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Joined: Jan 2005
Posts: 5,316
Cat Servant
Member
I have a 480v, 33-A (single phase) heater to control. As provided by the vendor, the heater is simply 'on' or 'off.' I'd like to find a way to control the output. The heater is nothing more than a bit of sheet metal holding a resistance element. Think "big toaster."

The heater is the first of many, that will be permanently mounted at work stations. As currently planned, the heaters are WAY too close to the operators and the equipment. I want to warm folks - not sear them! Having the operator cycle the disconnect between 'on' and 'off' strikes me as a (forbidden words deleted) means of control. Vendor does not offer any controls.

I imagine the 'perfect' control to be a simple dial .... turn it more, you get more heat .... plus a timer to keep it from being forgotten.

Any thoughts? I'm open to anything from a readt-to-use product, to some specific component suggestions.

Thanks.

Joined: Mar 2004
Posts: 251
W
Member
I use SSR (Solid state relays) to control the elements in large industrial dryers They can be cycled rapidly to regulate the elements controlled by a temperature controller or thermostat. Athena Controls as well as others make proportional control relays that can control the amount of current that passes. These are driven by a 4-20 ma controller. Thermocouple needed to send the temperature to a controller. Somewhat complicated for just a heater. I think a SSR with a standard thermostat would be better.

Joined: Nov 2005
Posts: 826
J
Member
If there's no fan integral to it, I would want to phase control that much power in close proximity to people. Phase angle would be driven by the difference between ambient and setpoint temperatures. But surface temperature would limit the maximum phase angle. That's what I would want/build and I would use 120VAC or less for my control power.
With no phase control, I would use a 2-pole 480VAC contactor with a 24VAC coil, small 24VAC control transformer, open-on-rise thermostat for the work space, and a fixed temperature, bi-metallic temperature switch for the heater enclosure surface.
Joe

Joined: Jul 2004
Posts: 783
L
Member
Quote
With no phase control, I would use a 2-pole 480VAC contactor with a 24VAC coil, small 24VAC control transformer, open-on-rise thermostat for the work space, and a fixed temperature, bi-metallic temperature switch for the heater enclosure surface.


With no phase control, that contactor will be banging on and off like crazy! 16 KW for bench heating seems to be a bit excessive. A PID temperature controller would make more sense for moderating versus a bang-bang control scheme. If it is grossly oversized, why not operate it on 277 or 240 if available? Less output, longer cycle times.

Joined: Jan 2005
Posts: 5,316
Cat Servant
Member
Neither 277 nor 240 is available. Supply is from an ungrounded 480 panel.

A smaller heater would be appropriate. Two similar instals in the plant have had issues; these things are simply being placed too close to the targets- and there's no easy way to add space.

I'd really like to give the operator a dial to turn up or down, as needed. Work stations are out of the weather (strictly speaking), yet fully exposed to outdoor temperature swings.

I'll call Athena from work tomorrow; their downloads come out in gibberish on my computer here.


Joined: Jul 2004
Posts: 9,672
Likes: 7
G
Member
You basically just need a big triac dimmer (phase angle controller) but obviously listed for that use. I looked around a little but no joy.


Greg Fretwell
Joined: Jul 2004
Posts: 783
L
Member
Try Watlow for temperature controllers. The DIN-A-MITE A looks like 25 A at 600 VAC single phase. The DIN-A-MITE B looks like 40 A at 600 VAC single phase.

Sales critters available at 1 800 watlow2.

Not associated with any heater or controller company.

LarryC

Joined: Jan 2005
Posts: 5,316
Cat Servant
Member
Looks like I might be able to do something with Watlow.

Their approach certainly sounds better than using a $3000 Variac!

Joined: Nov 2005
Posts: 826
J
Member
It just seems to me that someone is making you use the wrong heater(s) for your application. My latest crazy thought would help if they were being installed in 2's. You could use a dual diode module and drive each element from opposite half cycles. So you cut your power in half per unit and not even add any odd harmonics. A scope would be the only place where you might see a tad of crossover distortion. You could still add phase control upstream.
Joe

Joined: Apr 2002
Posts: 7,283
Likes: 3
Member
If Watlow isn't an economical solution, then perhaps finding a heater that is made to do what you intend, as opposed to the modifications.

Did anyone consider the costs of modifications, vs. buying an appropriate heater?

Reno, I know you love the challenge!!


John
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