Research has revealed that improving installation techniques for variable
speed drives can dramatically reduce the number of drives failures
recorded in the early life of a plant. Here, Geoff Brown offers his top
five tips for better installation
Start-up frustrations and teething troubles seem inherent with the
purchase of any new plant and equipment. Even variable speed drives, for
all their high reliability, sometimes come under scrutiny during the
commissioning of a plant or process. While some drives may fail through
component breakdown, these days this is rare. In many cases the cause can
be attributed to installation techniques. Here, we reveal the five most
common reasons why installation may go wrong and offer ways to avoid
misery:
Cable routing
Have you ever experienced test and measuring instruments giving readings
that are clearly inaccurate or sporadic? Surprisingly, the problem could
rest with your variable speed drives and the way in which their cabling
has been installed. Because of the power electronics used and the nature
of operation of variable speed drives, it is essential that the routing
of all cabling be very carefully considered, otherwise other electronic
equipment in their vicinity could suffer from interference.
Firstly, ensure that the motor cable is adequately separated from the
signal or control cables by at least 500mm and from other power cables by
at least 300mm. Secondly, ensure that all power cables are adequately
separated from the signal or control cables by at least 200mm, so as to
avoid interference. When control cables must cross power cables, make
sure that the crossover angle is as near to 90 degrees as possible. It is
also important to avoid mixing pairs with different signal types, ie
110Vac, 230Vac, 24Vdc, analogue, digital in order to prevent cross
coupling. For example, an unsuppressed relay coil switched at 110V can
transmit a surprisingly high transient pulse into an adjacent 24V dc or
analogue signal line.
Cable trays should have good electrical bonding between each section and
to the grounding electrodes (see ‘Grounding’ below). Stainless steel or
aluminium tray systems can be used to improve the local equalisation of
potential, and reduce the problem of corrosion. If plastic trunking is
used, secure it directly to installation plates or framework. Running
spans in mid-air should be avoided if antenna affects are not to be
incurred.
It is advisable to use twisted pair wires with equalising conductors
wherever possible in order to avoid differential mode disturbances, which
can lead to spurious signals. In some installations it may be beneficial
to use ferrite rings to avoid common mode disturbances. Common mode
disturbances do not distort the signal itself but can disturb the
receiving device. Installers should endeavour to keep wires twisted and
as close to the terminal as possible. Unused wires in the cable should be
avoided. They should be connected to ground or to another signal.
Cable type
Selecting the correct motor cable is more critical than the supply cable,
which can normally be selected from standard cabling without any problems
being incurred. The reason is that the motor side does not have a
sinusoidal voltage waveform and therefore suffers from fast rising
voltages and high frequencies. These effects can cause electromagnetic
disturbance to radios and televisions as well as causing PCs or process
controllers to perform erratically.
Incorrect cabling can increase the likelihood of voltage transient
problems, which in turn reduces the lifetime of the motor, gearbox and
driven machine bearings. To minimise these problems, and also to meet EMC
requirements, it is important to use only shielded, symmetrical (3 core
or 6 core), multi-core cables and also to ensure 360 degree termination
of the cable shield at both ends. Symmetrical, shielded cables reduce the
motor frame voltage, the effect being more significant with high motor
current. Thus, unsymmetrical cables can be used up to 10mm2 cable size
and up to 30kW motor power, but shielded cable is always recommended.
To be effective at high frequency, the shield conductivity needs to be at
least 10% of the phase conductor conductivity. One way of evaluating the
effectiveness of the shield is by measuring the shield inductance, which
must be low and only slightly dependent on the frequency. As the high
frequencies present in the inverter output also cause a current to flow
by capacitive coupling into the screen, the drive manufacturer will
always quote the maximum length of cable. If longer lengths are required,
this is normally possible using du/dt inverter output filters.
There are three common types of cabling that can meet the requirements
for variable speed drive operation. These are:
* Three-core cable with concentric protective copper shield. Here the
phase wires are equidistant from each other and from the shield, which
is also used as a protective conductor. For MICC types that meet this
requirement, as the internal clearances are relatively smaller than for
plastic insulation the permitted lengths of cable are reduced. CY
cables can also meet this definition.
* 3+3+Cu/Al shield + armour. This cable has three symmetrical
conductors for protective grounding. The aluminium shield is usually
solid corrugated armour. The shield is connected on the frequency
converter side to the PE bar and on the motor side to the PE terminal.
* Galvanised steel, close stranded armour/shield (SWA and SY types).
The shield is connected to PE at both ends. However, a separate
high-conductivity PE conductor is needed unless sufficient
cross-section of copper is incorporated in the strand, such as in
cables designed for the mining and quarrying industries.
Most standard brass cable glands used with these cable types will also
meet the requirements for 360° grounding. Using other cable types can
lead to difficulties. Four-core cables, high gradient/interlaced steel
plate armour cable and especially single core cables are not suitable for
motor use and should be avoided. Where the use of an unscreened cable is
inevitable, in applications such as submersible pumps, it may be better
to consider the use of a sinusoidal filter in the inverter output to
eliminate problems.
Grounding (earthing)
When using variable speed drives, close attention to the grounding
schemes are needed to avoid unwanted effects such as high frequency
bearing currents (caused by induced motor shaft and frame voltages) or to
reduce the effects of EMC. To meet the EMC requirements of variable speed
drives, high frequency earthing is used - a technique not needed for
standard installations such as lighting circuits or direct-on-line
motors. The reason is that variable speed drives emit high frequencies
through the fast switching of power components such as IGBTs and control
electronics, and this high-frequency emission can propagate by conduction
and radiation.
To prevent radiated emission it is necessary to ensure that the entire
drive system, including both converter and motor, forms a Faraday cage.
Using a shielded cable with 360° grounding ensures continuity of this
cage. In fact, 360° high frequency earthing should be undertaken wherever
cables enter the drive enclosure, auxiliary connection box or motor.
There are several ways to implement it and installers are advised to read
the installation manuals.
Always use special high frequency cable entries for grounding of power
cable shields and use conductive gaskets for high frequency earthing of
control cable shields. High frequency earthing in power cable entries can
be done by using a conductive sleeve around the cable shielding. The
sleeve is connected to the Faraday Cage by tightening it to the specially
designed collar in the gland plate. Grounding should always be
implemented through the motor cable screen.
The basis of good grounding starts from the well-structured building with
equipotential areas established on all structural levels. However, even
with the main grounding system in place, variable speed drives and motors
may still need special attention. The main problem can occur if the motor
and the drive have different ground potential. It is then important that
the grounding is done through the frequency converter to bring both the
drive and motor up to the same potential.
Location and environment
Many drives fail because they are used in environments that are too
dusty, too hot, too damp or suffer from too much vibration. The message
is simple - always check that the drive is designed for the environment
in which it is to be used. All drives carry an IP rating, which is a
measure of the degree of ingress protection designed into them. For
example, a drive may carry an ‘IP 21’ rating.
While modern drives are very efficient, they are not perfect. A typical
drive will be 98% efficient, and the 2% of losses must be dissipated.
Most drives tend to be air-cooled and they need large circulating air
volumes so that each kW of loss will need a flow rate of 200m3/h. As
such, it is essential that the IP category is not specified too highly,
or the filter materials used to improve the protection will deteriorate
rapidly. This will lead to the need for very regular maintenance, or will
produce overheating and unnecessary down time. Installers of drives
should not assume that the designer or specifier has already considered
the environment. Installers should always double check that the drive
they are handling has a suitable IP rating for that environment.
Personal safety
The point of good installation practice is not only to guarantee the
functionality of the equipment, but also to ensure the safety of any
personnel working within the vicinity of the plant and equipment. It is
essential that all personnel involved with the installation of variable
speed drives are familiar with local electrical installation regulations
as well as familiarising themselves with the user manuals and
documentation relating to the specific drive being installed.
For example, local rules may dictate PE grounding, but some manufactures
may stipulate the need to use shielded cables as part of the grounding.
However, the conductivity of the shield may not be good enough for PE
grounding, and if this occurs, the installer should first check that the
seal/ shield is good enough to act as a PE conductor and then follow the
local safety rules and regulations.
It is important to realise that these top five tips are literally that -
tips. Behind each one is a wealth of knowledge and detailed instructions
that can be found in the manuals and documentation of all reputable
manufacturers. Furthermore, totally competent and equipped personnel
should only carry out installations.
Geoff Brown is Principal Applications Engineer at ABB Automation