Transformer options to run 240V 1P motor on 120V 1P

[kwired]

That’s not the problem. The problem is that most single phase motors will have a centrifugal starting switch and if you slow them down below the activation speed of that switch, it puts the auxiliary start winding and capacitor (if any) back in the circuit. Start windings are not made to run continuously and with start caps there is a race to failure between the caps and the drive transistors.

There are VFDs available for single phase PSC and Shaded Pole motors but they are ridiculously expensive because the sales volume is really low. It’s usually cheaper to buy a 3 phase motor + single to 3 phase drive.

Included in what you mentioned there is the Drives with Franklin, Pentair, or a few others designed specifically to run single or three phase submersible pump motors. You tell the drive which kind of motor you are using and it adjusts output as needed to drive that motor type.

They are not cheap Maybe around $1000 for 1 HP drive compared to say a Automation Direct VFD that may cost you $300. They do however contain software already set up for the application and possibly even come with pressure transducer for the control signal. A generic drive you will need to come up with your own pressure transducer or even just simple on/off pressure switch, maybe other relays or logic components depending on what capabilities are built into the drive. May also need an enclosure to mount it in as many the lower cost ones are not intended to be used in the open. So you do spend more than just the ~$300. They won't normally work on a single phase motor at all so you would need to have a three phase submersible pump.

 

[kwired]

I will try to keep this simple for clarity. This particular project is personal, so I want to try a few things. We have a huge remote garden that will be watered off a water well, fed by a generator for now, but will end up moving this to solar. Inrush on these darn pumps has to be considered. They start hard. I am really looking for a better solution like a 3P pump motor and VFD, but those are nearly impossible to find, or very expensive.

Right now we will have to drive a 1P, 1HP, 2 wire pump. Already tested in the field at 9.6A/240V at load, and 40A inrush.

All this being said, I would like to employ an RV type generator and they are 120V only.

Some time back, we had a discussion here about slaving a buck/boost transformer rather than a true isolation transformer. ?? Basically feeding the 120V in the primary as a split phase config, then pull 240V off that primary?

By the math, it would seem you would need to double the size of the buck/boost to account for driving only one primary coil at 120V? I am not sure how inrush may play with these?

Commonly available buck-boost have 12/24 or 16/32 secondaries. This works for ~25% or less voltage changes but not so well for a 100% change. I suppose you could configure an autotransformer with 120 main winding and connect the aux winding in series, but the secondary winding still needs full amp rating of the load, as you are boosting the voltage by 120 volts. Half the load will be boosted through the transformer since you are starting out with only half the operating voltage.

 

[synchro]

You might consider using a 120/480V transformer with a 480V VFD. The VFD current rating would need to be increased by a factor of 1.73 when providing only a single-phase input.

A step-up transformer that can be configured for a 120V primary and 480V secondary may be difficult to source. A step down might be used in reverse but that should be avoided unless approved by the manufacturer. An alternative would be to use two readily available transformers which are connected such that each will contribute 1/2 of the total kVA rating.

For example, a pair of the Acme T279740S 1kVA transformers could have their 120V primaries connected in parallel, and their secondaries (each configured for 240V) connected in series to provide 480V across them. The connection between the two 240V secondaries would be grounded.