Rotary phase converter musings.

[Gareth J]

Having read the transwave thread, I got thinking...

I'm not ready to go into wiring diagrams or details just yet but had a few thoughts on my single to 3 phase system.

Currently I have an old, static 2hp converter connected to a small network of 3 machines. With a few clunks and hums, it starts and runs them fine, including one motor which is 2.5hp although I don't make it work at all hard. I don't run more than one machine at once.

-with a decent sized 3phase motor, could I use the static converter to make a rotary converter?
-if so, would this be "better" than just the static converter?
-indeed, would it be any different to running one of the machines on idle?

I don't intend to get anything that needs particularly sensitive electronics or is particularly huge. For now it will be a case of "ain't broke, don't fix" but will be interesting to consider future options.

Thanks

 

[MBB]

The reason for using a rotary converter it balances the phases to make them nearer to a true three phase, you can also use them to run smaller motors. I think static converters don't run motors under 1/2 hp without them overheating and burning out.
If you are running static converter on a machine with a power feed motor make sure the main motor is running before engaging feed motor.

MBB

 

[WorkshopChris]

-with a decent sized 3phase motor, could I use the static converter to make a rotary converter?
-if so, would this be "better" than just the static converter?
-indeed, would it be any different to running one of the machines on idle?

To try and answer some of your questions.

It would not be any different to running a machine on idle. The rotary converters are basically the same with a pilot motor.

The problem with fitting a bigger motor to the system is that the transformer will still be the limiting factor may depending on its specification may be undersized for the bigger load, leading to heating and premature failure.

The main advantage of a rotary converter is they will run 3 phase transformers so if you have a machine with a dc fed control system or dc spindle.
They will also run small fractional hp motors used on feeds and pumps more efficiently. Most static converters have a minimum motor size specification.

 

[Gareth J]

Thanks guys, well that's good to know. Obviously my 13amp plug will limit overall current draw but hopefully I can use existing motors on idle to take the hit off starting up if needed.

The reason for using a rotary converter it balances the phases to make them nearer to a true three phase, you can also use them to run smaller motors. I think static converters don't run motors under 1/2 hp without them overheating and burning out.
If you are running static converter on a machine with a power feed motor make sure the main motor is running before engaging feed motor.

MBB

Yes, I have a baby drill on a fractional hp. Luckily the static converter has different start/run options. Set too high and the little one doesn't sound good.

Excellent, nothing to alter for now.

The only thing I'd consider now is whether it would be possible, in the future, to use a 32amp supply. But I have to imagine that would mean a new box of tricks as the one with my 13amp plug would surely be seriously underrated.

 

[MBB]

Yes, I have a baby drill on a fractional hp. Luckily the static converter has different start/run options. Set too high and the little one doesn't sound good.

The alternative is to connect you motor to delta and use a run capacitor, small motors may start without a start capacitor if taking a small starting load like grinders or small drill if the belt isn't to tight.

MBB

 

[Gareth J]

Seems fine on this. Drill motor has a two speed switch (star/delta??) So needs proper(ish) 3 phase to work properly.

 

[bletchmonster]

Yes, connecting an idler motor to a static converter helps a lot as it balances the phases allowing easier starting of machine motors.
It also allows you to run small motors such as bench grinders without overheating and burnout issues.
I ran a 7.5hp motor idler on a 7 hp static converter for years and it worked well on everything from 5hp radial drill to 1/2 hp linisher. I only needed to use boost circuit to start idler, not needed once that was up and running.