Variable speed inverter drives (VSDs) have grown in popularity as they offer many advantages to the installer and also the user. According to the Lapp Group, in some applications, the correct VSD cable is also a crucial component to consider. It is important to appreciate that many inverter drive manufacturers do not provide enough guidance on cabling and leave it up to the installation engineer on site. Here, Lapp identifies four potential problem areas and how to address them.
Is that the correct VSD cable?
Electromagnetic Interference (EMI)
Industrial factory floor applications are producing overall high levels of electrical noise. It appears either by radiation or conduction as electromagnetic interference (EMI), and can disrupt the operation of some equipment. The steel braiding on SWA or SY cable offers mechanical protection but is transparent to electromagnetic energy. In fact, even some copper screened CY cable may not be suitable. In these situations, specific motor cable with at least 85% copper braiding (some cables only offer 60%), to combat the effects of EMI.
Reflected Wave Voltage
Some of the inverter output reflects back from the motor back toward the inverter drive. This can cause a standing wave to form (depending on the distance and switching frequency). Voltage from the inverter pulse combined with the reflected wave increases the voltage to the motor. At more than 5 meters, this can mean that an original 460V RMS output can exceed 2000V at the motor. Again, it is important to use specific motor cable with 600/1000V rating (standard CY cable is only 300V). Moreover, high voltage spikes cause strong electric fields and in long cable lengths, and some cable cores can cause an earth build-up.
Current-related bearing failures can occur due to a flow of current generated within a motor. Although low, the incidence of damage they can cause has increased due to the growing use of modern variable speed drives. Their fast-rising voltage pulses and high switching frequencies can cause current pulses through the bearings, and this repeated discharging can erode the bearing races. To avoid this damage occurring needs an earthing system that returns the stray current back to the inverter frame. Using a 3+3 symmetry earth configuration will achieve this.
Cable capacitance effect
As well as the peak voltage, consideration of the instantaneous peak current is also required. At each inverter output pulse, the distributed cable capacitance charges and discharges. For small motors with long cables, the cable charging currents may be of the same order as the motor rated current!
Cable charging currents may cause nuisance inverter overcurrent tripping. For each inverter frame size, there is a maximum cable length for both shielded (braided or armoured) or unshielded cables. These and may vary from 10 m on small drives to above 250 m on high power drives. Measures such as reactors, transformers or filters will help extend the maximum cable length.
Using the correct VSD cable designed for high power drive systems simplifies the selection of suitable cables by addressing these issues. Examples of these include the OLFLEX® Servo 2YSLCY-JB from Lapp Group. They are EMC optimised motor cables for power drive systems to EN 61800-3. Their low capacitance design allows longer cable runs and high-power transmission for larger drives. Finally, compatible compression glands from the SKINTOP® MS-M Brush are also suitable for use with the installation of variable frequency drives