Re: transformer

[Gary Norton <gnorto at compuserve_com>]

<lurk mode off>
Forbem at aol_com wrote:

> Sorry to go off the topic but I was wondering if anyone could help me with a
> transformer problem.I ordered dupla heating cables from J.P. burleson a while
> back but they had no transformers left so I brought one from an electrical
> supplier.The employee there told me a 120V primary to 24V secondary at 40VA
> would do the trick but I tend to doubt it because it looks awful small.The
> cables are the 100 and 60 watt cables plugged into each other totaling 160
> watts.I am not sure how to convert watts to VA to tell if I would be
> overloading it.Any possible sources of online transformer sources would also
> be helpful because it didnt seem like the guy at the electrical supply store
> knew too much.Thanks.

Conversion of VA to watts is problematic, as it depends on the nature of the load.
In your case, the load (heating cable) is resistive, so the conversion is simple:
watts = VA.

You say the cables are plugged into each other... If you have connected an end of
one cable to an end of the other, then connected the other ends of the cables to
the transformer (series connection), you are not going to get the amount of
heating you might think. The 100 watt cable has a resistance of 24 * 24 / 100, or
5.76 ohms, while the 60 watt cable has a resistance of 9.6 ohms. A series
connection gives a total resistance of 5.76 + 9.6 = 15.36 ohms, for a power
consumption of 37.5 watts (VA) at 24 volts. A 40 VA rated transformer would indeed
be sufficient to handle the load without getting too hot.

However, this may not be what you want. You're getting only 37.5 watts of heating
here. If you want 160 watts of heating, you should connect both ends of both
cables to the transformer (parallel connection). Now, the total resistance is
(5.76 * 9.6) / (5.76 + 9.6) = 3.6 ohms, for a power consumption of 160 watts at 24
volts. You would need a transformer rated for at least 160 VA to ensure that it
would not get too hot; 200 VA might be even better; the transformer would run
cooler. Your suspicion that the 40 VA transformer is too small is entirely
justified in this circumstance, it would probably burn out.

HTH,

Gary Norton, in Clifton, New Jersey, where Spring hasn't decided whether to hang
around or not.
<lurk mode on>

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