Replacing 3 phase drive w/ 1 phase drive

[ganutenator]

I work for an OEM.
The OEM has a test site.
We wanted to test a single phase vfd. (reason, they were offered in the 1/4 hp where as the 3 phase ones were not)

We left the original 3 phase Telemechanique mtr disconnect feeding the line leads of the vfd in place but only connected two phases.

The mtr disconnect kept tripping.

A panel builder supplier recommended looping one of the legs of the disconnect back up to the top like the auto cad drawing attached.

 

[chelton]

Wouldn't you only be using 1 phase from the 3 phase C/B and a neutral? Or is the drive single phase 380/400?

 

[ganutenator]

Wouldn't you only be using 1 phase from the 3 phase C/B and a neutral? Or is the drive single phase 380/400?

The drive is single phase 230V. Two of the 3 phase Lines to feed the drive.
One phase to neutral would be 120v.

 

[Rson]

I work for an OEM.
The OEM has a test site.


The mtr disconnect kept tripping.

.

I think you mean the circuit breaker (that may have a rotary disconnect handle tied to it). Is this upstream or downstream of the VFD? (should be upstream)

Yes - these will trip if it was originally 3 phase, you removed one of the phases and it no longer has current. For single phase, you will have to wire like your attachment shown so that current flows through all 3 poles.

Also keep in mind - it may pull a different current on the single phase VFD than the three phase. That circuit breaker may not be rated correctly for a single phase application.

 

[roxusa]

Use a 2 pole CB ?

 

[jraef]

Devices like that have what is called a "differential trip bar" inside that artificially LOWERS the trip threshold point of the thermal setting if you lose one phase. It works by being set up with balancing springs that counter against the mechanical force exerted by the bimetal thermal trip elements in all 3 phases. When there is no power flowing through one element, you have 1/3 of the mechanical force removed, so it takes less current to make the device trip. The purpose of this is to lower the trip threshold if a 3 phase motor loses a phase.

The solution is exactly what was proposed; you loop one line back through so that current is flowing on all three poles. It doesn't matter that it is the same current as on one of the other poles, it just matters that all 3 thermal elements see current flowing.

 

[ganutenator]

Devices like that have what is called a "differential trip bar" inside that artificially LOWERS the trip threshold point of the thermal setting if you lose one phase. It works by being set up with balancing springs that counter against the mechanical force exerted by the bimetal thermal trip elements in all 3 phases. When there is no power flowing through one element, you have 1/3 of the mechanical force removed, so it takes less current to make the device trip. The purpose of this is to lower the trip threshold if a 3 phase motor loses a phase.

The solution is exactly what was proposed; you loop one line back through so that current is flowing on all three poles. It doesn't matter that it is the same current as on one of the other poles, it just matters that all 3 thermal elements see current flowing.

awesome! thank you for this

 

[I_Automation]

I have worked on quite a few lines of tanks with 120V mixer motors and small Penguin pumps.



They ran 120V just like your drawing - Line looped through 2 poles and Neutral through the third.

 

[Gene Bond]

Realize that on 1phase, the drive will pull more current than if it has a 3phase input (for the same output current). You will need to turn up the trip setting, or use a larger MCP.

See the manufacturer's specs on input protection ratings.