"Al MacDonald" wrote in message
news:gTCwd.1$uj2.0@clgrps12...
I latched onto an older horizontal mill that runs on 3hp 440V 3ph power.
My plan is to hook up a 220V rotary phase converter to a 3KVa 3ph
transformer 220/440 and then to the mill. Does anyone see anything
obviously wrong with my plan? I suppose I could try and find a 220/440
single phase transformer and then a 440V single to 3ph rotary converter but
I think this way might be much more expensive. Thanks, al.
Before you proceed make sure the mill's motor can't be optioned for 220V,
3-phase operation. It is a pretty good bet the motor can be *strapped*
(just inside the junction box) for 220 V operation. If the motor can be
optioned for 220V it will say so on the name plate.

1) A 220V rotary phase converter connected to 3KVA, 3-phase transformer
will work fine. Be aware the transformer will pass everything from the RPC
so a *reasonably* well balanced RPC is essential. Three KVA is about 4
horsepower so there will be a 33% "reserve" power capability in the
transformer when / if it is loaded to a full 3 HP. Some design practice
says there should be a 100% reserve power capability in a transformer, but
it is unlikely you will ever work the mill near its full 3 HP rating. So I
think you will be alright with the 3KVA transformer.

2) If you have a 220/440 single phase transformer (at least rated for 3KVA)
and a 440V, 3-Phase motor of at least 4 HP, you can make a 440V RPC to run
the mill directly. Four hundred forty volt RPCs are easier to adjust for
voltage balance than 220V ones. The recommended values of balancing
capacitance goes down to around 7.5 microfarad per HP. If your capacitors
are for 220V service, they can be connected in series to achieve the higher
voltage rating. The total value of 2 capacitors in series is found by the
formula, C total = (C1 x C2) / (C1 + C2).

Bob Swinney