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# Is ATV apparent power important for its sizing?

Published date: 26 June 2019

Drive sizing, above all, depends on the motor. Motor rated voltage and rated current are more precise criteria for drive selection than the motor power, but motor power simplifies the selection of ATV because the ATVs are manufactured in sizes that to match typical value of motor power.
TO know the precise ATV limit, value of motor voltage and motor current are significant because these variables are repsonsible for majority of ATV losses and overheating.
As you know, motor voltage cannot be greater then supply voltage. Max continuous motor current is therefore the most significant parameter that describes the ATV limit. Note that max continuous motor current depends on switching frequency due to difference in ATV losses. Apparent power between ATV and the motor is not much important. Active power between ATV and motor is also specified, but its value is standardized to match typical power values of motor size. For example typical motor size is 75kW, 90kW 110kW, and to simplify drive selection we also use the same sizing of the drive. BUt to be precise 90kW is in fact the mechanical power on motor shaft. So drive electrical power at its output is of course greater, but no need to know how much. Max continuous output current provides it more precisely.

On the other hand, ATV input circuit can have totaly different parameters of voltage , current, apparent power because the two circuits (ATV input and ATV ouptut) are separated by DC bus.
Apparent power declared in ATV catalog or installation manual is not specified because of ATV limits. You know that the heat inside ATV is mostly created by IGBT losses and so depends strongly on motor current and voltage and switching frequency. Apparent power is usefull to design upstream installation.
Apparent power at the ATV input is calculated as Urms*Irms*sqrt(3), where Urms is rms value of suply voltage, Irms is rms value of input current.
Irms depends on suply voltage and little depends on harmonics, for example if line choke is used, the Irms is lower than without line choke.

ATV630D0N4 in ND has line current from catalog 156.2A at 380V supply. At 480V suply it is only 135.8A
If you apply Urms*Irms*sqrt(3), then at 380V you get S=102.8kVA and at 480V you get S=112.9kVA

Summary:
The value of apparent power S is used for sizing the upstream installation (supply cables, breakers, transfomers, etc.), though you can make sizing of upstream cisrcuit also based on Irms, instead of apparent power.