Band Heaters

[The Plc Kid]

Should a band heater pull the same amount of current when it is steady on ( heating up to set point) as it does when it pulses at temp and maintaining temp?

This is a pulse on/off heater system

 

[allscott]

As the heater heats up its resistance will increase a bit and therefore pull slightly less current. At least thats my experience.

 

[The Plc Kid]

I am trying to define a current range. when it is on is it pulling the correct amount of current. Trying to detect open heaters on the hmi.

If it shorts and blows a fuse i have alarms for that but if it's open i don't know several heaters in each zone so the other heaters will keep the zone up to temp but will still produce cold spots which Effect quality.

 

[jrwb4gbm]

Here's a simple (maybe not practical though) solution. Put a relay coil of the correct voltage for the open element in parallel so that when the element opens the coil will energize but not energize if the element is not open. Connect your monitoring system to a contact on each of the relays. Another option is to use a current sensor device on each element. Next configure your logic to only alarm if the controller is outputting (on) and the relay is energized or there is no current sensed with the current sensors.

 

[Lancie1]

This is a pulse on/off heater system

I assume that you mean that you have some type of switching transistor output (SCR or similar that produces a PWM pulse) that turns the heater On/Off, so that the pulse width is controlled by a temperature sensor? If so, then the average current will be larger during the heat-up mode, than it will be when it reaches setpoint and only has to procuce enough heat to maintain the temperature.


On the other hand, if your heater is being turned On/Off by a relay or contactor (with no curent control), then the "ON" current will be the same size ANYTIME the heater is on, in heat-up or steady-state mode.

 

[leitmotif]

Kid
This question is obviously related to your other thread. You would be better off to as the question there to at least maintain continuity. Will answer there.

Dan Bentler

 

[realolman]

.... Put a relay coil of the correct voltage for the open element in parallel so that when the element opens the coil will energize but not energize if the element is not open. .

I don't see why a 120 v relay would not pull in when in parallel with a 120 v heating element, regardless of the continuity of the heating element?

Can you monitor one element and divide the total current by the monitored current and see if it works out? 5 heaters total / monitored current = 5 = OK .... 4 heaters total / monitored current = <5 = not OK

 

[jrwb4gbm]

I don't see why a 120 v relay would not pull in when in parallel with a 120 v heating element, regardless of the continuity of the heating element?

Can you monitor one element and divide the total current by the monitored current and see if it works out? 5 heaters total / monitored current = 5 = OK .... 4 heaters total / monitored current = <5 = not OK

Current monitoring seems to be a simpler way. I can see where parallel relays might be confusing, sorry. What I was thinking about is, for example, when a bank of (4) 120v rated elements were connected in series and fed with a 480v source then if a 480v relay coil were paralleled with each element it would not pull in with the element OK but would pull in with the element OPEN.

 

[LadderLogic]

Put a relay coil of the correct voltage for the open element in parallel so that when the element opens the coil will energize but not energize if the element is not open.

Herr Georg Ohm may not agree with this idea...

 

[leitmotif]

Herr Georg Ohm may not agree with this idea...

Was that his first name??

The relay in parallel with heater idea could work BUT only if the relay windings could handle the current drawn by remaining heaters AND if you were willing to pay for the power lost in the relays being energized all the time.

Dan Bentler

 

[LadderLogic]

Was that his first name??

Georg, it was. And, him being a German gentleman, we would appreciate being addressed as "Herr"... Although I am not sure if he preferred to be called "Professor". Or "Doctor".

I also don't see any reason why a relay parallel to a heater would work. It really doesn't care if the heater is OK or broken: same voltage applied to the coil in each case. Heaters, for all intents and purposes, are just resistors; no complications related to inductance and capacitance are involved.

So Ohm's law for a heater in an AC circuit would be as simple as it is in a DC circuit: voltage, current, resistance. Resistance may vary somewhat depending on temperature, but that is about it. The only way I can think of to detect an open heater is to measure the current, as it was suggested above. Since a heater is a plain resistor, the current should change as fast as the voltage applied, virtually no delays usually caused by coils or capacitors are present. So as long as there is a device capable of measuring current as fast as the shortest pulse in the system, an open heater can be detected.

It would be a bit harder to do wherever a number of parallel heaters is involved: for too many heaters, a failure of one may change the total current insignificantly, within the range of the error caused by normal resistance variation. Some simple calculations would be required to make sure it will work in this case.

Edit: I should've looked at that other thread. Did not have to write a word, really.

 

[ablogix]

An alternative if you have the money is to switch to these
http://www.eurotherm.com/products/controllers/2200e.htm

 

[realolman]

Was that his first name??

The relay in parallel with heater idea could work BUT only if the relay windings could handle the current drawn by remaining heaters AND if you were willing to pay for the power lost in the relays being energized all the time.

Dan Bentler

...when a bank of (4) 120v rated elements were connected in series and fed with a 480v source then if a 480v relay coil were paralleled with each element it would not pull in with the element OK but would pull in with the element OPEN

Wouldn't the current drawn by the remaining heaters be limited by the high impedance of the relay coil in parallel with the open band heater?

 

[rootboy]

I am trying to define a current range. when it is on is it pulling the correct amount of current. Trying to detect open heaters on the hmi.

If it shorts and blows a fuse i have alarms for that but if it's open i don't know several heaters in each zone so the other heaters will keep the zone up to temp but will still produce cold spots which Effect quality.

Sounds like to me that what you want are CT's (Current Transformers) to monitor the current of the heaters.

Put a resistor in series with the CT and read the voltage across the resistor back into the PLC (via an analog input) to monitor your current.

The "uptown" method would be to use one analog input per CT. Me being cheap and all, I would multiplex the heaters into one analog input and then just use relays and a "round-robin" routine to read each of the heaters.

Keep in mind that the CT always has to have a complete circuit (in other words, switch the analog inputs, not the the CT's themselves).

Opening up a CT under load will cause the CT to burn up.


One place to look for CT's...

http://www.magnelab.com/products.php


We had Ube injection molders where I used to work, and the $%@&'ing mercury-wetted relays would weld shut causing the barrel heaters to cook (as in catch fire).

We had logic that would monitor the barrel temperature via a thermocouple, but by the time that the alarm was registered the barrel would have gotten so hot that it would take about and hour or two to cool off enough to resume production.

I wanted to install CTs but we didn't have the money. I was even willing to wind my own (all I cared about was on/off, not amplitude), but about that time I took the severance package from GM...

What made matters worse was one of the electricians was trying to Do The Right Thing by taking the bad mercury relay and replacing the one pole that had welded shut (he really did mean well). He didn't realize that the remaing two weren't far behind. So when the "rebuilt" relay would be put back into service, it would be a very short time before one of the remaining poles became welded shut.

My "bottom dollar" plan was to use one CT that was sized large enough to handle all of the heaters being on at the same time and periodically shut off all of the heaters. If I detected current flow during that time, then I knew that at least one of the mercury relays had welded shut. Then I would turn each of the heaters on one at a time and monitor the current. If the current didn't increase when I turned on a relay, then I knew that it was the one with the bad contacts.

Edit:Um, did I forget to mention that you will have to convert the voltage coming off of the resistor to DC?

 

[rdrast]

Take a look at some of these products as well...
http://www.tempco.com/Instrumentation/current_indicators.htm