Why should I use these cables over standard products?
Modern AC variable speed motor drives employ a method of motor
control called PWM (Pulse Width Modulation). Basically a
Kleenscreen takes incoming AC (50Hz) mains power, converts it to
DC, and then converts (or inverts) the DC back to AC but with a
variable or adjustable frequency. It uses newer technology
semi-conducting power switching devices known as IGBTs
(Insulated Gate Bipolar Transistors) that are capable of faster
switching speeds than those of the older SCRs (Silicon
Controlled Rectifiers) employed in DC controllers. Compared to
the older DC drives, PWM controllers enable reduced size,
improvements in operating efficiency and tighter control of the
motor’s speed and torque. As with any different technology,
there are new aspects and considerations that must be
incorporated in the design and installation process. The three
main considerations are:
- Electromagnetic Interference (EMI)
- High Ground Currents
- Reflected Waves
The higher switching speeds of the IGBTs inherently generate
more EMI that inevitably gets radiated into the environment by
the inverter-to-motor power cable. The EMI emitted from the
power cable can induce voltages, currents or crosstalk on
adjacent cables, which is potentially dangerous to equipment and
personnel. The risk of failure or malfunction in automated
plants is also considerably increased.
The presence of the system operating frequency (50Hz), the
frequency of the PWM carrier (800 to 3.5kHz), the PWM pulse
switching time and harmonics from all three, induce currents on
the ground plane. In some situations all these currents may not
add up to zero in the cable, and thus a common mode current
enters the system with its return path to the inverter not
necessarily through devices that were designed to carry current
(motor bearing damage is typical). Common mode currents can
cause havoc with control systems and mechanical parts, hampering
reliability and performance.
The third consideration is that of reflected waves. Whenever a
signal travels down a cable and hits a sudden increase in
impedance (eg: motor terminations), part of the signal will be
reflected back up the cable. The reflected wave will add and
subtract to other signals along the cable, giving rise to
standing voltages often in excess of three times the system
voltage. These high voltage spikes can quickly destroy cables
that are not designed to withstand this sort of abuse.
The challenge to cable makers
The Kleenscreen range of cables represents a solution to the
above problems in the following manner:
Reduce EMI emission from the cable.
Present a balanced system impedance to minimise Common Mode
Currents Offer a product that will reliably withstand the high
voltage spikes.
A heavy-duty copper tape screen is applied over the entire
core assembly to offer a very high level of EMI screening.
Multiple earths are placed in all interstices under tight
manufacturing tolerances so that the effective distance from all
phases to earth (affects the phase impedance) is identical, thus
producing an electrically ‘balanced’ cable. Additionally the
earth sizes combined with the screen offer a higher than normal
earth conductor size, offering a very low earth return impedance
which helps to minimise common mode currents. Materials used in
the construction of these cables are also specifically selected
to provide the highest level of reliability and safety. Only
pure dielectric materials such as cross-linked polyethylene
(XLPE) are used for the insulation, favoured for their tolerance
to high voltage spikes, superior overall electrical performance
to PVC, and improved high temperature characteristics. As an
additional level of safety for personnel, General Cable also
uses high quality insulation grade compounds on the cable
sheaths.