Inaccurate Variac and/or Kill-a-Watt

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swisher103
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#1: Post by swisher103 »

I just upgraded my power control for the Hottop B2K+ including a 20A Variac and Kill-a-Watt. When plugging the Kill-a-Watt into my wall outlet, I see a reading of approximately 123 V, with minimal change over time, although I'm not putting load onto the circuit. That seemed relatively stable throughout my house (12 years old, good wiring). When I then plug my Kill-a-Watt into my Variac, which is dialed to 120 V per the top dial, the Kill-a-Watt reads approximately 133 V. Unless I don't fully understand the Variac or the Kill-a-Watt, the power output from my Variac seems bad/faulty as compared to my dialed-in output of 120 V. I certainly don't want to send too much juice to my Hottop and accidentally damage the heating element. Am I missing something here or am I using these data incorrectly? Thanks

Scott

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yakster
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#2: Post by yakster »

I suspect that you're dialing your Variac not to 120 V but to 120 %. Many Variacs are wired to be able to boost the voltage at full scale, I use mine to make up for voltage drops. Post a picture of the scale and the model number.
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swisher103 (original poster)
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#3: Post by swisher103 (original poster) »


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yakster
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#4: Post by yakster »

Hmm, clearly not labeled in percentage like I thought. I would trust the Kill-a-watt over the dial of your Variac but it is odd that you're reading that much higher at that setting. You could try plugging in an incandescent lamp to your wall outlet and then your variac to see if you detect a significant difference in brightness but my experiences with the Kill-A-Watt's is that they're pretty accurate.
-Chris

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Marcelnl
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#5: Post by Marcelnl »

If your variac has a couple of additional windings on the secondary the output voltage will accordingly be higher, I'd suggest to fnd a decent multimeter and calibrate the dial and or killawatt.
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samuellaw178
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#6: Post by samuellaw178 »

The variac scale seems to assume an input of 110V. Your input as indicated by Kill-a-watt is about 123V.

When your variac is dialed to 120V ( 120/110*100%= 109% output), your output will be 123Vx109% = 134V.

Or it is just extra +10V over your input when you set to 120V...

Just my guess.

Nunas
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#7: Post by Nunas »

We used a lot of Variacs in one of my first jobs many moons ago. From the photo, I see what you have is not a Variac, which is a very high quality variable auto-transformer. What you have is a Chinese copy of a Variac, which are not made to the same standards and which vary from one another quite a bit. If you trust readings from it, I suggest that you go with your Kill-a-Watt and ignore the dial on your "Variac".

You are right to be concerned about over-voltage; it is the number one killer of resistive devices such as lamps and heating elements.

Frankly, though, from our description, you have a decent mains voltage (does not vary much and is about 120-Volts), I'd not bother with the autotransformer at all; what are you attempting to accomplish with it? Possibly a dumb question, as I don't use a HotTop.

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jfrescki
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#8: Post by jfrescki »

Looks like I have the same variac (or imitation that is). Although the dial on mine does not correspond exactly to the output voltage, it's pretty close (of course dependent on incoming voltage). However, I find that output voltage when read on my kill-a-watt is nearly identical to the ouput voltage on the variac's voltage meter on front. So I trust the variac's voltage meter corresponds to output voltage.

My incoming line voltage is usually around 115v, almost never 120v, so YMMV.
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JohnB.
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#9: Post by JohnB. »

I've got one of those Chinese variacs that I use with my Hottop. I never really look at the gauge, just set the voltage by the energy meter readout. If I had 123v at the outlet I'd have no use for a variac. My outlet voltage runs 117v & I set the variac output to 123v before turning on the HT.
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DaveC
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#10: Post by DaveC »

samuellaw178 wrote:The variac scale seems to assume an input of 110V. Your input as indicated by Kill-a-watt is about 123V.

When your variac is dialed to 120V ( 120/110*100%= 109% output), your output will be 123Vx109% = 134V.

Or it is just extra +10V over your input when you set to 120V...

Just my guess.

Absolutely correct, his Variac has voltages marked only for a 110V input.....so simply speaking 110V on the dial is 100% of input voltage. The other increases in input voltage would be as follows.

110V = 100%
115V = 104.5%
120V = 109%
125V = 113.6%
130V = 118%

So when he said the Variac to 120V it actually added 9% to the input voltage of 123V....giving his 134V. I think the main concept the OP needs to understand is that a variac can't be set to a particular voltage e.g. 125V, but only a % multiplier of input voltage...as input voltage varies, so does the output voltage of the variac

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