Heating Jacket of a glue vessel keeps on heating up crossing the set point ?

[munzir]

Heating Jacket of a glue vessel keeps on heating up crossing the set point ?

Its a heating jacket that is tighten up on the glue vessel to keep the viscosity of the glue as per the requirements.

Its a single phase (220 V) heating jacket

It has a thermostat (on the heating jacket) ....

A knob / selector switch is there to turn on (to supply 24 Vdc as the coil voltage of a contactor)

The contactor is energizing (passing the 220 V supply) to the heater continuously.

I think the thermostat is gone bad , What do you guys think ?

Because after setting the set point (value) the heating should be stopped when the heating raise to that setting point (value) and it should be done by the thermostat .

there's no digital temperature controller at all of any type

thanks
take care

 

[FboDigit]

What kind of controller do you have ?
An on/off controller let the temperature raise more then the set point
A PID controller also let the temperature exceed the set point.

 

[munzir]

there ain't any kinda controller , there's just a thermostat to control the heating of the heater jacket

 

[betwixt]

There are things to consider Munzir.

If the thermostat is a mechanical one (bi-metal) it may have a tolerance from new of 10% or more and this worsens at it gets older. If it's an electronic thermostat, it can either work on an upper and lower trip pont principle or it could use a constant fedback (PID like) method.

All of these systems attempt to keep a constant temperaure, either by turning the heating source on and off to maintain an average temperature which may at any time be above or below the setting, or by contnuously running the heater by controlling the heat it produces. Without knowing which system you have, it's impossible to diagnose the problem for you.

Questions:
1. when it is running, can you hear the contactor turning on and off?
2. if the temperature can be adjusted, does turning the control make the contactor turn on and off?


Brian.

 

[jeffrey samuel]

the first impression got is that the thermostat is not doing its duty that may be the prime reaason for the ckt to operate beyond the limiting temperature

 

[munzir]

when its running , no sound of turning off & on of the contactor .. because the contactor is getting coil voltage from the selector/knob switch ......

Its a bi-metallic thermostat

if the temperature can be adjusted, does turning the control make the contactor turn on and off?

No , nothing happens when turning the knob of the thermostat (i.e. control)

I also check its resistance and it displays 63.5 ohms at all different temperature settings except at 0 (where the knob of the thermostat is off) ..... though it's resistance should vary at all different setting points of the temperature , what do you think of that ???

thanks take care

- - - Updated - - -

because the contactor is getting coil voltage from the selector/knob switch...... which is a mechanical switch ... once it switched on it remains switched on unless the operator turns it off itself

 

[betwixt]

Without seeing it I would guess the thermostat is supposed to open when the set temperature is reached and this should stop power to the contactor and therefore turn the power to the heater off. It sounds like the thermostat has broken, most likely by welding it's contacts together. The only reliable solution is to replace it with a new one. If it can be opened, you might be able to clean and file the contacts to remove any 'spikes' that have grown on them but it can be difficult to do and in an case, you would have to re-calibrate it afterwards.

It's unlikely but possible that the thermostat has become detached from the heated vessel so it has less heat conducted to it and thinks the heater needs to be on for longer. This should be obvious by looking at it.

Bran.

 

[jeffrey samuel]

try changing the thermostat there may be a manufacturing defect in it
the range of the thermostat does not suit your appliance

 

[munzir]

Shouldn't the resistance be varied while turning the knob of the thermostat ? I also check its resistance and it displays 63.5 ohms at all different temperature settings except at 0 (where the knob of the thermostat is off) ..... though it's resistance should vary at all different setting points of the temperature , what do you think of that ???

 

[betwixt]

No, the resistance should stay the same and it should be almost zero so I suspect you are measuring the contactor coil resistance. You should measure directly across the thermostat while it is disconnected from the remaining circuit. The type of thermostat you have is a switch which is normally on (short circuit, zero ohms) but switches off (open circuit, very high resistance) when the trip temperature is reached.

Brian.

 

[munzir]

No, the resistance should stay the same and it should be almost zero so I suspect you are measuring the contactor coil resistance. You should measure directly across the thermostat while it is disconnected from the remaining circuit. The type of thermostat you have is a switch which is normally on (short circuit, zero ohms) but switches off (open circuit, very high resistance) when the trip temperature is reached.

Brian.

no i am not measuring the contactor coil resistance ......

I am just measuring across those wires which are coming out of the heater (out of the thermostat knob) and are not connected to any circuitry ...........

What should be the resistance values across the full turning & swing of the know from zero/starting/minimum position to maximum/full position.

i think its a 750 W heater because its resistance is 63.5 ohms

 

[betwixt]

That doesn't make sense. A bi-metal switch is just that, a metal bridge across two contacts which bends when heated and breaks the circuit. So the switch itself cannot be 63.5 Ohms, you are measuring something else. When the knob is turned, a screw thread enforces different mechanical pressure to the bi-metal strip so it takes less or more heat to bend it far enough to open the switch, the electrical resistance should be close to zero when closed and open circuit when open.

It is just possible, but unlikely, that you have a timed thermostat. one which does NOT measure the temperature of the vessel at all but instead has it's own heater built inside it. These are very inaccurate as they can't monitor any external temperature and they work on the principle of cyclicaly turning on and off by themselves. When the knob is set to a higher temperature, they just spend longer 'on' and less time in the cycle 'off' ao the average heating power is higher.

Your power calculation makes sense but if the thermostat alone measures 63.5 Ohms, it means it alone is dissipating 750W which is extremely unlikely (I'm assuming the knob isn't glowing red hot !) although the main heater possibly could be that resistance. In any case, you should measure the resistance when it is at operating temperature, it will be considerably lower at room temperature.

Brian.

 

[munzir]

but how to measure the resistance when its at operating temperature ?

- - - Updated - - -

ok fine i think i am just measuring the resistance of the heater ..........

but how to check/measure that the thermostat is working fine ??? Any cold test of it ??

 

[betwixt]

Run it up to temperature then disconnect it and measure the resistance as quickly as possible, It will take a while to cool down so the resistance will still be fairly accurate. In most cases, the resistance is higher when the heating element is hot so measuring it cold would give the impression it had low resistance and was rated at higher power.

There is no way to test a thermostat without heating it up. I'm not sure what temperature it is supposed to work at but you might be able to remove it, attach wires to an Ohm meter then put it in a container of water. Heat the water and make sure the switch opens at the right temperature. Make sure it is dried out before using it again!

Brian.

 

[munzir]

measure across the wires where phase (220 V supply Ac) & neutral is feeding in ???

the resistance should be higher when the heater is hot and vice versa , right ????

 

[betwixt]

I don't think the resitance is relevant to the problem but yes, that's the way to do it. The only problem is that you stated there is a contactor in circuit as well so if you measure across the power cables you will measure the contactor coil as well as the heating coil.

The heater will be a coil of wire around the vessel. Usually the wire is an alloy of Nicklel and Chrome metals ("Nichrome") which has a relatively high resistance. It also has a positive temperature coefficient so the resistance rises as the temperature increases. The heaters power rating is measured while it is in operation, in other words while it is hot and the resistance is high. If you mesure it when cold, the resistance will be lower and the power calculation will give a result which is higher than it should be.

Your issue is that the thermostat switch or the contactor switch doesn't open at the set temperature. I don't think it has anything to do with resistance or power ratings.

Brian.

 

[jeffrey samuel]

we find you drifting of from the title and first post you started

but the function of the thermostat is to turn on and of but the resistance has a lesser role to play in this issue


only on hypothetical grounds your statement is valid for you to measure the power rating

 

[munzir]

I would have to measure the resistance after removing the wires out of the circuit but as betwixt told i have to heat up the heater and then measure the resistance of the wires firstly to take them out of the circuit

and in a same fachion i measured 63.5 ohms resistance but that was a cold test ....... the heater was absolutely cold

 

[betwixt]

As I keep saying, and Jeffrey confirms, the resistance and power rating of the heater has nothing to do with the thermstat not operating.

The thermostat is simply an on/off switch that is operated by the temprature crossing the trip point. Below the trip point the thermostat switch will be closed and almost a short circuit (zero Ohms), above the trip point it will be open circuit completely. The current through the thermostat switch will energize the contactor coil which in turn enables the power to reach the heating element.

The thermostat resistance should not change as you turn it. It works mechanically and changing the setting just alters an internal spring pressure to make it open and close at a different temperatures.

Brian.

 

[jeffrey samuel]

i have a feeling that your are not in aposition to picturise your problem exactly kindly post some photos are stuff that will put us to light regarding the issue

the fact of your problem is puzzling