Importance of the Apparent Power parameter for Variable speed drive sizing.

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 variable speed drive (Altivar) because the Altivars are manufactured in sizes that to match typical value of motor power.
To know the precise Altivar limit, value of motor voltage and motor current are significant because these variables are responsible for majority of Altivar 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 Altivar limit. Note that max continuous motor current depends on switching frequency due to difference in Altivar losses. Apparent power between Altivar and the motor is not much important. Active power between Altivar 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, Altivar input circuit can have totally different parameters of voltage , current, apparent power because the two circuits (Altivar input and output) are separated by DC bus.
Apparent power declared in Altivar catalog or installation manual is not specified because of Altivar limits. You know that the heat inside Altivar is mostly created by IGBT losses and so depends strongly on motor current and voltage and switching frequency. Apparent power is useful to design upstream installation.
Apparent power at the Altivar input is calculated as Urms*Irms*sqrt(3), where Urms is rms value of supply voltage, Irms is rms value of input current.
Irms depends on supply 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, transformers, etc.), though you can make sizing of upstream circuit also based on Irms, instead of apparent power.

Đã phát hành cho:Schneider Electric Việt Nam