glensmith wrote:
Hi,

I am going to make a phase converter similar to the two stage unit you have.
One thing I have been unable to get a definitive answer on is sizing the
ider starter (mag). So... running a 10HP ider off single phase, do you size
the starter the same as if you were suppling it with 3 phase, or do you have
to up size it as if you were suppling a 10HP single phase.

Your set up is similar in that I what to start a 10HP idler with a size 2
starter (rated 15HP @ 230/3P or 7.5HP @ 230/1P)

Any clear opinions on whether the size 2 is fine or undersized????

Hi Glen,

You should size the starter for single phase as you have single phase
entering the convertor. Presumably you have a starter with three sets of
contacts, and you use two sets for single phase operation or three sets
for three phase operation?

The rating of the starter will be based on the current which the
contacts can handle. For a single phase motor the equation giving the
power in terms of line voltage V and line current I is:

P = I * V

For a three phase motor it is:

P = 1.732 * I * V

Given that I and V are the same in this case, one would expect the three
phase power rating to be 15/1.732 = 8.6 hp. The manufacturer has
obviously chosen a 7.5 hp, which is a nice round number. What machines
do you plan to run from your convertor? Technically you should probably
go for a larger starter, but if you're only going to run your convertor
lightly loaded and don't insist on doing everything by the book, you'll
probably get away with it. But if I was building the convertor myself I
would go for a larger starter.

Best wishes,

Chris