VSD cables in - Controls and Drives Ltd

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Efficiency in Automation
Cable
Connectivity
Cabinet
Control
Working with
VSD cables in
industrial & automation
applications
Description of a VSD System
A functional VSD system consists of at least
three components:
• VSD device
• VSD cable
• VSD motor
Figure 1: Overview of an entire VSD system
2.1 VSD Device
Figure 2: Block diagram of a VSD device
A typical VSD device consists of the drive
controller and the operator interface. In the
controller, the AC input power is first rectified
into a DC intermediate power (DC bus) and
stored in capacitors. An inverter circuit, which
typically contains a 6-diode bridge network,
subsequently transforms this DC bus power
back to a “Quasi” AC signal with adjustable
voltage and frequency. The DC bus voltage is
calculated by AC line voltage x 1.414.
Figure 1
Figure 2
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VSD Cable
Issues and difficulties during VSD
Operation
Figure 3: VSD cable
Variable Frequency Drive applications can
cause unique electrical issues that are unlike
other standard power transmission in machine
applications. There are higher demands on the
cable connecting the motor to the drive.
Standard multi-conductor cables rated for 500V
will most likely not meet the requirements of
VSD applications, and can cause operating
malfunctions and early failures. Cable is often
an afterthought in the planning process but
represents actually a very important component
in the whole application.
In VSD devices, semiconductor switches are
used to switch the DC bus power to the output.
These switches only have two states: “on” or
“off”. The PWM continually switches between
these two states with a constant frequency but
with variable pulse widths. The widths of the
pulses determine the effective output voltage.
Smaller pulse widths result in lower effective
voltage and larger pulse widths result in higher
effective voltage. Depending on the drive, the
frequency of these pulses is between 4 and 20
kHz. Instead of steep pulses, a sinusoidal
waveform of the voltage and current is desired
at the motor. The pulses are smoothed by the
motor’s inductance.
Instead of just any standard power cable
the use of a special VSD Power cable is
required because the construction and
insulation is designed to cope with the
harsh demands of a VSD Application.
Large pulse width
Few Drive manufacturers have detailed
specifications for VSD cabling. The following list
of common requirements can be found in most
Drive manufacturer’s manuals:
• Four tinned copper conductors shielded for
Drives up to 40-80 kW
• Three conductors with three symmetrical
split grounds for larger Drives and Motors
especially 80 kW and up.
• Low capacitance insulation
• Foil and braid shield combination (foil for
high frequencies and braid for low
frequencies)
• Ruggedized PVC jacket, preferably oil and
sunlight resistant.
The following explain the properties of power
cables for VSD applications.
Insulation and Capacitance
Small pulse width
Figure 4
Pulse Width Modulation. Voltage pulses of different widths and the resulting sinusoidal waveform.
Modern semiconductor switches (IGBTs) used in
Drives are very sophisticated and allow for high
pulse rise times of more than 3 kV/µs in VSD
applications with cable lengths of several
hundred feet. These constantly occurring steep
voltage impulses stress the cable insulation.
As a result of the fast binary switching, high
switching frequency, and fast rise time; the
length of VSD cable used in an installation
becomes an important issue. An electrical
characteristic called the cable capacitance
indicates how much electrical charge the cable
can store between its three power conductors,
and between the conductors and the cable
shielding.
The level of capacitance is determined by the
implemented insulation material, the insulation
thickness, and the shielding type. A higher
cable capacitance results in higher charging
currents. Therefore, low cable capacitances are
desired. However, even with a low capacitance
VSD cable, the capacitive cable charging
current can reach 0.6 A/m. For longer cables
this effect can easily cause a capacitive
charging current of more than 20 A. This
current stresses the cable without providing any
usable power to the motor. This issue becomes
especially serious for smaller drives with a
power of less than 10kW, for which shielded
cables are necessary. The drive voltage is also a
concern. Drives that operate at 460V lead to
higher charging currents than those that
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operate at 230V.
During the design process of machinery
with motors and VSD devices, the cable
lengths must be considered. The VSD
device should always be installed in close
proximity to the motor.
Another issue with cables that connect VSD
devices to motors is known as the reflected
wave phenomenon. Both the cable and the
motor have an electrical characteristic, which is
called the electrical surge impedance. The
electrical impedance applies to sinusoidal AC
currents and is comparable to the electrical
resistance in a DC circuit. When the motor
impedance is larger than the conductor cable
impedance, the voltage wave form will reflect at
the motor terminals, creating a so called
“standing wave” or also known as “reflected
wave”.
The cable insulation and cable construction will
have an effect on the cable impedance. It is
desired to use a cable with impedance values
as close as possible matched to the motor
impedance. Please note that especially for
smaller motors it is impossible to design a
cable that matches the motor impedance, but
the goal is to use a cable with the best possible
match to the motor’s impedance.
This reflected wave results in a voltage pulse
reflected back from the motor to the drive. Long
cable lengths between the motor and drive
increase the probability of the reflected wave. A
reflected pulse combined with a second pulse
coming from the drive may raise the voltage at
the cable to up to 2-4 times, depending on
pulse width, cable length and drive rectifier type
of its nominal voltage (DC bus voltage), even for
very short cable lengths. This over voltage
increases with the cable length.
In some cases voltage spikes have been
reported to peak values as high as 2150 volts
in a 460 V system. High voltage spikes can lead
to insulation breakdown on the motor or cable
insulation, resulting in short circuits.
Figure 6
Figure 6 shows the typical output voltage of a
460V variable-frequency drive. As expected, the
voltage reaches a level of approximately 650V
which is the DC bus voltage. (AC line voltage x
1.414 (sqrt. of2))
The following algorithmic chart shows the large
delta between motor and cable impedance but
also shows that XLPE insulation offers a closer
match than for example PVC. For that reason it
is recommended to use XLPE in particular with
Figure 7
Figure 7: Magnification of one voltage pulse at
the motor end
PVC
smaller motors.
Figure 5: Algorithmic chart and impedance
Delta between PVC insulated cable vs. XLPE
insulated cable.
In Figure 7 the magnification of one voltage
pulse is shown, as it appears at the motor end
of the cable. It can be seen that the voltage not
only reaches the 650V of the DC bus, but
spikes up to almost twice that value. Thus,
more than 1,200V would stress the cable.
Additionally these high voltage levels can result
in premature aging of the cable due to corona
discharge. Mechanical stress caused by tight
bends can also add stress to the insulation.
The biggest possible bending radius
should be chosen, especially for bends of
90º or more.
When choosing VSD drive cables use low
capacitance insulation such as cross linked
polyethylene (XLPE). This reduces cable
impedance.
The material of the insulation and its thickness
can also affect the CIV of the cable. A thicker
insulation results in a higher CIV, and thus
reduces the probability of a corona discharge.
It should be noted that the corona inception
voltage (CIV)declines over the lifetime of the
cable due to natural aging. The presence of
moisture will cause the CIV to decrease. While
the presence of moisture will affect the CIV of
XLPE cables by only a few percent, it can lower
the CIV of a PVC cable to drop to half of its level
than it would have in dry conditions.
In order to assure that a VSD cable
reaches its expected life span, the
insulation material and thickness are
factors that have to be considered.
Furthermore, the type of insulation material that
is used in a VSD cable affects the heat
generation. The insulation is a so called
dielectric material, a nonconductive material
within the electric field of the live conductor.
How much the insulation material is affected
can be expressed by the dielectric constant.
Since the cable acts like a capacitor, this
dielectric constant is the ratio of the amount of
stored electrical energy. For time-varying
electrical fields which are the case in VSD
applications, the dielectric constant becomes
frequency dependent (generally called
permittivity). The electric field polarizes the
dielectric material and due to the high
frequency this results in heat generation
(dielectric losses within the material).
The cable between drive and motor is a
contributing factor to the strength of
occurring reflected waves. The impedance
of the cable can help in reducing its
negative effect.
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Operating temperatures
Figure 8
The type of insulation affects the thermal
stress of the cable. The insulation material
will affect how much heat the cable is
able to generate at a given amperage.
Cables with higher operating temperatures
permit higher current carrying capacities. This
means that cable sizes can be reduced which
assists in cable installation. XLPE insulated VSD
cable has a conductor operating temperature of
90ºC.
Grounding System
The grounding system of a VSD cable is a vital
part of its construction.
Four-Conductor
Power Cable
On the four-conductor cable shown above, the current flowing in the earth conductor is due to the
differing voltages induced on the earth conductor, caused by the distance from each phase
conductor to the earth conductor. This distance directly affects the capacitance between the
different phase and earth conductors.
Common mode current
Symmetrical/Concentric Earth Design
Is sometimes known as current noise. It is
defined as any current that leaves the drive on
the primary motor leads and returns through
any ground path (including the cable grounds
and shield).
In a symmetrical earth design cable the earth conductors are located in the interces between the
phase conductors. This ensures the distance between the phase conductors and earth is consistant.
The resultant earth currents now cancel each other out minimizing the common mode current that
can flow between motor and drive.
Figure 9
The role of VSD cable is to provide the most
attractive path for these potentially harmful
currents to return to the drive with minimal
disturbance to the surrounding networks and
instrumentation. A cable that provides the
lowest impedance ground path will be most
effective in reducing common mode currents.
When the VSD cable provides the lowest
impedance ground path, it will mitigate
common mode current flowing in to other
devices/systems. Having a suitable ground
system means it is possible to control where
potentially harmful energy from the VSD goes.
In the simplified representation shown, the current
on the ground conductor is the resultant value of
the voltages on the phase conductors, allowing for
the 120 degree phase shift between them. On
high power applications the common mode
current that flows can cause motor bearing fluting
and premature motor bearing failure.
Minimizes:
Induced ground voltages
Induced ground currents
Motor bearing fluting
Motor bearing failure
VSD Cable
Three Plus Three
Concentric earth designs are often
recommended for applications of 30kw
and above.
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Cable Comparison Chart
SY
SWA
Lütze Drive Cable
Bend Radius
10 x O/D
8x O/D
5x O/D
Voltage
300/500V
(VSDs 560V DC @ 400 VAC)
600/1000V
600/1000V
Temperature Rating
80ºC
90ºC
90ºC
Yes (XLPE)
Yes (XLPE)
Low Capacitance Insulation No (PVC)
EMC Shielding Capability
Steel Armour
Steel Armour
(Not optimised for EMC Performance) (Not optimised for EMC Performance)
Copper Braid 80% Coverage & Foil.
Offers Low and High Frequency
Coverage
Flame Retardancy
EN60 332
EN60 332
EN60 332
Earth System
Single Conductor
Seperate Insulation
4 Core not recommended
above 30kW
Single Conductor
Seperate Insulation
4 Core not recommended
above 30kW
Concentric
Earth 3 Core System
desirable for AC Drives
Meets Requirements of
17th Edition (BS 7671) for
drive cable application
Only with appropriate
cable marking
Yes
Yes
Direct Buriel
No
Yes
No
Advantages of Lütze Drive Cables
• Cable is 10-15% lighter than other products - this helps with cable installation.
• Smaller bend radius means the cable is easier to route and terminate.
• Low capacitance of the cable aids compliance with EN 50598 as cable losses are reduced.
• Low capcitance, concentric earth system and effective shielding allows longer cable runs in
drive applications.
• Voltages withstand gives protection against transients produced by VSDs
• Low screen DC resistance affording protection agianst AC harmonics
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Shielding
In addition to an effective grounding system a
good quality VSD cable has foil and copper
braid shields. This combination of both shield
types ensures good protection against low and
high frequency interference.
The tinned copper braid must provide at least
80% braid coverage and have sufficient braid
fan angle and wire gauge to ensure a low
transfer resistance. The screen offers a low DC
resistance affording protection from low
frequency interference such as AC harmonics.
If the conductivity of the shield is less than 50%
of the conductivity of the phase conductor an
external earth conductor is often required.
Appropriate shielding and grounding of
the cable is required in order to achieve
proper functionality of the VSD system.
Figure 10
Correct (left side) and wrong (right side) connection of VSD Cable. Correct 360º connection of
shielded cable requires cable clamps or metal fittings.
Installation guidelines for VSD cables
• The length of the VSD cable has to be kept
within the limits set by the drive
manufacturer. Always avoid unnecessarily
long cable runs.
• Cable shielding (foil and braided shields)
must be connected at both the drive and the
motor end unless the Drive manufacturer
provides different guidelines.
• The shielding must be connected at a 360°
contact. Connecting only the drain wire to be
used for grounding and cutting off the
shielding does not provide sufficient EMC
noise protection.
• Where the cable has been stripped and the
wires are exposed, a conductive tape should
be used to improve EMC noise protection.
• The VSD cable should not be routed in the
same tray/conduit as signal, networking, or
communications cables. Always use
separate trays or tray dividers for power and
data cables.
• The cable should be stripped as little as
possible. It has to be assured that the
shielding is not damaged or interrupted.
• If the VSD cable has to cross signal or data
cables, this has to be done at a 90° degree
angle.
• The PE ground wire has to be connected at
both cable ends.
• Cable bends must be reduced to a
minimum. The biggest possible bending
radius should be chosen, especially for
bends of 90° or more.
Typical Current Rating Table for VSD Cables
No cores x cross sec mm2
3 x 1,5 + 3 G 0,25
4 G 1,5
3 x 2,5 + 3 G 0,5
4 G 2,5
3 x 4 + 3 G 0,75
4G4
3 x 6 + 3 G 1,0
4G6
3 x 10 + 3 G 1,5
4 G 10
3 x 16 + 3 G 2,5
4 G 16
3 x 25 + 3 G 4,0
4 G 25
3 x 35 + 3 G 6,0
4 G 35
3 x 50 + 3 G 10,0
4 G 50
3 x 70 + 3 G 10,0
4 G 70
3 x 95 + 3 G 16,0
4 G 95
Power ratings with 3 loaded cores in Amperes
23
23
23
32
32
32
42
42
42
54
54
54
75
75
75
100
100
100
127
127
127
158
158
158
192
192
192
246
246
246
298
298
298
Current quoted in ambient temperature conditions.
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LUK-MCablesPF-Rev2 9/15
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