S t e v e ,
{Snip}
1. Should there be a step-start resistor in both primary legs or is
just one leg adequate?
If the transformer has multiple windings for changing from 120 to 240
then putting a stepping resistor in both windings is the way to go, and
this translates into one resistor per power leg.
{snip}
2. Some sample circuits show the step-start relay across one side of
the mains supply (120vac) with an electrolytic across the coil, a series
pot for adjusting time delay, and a series diode. Now some samples
indicate a 120vac coil and others a 120vdc coil. It seems to me the
diode and pot provide enough rectification and smoothing to go with a dc
coil here.
There are some excellent commercial delay relays made by Crouzet
that work on a.c., D.C. 12 to 240 volts (all in the same package). Why
reinvent the wheel when a simple and inexpensive timer relay control
operating a convenient P&B open frame relay does the job?
{snip}
3. Am I assuming correctly that only a couple seconds of delay are
needed and that the typical 25 or 50 watt resistors are an adequate
rating for this short delay?
The time delay is the time necessary to charge the filter capacitors
without exceeding the current rating of the diode rectifiers. The
transformer
secondary will be looking into an essentially zero-ohm dead short for
a wee bit. Only that step-start resistor pack is limiting the shebang at
this
point. Some of my power supplies take a half-second, others take ten
seconds. You know your investment is worthwhile if your stepper
resistors get hot when they operate. Do not forget to fuse the stepper
resistor lines from the DPDT relay armature.
Hal
W4HBM
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