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THREE-LEVEL CONTROL IN GENERAL PURPOSE INVERTERS – A SOLUTION TO
SOME OF THE PROBLEMS WHICH RESULT IN PREMATURE MOTOR FAILURE
By W Law, Applications Engineer, Varispeed, a Division of Hudaco Transmissions (Pty) Ltd,
P O Box 4202,Halfway House, 1685, South Africa; e-mail: [email protected]
Abstract
This paper describes the application of a power control method known as three-level control to the inverter section of
general purpose DC-link AC variable speed drives. The method of implementation and the resultant improvements and
benefits to motor performance are described
Introduction
Most AC variable speed drives commercially available today are of the two-level control type and have a number of
inherent problems arising from the design of the inverter bridge, The results of these problems include premature
breakdown of motor insulation, electrolytic bearing corrosion and the generation of electrical and audible noise.
An alternative approach is to use an inverter bridge that is capable of performing three-level control of the output
waveform. While the variable speed drive itself may, initially appear to be more expensive, the need for output chokes,
filters or motor terminating devices is eliminated and motor failures are substantially reduced.
The operation of a three-level inverter is described in simple terms below.
The most common type of AC drive available today is the DC link converter with an inverter bridge made up of six
power devices. This configuration produces a two-level output waveform. An improvement on this type of design is the
three-level inverter which utilises twelve IGBT’s in the inverter bridge.
Two-level inverters
A DC link converter consists of an input rectifier, which may be passive or active, a DC link section with a capacitor
(and sometimes an inductor), and an inverter bridge made up of six power devices; normally IGBT’s or variations
thereof. This configuration generates a good approximation of a sine wave with a pulse width modulated two-level
output waveform. Fig. 1 below shows the basic power circuit of a two-level inverter.
The control mode of the inverter output could be a variable voltage/variable frequency (v/f) mode, a sensorless vector or
a flux vector type of control.
The waveforms applied to the motor by this type of inverter can have the following disadvantages:

Switching spikes with a peak of up to twice the DC bus voltage applied to the motor,
i.e. spikes of 1 300 volts on a 400 volt system and 1 800 volts on a 550 volt system.
These can cause premature insulation failure on older or poor quality motors.
To prevent such a failure the use of output chokes, filters or a termination device at the motor is required.

There is some leakage current that flows back to the inverter via the designated earth return path, the motor
frame and the motor bearings, shaft and driven machine. That part of the leakage current that flows through the
motor bearings can cause damage referred to as electrolytic corrosion and is due to the vibration caused by the
waveform on the steel parts.

The switched or modulated nature of the output waveform can generate a considerable amount of
electromagnetic interference.

The waveform will also cause audible noise due vibration of the laminations around which the stator windings
of the motor are wound.
+
Q1
Q3
Q5
+
L1
L2
L3
U
V
W
Q2
Q4
M
Q6
_
DC LINK
CAPACITOR
RECTIFIER
INVERTER
Fig 1
The Power Circuit of a Conventional Two-Level Inverter
Three level inverters
A three-level inverter differs from a two-level type in the design of the inverter bridge. A three-level output waveform
is produced when twelve switching devices are used in the inverter stage. A third voltage level in the DC bus is created
by using two capacitors in series for the DC bus capacitor as well as two additional clamping diodes in each phase of
the inverter bridge. The third voltage level is created at the centre point between the two capacitors and is referred to as
the ‘neutral’. The switching sequence for the power devices is described in detail later. Figure 2 below shows the basic
power diagram of a three-level inverter.
+
Q1a
Q3a
Q5a
Q1b
Q3b
Q5b
+
U
L1
L2
L3
V
W
M
NEUTRAL
Q2a
Q4a
Q6a
Q2b
Q4b
Q6b
+
_
RECTIFIER
DC LINK
CAPACITOR
INVERTER
Fig 2
The Power Circuit of a Three-Level Inverter
Comparison between Two-Level and Three-Level Waveforms.
In both two-level and three-level inverters similar forms of pulse width modulation are used. The difference with the
three-level inverter is that, with specially controlled switching of the twelve power devices in the inverter bridge an
additional voltage level appears in the waveform.
The diagrams below in Fig 3 illustrate the difference in appearance between the output voltage waveforms generated by
two and three-level inverters. The notable feature is that with three-level control the peak value of the modulated
Line Voltage
Phase Voltage
waveform is half of the DC bus voltage VPN , while in the two-level case the peak value is the full DC bus voltage. Even
though the combined three-level waveform is a series of “blocks” of pulse with modulated wave, the magnitude of the
voltage pulses that are applied to the motor measure only half of the peak value of the DC bus voltage.
VPN
VPN/2
VPN/2
VPN
Conventional Two Level Control
Three Level Control
Fig. 3
Comparison of Two-Level and Three-Level Wave Forms
Generation of a Three-Level Waveform.
The switching sequences of the power devices for one phase of a three-level inverter are shown in Fig 4 below.
In the descriptions below it should be understood that when a switching device is said to be ON, it implies that it is
under pulse width modulated control from the control section of the inverter.
+
+
Q1a
+
Q1a
+
Q1a
+
+
Q1b
Q1b
I
I
U-PHASE
U-PHASE
NEUTRAL
Q2a
NEUTRAL
Q2a
+
Q2a
+
Q2b
_
I
U-PHASE
NEUTRAL
+
Q1b
Q2b
_
Q2b
_
Q1a & Q1b are on
Q1b & Q2a are on
Q2a & Q2b are on
Positive Current to Load
Positive Current to Load
Positive Current to Load
Via Clamping Diode
Via flywheel diodes
+
+
Q1a
+
Q1a
+
Q1a
+
+
Q1b
Q1b
I
U-PHASE
NEUTRAL
Q1b
I
I
U-PHASE
U-PHASE
NEUTRAL
Q2a
NEUTRAL
Q2a
+
+
Q2a
+
Q2b
Q2b
_
Q2b
_
_
Q1a & Q1b are on
Q1b & Q2a are on
Q2a & Q2b are on
Negative Current from Load
Negative Current from Load
Negative Current from Load
Via flywheel diodes
via Clamping Diode
Fig.4
Switching Sequence for one Phase of a Three-Level Inverter.
Characteristics of a Three-Level Waveform.
When any switched waveform such as the pulse width modulated types from inverters are fed to a motor via a cable, the
natural capacitance of the cable is charged and discharged by the “DC pulses” in that waveform. Voltage spikes occur
which force the peak voltage of the waveform to rise above the DC bus voltage in the inverter. The longer the cable, the
higher the capacitance, and therefore the higher the switching spikes will be. Fig 5 below illustrates typical spikes that
could occur on a 400 volt inverter with various lengths of cable.
Fig.5
The Effect of Cable Capacitance on a Pulse Width Modulated Waveform.
Because the “blocks” of pulses in a three-level waveform have a magnitude of only half of the DC bus voltage the peak
of the switching pulse is considerably reduced, in fact it will be 1,5 x V pn as opposed to the 2 x Vpn experienced with a
two-level system.
A diagrammatic representation comparing two pulses in each type of inverter is shown below in Fig.6.
Suppresion effect
1,5xVpn
Level 3
2xVpn
Level 2
Vpn
Level 1
Two-Level Switching
Three-Level Switching
Fig.6
Comparison Between Two-Level and Three-Level Switching Waveforms.
It is clear from the diagram above that the switching spikes will be lower in a three-level inverter than in a two-level
inverter by half of the DC bus voltage. In a 400 volt system this is equal to about 270 volts and in a 550 volt system the
reduction is about 390 volts.
Advantages of a three-level inverter.
There are several significant advantages in using a three-level inverter. These are summarised below.
Motor Spike Protection Devices.
When two-level inverters are to be used on motors with long cable runs, i.e. in excess of about 50 metres, output
inductors, output filters or some form of motor termination device is normally used, particularly on 550 volt systems
and where older motors with perhaps poor insulation are to be used.
With a three-level inverter no spike suppression device is needed between the inverter and the motor thus saving on
capital cost, space and installation time.
Leakage Current and Bearing Current
There are three paths via which leakage currents return to the inverter.
As shown in Fig. 7 below these are:

The installed earth wiring.

The motor mountings and earth.

The motor bearings (and possibly the bearings of the driven machine) and earth.
The bearing current can cause damage to the bearings due to arcing across the bearing surface. The current is
concentrated through the contact points of the bearings and wear occurs. Arcing also causes the lubricating grease to
loose its properties compounding the problem.
A three-level inverter will therefore improve bearing life, due to reduced leakage current. The leakage current is
reduced because the three-level waveform is a closer resemblance to a sine wave and thus dv/dt is considerably reduced.
Audible Noise.
Inverters cause audible noise in a motor due to the dv/dt of the pulse width modulation at carrier frequency. The motor
laminations tend to vibrate at this frequency. As the three-level waveform is significantly closer to a pure sine wave
with low dv/dt the total noise level can be reduced by up to 5 dB. Selecting a carrier frequency well out of the audible
range, say 12-15 kHz will further improve the audible noise.
Performance.
Depending on the design of the control algorithms in the inverter a three-level type is capable of providing higher motor
torque at low speeds, typically 150% at 0,3 Hz in open loop vector mode . Faster torque response is also achieved.
These improvements are mainly due to the improved sine wave output.
Conclusion.
The three-level inverter, although more costly than a conventional two-level type due to the increased number of power
devices, can in fact produce overall savings when all of the factors discussed above are considered.
References.
Information from Yaskawa Electric Corporation.