VTdrive Frequency Converter Line Reactors0 pages

Line Reactors and
Adjustable Speed Drives
Introduction
A Line reactor is an electrical device that is used to filter out high frequency voltage
transients and is installed on each phase in series with an electrical load. A typical
line reactor application (see illustration below) is to reduce harmonics and transient
voltages on the line side of adjustable speed
drives. Line reactors are specified by voltage, power ratings (in horse- power), and
percent impedance. Line reactors reduce the line voltage by their rated percent
impedance. For example, a 3% line reactor reduces the voltage by 3%.

Transient Voltage Mitigation
A1

A2

B1

B2

C1

C2

Line Reactors

Controller

MOTOR

Voltage transients can occur for a variety of reasons including
utility capacitor bank switching or equipment switching. Sometimes, these transients are large enough to elevate an adjustable
speed drive’s DC bus voltage such that the drive’s control circuitry
will sense a trip condition and shut down the drive. In many cases,
the installation of 3 – 5% line reactors may reduce these high
frequency transients such that DC bus overcurrent trips are
reduced or eliminated altogether.

Reduced Current Harmonics and Line Current
Adjustable speed drives are known to increase current harmonics. These harmonics are sometimes injected back
through the electrical supply and may interfere with a building’s electrical distribution system. Harmonics cause the
overheating of conductor insulation, which can lead to premature insulation failure. Line reactors will actually
attenuate some of the current harmonics, and also reduce the overall line current.

Are Line Reactors The Right Application?
Before installing line reactors, check the following:
1. Assure that the drive is at least 70% loaded. Line reactors installed on lightly loaded adjustable speed drives will
not alleviate the transient voltage trips.
2. Assure that the adjustable speed drive can tolerate a reduction in steady state line voltage based on the rated
percentage impedance. Most utilities will operate within a + or – 5% band above and below nominal voltage
levels. If the supply voltage swings down to –5% at some parts of the day, a 5% line reactor will effectively
reduce the voltage at the drive input terminals by 10% of nominal.
3. Some adjustable speed drive manufacturers install DC link reactors as a standard component of their products.
If so, then a 3% link reactor may be enough to mitigate line voltage transients. Higher impedance ratings may
be required for drives without integral DC link reactors.
4. Customer site power factor correction capacitors may have a resonant tuned interaction with utility-switched
capacitors that actually amplify the voltage transient magnitude by as much as 2 – 3 times. Generally a
10 – 20:1 ratio between utility capacitors and power factor correction capacitors are the range of concern for
this resonance effect.
5. Have a consulting electrical engineer evaluate your specific facility to determine if line reactors can be beneficial.

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