Jim Eichner asks several questions about transformer insulations.
1. Is a ferrite core considered conductive since it measures several
thousands of ohms?
Consider that:
Conductors are never zero ohms, but approach zero.
Insulators are never infinite ohms, but approach infinite.
Many safety standards include an insulation resistance spec.
Typically, this is at least 7 megohms at 500 volts. Ferrite cores
are much less than this value, and therefore must be considered
conductors for the purposes of safety.
(If you have ever failed a transformer hi-pot test, and the core
played a part in that failure, then you would have no doubt that the
core is a conductor!)
2. Floating core.
(a) Can the core be simply treated as an interspersed conductive
part which counts for zero in creepage and clearance measurements
from primary to secondary?.
Yes.
(b) Does it matter how the total is divided - can most of it be on
one side or the other, or do you have to meet at least Basic from
each winding to the core?
Depends. The problem here is that the core will float at a potential
established by the stray capacitances of the total insulation system.
If you assume that the core is floating at a potential different than
the value at which it actually floats, then you will stress at least
one of the two insulations.
The most reliable practice is to assume the core is floating at the
midpoint, and establish Basic insulation from each winding to the
core.
Or, assign the core to either the primary or secondary, and establish
Double or Reinforced insulation to the other winding.
4. Does our varnish impregnation help us in any way?
No. For the purposes of safety, varnish is not considered an
insulation. This is because (a) its thickness is not controlled,
(b) its coverage is not necessarily 100%, and (c) it is considered
fragile and subject to damage from handling.
5. Is hipot testing required from either winding to the core or just
between the windings?
Between windings. However, during the design evaluation, because
you are desiging two, independent insulations, it is a good idea to
check each one independently.
Best regards,
Rich
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|Richard Nute |Quality Department |
|Hewlett-Packard Company |Product Regulations Group |
|San Diego Division (SDD) |Tel : 619 655 3329 |
|16399 West Bernardo Drive |FAX : 619 655 4979 |
|San Diego, California 92127 |e-mail: [email protected] |
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