Heater circuit using a DC contactor

unsaint33

Member
Join Date
Sep 2019
Location
MInnesota
Posts
114
Our industrial oven heater is powered through two contactors in series. The 1st contactor's coil is AC. The 2nd contactor's coil is DC. AC power goes through a couple of devices (probably rectifier and filter) that change the AC power to DC before it feeds the DC coil. One day the DC coil contactor went bad and we didn't have a replacement, so we just bypassed the rectifier circuit and used another AC contactor. It's been a month and the oven runs just fine with two AC contactors in series. I am wondering why the DC coil contactor was used in the 1st place in the original design?
 
If it’s a heater I would have to wonder why the AC to DC rectification in the first place
It seems like a waste to me most heaters are resistive and they would work just as well with either.
Induction heaters would need to be AC only.
If you are saying you have 2 contactors for the heater and one has an AC coil and the other has a DC coil. It would think that they are controlled from 2 different controllers maybe one is for a high level safety. Maybe the DC coil is on the high level. DC coils can reduce power consumption and extend the coil life by reducing the voltage after the contactor pulls in, usually it’s about 50% reduction.
If you are using the contactors are switching the heater load then I would expect that one contactor would be switching the AC power before the rectifier and the other would be switching the DC power between the rectifier and the heating elements.
But be careful there
I have seen places where they use a standard AC rated contactor to switch a DC load. It will work short term but that’s not a good idea. Switching DC requires a much larger air gap in the contactor to beak the arc in the dc load. While if the DC is generated from AC rectified the contactor will hold up better they still will burn the contacts when opening under load. Consider the usage carefully when selecting contactors.
 
...
If you are saying you have 2 contactors for the heater and one has an AC coil and the other has a DC coil. It would think that they are controlled from 2 different controllers maybe one is for a high level safety. Maybe the DC coil is on the high level. DC coils can reduce power consumption and extend the coil life by reducing the voltage after the contactor pulls in, usually it’s about 50% reduction.
...
I interpreted the question to be about the coils of the contactors, in which case this is the salient point. In some types of heat control systems, there is a High Limit Safety Relay involved to prevent a runaway over heating situation if the main control contactor welds. A special controller is tied to a separate contactor from the main one that is controlled On-Off by the PID control system. That safety contactor is basically always held closed unless the Safety Relay drops it out, so they will use a DC coil for the reasons cited by GaryS. An AC coil will work fine for a while, but is considered less reliable when continuously energized like that.
 
I made a simplified drawing of the circuit. It sounds like I should order a replacement DC coil. Thank you so much.

It appears that the AC contactor is for on/of control of heat.
The DC contactor is for high limit condition. You should be okay to use an AC relay for high limit condition. Little gain from using the DC relay.

Using 2 relays, one for control and one for high limit is normal.
The reason for the separate relay for high limit?
The constant on/off of the heat control relay (C1) can cause contact damage and eventually the contacts may "weld" together.
Then the high limit relay (C2) will open to solve C1 failure.

Since the high limit relay is no going on and off all the time, it should have good contacts.
 
In some types of heat control systems, there is a High Limit Safety Relay involved to prevent a runaway over heating situation if the main control contactor welds. [...] That safety contactor is basically always held closed unless the Safety Relay drops it out, so they will use a DC coil for the reasons cited by GaryS. An AC coil will work fine for a while, but is considered less reliable when continuously energized like that.
Thank you for the information. However, as you can see in the drawing I posted, it is the AC coil that is for the high limit safety. I double checked it. (the DC coil is the one that opens and closes by a PID controller) So, if you were me, would you swap the contactors so that the safety high limit will maintain the DC coil power?
 
Keep in mind that the AC/DC coil longevity argument is contingent on additional control of the the DC coil current. If you don't have an additional DC current controller that will reduce the coil current after the contactor is pulled in, there is no benefit to using a DC coil in a continuous on application.

I have always been told the opposite when it comes to coils. The majority of an AC coils impedence is the result of inductive reactance. This is why an AC will coil draw HUGE amounts of current when first energized. The coil needs the armature in place to increase the inductance and limit the current. This is also why an AC coil will burn out in short order if the armature is frozen. The larger initial current allows for larger initial magnetic fields from an AC coil, which can result in faster actuation. Once the armature is in place the current through the coil naturally decreases to a much lower level due to the increase in inductive reactance.
A DC coil doesn't act that way. The current through a DC coil is constant based on the voltage applied regardless of the position of the armature. So you need to apply a larger current through the coil to close the contactor than to keep it closed. However, this reduction in current doesn't happen automatically. The user has to control that outside the coil. This is likely not true with the electronic coil variants but with larger contactors it is.

All that said, the way the DC coil contactor is controlled in your drawing doesn't seem to provide any benefit UNLESS there is more in that "rectifier" than you have drawn.

Keith
 

Similar Topics

We have a new extruder that came in a couple weeks ago and it has a ferraz semiconductor fuse for each of the barrel heater circuits. This is a...
Replies
9
Views
5,171
It has been a long time since I have had to figure this calculation, so maybe someone could refresh my memory. I am hooking up some single phase...
Replies
3
Views
1,441
This is something I figured out that electrically doesn't make sense to me. An electric blanket with a single heating wire looped through it and...
Replies
12
Views
983
Hello everyone! I'm new here and this will be my first post so hopefully I can get some help, I'm having a hard time wrapping my head making some...
Replies
11
Views
4,642
Not directly PLC related, sorry, but I believe I'm ok to ask this question at this forum. I'm a Controls Engineer, programming PLCs, HMIs etc. I...
Replies
19
Views
9,303
Back
Top Bottom