VTdrive Technical note Reducing harmonics caused by variable speed drives(VSD)0 pages

VTdriveTechnical note
Reducing harmonics caused by variable frequency drives
1.5

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0.5
1st
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5th
1st + 5 th

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-1.5
The total current as the sum of the fundamental and 5 th harmo nic.

What are harmonics?
Harmonic distortion is a form of electrical pollution that can
cause problems if the sum of the harmonic currents
increases above certain limits. A harmonic current is one
with a frequency at a multiple of the fundamental frequency,
for instance a 250 Hz current on a 50 Hz network is the 5 th
harmonic. The 250 Hz current represents energy that cannot be
used by devices on the network. It will therefore convert to heat.
According the International Electrotechnical Commission
(IEC), the level of harmonics is described by the total
harmonic distortion (THD) and is expressed as a percentage
of the total voltage or current. In the standard IEEE 519 it is
refered to as total demand distortion (TDD).
What is the effect of harmonics?
Harmonics may cause cables to overheat, damaging their
insulation. Motors may also overheat or become noisy and
torque oscillations in the rotor can lead to mechanical resonance
and vibration. Capacitors overheat with, in the most severe
cases, the risk of explosion as the dielectric breaks down.
Electronic displays and lighting may flicker, circuit breakers can
trip, computers fail and meters give false readings.

How are harmonic currents created?
Harmonic currents and voltages are created by non-linear
loads connected on the power distribution system. All power
electronic converters used in different types of electronic
systems can increase harmonic disturbances by injecting
harmonic currents directly into the supply network. Common
non-linear loads include motor starters, variable speed drives,
computers and other electronic devices such as electronic
lighting, welding supplies and uninterruptible power supplies.
How can the effect of variable speed drives be reduced?
Harmonics can be reduced by modifications to the supply
network, drive system and by using external filtering. The
current harmonics depend on the drive construction and load.
Factors that increase current harmonics include:
− No network choke
− Higher motor load
Factors that decrease current harmonics include:
− Greater DC or AC inductance
− Higher number of pulses in the rectifier
− Active rectifier