Re: MOV selection and specs (fwd)

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Original poster: List moderator <mod1@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Fri, 25 May 2007 17:53:35 -0700
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>, tesla@xxxxxxxxxx
Subject: Re: MOV selection and specs (fwd)

At 04:00 PM 5/25/2007, Tesla list wrote:
>Original poster: List moderator <mod1@xxxxxxxxxx>
>
>
>Subject: MOV selection and specs
>
>I’m trying to order some MOVs to protect my sync motor and for my
>transformer input.  I’m using a 120v 15/60 NST.
>
>Is DigiKey P7231 the right one?
>
>The spec sheet says the Maximum Allowable Voltage is 150V AC and 200V DC ­
>is that 150V RMS?  But I’m also confused about a couple of other specs.
>
>Max Clamp Voltage at Test Current.  That says 395V at 100A (yet the "maximum
>allowable voltage" is 150 and the surge current is 6500A??)
>
>Then there’s Varistor Voltage +/- 10% and that says 240V.


I would be much, much more liberal with your 
voltages.  A transient of a couple hundred volts 
on your 120VAC isn't going to cause a problem, 
and you really, really don't want to have 
something trying to clip a momentary small surge 
(or more properly a swell).  You should probably 
assume that you want to not clip on a 30 or 40% 
overvoltage.  Plan for your voltage at the max 
(117+20%) and the varistor at the minimum (threshold - 20%)

The clamp voltage at test current takes into 
account the IR losses through the device..  i.e. 
it starts to turn on at 240V, and at 100A it has 
about 1 ohm of resistance, so there's 350 odd volts across the device.

Also, if your MOV is on your AC input, then it's 
also essentially in parallel with the rest of 
your house.  You don't want the MOV in your TC to 
be serving as a whole house protector.


>Can someone shed some light on going about selecting the right ones for the
>job?  It seems like I would want to pick something that clamps at about 120
>x 1.414 which is 170V with maybe a little more headroom.
>
>Once I have them, they go between all the pairs of wires on the line filter
>­ hot/neutral, hot/gnd, and neutral/gnd ­ right?
>
>Thanks for all your help!
>
>Andrew in Fort Worth