A choke is a device for blocking high frequencies (e.g. #RF) while allowing low frequencies (and/or #DC) to pass. Two basic types are the AC choke (or line reactor) or the DC choke. The simplest form of a choke is a ferrite bead on a DC power cable (like a laptop charger) which is intended to smooth out AC noise from the DC power supply.

chokes in VFDs

From the HoneyWell Technical Reference Manual:

The main reason for using chokes is to add series impedance to reduce harmonics and stresses on capacitors. This is achieved with both types of chokes.

In the context of variable frequency drives (VFDs):

• A line reactor would be placed on the input side of the drive.
  • between the line voltage and the VFD
  • This provides overcurrent protection (e.g. to limit the inrush of the starting current).
• A #common-mode choke should be placed on the DC Bus of the VFD
  • To filter out noise and ripple from the rectifiers.
    • Without a DC choke, we can expect a +/-5% DC voltage ripple on the DC bus. By adding a DC choke, the ripple will be reduced to +/-1.5% or smaller.
    • Smaller ripple will result in lower heat losses in the bus capacitor and hence longer life. There is usually a slight reduction in DC bus voltage with the addition of DC choke, which can be considered minor considering the other potential benefits.
  • Controlling line side harmonics by limiting the peak value of the line current.

According to DC Choke for VFD – Voltage Disturbance, you want a DC choke on your VFD rather than an AC line reactor. A DC choke provides:

1. Reduced AC voltage drop compared to a #line reactor.
   1. The voltage drop of an AC line reactor is higher than that of a DC reactor with similar % impedance.
   2. Using a DC choke reduces the ripple voltage and does not have much effect on the overall DC bus voltage.
2. Reducing the transient switching current magnitude in to the DC capacitor after a voltage sag (dip) event.

Keep in mind that a choke on the DC bus does not provide overcurrent protection to most of the electronics and your design will need to have other ways of mitigating current spikes.

common-mode choke

Common-Mode Chokes: From Working Principles to Important Performance Parameters - Technical Articles

A common mode choke has a high impedance to currents in the same direction on BOTH lines, but because the windings are in the same direction

differential-mode choke

By wiring the inductors around the iron core with the opposite polarity to the common-mode choke, you create a differential-mode choke which functions in the exact opposite manner.

line reactor

A current limiting reactor (also referred to as a line reactor) is an inductor placed on a powerline. These are used with #AC power distribution with the intended purpose of:

• smoothing out spikes in current (e.g. to protect sensitive equipment)
• and of delaying the hazardous changes in current for as long as it takes the circuit breaker to actuate.

Generally, line reactors are air core inductors (because you do not want the inductor to become magnetically saturated).

sources

HoneyWell Technical Reference Manual
Wayback Machine - NWL Technical bulletin #22
Choke (electronics) - Wikipedia
DC Choke for VFD – Voltage Disturbance
Current limiting reactor - Wikipedia
Electrical reactance - Wikipedia