Rotary Converter Efficiency

• By Electram

Rotary converters have idle currents that can be quite high when the converter operates unloaded.  

 

As example, a PhaseMAXX - 30T has idle amperage that is approximately 23A at 240v, or 5520w.

 

Where the converter operates continuously without load, power consumption costs (assuming $0.07

per kwh) would be approximately (5.52kw x 0.07) = $0.386 per hour, or $9.26 per day.

 

And of course, $9.26 per day is $3380 per year.

 

Rotary converters though, are inherently power factor correction devices. At little or no-load their efficiency is very poor. Efficiency of the converter increases as load is added.

 

Consider a PhaseMAXX - 302T that only operates a 2 HP, 230v, 3-phase motor rated 8A running at its full-load rating. The 1-phase current can be calculated as the 3-phase amperage x 1.73, or (8A x 1.73) = 13.84A + the idle current of the converter. In essence, the 1-phase amperage would be closer to (8A x 1.73) + (converter idle current of 23A) = 36.84A.

 

Because the converter is a power factor correction device, the total 1-phase current is the vector sum of (converter idle amperage + load amperage). At little or no-load however, the converter’s capacity as a power factor corrector is negligible. As load is added to the converter, the total 1-phase current is increasingly the vector sum of both converter and load amperage; hence the combined converter/load efficiency increases. Given the above example for the 2 HP motor operating from a PhaseMAXX – 30T, the 1-phase amperage would be slightly less than 36.84A - extremely high for such a small load. The efficiency of such a large converter operating this small load is dramatically poor.

 

Consider a different example, where the PhaseMAXX - 30T is operating two (2) 20 HP motors each rated 50A at 230v, and where those motors are operating at their full-load amperage.

 

We could accordingly determine that the 1-phase amperage should be (50A + 50A) x 1.73 = 173A + (idle current of the converter), which would be 196A.

 

However, at this higher load the converter operates fully as a power factor corrector, and the resulting total 1-phase current is closer to (50A + 50A) x 1.73 = 173A, where the converter’s idle amperage is essentially reduced to a (nil) value for all practical purposes - rendering this converter and its connected load highly efficient. Multi-stage converters provided for on-demand operations where individual loads are applied to the converter’s (sectional) output are designed so that each stage of the converter operates the specific load where the converter stage has its highest efficiency – that is, with almost (nil) converter idle current.

When considering the (maintained) operation of a rotary converter versus the (on-demand) operation of that same converter for a cyclic load - particularly where the load operates for only short time periods with long stand-still periods in between, the reduced Utility power supply cost of the on- demand converter becomes readily apparent. One further consideration that may be given to the on-demand type converter is the substantial extension of the converter’s useful service life.