Variable Transformer Fuse Question

Discussion in 'The Projects Forum' started by PGB1, Jan 17, 2013.

  1. PGB1

    Thread Starter Member

    Jan 15, 2013
    Hi Folks!

    I hope I am not asking something that has been answered here previously. I could not find a definitive answer, but I am not the best searcher-of-things sometimes.

    My questions concern proper fuse type, sizing & placement for a Variac.

    I have a variable transformer. I believe it is really an inductor, as opposed to a true transformer. But, I may be wrong.

    It is Variac brand Type 100-R. The rating plate states 230 volts 2kVA Max current 9 amps.

    It is to be connected through the isolation transformer which powers my bench. The unit will be tapped to supply 0-140vac secondary. Preliminary bench testing confirmed 120vac pri = 0 to 140vac sec (At 230 primary, 0-291vac sec). I plan to use nominal 120vac primary so I can plug into an existing GFI protected receptacle (from the iso trans).

    Primary Fuse:
    For a normal control transformer, I would size the primary fuse for up to 125% of 9amps (NEC 450.3(b) FN-1 (Guess my trade?)

    Is 9 amp slow acting fuse proper for the primary? Since there is no laminated coil involved, I would expect the inrush be much smaller than on a normal transformer. Note, however, that loads will not always be 'ramped up' to full voltage. Sometimes I'll energize the load at the pre-selected voltage.

    If I used "next standard fuse size" to 125% of 9amps, which would be 11.5 amps, is the winding still protected?

    Secondary Fuse:
    Single or multi-tap secondaries are easy to size.
    But, since the device's output voltage is variable, properly sizing a secondary fuse becomes a challenge. Am I correct that I should protect from the primary side only?

    I sure appreciate any ideas & thoughts you all have about properly protecting this device. Thanks Again!
  2. Duane P Wetick

    Senior Member

    Apr 23, 2009
    Probably 10% higher than the nameplate fuse rating is a good choice and also a T.D. (time delay) fuse should be selected. The fuse action should be such that the bonding wire (slider) will be protected, ie; the fuse link will melt first.

    Cheers, DPW [ Everything has limitations...and I hate limitations]
  3. PGB1

    Thread Starter Member

    Jan 15, 2013
    Thank you for the advice Duane.

    After reading your post, I went 'science-project' on it.

    Good call for 10% slow acting. With a 10 amp fuse on the primary, slow or fast, the no-load inrush opened the fuse if the secondary was pre-set anywhere above 50 vac. I guess there is enough steel & winding wanting to be saturated all at once. Don't know (yet) why below 50 volts the fuse lived.

    Interesting Side Note- When energizing un-loaded pad mount transformers I've occasionally seen them jump 2 or 3 inches off the ground due to inrush. But, these are in the multi mega-watt & high primary class.

    One follow up question I have is:
    You mentioned putting the fuse to protect the slider. I'm guessing this is the moving wiper (shunt?) terminal.
    This placement is protecting on the secondary side, since on my unit the Pri-Sec Common tap isn't the sliding contact. It's an end-of-coil terminal.

    Did I get it right, or misunderstand completely? (I attached a quick sketch)

    Thanks Again for helping,
    • V.jpg
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  4. crutschow


    Mar 14, 2008
    A Variac is indeed a transformer, just one that shares a primary and secondary winding. It can have a high inrush current, the same as a normal transformer.

    The fuse at H1 would be better at H2 although either would work.

    The fuse at X2 should be rated for the maximum current at the maximum voltage and power. The same value as used for the H1/H2 fuse should work.
  5. PGB1

    Thread Starter Member

    Jan 15, 2013
    Thank you Crutswchow for the advice. Your words are appreciated.

    I goofed on the drawing- I mirror imaged the picture by mistake. (Dyslexia rears its ugly head once in a while. Good thing I'm not a neurosurgeon.)

    Thanks for your thoughts about the secondary fuse sizing.
    As wired, I can have a step-up to about 140 volts across X-1 & X-2.

    With 120 volt primary, this gives me a maximum of approximately 1.26KVA with 140v secondary while impressing 9 amps on the windings, just as is the case with a 'normal' auto transformer (a type I am familiar with).

    My Thoughts:
    If I use 230v for the primary, I can load the unit to 2KVA (per the plate). I'll have about 8.7 amps on the primary conductors.
    Since it is all one winding, if I impress a 2KVA load at over about 220 volts secondary, I will exceed 9 amps through the single winding & wiper conductor.
    So the fuses will both be ca 9 amps. (When I read your message, it became an "Oh Yeah! Same Winding!" moment.)

    For this time, I plan to use 120 primary. Should I ever find the need for 2KVA from this unit, I'll address installing proper ground fault protection & using (isolated) 230v for the primary.

    Why isolation & ground fault? Isn't that overkill? No!
    I'm an extreme electrical safety maniac. I always remember electricity is Good Employee but a Very Bad Boss.
    I started in the electrical trades in the early 1970's as a high voltage linesman, learning respect for power. As time passed & others were under my tutelage, their safety became paramount. Add dyslexia to the mix & I have a higher-than-average chance of an accident. (None so far.)

    As I (try to) learn electronics, safety practices will follow me.

    Thank you all for helping me protect this piece of equipment. My next trick will be to determine exactly what the no-load inrush load is. Then, back to learning electronics!