Thyristor Protected Fuse Problem.

Never heard of a thyristor protected fuse, but more likely they are thyristor protection fuses. They would be high speed fuses used to help protect the thyristors (SCRs) in your rectifier.

No circuit breaker is fast enough to provide equivalent protection. If your drive was designed to be protected by these fuses, making a change is exposing your system to increased risk of a very expensive failure.

Long ago I contacted one manufacturer who claimed that they manufacturing Semiconductor Protection MCBs. But this fellow did not reply any of mails.

http://cr4.globalspec.com/thread/8050/Semiconductor-grade-MCB

MOSFET or IGBT switch with current monitoring trip switch is very well possible for 400A and few hundreds of Volts DC, however they are going to be expensive.

I use 10A to 100A peak current 1kV DC switches and making these for other higher current rating is all possible. My designs are experimental and research type.

Sources for high voltage high current switches are www.behlke.de www.vsholding.com and perhaps other integrators of MOSFETs and IGBTs.

Ever heard of a "Crowbar" with regard to power supplies and the like? This is most probably what they are talking about.....its a useful circuit to know, I use it in my battery chargers....

If not, look here for a good explanation, which also takes into account why the Crowbar was invented = to make sure that no over voltages can get to the equipment to be protected.....

http://en.wikibooks.org/wiki/Practical_Electronics/Crowbar_circuit

This is in the Wiki link......

Hi Andy_Germany

This circuit shown by you is actually a over voltage crowbar protection and not an over current protection we are discussing now.

Its actually dead easy to make it react to either...........usually the fuse will blow on over current all on its own......which is part of the reason its there......but you are quite correct......I was just demonstrating an electronic crowbar of any sort.

Well, it is possible to have a crowbar circuit for over current fuse blowing that will force excessive current through fuse and will make it to blow. Perhaps this idea is much better than to rely only on fuse property to blow off on excessive current.

Regards.

This will protect the "Load" not the Thyristers.

The thyristors are there in the event of an over voltage (in the example shown) to short out the power, and holding the voltage down, to run a high current, blowing the fuse.

An MCB fuse will not blow before blowing-out of Thyristers.

dear sir,

thank you very much for your reply,

but please give me solution of this problem. I want to remove these fuses from this drives same like new generation drives. can be possible?

Has the drive run OK until recently? If yes then something has changed. It's up to you to find out what.

Do not change the fuses for a MCCB you will toast the drive unit

OK Sir,

I will never replace fuses with MCB but can you give me some tips for how to reduce fuse blown problem of drive. I didn't found reason of fuse blown? because fuse blowing after 8hrs or some time after 2 to 3 days or some time after a week?

Look at the mechanical drive line.

Thyristor drives do not have the luxury of reacting to high-current faults within a few tens of micro seconds like the newer IGBT controlled drives.

Over-current faults can easily blow fuses on thyristor drives - like a certain refrigerated product extruder that had the refrigeration left on during a coffee break (not too complain about it...).

Reduce the current limit on the drive until you don't have enough torque from the motor then increase it back a little. Reducing the controller speed (PID) can also help. This is about the best way to avoid hitting the fuse melting point.

Another reason for fuse failure can be mis-firing caused by intermittent connections at the gate drivers. Make sure it operates properly.

Use a megger to check the motor's insulation. Arcing to ground once in a while is not good and can cause fuse / thyristor failures.

Good luck.

You did not say if these are new fuses blowing ? are they different surge ratings? Thyristors 400 Amp rating is that that the average capacity if so they should take momentary peak loads classified as non repetitive of at least 3 times that. You could upgrade your fuses to more than 400Amp depending on your motor demand. and the system rating. you should do some logging to see what happened before and during the " fault " condition. I would not use breakers they are to slow.

It looks that system is critically designed where power source and drive both are protected by fuse system and it is expected to be in this way most of the time. Over heated fuse may blow in shorter time when machine is in continuous use for longer time.

Mostly fuse action is temperature dependent so working hours and environmental conditions may also affect. Some fuse may have under ratings or over ratings. Look for table in the catalog for xxxA rating for fuse blowing. As fuse are blowing in shorter time their operating condition of blowing can be recorded and analyzed easily.

are you suggesting transient suppression from AC source?

Such a Choke may not protect from transients but it protects Fuse before crossing I²t. Semiconductor fuses are normally rated at 8 to 10kA @ 25'C (whereas HRC are rated at 80kA) and one needs to consider de-rating if ambient temperature is high at fuses. If we go through SCR data sheets they can tolerate high current surge 20 to 30 times of the rated current for one-half-cycle. Normally manufacturers (distructive) test three-half-cycles. Normally high TRR rating SCRs and fuses can tolerate transients.

I think you have good points.

Perhaps choke can reduce the start up peak current from source, and will deliver it back with some delay. As Load is motor, they also have large inductance, hence, current delivery process is almost identical. Fuse here are not blowing in single cycle action but in an average acquired energy and above that added transient action. If temperature of the fuse box is somehow regulated through proper ventilation or forced air, then life of the fuse may be much better.

What is a chance that machine is bad or its bearings are down and when it exerts excessive force then fuse blows. This condition may come on over use of machine for which it is not designed, such as fitting extra load assemblies to the system then permitted and then they are peaking out at some time casing fuse to blow. I think, a record of current into load waveform must be used to analyze the problem, One can avoid forcing many loads in start up condition and also keep a tag on running added load within permissible limits.

Power factor and harmonics in the power source also to be considered.

What makes you not to limit the maximum current into your load? Does your load require more than 400A for short periods in order to remain functional? Do you have any idea of the actual current that is going through fuse? 400A fuse can survive 1000A for a very short time but will blow off if you keep 400A or prolonged time or Give a short time high current shock and then apply lower than 400A. Well, it is the power history that makes it time and power used up dependent.

As you have said that it takes several hours before fuse blows up and there is random time in which it blows up, makes me feel that your power consummation is also random. If you record power consumption rate then sure it will tell more when fuse is blowing off. If you give some rest to machine in between then there is a good chance that fuse may survive for much longer time.

There is non-linear response to power and time to which power can be used for fuse to sustain. 400A for 1 hour and 800A for 30 minutes are not same here. I think, gained temperature and susceptibility of the fuse on that temperature to peak current are critical. For example, if machine was ON for for few hours and after that there is jerk in current even for short time then fuse will blow off as fuse are already hot and just waiting to be triggered up.

It is not that fuse will blow off just above 400A current level in a fraction of seconds.

You have to tell us about the load current and ratings of the fuse in use.

If you use over rated fuse then if fuse is not blowing off then something else may blow off. However, your fuse must be of the right rating and you need to know which system you are protecting. Sometime you are protecting power source and some time you are protecting load and some time you want to protect both at the cost of fuse.

Now that you want to protect the fuse, then you can use higher rated fuse or no fuse at all. Fuse is doing what it is supposed to do.

All semiconductors are protected by Semiconductor-Grade fuse which are superfast blow time to protect semiconductors which are fast to damage. Semiconductors-grade fuses are much cheaper than semiconductor device and save service time.

One thing to note that semiconductors are not protected from over-loads but short-circuit protected.

MCB fuses are overload protection and very slow in cut-off.

The suggestion by one poster [GA] is really a good step to be considered.