Semiconductor grade MCB

I never came across such an animal, and I have been invloved with SCR motor drives, design, application etc for 28 years. Now these have all been larger HP industrial drives, so for small stuff I don't know. We always protected our SCR's with fuses with an I2t rating that protected the SCR's. One exception was a 1200HP bridge and we used a P frame Tmag MCCB. If the SCR's blew, the breaker cleared the fault, and then the unit was repaired with new SCR's. Reason for this aproach, SCR's were far cheaper than the fuses!

For 4 Quad SCR bridges we would use a MCCB with a mag only trip unit set at about 250% of motor rated in the DC circuit. If an inverting fault occurred the motor was protected. We felt the scr bridge and fuses on the line were sacrificial. The interesting thing is when the inverting fault breaker tripped, we usually also tripped the tmag MCCB ahead of the bridge and maybe blew fuses. Far less damage occured than was anticipated. Motor usually survived, and that is good when a 300Hp dc machine is worth abot $30k.

We also tried using DC fuses in the armature. Worked OK for small motors and 240vdc. Around 100Hp the fuses tend not to clear with DC currents and would end up with a plasma arc and all sorts of molten metal was created. MCCB worked far better.

Dear GW thank you. I am using 1200 units of 4Q SCR battery charger from 2 KW to 20 Kw range. I am also using Fast acting Semiconductor fuse only at present. My problem is when ever there is a power fail and if my charger is in Re-generative discharge mode the fuses fail thus protecting SCRs. Two to three times a day mains fail in my country and fuse failure is very high. This I have solved to some extent by introducing a Crash resistor in the discharge path. While surfing I came across the following company: United Automation 1 Southport Business Park Kew Southport PR8 4HQ; Tel:01704 516500 www.united-automation.com Publication No: X3-00-001 Refer page 7 of the PDF file; This company claims that they got Z type MCB that can replace Fast acting fuse; but not responding to mail: that is all the story. the funny animalis my pet dog Josh! have a nice day...

Have you considered using a power mosfet to shut off the current?

I am thinking along the line of a regenerative AC drive.

In the same idea, Rockwell has an "RGU" unit that will regenerate the DC buss voltage of an AC drive back into the AC line. Using IGBT circuits they have protection for overcurrent built in, and "turn off" if the current is too high.

Thanks for your comment. Power MOSFET is the ultimate solution. I am trying cost effective and simple solution. That is why trying for a Type "Z" MCB. The discharge air break contactor takes 50 to 100 milli seconds to open the circuit in the event of power failure. At present to reduce fuse failure I have introduced a Crash Resistor (0.05 Ohms/ 22 KW) and it is working ok, but wasting 300 to 500 Watts of power. The other solution I have found is to have series Choke of 15 to 20 milli Henry 100 Amps. This is costly in my country and also bulky.

We have a developed a new type of solid state relay which produces very little conducted noise and therefore complies with all EU and US EMC limits at full load of 80amps (240V). You therefore get the advantage of a MCB without running into EMC filter problems. Other advantages are: They work extremely well with inductive loads, can be wired in parallel to switch any size load and the Thyristors used (Ixys) have an I2T of 1,600A/10mS which means that the fuse ratings aren't so critical. The SSR is the same physical size as a standard 'puck'. Don't know if this would be of any help.

We also have a 530V version in test.

You can read more at www.root2.ltd.uk

I will contact United Automation on your behalf on Monday (they distribute our SSR)

Sorry this wont work. I need to protect Fuse with fast acting MCB. It is possible to have IGBT as switch but costly. I want protection within 2 milli seconds. Thanks for your comment.

You may try to use ABB Make MCB ( S.280 UC - Z ). This will break with in 0.1 seconds on five times rated current. Pl check up weather this is available in India now .I remember to have seen this in some of our equipments. Check up with some firms in Bangalore weather they will be able to supply this if the range is now available in manufacture. Bangalore 22949650 or Mumbai 26730852 is the telephone number seen as ABB contact in my diary . Since I am right now out of country I can't give any better suggestions .To best of my feeling a suitable rated Fuse back up should work with readily available MCB with 0.2 seconds trip time . Best of Luck.

You are right. ABB do make such MCBs. ABB marketing and application engineers do visit our place frequently. I have already mailed them about this requirement but they did not reply till now. As you mentioned 0.1 seconds is high. If it is 2 milli seconds then it will workout. Most SCRs can take 3 to 6 times of rated current surge for one half cycle, that is 10 milli econds and more than three surge cycles will damage the SCRs permanantly. I am doing some experiments and I shall share with you in future. Thank you all for your comments.

Semiconductors are protected by "R" grade Fuses only as their burning time ISquared x t

is in μSec. <10μS

MCBs are only for over-load protection