Sizing Phase converters input currents

[Rob Fitzgerald]

Recently I was posed a question regarding sizing a phase converter. The converter in question has a single phase 240V input, the output is 3 phase 480V serving a 15HP motor. The code is specific about sizing the ampacity of the input conductors. It?s my understanding the line side conductors need an ampacity of (21 x 250% x 2) = 105A. The overcurrent protection for converter would be sized for ampacity of the conductors. My question is does the code provide for a means of calculating the input current? Wouldn't the input current be necessary for sizing the converter? Thanks Rob

 

[infinity]

What calculation are you looking for, something beyond what is stated on the nameplate of the PC? 455.4 requires that the nameplate provides the rated single-phase input full-load amperes.

Welcome to the forum.

 

[Rob Fitzgerald]

What I'm looking for is a calculation for sizing the converter when the only information given is the load data and input voltage. In this case a 3 phase 480V a 15HP motor. Can I take the load current multiply it by the voltage ratio and say my converted needs to be rated for 42A or would I also have to apply 250% being its a fixed load. I'm looking for a defined calculation used to size a converter based on the connected load. Thanks Rob



What calculation are you looking for, something beyond what is stated on the nameplate of the PC? 455.4 requires that the nameplate provides the rated single-phase input full-load amperes.

Welcome to the forum. [/QUOTE]

 

[infinity]

Here's some info based on the motor and type of load that the PC is supplying.

For TYPICAL Single Motor Loads (not CNC or Resistive):
Including milling machines, blowers, band saws, lathes, pumps, etc.
The total running current should be less than 75% of full load 3-phase current.

• Easy - no flywheel, clutch, little resistance/inertia, 1-2 times full load current upon start-up
  Choose the rotary phase converter HP that is one size larger than the 3-phase load.
• Medium- machine inertia, medium resistance, 3-4 times full load current upon start-up
  Choose the rotary phase converter size by multiplying HP by 1.5 and round up.
• Hard - flywheel, no clutch, starts against load, 5-6 times, or locked rotor current upon start-up
  Choose the rotary phase converter size by multiplying HP by 2.0 and round up.
• Very Hard / Frequent Start- elevator, hydraulic pump, equipment under continuous load etc
  Choose the rotary phase converter size by multiplying HP by 2.5 and then round up.
• Increase rotary phase converter by one extra size for motors that are foreign or high efficiency or that have locked rotor currents greater than 6 times full load current.
• Increase rotary phase converter size by the ratio of load RPM to 1800 (common for 2-speed motors).

Similar info:
http://www.northamericaphaseconverters.com/how-to-size-a-rotary-phase-converter/