What is more important in a drive? Ac line reactor or a DC choke? If Ac line reactor is missing what are its possible impacts? What if DC choke is missing?

AC line reactors reduce current harmonics on AC line which is caused by rectifier, while DC choke deal with DC bus current.

Manish,
Reactors add impedence which has the effect of reducing voltage when power is flowing. A DC link reactor will have the same effect on the operation of the VFD as an AC reactor so long as the impedence is roughly double that of the AC reactor. The best use of an AC reactor is when the ratio of the mains supply and the VFD kVA is > 20. Example: You have a 7.5kW (~11kVA) inverter and a supply line capable of 1000kVA. The resultant ratio is almost 100:1 and results in a 'stiff line'. A voltage transient (voltage goes up) at the supply transformer can have a severe impact on the diode bridge, bus caps or both. An AC reactor will limit transients and prevent nuisance trips.

A line reactor or a DC Choke (aka DC Link Reactor) performs roughly the same function if you are just looking at reducing line side harmonics a bit. Both will take the harmonics from 80-100% down to 40-50%, which is OK for a lot of applications. As noted on an earlier comment, the line reactor will help limit transients on the line side and reduce nuisance trips, while the Choke will not. Some existing drive designs come with a DC Choke as standard. It is either built in on the smaller ratings, or is added as a separate component typically on top of the drive for the larger ratings. On these drives, you MUST USE THE CHOKE as the drive's impedence without it is extrelemy low and it will not operate well without this additional impedence. (I don't remember if anyone ever told me what would happen if you leave out this additional impedence, but it must be bad. Robert Bland's comment above about having a severe impact on the diodes and caps sounds good to me. Is that my old friend Robert Bland from Toshiba?) Toshiba's AS1 and Schneider's ATV61/71 are like this. You can eliminate the DC bus choke if you add a line reactor on the front end, but the drive literally needs one, or the other, to operate well.
I hope this helps.

Michael

It is I!

My personal preference is for an AC line reactor simply because it does a better job of protecting the drive from the line and the line from the drive. I've always viewed a DC bus reactor on a PWM drive as a cheap attempt at filtering to save space and money over the better protection afforded by a line reactor.

When not to use a line reactor or use a line reactor will depended on the applicaton of the drives relative to the utility power system to prevent excess current that may possibly damage drives during events such as line disturbances or certain types of ground faults. Drives should have a minimum amount of impedance in front of them but an evalutation of the application and where it is located is just as important. Impedance comes from the transformer(s) and line side power cables. Some cases a line reactor is recommended in front of the drive when some application has switchng power factor correction capacitors. The drive maybe installed in an area that has a large potential lighting strikes in the area. Maybe there are large power interruptions or voltage dips that could potential damage the drive that may need to be evaluated. In addition the primary utility transformer may be too large in comparision to the drive thus an impedance matching evaluation may be calculated realtive to the frame size of the drive. Ask for the recommendation of installation by the drives manufacture. They will advise how the proceed and what and when to provide a line reactor realtive to the application mounted in an motor control center or a wall mount design.

Using either depends on what you what to achieve. A reactor (AC or DC ) is mainly used to reduce harmonics in the input current - as there usually are regulations in place to minimize the disturbance caused ( IEC 61000-3-12- and 3-2, for example). Either will work fine - in order to reduce the harmonics to an acceptable level a DC choke is somewhat more efficient- given the space and weight and cost constraints for a specific product - but the difference is small. Theoretically - a DC choke, if infinitely large - will reduce the current distortion to a pure six step waveshape - a THD of about 25 %, whereas a similarly sized AC choke will reduce it to zero ( if you can get the current through an infinitely large inductance..). Various manufacturers prefer either the one or the other - both solutions work, so forget debating and specifying it. Using a DC choke necessitates better protection for the rectifier, but this is not a significant issue today.
Using either also helps with capacitor life - if you rely on transformer and cable impedance to reduce the ripple currents and capacitor stress you're in trouble with long life requirements

Thanks All. This really helped me a lot.
Can we use a separate line reactor if it’s not built in with the drive? What all parameters I would have to look into, if I want to add the line reactor? Is there any sizing criteria? How would I have to install it?

To add an AC choke to a Drive, you have to watch out for netowrk Rsc (Rated Short Circuit) and power losses inside the cabinet, they will surely increase. Each drive has its own installing criteria.

AC Input Reactor will drop some voltage to the input voltage, as the % Impedance goess up so does the drop,which lowers the DC Bus which ultimately limits how much output voltage is possible. DC Bus Choke does not lower the DC Bus because there is no average voltage across an inductor. DC Bus Choke allows the most output voltage. If you have a low line this may be beneficial.

If the source transformer has high enough impedance you can get by with no magnetics. However, this is rarely the case. Effectively you would need as a bare minimum 1 to 2 % impedance on the transformer at the same base as the VFD.Meaning 2% impedance on a 100kva xfmr supplying a 100kva VFD. If the transformer is 500kva then it would need a 10% impedance for a 100kva VFD. Reactors and chokes offer a more convenient and cost effective means of providing impedance. So generally a 3% reactor is a solution that always does good and is usually sufficient. Double that for DC. The impedance does indeed reduce harmonics. AC reactors are bit more effective at this. But reducing harmonics is not always essential. What is essential is reducing or limiting charging current into the bus caps. With no reactor or choke, the only thing limiting this current is the wire resistance and source xfmr impedance and cap current. The caps can draw very high currents and kill the diodes with no impedance.

One aspect that has not been mentioned. The DC link has a fully developed magnetic structure all the time. From 0-100% load because it is supplied bus voltage. The AC line reactor is less effective when lightly loaded which is generally the case in variable torque applications when the speed is reduced.