3ph kw rating conversion to 1ph kw rating

[lesmar96]

Hello Pros,

This question may be simple. Can someone explain the conversion between 3ph kw and 1ph kw.? Our staff here doesn't quite all agree.

What we are trying to do is to run a 50hp on a single phase supply, using a 100hp vfd as the phase converter. We are trying to figure out what kind of single phase amps it might pull. If we take the FLA and run the standard formulas with effieincy and power factor, the answer comes out reasonable. If we take the Kw rating of the motor and run the formulas, it does not near agree.

What we are trying to figure out is do we nned to take the kw rating of teh motor * 1.73 to ge the 1ph kw?

Onyone wth suggestions. Plese let us know.

 

[jraef]

3 phase values x 1.732 is the correct formula to use (for kW or Amps). But understand that it is the MEASURED values, not the nameplate values. Nameplate kW is a mechanical value, nameplate Amps are only valid at full load and nameplate voltage.

Also, your input Amps to that VFD will be slightly higher (maybe 1%) in this case because there will be slightly higher losses (as a percentage of connected load) in the 100HP drive feeding a 50HP motor. That's still less in losses than any other way of getting there however.

 

[lesmar96]

So are you saying that since our 50hp 230V motor is rated on the spec sheet from the OEM at 45Kw, that when done with phase conversion, the kw requirement on the single phase line (excluding losses) will be a minumum of 78Kw?

So simply running this motor on single phase will make the meter record approx. 33 more kw/hr per hour than running it on three phase?

 

[boneless]

Came across this:

http://www.dartcontrols.com/wp-content/uploads/2012/01/De-Rating-VFD-for-Single-Phase-Power.pdf

Makes sense once you think about it.

 

[Mill_Control]

I think the kW stays the same but the amps required to achieve said kW increases by 1.73 if nominal voltages are the same. Motor kW is basically just metric horsepower.

 

[lesmar96]

I think the kW stays the same but the amps required to achieve said kW increases by 1.73 if nominal voltages are the same. Motor kW is basically just metric horsepower.

We tend to agree with you, but it does seem like every one does. We ran some calculations on the kw of a 1ph motor and a 3ph motor of same hosepower and they came out very similar.

It just doesnt seem to work to multiply the 3ph kw rating by 1.732. that number is way too high.

 

[mellis]

Power is power. It doesn't matter if it is single phase or three phase or a diesel engine or actual horses.

 

[jraef]

So are you saying that since our 50hp 230V motor is rated on the spec sheet from the OEM at 45Kw, that when done with phase conversion, the kw requirement on the single phase line (excluding losses) will be a minumum of 78Kw?

So simply running this motor on single phase will make the meter record approx. 33 more kw/hr per hour than running it on three phase?

Sorry, I misled you by conflating the two issues. I started out saying Amps, then saw that your question was for kW and added that in without thinking it through, but that was incorrect (as others have now pointed out anyway). kW is an absolute (within the confines of what I said about nameplate vs measured) value, it doesn't change for anything. Amps will however change by a factor of 1.732.

It's worth noting however since you are in the US, the input conductors feeding that VFD must be sized for the nameplate input amps of the VFD, not the motor current, as per article 430.122. So really, there is no need for conversion; you are buying a 100HP VFD, likely rated for 124A input or so, therefore your minimum feeder conductors must be rated for 125% of 124A (or whatever the value ends up being). As far as the OCPD for the VFD, that is typically spelled out in the VFD installation instructions based upon what it was UL listed with. Generally that is listed as a maximum value of CB or fuses, but be careful, because some VFDs are not UL listed behind anything other than fuses.

 

[lesmar96]

thanks. Good Conclusions.