Straight Talk About PWM AC Drive Harmonic Problems and Solutions
Rockwell Automation Mequon, WI
Abstract:
Though much has been written about harmonics and
related issues with respect to AC drives, many drives
users still seek clear answers to some basic questions.
The purpose of this paper is to provide the interested
reader with some basic information regarding AC
drives and harmonics with a simplified explanation of
harmonics and power factor, showing how both can
affect a distribution system. It is the intention of the
author to dispel some of the myths as well as point out
legitimate concerns, show some viable solutions and
their pros and cons.
Drive basics:
Before we can have a meaningful discussion on
harmonics with respect to AC drives, first it is
necessary to have a good understanding of the basic
workings of a modern PWM AC drive, specifically how
it draws power from the utility line. Figure 1 below is a
schematic diagram of a typical “voltage source” AC
drive power structure.
A modern AC drive power structure consists of three
basic stages. It is ironic that while most DC drives run
on AC, most AC drives run on DC. This is because the
inverter section shown in yellow in figure 1 requires a
stable DC source to operate. Therefore, the first stage of
the drive must convert three-phase AC to DC. The first
stage is known as the converter section.
In an AC drive, the converter stage consists of a threephase,
full wave diode bridge, though SCRs (Silicon
controlled rectifiers) are sometimes used in place of diodes.
If this stage were isolated from the rest of the power
structure, we would see a DC voltage with a 360 Hz ripple
at the DC bus connection when 3 phase power is applied to
the input (see figure 2).
A filter is required to smooth out the ripple on the DC bus in
order to run the IGBT inverter. Therefore, a second or
“filter” stage is required. Primarily, this consists of a large
capacitor bank shown in green. Often an inductor or “link
choke”, shown in orange, may be added. The choke, when
used, helps buffer the capacitor bank from the AC line and
serves to reduce harmonics. We will discuss why as we go
on.
The third stage, shown in yellow, is the inverter section.
This section uses high-speed transistors as switches to apply
a “Pulse Width Modulated” or PWM waveform to the
motor. Taking advantage of the fact that a motor is basically
a large inductor, and that current does not change very fast
in an inductor, the DC bus voltage can be applied in pulses
of varying width in order to achieve current in the motor
that approximates a sine wave.