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Old 28th Oct 2017, 4:20 pm   #56
broadgage
Nonode
 
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
Default Re: Variac help please

I made a useful soft start unit for a large transformer.
It is a relatively simple matter to insert a series resistance for starting that is bypassed by a relay after a short time. However as has already been said, that leaves the risk that a failure of the relay or timer would leave the resistance in circuit indefinitely with consequent risk of fire.
I resolved this by using a standard electric kettle as the resistance, this is connected in series with the transformer for about a second and then bypassed.
Any failure that leaves the kettle connected continually can do no worse than boil the contents and then trigger the auto shut off of the kettle.

A 1KW or a 3KW kettle may be selected according to the size of the load.
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