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 VSD 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
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 VSD 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.