480V Tank Heating Application

Hi Everyone,
Got a "Oh by the way" added to my latest project. They want to add 480V 12KW Heater to one of the tanks. Since I have never done a heating application I thought I would run my ideas by everyone and see what you all think.

Panel Design: I plan on using a 3 phase SSR (connected to the PLC output) to control the heater. In front of the SSR's I will install a Contractor. The Neutral from the coil on the contractor will run through the High Temp Relay that is installed on the Heater. So that way if the tank level gets too low and/or the heater gets too hot the contractor will drop out. Of course this shouldn't happen in normal operation because my system will see the level and tank temp and not turn on the heater if the level is too low or the temp is too high, but just in case I want the safety

Control Philosophy: There will be a Temp Transmitter in the Tank. I was thinking of just using a On/Off style of control like a home thermostat. Where I would turn on the heater at a certain Temp and then turn it off at a higher Temp.

One question I have is about the SSR's. Do I have to worry about the heat that they will generate? To take care of this should I just go with a larger SSR's than required? What is the best way to handle the heat?

Let me know what you think of this and if you have any ideas I would be happy to hear them!!
Thanks!!

You can get SSRs with built in heat sinks - that is the way to go. The SSR specs should identify the heat dissipation.

Thermostat control is probably the way to go unless you need very precise temperature control.

What is being heated to what temperature - Safety assement

What are you heating? What are the risks?

Yes, solid state switching is a good, long term method of switch power to electric heaters.

You can do this through analog command to variable voltage solid state device, or (TPO) time proportioned output to digital solid state device.

It is recognized that solid-state devices may fail in a closed state, and cause thermal run-away. What are the dangers of your media being over-heated. If no risk, then no worry, but if human injury is a possibility due to thermal runaway, the it is prudent to install secondary “HiGH-LIMIT cutout device coupled to an electro-mechanical external cut-out contactor.

If PLC is in place, then either ON-OFF control or PID (analog) control is readily achieved.

If no PLC is not present, the ON-OFF control via panel mount instrument is acceptable, if it meets your customers bandwidth tolerance of the process requirements.

How often you need to switch would determine the need for solid state relays, yes? So how big is the tank and what goes in and out? Also heat sinking - without actually checking but isn't it the switching that causes losses?

All SSRs will reject about 1-1/2W of heat per amp flowing through them, per phase. Buying a larger SSR does not change the heat rejection into the panel, but it does decrease the thermal stress on the SCRs themselves. But if your panel is sealed (not vented), that ultimately makes no difference. They will eventually cook themselves and or everything else in the panel.

Just wanted to chime in about wiring the neutral for safety, would this be considered safe? We always break the hot wire. In fact, we usually trip our breaker with an undervoltage relay, such that (local) manual intervention is needed. (as you mention, this is not supposed to happen during normal operation).

Also, if possible, I would recommend dividing your heater into groups.

With SSRs you probably don't even need a dead band. Just turn on if below temp, turn off if above. Remember to use fast blow fuses, sized about 15% higher than the heater amperage.

Two single phase ssrs will accomplish the same thing and put less heat into the panel than a 3-phase ssr, if it matters.

Also you can size the contactor by its AC-1 rating, not the AC-3 rating that is usually shown.

Good luck.

If switching the neutral can be avoided, it should be. Having a contactor or shunt trip breaker upstream of the SSR is a very good, if not essential, practice since the SSR will tend to fail shorted if it does fail. Using 2 single phase SSRs would mean that one terminal of your heating element can have voltage present with a high current potential at all times. No one should be touching those terminals without killing power, but something to think about.

Thanks for the info everyone,
On the switching neutral, yea that is not a good way to go I know that. I was just thinking of a motor starter with the overloads and how that works when I said that. Bad Controls Engineer, Bad (said as slapping my hand).

About the SSR and the heat: Would it just be better to use the contractor to start and stop the heater and avoid the SSR's all together? I dont need tight temp control. We are just trying to keep this stuff from gelling up in the Winter. That is the only time the heater will be used anyway.

Give me your thoughts.
Thanks,

have bee using Crydonm stuff for years with no issues.
With the proper heat sink i would use SSR


http://www.crydom.com/en/products/catalog/53tp-series-ip00-ac-panel-mount.pdf
http://www.crydom.com/en/products/catalog/hs053-heatsink.pdf

jimtech67 said:

have bee using Crydonm stuff for years with no issues.
With the proper heat sink i would use SSR


http://www.crydom.com/en/products/catalog/53tp-series-ip00-ac-panel-mount.pdf
http://www.crydom.com/en/products/catalog/hs053-heatsink.pdf

Click to expand...

+1. I have these SSR's running 24/7/365 for a few years with no issues.

Bullzi said:

Thanks for the info everyone,
On the switching neutral, yea that is not a good way to go I know that. I was just thinking of a motor starter with the overloads and how that works when I said that. Bad Controls Engineer, Bad (said as slapping my hand).

About the SSR and the heat: Would it just be better to use the contractor to start and stop the heater and avoid the SSR's all together? I dont need tight temp control. We are just trying to keep this stuff from gelling up in the Winter. That is the only time the heater will be used anyway.

Give me your thoughts.
Thanks,

Click to expand...

The reasons for a starter coil ending up on the “neutral” side are steeped in history and the original reasons are no longer valid, but are stuck in “convention”. The practice stems from when OL relays were separate on each phase so their contacts were wired in series. Plus people primarily used the line voltage as the control voltage, so that side of the circuit was not a grounded neutral therefore it made no difference and was simply convenient. Then for consistency, starters were built this way at factories so that regardless of the coil voltage used, the factory wiring was always the same.

As a general rule though, it’s not a good practice on a control circuit with a grounded neutral. The wire between the coil and the contact could go to ground and then the contact could open, but the coil would remain energized. The reason it was accepted on factory built starters was that the original wiring was considered “protected” within the starter assembly and unlikely to go to ground. Doing it in the field though is considered a bad practice. I’m not saying it isn’t done, just that it shouldn’t be.

We use zero-crossing SCRs for our heater controls. This way you are only switching a zero current, so there's little to no harmonic distortion to worry about.
You may not need this.

Do not switch the neutral, switch the line.
Be sure to put a current limiting fuse on each phase in front of the SCR.

For heat dissipation, we usually figure 5% or 1 watt per amp of heat dissipation as rules of thumb. You can calculate the approximate amount of heat you will be producing. You'll probably want to make sure the SCR is heat sinked, and might penetrate the heat sink through to the outside of your enclosure.

If there's no process control to worry about, you can definitely just use a thermostat for control. We usually use dual loop PID control, because we are controlling a flow process for pressure and temperature.

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jraef said:

The reasons for a starter coil ending up on the “neutral” side are steeped in history and the original reasons are no longer valid, but are stuck in “convention”. The practice stems from when OL relays were separate on each phase so their contacts were wired in series. Plus people primarily used the line voltage as the control voltage, so that side of the circuit was not a grounded neutral therefore it made no difference and was simply convenient. Then for consistency, starters were built this way at factories so that regardless of the coil voltage used, the factory wiring was always the same.

As a general rule though, it’s not a good practice on a control circuit with a grounded neutral. The wire between the coil and the contact could go to ground and then the contact could open, but the coil would remain energized. The reason it was accepted on factory built starters was that the original wiring was considered “protected” within the starter assembly and unlikely to go to ground. Doing it in the field though is considered a bad practice. I’m not saying it isn’t done, just that it shouldn’t be.

Click to expand...

Interesting. I've always that that breaking the neutral via the overload was a bad practice, but nearly every motor starter I've seen has done that. I did not know why the practice started/persisted.

One more reason that you may see the OL contact in the neutral circuit is when you have reversing starters with one OL relay. You can't always break both coil circuits on the "hot" side with one contact, but you can interrupt them both on the neutral side.

https://www.doityourself.com/forum/...-delay-limit-switches-3ph_forward_reverse.jpg