle the unloaded transformer surge.

Of course for a piece of lab gear, to be used by people who in theory know (some) what they are doing, you can just put another switch after the isolation transformer. Turn on the transformer, which has an NTC or relay, or both to keep it form tripping the breaker. Then hook up your load and turn on the load switch by hand, or ramp it up with a Variac, etc.

If you are going to make a product then it needs to be idiot proof. If you are making something for use in your own lab then it only has to be you-proof.
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Offline raptor1956

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Re: Add "soft start" to Isolation transformer
« Reply #64 on: September 09, 2016, 02:25:54 am »
Here's a first pass on what I'd do to build a soft start isolation transformer.  As you can see (hopefully) until input time delay relay K1 turns on at, say, 0.5 seconds the toroidal transformer is current limited by R1 -- a 20 ohm 25W resistor.  Additionally, until K1 picks the output relay, K2, is inhibited from turning on because it needs K1-2 to be closed in order to turn K2 on and K1-2 is open until K1 times out.  Once K1 times out K1-2 closes and the output relay can be turned on.  To turn K2 on K1-2 must be closed as above and momentary switch "Output ON" is pressed.  Once that happens and relay K2 picks the output as activated and relay K2 is latched with K2-3.

So, with this arrangement the output can not be on until K1 picks AND the output is manually turned on with "Output ON".

As I mentioned in a prior the output latching arrangement is very like an EMO circuit.


Brian

 
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Offline Ian.M

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Re: Add "soft start" to Isolation transformer
« Reply #65 on: September 09, 2016, 03:25:17 am »
IMHO that's *Waaaaaay* over-complex.  Also heavy duty relays/contactors with auxiliary contacts don't come cheap.   As long as your time delay relay has sufficient isolation between its poles not to compromise the transformer's isolation rating, simply use a DPST time delay relay, one pole to short the resistor, the other to connect the load to the secondary.   If the relay's pole-to-pole isolation is doubtful use separate SPST relays, either both time delay or driven by a master delay relay or circuit.

I also don't like the idea of pushbutton output switching much - simply use switched outlets or if you want something that's actually safe to work on the load without total manual disconnection, use an isolation switch designed for lockout-tagout.
 

Offline FlyingHacker

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Re: Add "soft start" to Isolation transformer
« Reply #66 on: September 09, 2016, 04:14:16 am »
Looks pretty cool if you are making a product to sell. However, Installing an NTC takes about five minutes and one part (plus some heat shrink perhaps). It may not work well for the once a year flickering power outage, if that turn on happens to line up with the wrong phase of the AC cycle, but otherwise it just works, and lets you move on to other projects. Plug a power strip into the output and you have your DUT power switch, and can leave the transformer on for your whole work session.

Do get me wrong; I love over engineering stuff too.
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Offline SteveyG

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Re: Add "soft start" to Isolation transformer
« Reply #67 on: September 09, 2016, 07:54:58 am »
OTT soft starter - although the soft start bit only consists of resistors for current limiting:


Offline julian1

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Re: Add "soft start" to Isolation transformer
« Reply #68 on: September 09, 2016, 08:43:26 am »
I've experimented a bit with NTC's and toroids. There's a couple of problems with using them by themselves.

- if power is cycled or if there's a short interruption to power, the NTC stays hot, and therefore offers no unrush protection. In fact it would take a minute or more for mine to cool.

- With no load on the secondary, there's not enough voltage drop, to even keep the thing warm. That means it will thermally-cycle between times the transformer is loaded and when it's not.

Offline FlyingHacker

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Re: Add "soft start" to Isolation transformer
« Reply #69 on: September 09, 2016, 03:26:32 pm »
I've experimented a bit with NTC's and toroids. There's a couple of problems with using them by themselves.

- if power is cycled or if there's a short interruption to power, the NTC stays hot, and therefore offers no unrush protection. In fact it would take a minute or more for mine to cool.

- With no load on the secondary, there's not enough voltage drop, to even keep the thing warm. That means it will thermally-cycle between times the transformer is loaded and when it's not.

Yes, it takes a while for the NTC to cool and reset. That is why you pit the DUT on a separate switch or on a power strip. You just leave the transformer on the whole time you are at your bench. This is like old tube gear. You need to wait a little while before turning it back on.

Personally I run a Variac after the isolation transformer. Then that goes into a switch box with a volt/amp meter. It's certainly not foolproof, but it was easy and cheap (free sample NTC).

If I were going to make something more complex I think I might make it as a separate outlet box, with its own plug. That way I could use it to limit inrush on anything I plugged into the box, and I could change the order around or remove it per application. In that case the added complexity would be useful, as it would be more of a generic inrush limiting tool. Now that I might consider building!
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Offline raptor1956

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Re: Add "soft start" to Isolation transformer
« Reply #70 on: September 09, 2016, 08:37:39 pm »
IMHO that's *Waaaaaay* over-complex.  Also heavy duty relays/contactors with auxiliary contacts don't come cheap.   As long as your time delay relay has sufficient isolation between its poles not to compromise the transformer's isolation rating, simply use a DPST time delay relay, one pole to short the resistor, the other to connect the load to the secondary.   If the relay's pole-to-pole isolation is doubtful use separate SPST relays, either both time delay or driven by a master delay relay or circuit.

I also don't like the idea of pushbutton output switching much - simply use switched outlets or if you want something that's actually safe to work on the load without total manual disconnection, use an isolation switch designed for lockout-tagout.


Yes, higher current time delay relays are fairly pricey and that would drive the cost up.  The push-button output is kind of driven by the need to make sure there is no output load until the input resistance is bypassed and that means relays.  The thermistor approach doesn't make sense to me given the potential cycling issue while there still hot.

The cheapest solution would be a couple of DPDT switches with the necessary rating and then manually throwing them in the right sequence at the right time -- throw the first one after a second to bypass the series resistor then throw the second one when you want to turn the output on.  Doing that to avoid the need for more expensive relays could save $150 or so.  That approach should be fine for the guy that knows what there doing but it has risks that limit it ability to be sold or even given to someone.

Perhaps a cheaper approach, if you still want to use relays, would be to add a third relay.  Use a cheap, low current ON delay relay to pick a higher current relay to bypass the resistor.  So, instead of buying a high current ON delay relay at, say, $150, buy a $20 ON delay relay with lesser current rating but only use that to pick the input relay and enable the output relay.  Both the input and output relays are simple dumb relays that only need to have sufficient current handling and no smarts and would therefore be much cheaper.  With this approach the relay cost in total could be kept below about $60 and perhaps quite a bit below it.


Brian
 

Offline FlyingHacker

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Re: Add "soft start" to Isolation transformer
« Reply #71 on: September 09, 2016, 09:41:56 pm »
The cheapest solution would be a couple of DPDT switches with the necessary rating and then manually throwing them in the right sequence at the right time -- throw the first one after a second to bypass the series resistor then throw the second one when you want to turn the output on. 

How about a four position rotary switch. OFF -> START (resistor in line) -> ON (no resistor) -> LOAD (turn on load).

That is actually a simple, elegant solution that is fairly foolproof unless you whip the switch around crazy fast. Just get one with enough clicking force in each stop that you have to move it slowly. Would just have to be sure the resistors could handle being left switched in permanently.

Quote
Doing that to avoid the need for more expensive relays could save $150 or so.  That approach should be fine for the guy that knows what there doing but it has risks that limit it ability to be sold or even given to someone.

Holy crap! Those are expensive relays.
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Offline The Electrician

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Re: Add "soft start" to Isolation transformer
« Reply #72 on: September 09, 2016, 10:57:57 pm »
 


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