The ampere reading on the input side (L1 & L2) of the H-A-S Static Converter will read as expected with any single phase equipment. That is, as the load increases, amperage will increase and both lines will be carrying the same amount of amperage. It is important to remember that these two lines will be carrying more amperage than the nameplate of a three phase motor will indicate. This is true because it will be carrying the same total power on two lines that it would be carrying on three lines when operating on three phase. The current required from single phase lines times 1.73 delivers the same power as three phase provided that the system efficiency and power factors are the same. For an H-A-S Static Converter-motor combination, the exact full load amperage taken from the single phase lines is calculated as follows:
$$(Converted \hspace{4 pt} Motor \hspace{4 pt} FLA = \frac{1.73}{PF_{H-A-S}}* PF_{3Phase}* \frac{Eff_{3Phase}}{Eff_{H-A-S}}* FLA_{3Phase})$$
Where:
- \(PF_{H-A-S} \hspace{4 pt}=\) Power Factor of H-A-S Static Converter and motor combination
- \(PF_{3Phase} \hspace{8 pt}=\) Power Factor of H-A-S Static Converter and motor combination
- \(Eff_{3Phase} \hspace{4 pt}=\) Efficiency of three phase motor from nameplate or motor data
- \(Eff_{H-A-S} \hspace{1 pt}=\) Efficiency of H-A-S Static Converter and motor combination
- \(FLA_{3Phase} \hspace{2 pt}=\) Three phase full load amps from motor nameplate
At full load conditions, it has been found that the power factor of the H-A-S Static Converter – motor combination is approximately .95 and its efficiency to be very nearly the same as when the motor is operated on three phase. The ratio of Eff3Phase/EffH-A-S then becomes unity and our equation simplifies as follows:
$$(Converted \hspace{4 pt } Motor \hspace{4 pt} FLA = \frac{1.73}{.95} * PF_{3Phase} * FLA_{3Phase} = 1.82 * PF_{3Phase} * FLA_{3Phase})$$
The above relationship should be used to determine maximum L1 and L2 heater coil and fuse sizing. The amperages from the tables on page 4 for L1 and L2 are based on the National Electric Code and are conservative values for protection. Although L1 and L2 values selected from the tables should not allow a motor overload, selection of heater coils and fuses from those vaules may not permit an output of 100% of the horsepower rating of the motor. This is because of the large range of design characteristics from motor manufacturers. The T1 amperage to the motor will be the same as the L1 and L2 current taken from the line. At first thought, it would appear that this amperage is excessive; but it must be remembered that due to the winding connections, the I2R losses are spread out over all the motor windings.
The T3 amperage may read higher than T1 amperage at no load or partial loads. This condition is normal and will not damage the motor or the converter. The T3 amperage will decrease as the load on the motor increases, while T1 and T2 amperages will increase as the motor approaches full load conditions. Although the actual amperages for L1 and L2 may be easily calculated as shown above, the amperage to use for the proper heater coil sizing for T3 is not so easily obtained. For practical purposes, however, the maximum T3 amperages should be calculated as follows:
$$(T3 \hspace{4 pt} Amperage = .75 * FLA (230 \hspace{4 pt} Volt \hspace{4 pt} converter-motor \hspace{4 pt} combinations))$$
$$(T3 \hspace{4 pt} Amperage = .90 * FLA (460 \hspace{4 pt} Volt \hspace{4 pt} converter-motor \hspace{4 pt} combinations))$$
The overall input wattage (I2R) of the motor at full load when operated with an H-A-S Static Converter does not exceed the overall input wattage of the motor when operated on three phase. For this reason, at full load conditions, the motor will have the same approximate temperature rise as if operated on three phase power.
