Surge Suppression Circuit

As far as I can tell from a cursory glance it's not a surge suppression circuit, it looks like some sort of horrible high voltage supply.
The diodes probably blow due to the inrush current to those capacitors.
Ah I've looked a bit closer... a diode and capacitor across the contacts may help?
I still can't see any supression, ok the resistor across the contacts does charge the caps slowly to help stop the inrush, but it all looks a bit iffy.
It doesn't show what the actual load is.... unless it's just some hot resistors, but they presumably have some inductance so there will be back EMF and stuff floating about when the switch opens.
Need a better explanation of the purpose of the circuit.
Del

Del is correct, there is no voltage surge suppression circuitry shown in this circuit. Most surge suppression designs act on the incoming voltage exceeding expected levels for brief amounts of time. Therefore a voltage surge could easily be the culprit to toast those diodes.

There is a very simple current surge circuit of the relay coil C1 and its contact bypassing the 10 ohm resistor. If that relay is a time delay relay for multiple cycles of the incoming power then I can see it having some useful function at limiting the inrush current. The unlimited inrush current might damage the diodes.

I dislike this current surge approach. The minimum time delay of a mechanical relay from coil energizing to contact closing is not a guaranteed attribute unless that relay is a time delay relay. Also the coil of this relay is sensing the incoming AC voltage and not the DC voltage across the capacitor. It is therefore indirectly responding to the inrush scenario instead of directly measuring the voltage that creates an inrush current. Even if this relay is a time delay relay I still dislike this approach. If the contacts ever weld together(unlikely) or the current limiting resistor fails open (likely) then full inrush current will happen. The inrush current might be anything up to the arc flash current. Also an open 10 ohm resistor path will never be detected because the relay contact will eventually bypass it.

The diodes blow up when there is either too much reverse voltage, or too much forward current, for them to sustain.

1. C1 provides a current surge limitation, by keeping the 10 ohm resistor in circuit until the 1500 MFD capacitors have charged to near normal voltage.
2. The time to close of C1 contact varies according to the time in cycle when main 3 phase contacts close. However, C1 likely takes over 20 milliseconds to close, 2 half cycles. The R x C time constant of 10 ohm & 500 MFD is 5 milliseconds, so initial charging surge current will be negligible by 20 ms.
3. I cannot read, with certainty, load resistor values, but wattage makes current about 3 amps [250 ohm load?] - so diode rating is decided by 600V x √2/10ohms = 90 amp surge current.
4. Have you checked all 3 caps of 1500 MFD are seeing equal voltage?
5. Diodes might fail due to reverse voltage spike - but circuit effectively keeps 20MFD//500 MFD connected to line through forward biased diodes all the time, which is a big spike limitation. Voltage dependent resistors VDR could be added to clamp spikes of very short duration.
6. Diodes might fail due to current surge caused by line overvoltage, but as noted in 2 above the capacitance charging time is only 5 ms. You could add semiconductor-type fast fuses, which should blow before the diodes - but if this is an external problem repeated fuse failures will occur.
7. One diode open-circuit reduces output, but one short circuit will cause other diodes to fail. How many diodes failed? Were all replaced, including survivors?
8. The usual questions apply a) has this gear been working for years without trouble till now b) has failure happened more than once recently? c) has some change to plant or local loads been made, since when faults occur? d) has there been trouble with something else, which might disturb rectifier, recently?

Nice post but I'd suggest that on your point number 5 those big caps will do nothing to suppress fast spikes.
I think their high frequency impedance will be too high.
I reserve the right to be wrong as it's not may area.
As you say a VDR is prob' a good idea .
Del

Hi Del,

I agree item 5, caps may not have low inductance [good 10 mmF film capacitors for AC connect the whole end of the foil to the terminal, which minimises inductance], but I feel this is proven equipment well able to survive connection to 600V 3 phase, so those diodes are robust.

All diodes are a "reverse avalanche breakdown" diode and those probably have a reverse breakdown energy minimum withstand as part of their spec.

It may all depend on the actual high frequency impedance of the source supply and what loads are switched on it and by what. Replacing an old breaker by a much faster new one may have increased the magnitude or number of spikes on a common source.

This may be equipment which was fine with a 50 metre cable run from the isolator, but now connected close to the supply transformer and frequently operated breakers.

Are you saying that C1 will take some time to close even if 600 VAC is directly applied on to the diode Bridge, the AC voltage will not be 600 until after some time (20 ms) and the coil will not activate waiting it to reach 600 VAC . This is interesting to note

but i thought the coil should pickup and close the contact the moment 600 is applied to the bridge as it is also connected to the coil

"...and close the contact the moment 600 is applied..."
Err, how long do you think a "moment" is exactly?
Have you never wired up a relay so you can watch the contact bounce on a scope?
If not I strongly suggest that you do it.
Del

Minggrabber, you have some misunderstandings.

As example, following is from Siemens 3RT contactor catalog, the time to close contacts after applying AC to coil.....

There is a large spead of times, 9 to 22 ms (partly because voltage of sine wave at instant of connection can be zero to peak) - your contactor would be selected on its minimum time.

The AC voltage into diodes will reach whatever voltage the source sine waves have at the instant the main contactor closes - it will then follow the sine curves. The ideal output of a 3 phase bridge fluctuates at 360 (300?, 6 x mains frequency) Hz from peak sine voltage to peak x 0.92. The voltage on the capacitors will rise as they charge - limited by the 10 ohm resistor - approximately 85% peak in 10 ms, 98% in 20 ms. By design, the capacitor voltage, when 10 ohm is shorted, should not cause excessive diode surge current - this will depend on...

Source supply impedance - unknown - suppose zero.

Capacitor internal resistance ESR - about 0.75 ohm for 3 x 470 mmF 400V in series*.

Difference between supply voltage and capacitor voltage = D.

* suggests that for 90 amp (peak) surge, D = 67 volts, note at 85% peak volts on capacitor D = 140V.

If you confirmed the load resistance, supply frequency and gave the C1 contactor, capacitor and diode make, rating and type code, one could go further. Length, cross section, material (copper?) of AC cable feeding this box would help.

67model

What I see here is a 3 phase, full wave bridge rectifier which charges capacitors on the DC side with a voltage equal to the peak-to-peak voltage of the AC. After they have charged, there should be no more current drawn through the diodes until a spike occurs.

When a voltage spike comes along, the appropriate diode will be forward biased and its energy will hopefully be absorbed by the capacitors and resistors.

If your diodes burned out, I would suspect that a capacitor (or capacitors) shorted out and the overcurrent killed the diodes. (A failed capacitor may seem good until it has the circuit voltage impressed across it.)

JRaef,

I give you a good answer mark. I expect the OP would at least check resistor OK with ohm-meter.

If I was designing this unit, that 10 ohm resistor would be the biggest problem. The specified item hits 20x rated current.

Bare resistance wire on a porcelain rod, old firebar style would work - expect rate of temperature rise & expansion will crack-up anything encapsulated quickly.