Phase angle controler before or after furnace transformer?

[cci]

I am building a small glass melting furnace using 4 MoSi heating elements in series. Total load 10 kw at 60 volts.Transformer pri is 240v sec 60v Since I need to limit current on start up I plan to use a phase angle controler. My question is should I buy a 50 amp scr for the inductive pri side of the transformer or a 200 amp for the sec. side heater load. My concerns are line noise, efficiency, transformer heating ,power factor ,life of the scr ect. I think that once this unit gets to temp it should run at 20-30% of its kw max. One other though was to convert the transformer out put to dc and use a PWM control. Thank you for any info or tips ,Tony

 

[Skogsgurra]

I would put the SCR on the primary side.

50 A thyristors are smaller, will have less losses and cost less. There are also standard trigger units available.

One little problem could be that the transformer primary is inductive and it can be difficult to have a clean operation over the whole firing angle range. Snubbers parallel to the primary will help getting up to holding current (usually 20 - 50 mA) before the firing pulse disappears. And since you will never be close to 180 degrees delay (20-30% is far enough from the critical point), the DC problem (when firing is sucessful only in one of the thyristors) will probably not be a problem.

 

[Warpspeed]

I would also put the SCRs in the primary as Skogs suggests.

As the transformer has a resistive load, the primary will look pretty much like a resistance too. I have successfully used this myself to control the dc load current in an eddy current dynamometer, and also for very high current battery charging applications. For high power it is a much more efficient arrangement.

The only thing you might need to watch is that at initial startup the SCRs must be brought in slowly with a soft start circuit, otherwise flux doubling in the transformer core can cause a massive current spike.

Voltage spikes and transients on the mains might sometimes cause the SCRs to erratically turn on for the odd half cycle (dv/dt switching). But with a heating element load that should not be a problem. The SCRs just turn on to protect themselves from applied high voltages.

 

[Mike2468]

The correct way is a current-limited option on a phase angle scr power controller installed on the primary side. For noise issues, there are CE noise filters available if you required to address this (such as for European installations). The filters may cost as much as the phase angle controller. For SCR life, just follow the standard precautions of avoiding over-amperage, over-voltage, and over-temperature conditions.