Continuous Current Derating
Derating based on Single Phase Input
A Single-phase power source can be safely applied to three-phase rated inverter’s provided that
care is taken to properly oversize the inverter. NOTE: 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 in order 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% THDi
and greater compared to approximately 40% with a three-phase power source. The result is 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 of 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 the Technical Specifications Chapter. 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 (Pr.05).
• Set Motor Data and Protections - Set the parameters that are related to motor
• information (dr.14, bA.11 ~ bA.16), overload trip (Pr.20 ~ Pr.22) and E-thermal functions
(Pr.40 ~ Pr.43).
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