Variable Frequency Drives - What Cable to Use?

A.C. inverters (also known as inverter drives) have become increasingly popular as they offer a number of advantages, including:

Co-ordination of speed and acceleration, where multiple motors are working together in one machine.

Limit acceleration, speed, start-up currents, torque or inrush currents, e.g. in elevators or when handling fragile goods.

Energy saving, e.g. by reducing the speed of ventilation fans.

Controlling air or water flow with electrical pumps or fans.

In these applications, the type of cable is a crucial component to consider; however, many inverter drive manufacturers don't offer sufficient guidance on the correct type of cable to use, often leaving it up to the installation engineer on site.

Issues for Consideration:

Electromagnetic Interference (EMI)

Industrial factory floor applications are typically electrically noisy environments. Electrical noise, either radiated or conducted as electromagnetic interference (EMI), 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 a minimum of 85% copper braiding (some cable manufacturers only offer 60%), is required to combat the effects of EMI.

Reflected Wave Voltage

Some of the inverter output is reflected by the motor back toward the inverter drive, which 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 a distance of more than 5 meters, this can mean that an original 460V RMS output can actually 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). In addition, high voltage spikes cause strong electric fields and in longer cable lengths, eccentric cable cores can cause an earth build-up.

Bearing Currents

While bearing currents have been around since the advent of electric motors, the incidence of damage they can cause has increased, this is because modern variable speed drives with their fast rising voltage pulses and high switching frequencies can cause current pulses through the bearings, this repeated discharging can gradually erode the bearing races. To avoid this damage occurring it is essential to offer 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.

Recommendation

Considering the above, we would recommend that OLFLEX? Servo 2YSLCY-JB as the correct cable & SKINTOP? MS-M Brush as the compatible gland be used for the installation of variable frequency drives