44 August 2019 http://www.designworldonline.com DESIGN WORLDConnector Notes
Proper insulation material
for capacitance
Although the insulating material plays
a fundamental role in capacitance,
many power, motor and servo cables
are still being made on the market with
PVC insulation. This material has many
good specifications: it is cost-effective
and easy to process, but also has an
extremely high dielectric constant. A
higher dielectric constant will increase
the capacitive effects, therefore requiring
more energy to charge and discharge
the cable. This ultimately leads to higher
losses and higher switching inaccuracies
of the overall system.
In short, the higher dielectric constant
of PVC has a greater capacitive effect
than materials with a lower dielectric
constant. The cost savings gained by
using PVC insulated cables are lost on
problems created with power cables from
a drive to the motor.Safe and reliable cables for
high-frequency drives
Today’s generation of VFDs change their
speed by changing the frequency of
the drive voltage. If a high capacitance
cable is used in this control system, a
significant amount of energy is lost for
the capacitance of the cable alone. This
can lead in the worst case to unwanted
reflections and voltage overlaps.
Hence the growing need for low-
capacitive insulation materials. For morethan 15 years, igus has been offering
materials made of high-performance
polymers for all of its chainflex motor and
servo cables used in the energy chain.
These polymers have a lower dielectric
constant with a high electrical resistance.
The goal is to offer customers a cable
that is tested to withstand continuous
movement inside an energy chain and
provide power from a VFD to the motor
safe and efficiently.Smaller motors require a new
electrical supply
Maximum current carrying capacity
of an electric cable is defined by the
permissible maximum conductor
temperature (usually marked on the
cable) at a given ambient temperature.
Depending on the insulation material
used, this can range for PVC from 70°- 80°C and 90°C for TPE materials.
The data for the cable temperature is
calculated independent of the connector
used to terminate the cable.
Old generation motors were usually
larger than today’s with the same power
level, so the metallic round connectors
used were relatively large. As a result,
both the motors and connectors served
as heat sinks.
Today, users are demanding ever
smaller, more compact, and more
powerful motors. Drive manufacturers
are already coping with this demand
by modifying many of the structural
Replacement circuit diagram for a cable element of a two-wire cable of length dx. For a
required length, L’ = Inductance, R’ = Resistance, G’ = Dissipation factor and C’ = Capacitance are
used for simplicity.components of the motor including
smaller connectors.
The trend is moving from larger
motors, metallic round size M23 or larger
connectors, toward smaller motors with
M18 or M16 round connectors with
additional plastic housing insulation. Due
to their small size, the motors get hotter
when they run at the same power level
as their predecessors. This means that
a significant amount of waste heat is
driven directly into the first 3-4 ft of the
cable.
The large metallic connectors of the
past would help to absorb this heat, but
these new motors use the connecting
power/servo cable to absorb more heat.
Although there are no consequences
for the motor and the connected
components in the short term, serious
problems may be the outcome in the
long term. If the insulation material
of the cable does not withstand the
elevated temperatures in the long term,
short circuits can occur inside the cable
and in the vicinity of the connectors,
which in the worst case can lead to
fires. It is therefore important for cable
manufacturers to respond to market
developments with new insulation
materials that can withstand the current
requirements.Insulation material for the latest
servomotors
To prevent damage to the compact
motors, igus now offers a new insulation
material for its chainflex motor and servo
cables. This new insulation was qualified
after five years of testing in the igus
laboratory and extensive practical tests.
It has a low capacitance and withstands
the increased thermal requirements of
the latest generations of motors.
The igus range of cables undergoes
extensive testing under real conditions in
the company’s 30,000 ft^2 test laboratory.
For example, the servo cable CF29 was
also tested with the new insulation
material. In test number 5034, the cable
with a bend radius of 5.4 x d withstood
more than 45 million strokes inside an
energy chain. DWigus | igus.comConnector Notes 8-19_Vs4.LL MG.indd 44 8/5/19 11:20 AM