7/30/2019 AC Chokes & DC Link Chokes

1/4

TECH TIP

VFD LINE HARMONIC ISSUE: AC LINE REACTORSOFFER MORE PROTECTION AND FEATURES THAN DC

LINK REACTORS.

VFD manufacturers commonly offer reactors with their drives.

Reactors use coils of copper (or aluminum) wound around a core of

magnet steel to help smooth current flow from the AC line into the

VFD. Smoother current flow reduces stress on the input diodes in the

VFD and helps minimize input harmonics. (Refer to Reliance

publications D-5062 Dealing with line harmonics in PWM variable

frequency drives and D-8308BPWM Harmonic Line Distortion

Software Estimator for more input harmonics information.)

VFD designers use two different types of reactors. Some engineers

build DC link reactors into their drives. Others design their drives so

that AC line reactors may be used as an option. Reliance chose the

latter approach AC line reactors. Well tell you why we chose this

approach and why its best for our customers.

These diagrams show the difference between DC link reactors and

AC line reactors:

Figure 1, AC Line Reactor

AC LINE

REACTORIGBT

INVERTER

SECTION

INPUT

DIODES

DC BUS

CAPACITORS

VFD

MOTOR

AC LINE

INPUT

7/30/2019 AC Chokes & DC Link Chokes

2/4

TECH TIP

Figure 2, DC Link Reactor

IGBT

INVERTER

SECTION

INPUTDIODES

DC BUS

CAPACITORS

VFD

MOTOR

AC LINE

INPUT DC LINK

REACTOR

VFD LINE HARMONIC ISSUE: AC LINEREACTORS OFFER MORE PROTECTION AND

FEATURES THAN DC LINK REACTORS.

Note the simi lari ties and the advantages of each of the two approaches:

Similarities:

Both styles increase the reactance between the AC line and the DC bus

capacitors, causing current flow to be smoother than it would be without

reactors. They do an equally effective job in this respect.

Both are hard wired to the drive taking up some space in the enclosure and

emmit heat resulting in lost efficiency.

Both are passive devices in that they require no control circuitry.

2

7/30/2019 AC Chokes & DC Link Chokes

3/4

TECH TIP

VFD LINE HARMONIC ISSUE: AC LINEREACTORS OFFER MORE PROTECTION AND

FEATURES THAN DC LINK REACTORS.

DC Link Reactor

DC Link Reactor Advantages:

DC link reactors can be smaller and less expensive than AC line reactors and still

give an equal smoothing effect. (Explanation for Techies only: Since DC link

reactors see pulses of current from all three input phases through one coil pair,

link reactors can have lower inductance and use two coils instead of three for line

reactors.)

Including DC link reactors in every drive allows the engineers to use smaller,

lower-rated diodes in some cases.

VFDs with DC link reactors can cost less to build than the same VFD with an AC

line reactor, since the diodes and the reactor can both cost less.

3

7/30/2019 AC Chokes & DC Link Chokes

4/4

TECH TIP

VFD LINE HARMONIC ISSUE: AC LINEREACTORS OFFER MORE PROTECTION AND

FEATURES THAN DC LINK REACTORS.

AC Line Reactor Advantages:

AC line reactors protect the input diodes from line spikes and transients.

Some installations (for example, those with isolation transformers or very small source

transformers) should not use reactors at all. Use of AC line reactors makes it easy to

eliminate the reactors in these cases.

AC Line Reactor

Summary

DC link reactors cost less and smooth current flow into the VFD, but only AC line

reactors can help protect the input diodes from voltage transients and spikes!Customers get benefits by specifying AC line reactors for diode protection, even if the

drive has a DC link reactor!

D-140 4