A n input AC reactor (also called an input choke) can be required for two possible reasons:

1. If the supply is very low impedance the AC reactor will be required to limit current into the inverter; in particular the input rectifiers and capacitors.

Additional notes.
A low impedance supply is one where, when the inverter is running at full load, the voltage drop measured at the input terminals is 1% or less. Put another way, if the short circuit current of the supply is 200 times greater than the full load input current of the inverter, the supply impedance is low. A low impedance supply will cause high currents to flow in the input rectifiers and DC link capacitors, and may cause overheating.

2. If the supply is subject to transients, voltage peaks etc., the AC reactor will reduce these and protect the inverter from damage.

Additional notes.
Voltage surges and transients can be caused by power switching, lightning strikes, or badly regulated supplies. In an industrial installation, transients may be caused by old DC drives, welding equipment, load switching, and especially power factor correction equipment. The inverter is protected from transients internally, but where frequent transients of high energy are expected, an input AC reactor will limit the magnitude and energy adsorbed by the inverter. Note that an EMC filter will not offer protection against these types of high energy, low frequency transients.

Recommendations:

An input AC reactor is recommended where:
a) The supply impedance is less than 1%.
b) The supply is of poor quality, or subject to transients.
c) Nearby equipment may cause frequent transients of high energy.AC reactors with an impedance of 2% and 4% are offered by Toptek, but any suitable AC reactor may be used provided it is rated for the input current of the inverter. A 2% AC reactor is usually adequate for inverter protection in most installations.

Selecting an AC reactor

A 2% AC reactor is defined as one that drops 2% of the supply voltage at full load current.
i.e. Impedance = (Voltage Drop)/(Full load current)
or = (Supply voltage x 2/100)/(Full load current)
(Note that this is also the definition of supply impedance)
The impedance of an AC reactor = 2 x P x Supply Frequency x L
where L is the inductance in Henries.
It is therefore possible to calculate the required inductance.

Example:

For a 7.5kW 400V inverter, the input current is 22A.
Required impedance = (400 x 2/100)/22 = 0.36 Ohms
for a 50Hz supply, the inductance, L = 0.36/(2 x P x 50) = 1.15mH, 22A rated

AC reactor for 230V 1 phase Drive (TIAR-21xxx)
AC reactor for 230V 3 phase Drive (TIAR-23xxx)
AC reactor for 460V 3 phase Drive (TIAR-43xxx)
AC reactor for 575V 3 phase Drive (TIAR-63xxx)