13.6 Continuous Rated Current Derating
13.6.1 Derating for Single Phase Input
A Single-phase power source can be safely applied to three-phase (240V and 480V only) rated
inverter’s provided that care is taken to properly oversize the inverter. The output is always three
phase. Below is a summary of operating conditions that occur in the inverter when powered with a
single-phase power source compared to a three-phase source. For more detail, see Benshaw
Application Note - Sizing inverters for use with a Single-Phase Power Source.
Frequency - The DC bus ripple becomes 120 Hz vs. the normal 360 Hz. from a three-phase power
source. The result is the DC bus ripple voltage is higher and the DC Bus circuit is subject to higher
stress for the inverter to deliver equivalent power. Output current ratings are valid for a 60 Hz
single-phase power source only.
Input Current - The input current through the two phases of the diode bridge converter will
approximately double.
Harmonics - Input current harmonics increase resulting in current distortion levels of 90% THD
i
and
greater compared to approximately 40% with a three-phase power source. The result is a lower input
power factor. A line reactor is always required. Size the reactor based on inverter rating.
Voltage - A stricter input voltage tolerance of –5% applies compared to -15% when powering the
inverter with a three-phase power source. The average bus voltage will be lower than the equivalent
from a three-phase power source. The minimum input voltage must be no less than 228Vac for 240
volt models and 456Vac for 480 volt models. It will be necessary to maintain a rigid incoming line
voltage so that adequate motor voltage can be produced. To minimize the effect of voltage deprivation
at the motor, consider operating the motor at reduced speed (reduced power) or using a motor with a
base voltage that is lower than the incoming AC power source rating (EX: 480V source, 415V motor).
The result of all the above is that derating the inverter’s output current and horsepower is required.
Improper selection of the inverter will result in poor performance and premature failure. Refer to the
ratings table in 13.1 Input and Output Specifications on page 357. Identify the inverter’s rated
output current with single phase input conditions. This rating must meet or exceed the motor
current rating.
Precautions
•
Add a line reactor matched to the inverter rating. A three phase reactor can be wired as single
phase. Connect single-phase power source to R(L1) and T(L3).
•
Output current ratings are valid for a 60Hz power source only.
•
Verify minimum input voltage.
•
If an input phase open fault occurs, turn off the input phase open protection (PRT-05).
•
Set Motor Data and Protections - Set the parameters that are related to motor
•
information (MOT Group), Overload trip (PRT-20~22) and E-thermal functions (PRT-40~43).
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