controlling 250V DC/AC with solid state

[Lurch]

the customer will have to pay for testing two solutions although I'm trying to avoid it.

Why use 2 separate products? Maybe this is me not understanding exactly what you are testing or why but can't you just make a single board with a relay and MOSFET's and then just test that rather than making 2 separate boards?

[Simon]

the customer will have to pay for testing two solutions although I'm trying to avoid it.

Why use 2 separate products? Maybe this is me not understanding exactly what you are testing or why but can't you just make a single board with a relay and MOSFET's and then just test that rather than making 2 separate boards?

We are making units for trains so they have to undergo stringent tests. The power available in trains seems to vary from 110V AC/DC to 240V AC/DC or at least that is the range we want to cover to try and make one control board that will cover all possibilities. If we can't use the same circuit for both AC and DC we may have to make separate power switching boards that get swapped for AC or DC versions with a common main control board. As it is we can't have that sort of current on a PCB so need off board switching anyway.

[Codemonkey]

You've not said how much money you consider to be too much. There are plenty of relays and contactors that will switch a 140V 20A DC load without any problem, just as there are many solid state relays that will do the job. I can't imagine trying to re-invent the wheel here being any cheaper than just buying a mass produced off the shelf component.

I used to design emergency lighting systems that ran at ~110VDC or AC, with loads of anything up to about 200A and we never had any problem finding a relay/contactor to do the job.

[Simon]

Well the current AC design for 240V has standard relays, I guess they cost at most a couple of pounds. I''m after the next cheapest option.

[AlfBaz]

Series connection is actually a technique to use but you won't turn a regular AC contqctor into a DC one quite that easily. A proper DC contactor is qualitatively different in that there are special arrangements to extinguish the arc when contact breaks.  You won't find those in a regular contactor.

I must be missing something. Whilst some contactors don't have major arc drawing shields most of them have some sort of metallic flutes to achieve some degree of it. Secondly you are breaking the load in 6 places, 2 on the (pick one) negative leg and 4 on the positive. Are you suggesting that at 120V 20A DC the energy will be sufficient to keep arc's going on all six break points or are you envisioning the arc drawing out of the contact, ionizing the external air and flashing across the outside of the contactor?

I'm not trying to be obstinate I am genuinely interested. I've seen this technique used more often than I care to mention and am puzzled at these concerns

[Simon]

I had considered a capacitor between contacts so that the "pulse" from breaking contacts goes through/charges the capacitor as an alternative path to the breaking contacts but what do you do with the charged capacitor, as soon as you close the contacts again you will have a massive short on the capacitor and probably do more damage unless extra contacts/relays can then cut the dead load off and discharge the capacitor ready for the make.

[Lurch]

I must be missing something. Whilst some contactors don't have major arc drawing shields most of them have some sort of metallic flutes to achieve some degree of it. Secondly you are breaking the load in 6 places, 2 on the (pick one) negative leg and 4 on the positive. Are you suggesting that at 120V 20A DC the energy will be sufficient to keep arc's going on all six break points or are you envisioning the arc drawing out of the contact, ionizing the external air and flashing across the outside of the contactor?

I'd say a little of both. There's also the inrush currents to consider, which under fault conditions could be pretty major and you'd end up with a massive burning thing where your contactor was. There's also the utilisation category, and if you just slap a standard AC-1/AC-3 contactor in and run 120VDC@20A through it and anything ever goes wrong it will be your fault regardless. You could technically get away with doing it as long as nothing ever drifted outside of the perfect conditions you tested it in, but I wouldn't want to put my name to it.

[AlfBaz]

I'd say a little of both. There's also the inrush currents to consider, which under fault conditions could be pretty major and you'd end up with a massive burning thing where your contactor was.

Inrush currents? Not quite sure how they pertain to the problem at hand which is using a contactor designed for AC to break DC and under fault conditions the problem is not merely constrained to dc and should be handled by protection up stream

There's also the utilisation category, and if you just slap a standard AC-1/AC-3 contactor in and run 120VDC@20A through it and anything ever goes wrong it will be your fault regardless. You could technically get away with doing it as long as nothing ever drifted outside of the perfect conditions you tested it in, but I wouldn't want to put my name to it.

I was hoping for a better explanation. You've simply expanded on "Bad idea" without adding the reason why. If you choose the appropriately size contactor for the application I still cannot see where the problem lies
As for the "utilisation category" it's used to de-rate  the device depending on the load and again if something goes wrong you need to analyse the hazards wether you are use AC or DC and are going to have to implement other protection features such as monitoring for voltage at the contactors output when no power is applied to the coil and possibly energising  a shunt trip on an upstream CB that's feeding the rectifier.

[SeanB]

Just remember the fuses and circuit breakers you use have to be DC rated as well. For breakers that means big and massive arc chanbers and for fuses HRC sand filled units. AFAIK the only ones rated for DC are Diazed fuses, and a few ABB motor control fuses. In any case they are not the cheapest units.

[peter.mitchell]

I'd say a little of both. There's also the inrush currents to consider, which under fault conditions could be pretty major and you'd end up with a massive burning thing where your contactor was.

Inrush currents? Not quite sure how they pertain to the problem at hand which is using a contactor designed for AC to break DC and under fault conditions the problem is not merely constrained to dc and should be handled by protection up stream

There's also the utilisation category, and if you just slap a standard AC-1/AC-3 contactor in and run 120VDC@20A through it and anything ever goes wrong it will be your fault regardless. You could technically get away with doing it as long as nothing ever drifted outside of the perfect conditions you tested it in, but I wouldn't want to put my name to it.

I was hoping for a better explanation. You've simply expanded on "Bad idea" without adding the reason why. If you choose the appropriately size contactor for the application I still cannot see where the problem lies
As for the "utilisation category" it's used to de-rate  the device depending on the load and again if something goes wrong you need to analyse the hazards wether you are use AC or DC and are going to have to implement other protection features such as monitoring for voltage at the contactors output when no power is applied to the coil and possibly energising  a shunt trip on an upstream CB that's feeding the rectifier.

Yeah, i'm not seeing it either, I mean, Inrush currents? At the same RMS voltage, AC would provide significantly more inrush current, if it closed near the peak of the cycle. As for current derating, it isn't an issue because the RMS heat would be the same... The only issue i can see is quenching the arc, which was the purpose of using 3 phase contactors with the contactors in series.
THAT SAID, if you are using a 3 phase contactor, and in series, i would say use one rated for 3x the current, as generally, current is expected to be shared across the phases/contacts equally, rather than the total current going through each contact individually.

The loads you have specified, a heating element, and a DC brushed motor should have no issue being switched in this method, however, as you will need approval from the rail regulatory body, you probably won't pass with this solution.

Over rated, DC capable SSR is the solidstate way to go, but they are very expensive, a massive DC contactor would be cheaper, by far.

[Lurch]

Yeah, i'm not seeing it either, I mean, Inrush currents? At the same RMS voltage, AC would provide significantly more inrush current, if it closed near the peak of the cycle. As for current derating, it isn't an issue because the RMS heat would be the same...

I thought I saw some mention of an inverter somewhere, maybe I have misunderstood something along the way so I'll retract that if it's all wrong!

however, as you will need approval from the rail regulatory body, you probably won't pass with this solution.

I think that's what I was trying to get at.

[Kremmen]

Series connection is actually a technique to use but you won't turn a regular AC contqctor into a DC one quite that easily. A proper DC contactor is qualitatively different in that there are special arrangements to extinguish the arc when contact breaks.  You won't find those in a regular contactor.

I must be missing something. Whilst some contactors don't have major arc drawing shields most of them have some sort of metallic flutes to achieve some degree of it. Secondly you are breaking the load in 6 places, 2 on the (pick one) negative leg and 4 on the positive. Are you suggesting that at 120V 20A DC the energy will be sufficient to keep arc's going on all six break points or are you envisioning the arc drawing out of the contact, ionizing the external air and flashing across the outside of the contactor?

I'm not trying to be obstinate I am genuinely interested. I've seen this technique used more often than I care to mention and am puzzled at these concerns

Of course any relay or contactor will make and break any voltage/current until it is destroyed or decayred beyond repair. Unless you grossly overload the unit, breakage won't happen right away, and maybe not for a long time. A relay / contactor is an electromechanical device so it doesn't have a similar abrupt breakdown behavior as say a semiconductor switch. So you may get away with abusing a device for some considerable time. That doesn't mean it is a sound design principle however. When i was in that business it was a strict no-no to even contemplate applying devices beyond their approved ratings. Depending on the design culture YMMV and so on.

A proper DC contactor has in addition to the arc snubber chambers, a set of permanent magnets to guide the arc for maximum snubbage. These are significantly lacking in an AC contactor where they would not function as intended due to the oscillating polarity of AC currents and voltages.

And yes, they will be expensive - even exorbitant in price compared to the cheapo €1.25 Chinese AC contactors that barely manage the decent minimum in AC, for a time. But then we are not playing in the same ballpark either. Whether a DC SSR is any cheaper i have no idea - this is a question that should be evaluated early in the product life cycle to avoid expensive/painful dead ends halfway through the design schedule.

[peter.mitchell]

Well, it looks like you'll probably have to roll your own SSR if you want to use an SSR; if it has to do 25A, 150~VDC, 240VAC I can't find any.
The back to back FET ones only go to 300VDC, which isn't enough for 240VAC, and the higher DCV ones are IGBT, which don't work with AC.
This is a digikey listing of relays/contactors, it is weird though; that OMRON one looks like a bit of an outlier...

[Zero999]

Are you intending to use the same component for both 120VDC/AC and 230VAC?

I found a 200VDC 40A SSR in RS, otherwise if you want the same component for all systems, you'll have to make your own.
http://uk.rs-online.com/web/p/solid-state-relays/7360879/

[peter.mitchell]

Are you intending to use the same component for both 120VDC/AC and 230VAC?

I found a 200VDC 40A SSR in RS, otherwise if you want the same component for all systems, you'll have to make your own.
http://uk.rs-online.com/web/p/solid-state-relays/7360879/

I believe he was intending to use the same device/setup with both supplies, to save manufacturing steps and part selection confusion thus the need to work at 110-140VDC and 240VAC,

[Simon]

Uh i seem to have missed some replies/emails. Yes as you see a solid state device that may do two costs a lot: £63, we need two, I have found a mosfet that will do 250V with about 0.0175 ohm of full on resistance that raises to 3 times that when heated and it costs under £5. We have spoken to the subcontractor and they are going to redesign the whole thing, we will have one main control board and then separate switching gear so that we don't have large currents in the board and we can change the switchgear as we get new project options thrown at us. If they want AC again a simple relay solution will do it. They seem to think that they can do a universal AC/DC solution....... I'll let them think about it before finding out they came to the same conclusion as me.

[oldway]

I'm powering a 1.5KW heating element and a 1 KW brushed motor both at 110-140V DC

I don't think it's possible to switch on a 1KW brushed motor at 110-140V DC without a current limiting resistor.
Inrush current would be very high and would probably damage the motor and the SSR.

[Simon]

it is something at the back of my mind, it is a vacum cleaner motor, do they put starting resistors in vacum cleaners ? or is the DC a problem ?

[SeanB]

Dc is the problem. Take your 1kW motor and measure the DC resistance and calculate the current it draws until it gets up to speed. The current will be in the multi kW range for the first second or so if the supply allows it. With an AC motor the coil inductance limits the peak current to the current flowing for the first half cycle to magnetise the core.

[Simon]

hm, would an inductor in series be any use ? as the current tries to rush in it will opose it and effectively give some "soft start". Alternatively can gradually increasing PWM be used to soft-start ?

[SeanB]

PWM would work as soft start. Inductor would be bloody big and even more expensive.

[Zero999]

For a soft start, you could connect a resistor in series for a second, then bypass it with a suable low resistance switch.

Isolation can be achieved with a photovoltaic opti-coupler.
http://www.vishay.com/docs/83469/vom1271t.pdf
http://uk.mouser.com/ProductDetail/Vishay/VOM1271T/?qs=%2fha2pyFaduiCbdPYawMPDbvzpZNW3JFS40Vo2Z60Tow%3d

[Simon]

Well I suppose given that the power mosfet I found or something similar that our supplier may want to use is less than a fiver so no harm in having 2, one with a resistor in series and one that gives a direct connection once it has started up.

[oldway]

The power on must be done in 2 steps:
- first, power on with a current limiting power resistor in serie with the motor.
- second, after current drops below a reference value, a second Mosfet will short de resistor.
As the power resistor cann't stay in circuit fot a while without overheating, you must have an overload trip after several seconds if current did not drop below the reference value.
You should then wait a time sufficient to provide enough cooling of the resistor, then try to power on again.
After three times, you should stop and show a warning.

[IanB]

it is something at the back of my mind, it is a vacum cleaner motor, do they put starting resistors in vacum cleaners ? or is the DC a problem ?

While we are talking about trains, they certainly do (did) put series resistors on DC traction motors on trains when starting up. There are various steps of series resistor when going from standstill to full power. I don't think you could apply full power to the traction motors when the train is standing still. There would be fireworks.

(I say "did" above because modern train designs no longer use DC traction motors. They rather use AC motors with electronic drives.)