AC and DC chokes 3

[gsjhand]

Hello Guys,

I was wondering what is difference between AC and DC choke? Why some VFD manufacturers recommend AC choke and some recommend DC choke on input side of VFD (3Phase, 600 Volts)? What effect they will have on VFD input voltage?

 

[DickDV]

An AC choke or reactor and a DC bus choke both deal effectively with input harmonics but the reactor has the added benefit of protecting the input of the drive from line surges and spikes. The reactor also has the disadvantage of reducing the incoming AC voltage.

On the other hand, the DC bus choke has the advantage of not reducing the incoming voltage and the disadvantage of not protecting the drive input from line surges and spikes.

In practice, at least in my world, they are considered to be about equal in total protection.

 

[CJCPE]

I agree with DickDV. AC reactors and DC chokes can be sized to provide equal performance in their primary benefit, the reduction of line harmonics. If the DC choke is divided between the pos and neg bus it can provide ground fault impedance which may or may not be needed. Various manufacturers have used one approach in one model and the other approach in other models. At least one manufacturer has use both approaches in the same model series.

If the choke or reactor is an add-on rather than a standard feature built into the product, it is usually easier to add a line reactor.

 

[aolalde]

To me the basic construction of a choke or a reactor is the very same. It is an inductance or a magnetic core with a multiple turns coil to storage energy. The reactive power Q (kVA) is:
Q= 2E-3*PI*f*L*I^2
Were: PI = 3.1416, f= frequency (HZ), L= inductance (Henry), I= RMS Current (Amperes)
Certainly the specific inductance and current capacity will be matched to the position in the circuit and purpose of the reactor (harmonics frequency, ripple frequency, etc).

 

[ozmosis]

There is a perception that AC reactors will protect a VFD from a high energy transient. They don't. Low energy surges are handled quite well by an AC choke but fast transients will go through them and into the drive. Over-voltage protection in the drive will usually trip once it reacts quick enough to measure and do something about it but by this time the transient has passed through the rectifier and if damage is to be done, it will have been. Over-voltage measurement is carried out on the DC link of a drive.
AC reactors will reduce the effects of harmonics but they do have a number of disadvantages, some already mentioned about voltage drop (not too much of a problem in N.America but in Asia where the line voltage[if you are lucky] is 380Vac, then it can be a problem). The others are:
*An AC reactor introduces an efficiency drop in the overall system. Not huge but over the lifetime of a system, the costs add up. However, in these times of energy efficiency, it seems strange to be shaving losses from motors, cables, introducing higher efficient VFD's and then plonking a lump of copper/iron that drops your overall efficiency by a far degree
*the efficiency losses typically mean heat in the reactor and this requires careful design of where the reactor should go either in a cabinet or wherever.
*A large proportion of drives are NEMA12(IP54) and this is good for mounting the VFD close to the application but falls down when you find a (typically) open reactor (IP00) that needs a home somewhere.

DC chokes are more effective in reducing harmonic distortion, especially where they are specifically designed on both legs of the DC link. They obviously do not provide any direct protection to the incomming supply to the VFD but, as mentioned, nor do AC reactors as far as fast transients. DC chokes are also an effective way of offsetting EMC filter design. So rather than introducing the inductive elements (L) totally into the filter itself, you can balance this out with your DC chokes in combination with the Capacitance(C) of the filter to produce a more cost effective and compact design.
There are other designs on the market for VFD's that are effective in reducing harmonic distortion by reducing the source of the problem rather than expensively solving it afterwards.