Transmitter Project, more early stage musings

[N0WEK]

About sagging voltage using a variac. Me thinks that if you used one with double the KVA rating of the plate transformer, the sag would be very low. The only problem is finding one with about a 5KVA rating or more and then the space to put it.

I guess you could just mount it on the wall or get one of those fancy motor driven types.  I've seen some big three phase jobbies that perhaps could be put in parallel.

I think a 3 phase 15 KVA variac would weigh about 500 pounds. That should do it
 

Complete with one of those Buzzardly 12" spoked wheels to turn it and a big old cast iron and brass AC voltmeter next to it!

[KM1H]

I did a little test today, hooked a 10A 240V variac to a SB-220 I just finished converting to 6M for a customer.

Without the variac and at 2065W DC input key down the sag from 2995V is 413V or 8.7% regulation. With the variac at 240V it is unchanged as expected. At 220V the sag from 2745V and 1840W DC input is 441V or 8.4% regulation due to less xfmr and variac loading. I didnt go any lower as the filament xfmr is off half of the primary winding. Thats not too bad for a variac of just about equal (2400W vs 2150W) rating to the SB-220 when the 150W filament is factored in. I was rather suprised.

The SB-220 transformer is rather wimpy while the variac is CCS.

[ke7trp]

Thats not to bad really.

C

[K1JJ]

Yep, makes sense when run within its ratings.  Variacs are FB on smaller rigs where the weight is reasonable. I use one (about 40 pounds) on my own pair of 3-500Z's linear amp and find it a blessing when using the amp at 1500V, loaded heavily.    

Scaled up, they should work just as well, why not?

However, if the amp gets really big and current requirements soar, you soon need a fork lift...   If one can design the big amp / supply without it, it saves a lot of space and weight.

T

[ke7trp]

I read in one of my handbooks that there is a line you should nto cross when using a variac.  An example would be when you use the variac to cut the voltage down in half or less for the designed transformer. 

C

[KA2DZT]

I did a little test today, hooked a 10A 240V variac to a SB-220 I just finished converting to 6M for a customer.

Without the variac and at 2065W DC input key down the sag from 2995V is 413V or 8.7% regulation. With the variac at 240V it is unchanged as expected. At 220V the sag from 2745V and 1840W DC input is 441V or 8.4% regulation due to less xfmr and variac loading. I didnt go any lower as the filament xfmr is off half of the primary winding. Thats not too bad for a variac of just about equal (2400W vs 2150W) rating to the SB-220 when the 150W filament is factored in. I was rather suprised.

The SB-220 transformer is rather wimpy while the variac is CCS.

Carl,

I'm having some problems trying to follow your math.  I'm not seeing the same % regulation of the overall plate supply voltage.

Unless I'm wrong,  I always thought % regulation was;   no load V - full load V (sag) divided by the no load V times 100.

From your numbers;  I see in case one,  413V/2995V x 100 = 13.79%

                             and in case two,    441V/2745V x 100 = 16.07%

My results show a poorer regulation with the variac set at 220vac,  which is what would be expected.  Your results are showing a better regulation.  In addition, I'm not seeing how you arrived at those overall lower % numbers.

Maybe you define % regulation in some different manner.

I'm going to check a Handbook to see how the ARRL defines % regulation.

Fred

[Opcom]

About sagging voltage using a variac. Me thinks that if you used one with double the KVA rating of the plate transformer, the sag would be very low. The only problem is finding one with about a 5KVA rating or more and then the space to put it.

I guess you could just mount it on the wall or get one of those fancy motor driven types.  I've seen some big three phase jobbies that perhaps could be put in parallel.

I think a 3 phase 15 KVA variac would weigh about 500 pounds. That should do it
 

The 12.5KVA powerstats around here are about 120 lbs each. dual section 28A/240V each.  Real power supplies have handwheels. 8-)