Sizing Rotary Phase Converter For Multiple Motor Lathe

Hi everyone. I have acquired a Schaublin 150 lathe from the Canadian military. The lathe is setup to run 575V 60Hz and has 4 motors(Main spindle motor, lubrication motor, rapid feeds motor and cutting fluid motor). The total input power is 4.5KW as stamped on the outside of the power cabinet panel. I have 220V single phase in my shop.
4.5KW is approx. 6HP according to my calculations. The lathe is the only machine that requires a rotary phase converter. My other machines are running with VFDs.
What size rotory phase converter and idler motor should I be looking at?

The rule of thumb is twice the horsepower of the motor, 6hp is an odd size in the states. I would recommend a 15hp phase converter. They can typically start a 7.5 hp motor, and run a total of 15hp. If you can afford it go bigger, especially if plan on purchasing larger tools. I made the mistake of buying a 20hp, wish I would have bought the 40hp converter. Hope this helps. American Rotary units work real well.

For normal loads, the usual rule is 1.5x the load motor HP. For a hard starting load, 2x to maybe 3x the load motor HP is more realistic.

However, in your case, if the other smaller motors will always run with the main motor running, you probably do not need to consider most of them. This is because with the main motor running, it will tend to act as an additional idler, and assist in starting any other motors. The machine sounds like one which will have a clutch, so the main motor presumably runs continually.

The lubrication pump will probably start first, and will be no help starting the big one, most likely, so you can consider it an added load when sizing the RPC, unless it is small. The rapid will likely be no issue, just due to size and when it is likely to run. Coolant pump is also small and likely to be run only if the spindle motor is already running.

So, with 6 HP, either a 10 HP or a 15 HP RPC idler is likely to work well. Use the 15 size if the main motor is a known hard-starting "pig" such as a WEG motor.

My shop is tiny and every inch is a premium. I would like to keep the size of the idler motor to a minimum. Ideally I would like to separate the phase converter and the idler motor.
The lathe’s main spindle motor does not have a HP rating, it states 5.2 amps. I have attached a pic of the data plate

Not all companies size their units the same way. Kay and Arco size by the largest motor they can start so a 5 hp Kay is similar to a 10 hp American Rotary. A 5 hp Kay would probably take care of that lathe but undersizing a RPC will cause regret later as you will find a need for a larger one, assuming you have the input amps. Do you have a transformer for it? Dave

beckerkumm said:

Not all companies size their units the same way. Kay and Arco size by the largest motor they can start so a 5 hp Kay is similar to a 10 hp American Rotary. A 5 hp Kay would probably take care of that lathe but undersizing a RPC will cause regret later as you will find a need for a larger one, assuming you have the input amps. Do you have a transformer for it? Dave

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I haven’t sourced a transformer yet. I plan to buy a new 15 or 20HP rotory phase converter and perhaps try a used 10HP idler motor. I might have to move the idler motor around.

Should I be looking for any particular voltage and RPM idler motor? I was going to look for a used 10HP or 15HP motor and I’m not sure what voltage/rpm.