In a recent post, we looked at motor chokes — inductive devices installed between a drive and motor — and why they’re used in servo systems and variable frequency drives.
But in variable frequency drives (VFDs), inductors can also be placed after the drive’s input diodes, between the input rectifier and the DC bus link. In this configuration, the inductive device is referred to as a DC bus choke, DC link choke, or DC reactor. DC bus chokes are typically built into the drive and can be installed in pairs, with one on the positive bus and one on the negative bus, or with just one device on either the positive or negative bus.
As an impedance device on the input side of a variable frequency drive, a DC bus choke behaves much the same as an AC line reactor, but with several additional benefits and one main drawback.
Like an AC line reactor, a DC bus choke limits the peak value of the line (supply) current, which mitigates harmonics transmitted from the line — especially the 5th and 7th harmonics. This is important because the 5th and 7th harmonics are the main contributors to total current harmonic distortion, and DC bus chokes do a better job of attenuating these harmonics than AC line reactors do. And DC bus chokes add the necessary impedance to reduce harmonics without causing the noticeable drop in voltage on the DC bus that AC line reactors typically cause. (Lowering the voltage on the DC bus limits the maximum voltage available to operate the motor, which can increase motor current and cause the motor to slip.)
In some applications, it can be beneficial to use both a DC bus choke and an AC line reactor for higher overall impedance and better harmonic mitigation. Harmonic mitigation is a focus of the IEEE 519 standard, which establishes “waveform distortion goals” for electrical systems.
DC bus chokes also help mitigate the negative effects of voltage disturbances — especially voltage dips, or sags. After a voltage sag occurs, the DC bus capacitor needs to be recharged to match the level of the source voltage. But voltage cannot change instantly in a capacitor, so an immediate inrush of current attempts to stabilize the capacitor voltage. Normally, there is a precharge circuit that limits this current, but after a voltage sag or short interruption, the precharge circuit isn’t available. The DC choke resists this high current inrush and protects the rectifiers and DC bus capacitors.
The main drawback to a DC bus choke — especially in comparison to an AC line reactor — is that a DC bus choke’s location after the input diodes prevents it from protecting the rectifier against voltage surges from the AC line supply. Hence another reason why using an AC line reactor (which does protect the rectifier from AC line surges) in combination with a DC bus choke can be beneficial.