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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I have a 240vac/20A variac. The breaker I believe is attached to the input line. If I have a large load the current going out will be greater than 20A. While testing the induction power supply, I have 9A going into the variac and 36A coming out going to my rectifier.
How can I be sure that the wire will handle the higher output current if I don't exceed the input current?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
The nameplate current rating is not just for the input. With 9 A in and 36 A out, 1/4 of the turns are carrying 27 A and the brush is carrying 36 A. This is overloading the transformer and it will eventually overheat.
I met a guy who was proficient at repairing Variacs with burned windings -- he said it happened all the time in stage lighting for an on-campus theater.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Typically, you can run a variac for about 200% of its maximum nameplate current for SHORT durations. Just keep in mind, if you do have a fault, and you are using a bigger fuse than the rated current of the variac, you can flat spot the contact and/or coil.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
So how do I provide variable power to an induction heater at these power levels? Do I have to make a variable duty circuit that will chop up a 240vac input with a duty from 0 to 100%? If so, is it as simple as having a MOSFET or IGBT driven by a variable-duty gate signal? Do I need a capacitor to even out the output?
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Just purchase a [b]240/36 volt transformer rated at 25 amps [/b]. Connect this between variac and load. It will be hard for you to damage the variac using this.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
It will allow you to adjust voltage between 204 and 276 volts with ease if you connect the 36 volt secondary, buck/boost series with the input to the variac and drive the load that way.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
The connection is the same. Variac---> 240/36 transformer.
output voltage boost 240-276 volts output voltage buck 240-204 volts
note: your variac now only controls the boost/buck trim and main load current is carried through the secondary of the 240/36 transformer. Do not allow the primary of the 240/36 transformer to become open circuited.
This trick would rate a BAD iDEA in a text book but it works! See Dwg.
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Variac limits
