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Considerations that need to be made when selecting a Variable Speed Drive cable

A.C. Inverters (commonly known as Variable Speed Inverter Drives) have earned their place as the most popular type of industrial drive system by offering a number of key advantages to the industrial engineering community. According to the LAPP Group, in certain application, the correct VSD cable is also a crucial component. Yet, they point out that many inverter drive manufacturers don't offer sufficient guidance on cabling, leaving it up to the installation engineer on site. Below are four such problem areas that require consideration and how you can easily avoid these common VSD cabling mistakes.

Are you using the correct VSD Cable?

Electromagnetic Interference (EMI)

Industrial factory floor applications are notorious for creating high levels of electrical noise, appearing as either radiated or conducted as electromagnetic interference (EMI) which can seriously 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 is required to combat the effects of EMI. This is something that should be examined as some cable manufacturers only offer 60% copper braiding on their cabling.

Reflected Wave Voltage

Some of the inverter output is reflected by the motor back toward the inverter drive, causing 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 rated at 300V). Furthermore, high voltage spikes cause strong electric fields and in longer cable lengths, eccentric cable cores can cause an earth build-up.

Bearing Currents

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. However, the occurrence of this form of damage can be avoided by using an earthing system that returns the stray current back to the inverter frame. This is achieved by the use of a 3+3 symmetry earth configuration.

Cable Capacitance Effect 

As well as the peak voltage, the instantaneous peak current also requires consideration. 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 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.

Recommendations

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 ocapacitance design allows longer cable runs and high-power transmission for larger drives. Finally, compatible compression glands from the SKINTOP® MS-M Brush are the most suitable choice for use with the installation of variable frequency drives. 

 

 

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