Impedance Heating Power Control

[roydm]

I'm fairly familiar with using Zero crossover SCR controls for large resistive heating loads but what is the best method of controlling a transformer supplied heat load, I seem to recall that the transformer should be phase angle controlled. I am looking at 400 - 500 KW load.
Thanks in advance.

 

[Compositepro]

I think you have it backwards. Resistive heaters do not care about the difference between phase angle and zero crossing unless you have something like an incandescent lamp where you will see flicker unless you use phase angle. With inductive loads you will generate harmonics if you use phase angle because the input will be far from sinusoidal.

 

[Skogsgurra]

There is a problem with on/off control and zero crossing when transformers.

Standard transformers are designed to absorb a certain volt-second "area" and since the flux moves from -Maxflux to +Maxflux, the maximum area will be half of the half-period area.

If yo use a zero crossing thyristor (triacs are not available in the size you need) then the core will see twice the volts-seconds it is designed for and thus produce maximum inrush current, which may easily be ten times rated current, on every switch-on.

That is not good for the grid (constant deep voltage drops) or the thyristors, which need to be designed for that inrush current.

It isn't good for the transformer either. Especially not if your on/off cycle is short. Which it probably is. If you switch on/off repeatedly in around ten minutes - it is OK. But if you switch several times per minute, you will have heat in the transformer.

So, you are right: use phase control.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

Your phase angle controller will need to be evenly matched on the positive and negative half-cycles if you're driving a transformer, otherwise you'll end up with a DC offset in the core and asymmetric saturation. Most transformers aren't exactly generous with the core iron in the first place, so anything which tips them toward saturation is definitely a bad thing. Or use a special transformer which is designed to tolerate a DC flux.

 

[roydm]

Thank-you Gentlemen, that's the sort of input I was hoping for.
An experimental system we built years ago used phase angle control with a period of seconds rather than minutes. I believe we used standard off the shelf 24 Volt transformers of about 5 KVA each but it sounds like we should be looking for transformers designed specifically for this application.
Is there a standard way to convey to a prospective manufacturer what we need so they don't just assume we are going to feed it from a regular AC supply?

We are hoping to purchase suitable power controller/s off the shelf, the Transformer/s I understand will likely be a special build

BTW our regular MCC supply is just under 700V 50 Hz, obviously we will be looking to supply the transformer with that.

I appreciate your valuable input.

Roy

 

[electricuwe]

Have a closer look on these products:

https://www.advancedenergy.com/prod.../thyro-scr-power-controllers/thyro-px-series/

A few years ago Advanced Energy took over the thyristor controller business from AEG Power Supply Systems in Warstein-Belecke in Germany, being on the market with such products for decades.

 

[Compositepro]

This is interesting:

https://ccipower.com/support/resour...rove-power-factor-zero-cross-transformer-mode

 

[LionelHutz]

Are you in British Columbia? We been building a soft-starter in Ontario for years and a standard RC2 soft-starter could be set to current control to do exactly what you want.

The transformers might need to be custom built for the voltages and power you need. Otherwise, a suitably de-rated "off the shelf" unit would likely work fine too. Off the shelf in quotes since 5kVA, 3-phase 700V:24V doesn't sound very off the shelf or stocked.

One way you could tell the manufacturer what you want would be to specify a K rated transformer for the harmonics it will see.

 

[roydm]

Lionel, Yes BC
We are currently looking at 3500 Amps at 48 VAC for the secondary windings, a single phase transformer per phase.

I'm trying to get my head around the size and form of the conductors in the winding.

 

[waross]

Hello roydm;
This sounds like an interesting application. Are you able to share with us the actual application?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

A number of years ago we did a setup for a company to help with testing bus bar. It was 3 single phase current controllers which powered their TX. The idea behind 3 single phase controllers was to keep the current on all 3 input phases balanced so that the outputs from their step-down TX were all balanced and all the bus bars would get the same AC current. I never witnessed it working, but the feedback was positive.

 

[itsmoked]

but the feedback was positive

I hope this wasn't a pun..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[thebard3]

On a project long,long ago in a state far, far away, we once installed 2 bonding autoclaves, each with a 3 MW 480VAC electric heater. Each heater had 5 3-phase circuits, power was controlled by zero-crossover SCR (thyristors) on 2 of the phases, the third phase being directly wired. When the thyristors were switching, there was an obvious flicker of the plant lighting, which we thought might be a problem. There was a nearby town where a small hydroelectric generation station was located. Once the system was put into production and operation continued into the night, it didn't take long before we realized that the street lights in town flickered also. The electric supply to the plant was 69 KV, stepped down to 12470 which fed 2 double ended substations. So the transients were getting all the way to back to the grid. We were thus faced with the option to replace 10 zero-crossover SCRs with phase-angle type(big bucks), or to find another solution. We found a device that allowed the zero crossover SCRs to function like solid-state contactors, using time-proportional on/off control, and it also staged the outputs so that each SCR would share an equal duty cycle. This saved the day, at least as far as THAT issue was concerned.

Brad Waybright

It's all okay as long as it's okay.

 

[waross]

Be careful using time proportional control on a small plant.
I know of an instance where a large mine mill was run on diesel power during construction.
The main bearings went out on the new diesel generator.
The main bearings went out on the replacement new diesel generator.
It was found that the electric heating in the main office was time proportional controlled.
The repeated block loading every few cycles was taking out the main bearings in a matter of weeks.
Be wary of time proportional on a relatively small supply.


Bill
--------------------
"Why not the best?"
Jimmy Carter