Thread: 5.5 hp three phase air compressor on a vfd?

I presume you mean a single phase to 3 phase VFD

The name plate indicates it is a 4kW /5HP star (Y) connected motor.
For it to work the motor will need to be converted to a ∆ connection.
If you are not comfortable with opening up and doing mine motor surgery then this can be done for you by a motor rewind.

Thanks BobL. Yes i do mean a single phase to 3 phase VFD.
What would be involved in a said minor motor surgery?
If i was to replace the motor with a 5.5hp ∆ type would it be as simple as to just wire up the vfd and pressure switch to stop start it?

Originally Posted by brendon17

Thanks BobL. Yes i do mean a single phase to 3 phase VFD.
What would be involved in a said minor motor surgery?

I'm not qualified to tell you specifically what to do but you might get an idea of what is involved by looking at these threads.
Unfortunately I only post the tricky examples - there have been many which are more straight forward

This is for a multi speed motor - looks ugly and it was
https://www.woodworkforums.com/f271/wiring-arrangements-motor-183910

If the Y point is buried like this one it can get VERY tricky
https://www.woodworkforums.com/f271/tricky-common-excavation-188056

If i was to replace the motor with a 5.5hp ∆ type would it be as simple as to just wire up the vfd and pressure switch to stop start it?

Yes replacing the motor is an option. Switching via the pressure switch is not as straightforward is it seems.
My (unnecessarily complicated) compressor circuit is shown here https://www.woodworkforums.com/f271/compressor-control-circuit-184962
It has an auto off at set time feature, auto tank venting, and as my compressor is outside my shed in an enclosure it has a cooling fan which is controlled by the same circuits.
I don't recommend you look at it in the first instance

Basically you connect the VFD DIRECT to the motor and put the connectors of the remote VFD on/off switch into just one of the contacts on the pressure switch.
This works a treat.

Would this motor run on a vfd without doing any surgery on the motor? The id plate is a bit worn but this is all i got It is a 7.5hp teco motor

Nope - that is a 380-415 3P ∆ connected motor and running it on 240V 3P will only produce around half the power.
It will be unlikely to even have enough grunt to get to the end of the PSI range on the compressor Pressure switch.

Two other complications
a) it's only a 1440 rpm motor the other is 2800 rpm
b) @ 7.5HP this is a bit out of the range of most single to 3P VFDs


A 415V Y connected motor can be converted to ∆ to run under 240 3P and still have the same power.
A 415V ∆ connected motor cannot be converted to ∆ to run under 240 3P and still have the same power.

Thanks bob. would this be the same as before and only running on half power?

Originally Posted by brendon17

Thanks bob. would this be the same as before and only running on half power?

Yep - you need to target a 415V Y that can be converted