a bit out of tune..sooorry:(

im sorry to be off the hook but i really need some advise regarding electrical matter..

we have inspected a machine, a press molding machine to be precise.. the owner informed us that the machine was working well before an incident happened..one time when the machine was on operation the loaded materials were burned, on fire to be exact..after this incident, the machine could no longer reached its temperature setting..when we checked, numerous heaters were burnt out so we pull-out all the 'bad heaters' to be replaced with 'new & good heaters'..we were then asked what was/were the possible cause/s of this incident..we have a number of findings written as follows:

1.) out of standard wiring i.e., feeders not crimped very well to the lugs, use of smaller lugs with respect to the size of the wire.
2.) by power analyzer we found out that current going in to the machine abruptly changes from time-to-time i.e., for the 1st 5 min reading is 38A then after 5 min the reading is 19A.
3.) the machine is supplied with 380V but the components only need 200V, so what they did was to supply the components, including heaters, with Line-to-Neutral.

Questions:
1.) Does supplying the machine with Line-to-Neutral affects the current going in to the machine? Would it be neccessary to use step-down transformer for the components?
2.) Does the out of standards wiring affects the current going in to the machine?
3.) Which do you think greatly affects this phenomena?

Kindly help


best regards,
theripley

I am not familiar with the machine but..
1. Yes, if the wiring is too small, this will cause overheating of the lugs, although not explain why the material caught fire.
2. The current dropping sounds like all the heaters have a common controller and the unit is up to set temperature.
3. Yes, it is perfectly acceptable to use phase - neutral to get the desired voltage. It is good practice to have these "Balanced" so perhaps your temperature controller switches the heater elements through a contactor?
4. Why did the machine catch fire?. Was the temperature probe faulty, or perhaps the machine has a "Maximum heating time" to prevent the material catching fire!!!

hi markie,
there are more than a hundred heaters installed at the machine, all have their individual relays that turns on almost at the same time..these are single phase ceramic heaters..
one time materials burned because of overheating of heater..
i haven't verified how many sensors they have at the machine..
regarding current reading, the current fluctuates rapidly, every second it abruptly changes..

theripley said:

hi markie,
there are more than a hundred heaters installed at the machine, all have their individual relays that turns on almost at the same time..these are single phase ceramic heaters..
one time materials burned because of overheating of heater..
i haven't verified how many sensors they have at the machine..
regarding current reading, the current fluctuates rapidly, every second it abruptly changes..

Click to expand...

The description you give so far indicates to me that this machine has a 25 KW load if it is fed across 3 phases with a neutral.
You say it has about 100 single phase elements so that means each element is about 250 watts.
The rapid switching and changing of current indicates that there is a solid state controller feeding the elements probably 3 phase Triac control that start switching near or at setpoint, or a primitive control that uses diodes or Thyrister to reduce the heating load by putting half wave rectification onto the elements.

Possible causes of fire are numerous, a few that come to mind the moulded product was not unloaded from machine, the temperature controller failed keeping all the heater on full power, a grouping of heaters near the temperature probe were not working thereby giving false readings to the controller, the temperature sensor is faulty or has intermitant connection.

In regards to point 2, it is also possible that the resistance of the heating elements go up dramatically after they are heated. But as another responder to this thread noted, it could very well be the temperature controller responding to the increase in temperature.

Here some guesses. Improper fit between the heating element and the hole it goes in, too much or too little clearance. Maybe you need some type of thermal compound to transfer the heat from the element to the mold. Maybe the relays, if mechanical, are sticking if they are slightly undersized. If they are solid state, then maybe voltage spikes or current overload are leaving them turned on when they should not be on. The point is you should make sure that the relays are rated for peak load plus a large safety factor to insure they are not overloaded. You might even need a line input filter on the incoming power to keep out the spikes. Also if the neutral is not connected properly it would be possible that grounds developing due to an element failing could cause excess voltage to be applied to the other relays or elements. One more thing, you said that the elements were 200 volt rated on a 380 volt system. That calculates to 219 volts to neutral being applied. If the incoming line voltage is a little high, say 400 volts, that might cause element failure. All of this is just a guess so none may apply. I hope this helps.

Can you provide a little more information? How are the heaters arranged? Are they all encased in a 2 part mold or open air? How many temperature sensors are there? Where are they located. When is the heat applied? Is it on all the time or does it cycle? Is there a temperature controller(s) or is the temperature controlled by a PLC? It would really help.

jrwb4gbm said:

Improper fit between the heating element and the hole it goes in, too much or too little clearance.

Click to expand...

what would be the standard clearance? does this affects heater operation?

jrwb4gbm said:

Maybe you need some type of thermal compound to transfer the heat from the element to the mold.

Click to expand...

sorry, for my ignorance but what exactly is a thermal compound and what is its function?

jrwb4gbm said:

Maybe the relays, if mechanical, are sticking if they are slightly undersized. If they are solid state, then maybe voltage spikes or current overload are leaving them turned on when they should not be on.

Click to expand...

the ssr is rated for 20A, heater is 400W @ 220 Vac supply.. Perhaps there are voltage spikes or current overload because when we analyze its power using power analyzer current is very inconsistent and only two phases i.e., L1 & L2, have reading, the 3rd phase reads very small..im suspecting that the load is unbalance..

jrwb4gbm said:

You might even need a line input filter on the incoming power to keep out the spikes. Also if the neutral is not connected properly it would be possible that grounds developing due to an element failing could cause excess voltage to be applied to the other relays or elements. One more thing, you said that the elements were 200 volt rated on a 380 volt system. That calculates to 219 volts to neutral being applied. If the incoming line voltage is a little high, say 400 volts, that might cause element failure. All of this is just a guess so none may apply. I hope this helps.

Click to expand...

the voltage reading is consistently playing at 400V average..what filter would you recommend, if you have any?

thanks a lot

I would suggest you contact the element supplier or manufacturer for exact application recommendations for fit tolerances,and if a heat thermal transfer compound is needed or not. I would google "VFD input line filters" or something like that to find something to filter the incoming lines. I think you may need to somehow need to adjust the 400 volts incoming power for the heaters down to near 350 volts to keep the elements voltage near 200 volts. Buck-boost transformers can do this.

Improper isolation of the input signals to the SSR 20A relays could be causing problems.

Without seeing the equiptment and wiring diagrams it will be tough to be able to give specific answers to your questions, maybe some of these suggestions will help.

ATU said:

Can you provide a little more information? How are the heaters arranged?

Click to expand...

heaters are arranged in parallel manner concentrating on L1&L2 of the three phase supply.

ATU said:

Are they all encased in a 2 part mold or open air? How many temperature sensors are there? Where are they located. When is the heat applied? Is it on all the time or does it cycle? Is there a temperature controller(s) or is the temperature controlled by a PLC? It would really help.

Click to expand...

heaters are exposed to open air as they are use to mold plastics..i haven't verified how many sensors are installed and their respective locations..the control system is integrated via a PLC which then gives signal to the PID controller..heaters will on when a manual pushbutton is pressed and controlled by controller via signalling of the PLC..

Do you think its vacuforming, where a thin sheet of plastic is heated until it's pliable and then its draped on top of a mold. Then a vacuum is applied, forming it into the mold.

I think you have a runaway condition. If they are using a temperature controller with a PID loop, there has to be some kind of feedback for that to work. The PLC is probably giving it a setpoint reference. I would start at the temperature controller and go from there. Verify the feedback and where the setpoint is comming from. The setpoint could be being set from a serial or digital connection, however you should be able to view both the current setpoint and the current temperature on the PID controller. Like GIL47 stated, the actual temperature reading could be faulty. Perhaps the fire damaged the sensors? Also the setpoint could be set so high that the controller is maxing out all the time.