What causes Corona Discharge?

Every insulation material has a voltage breakdown level. VFD applications cause very high voltage levels which switch constantly from zero to peak voltage. If the Voltage rises beyond the voltagebreak down level, the insulator can become conductive and a Corona discharge can occur. This is called the Corona Inception Voltage. A Corona discharge causes a micro arc and produces large amounts of ozone and ultraviolet light. Ozone and UV light are known to break down an insulation causing premature ageing of the insulation. This process starts a vicious cycle leading to premature cable failures seen with THHN type insulation.

What is rise time?

Rise time in a Variable Frequency Drive application is the time it takes to go from a specified low voltage to a specified high Voltage in order to create a Voltage pulse. Fast rise times and high Voltages create high switching frequency and stress the cable insulation much more than a standard power application.

Do all Drive manufacturers have cable specifications available?

We find that most Drive system manufacturers do not offer enough guidelines on what type of cable should be used to optimize the application. But the cable is actually a very important component to the proper and safe function of the Drive-Motor system. Rockwell™ for example does publish minimum cable specifications in their Drive manuals and we have made sure that Lutze DRIVEFLEX® cable meets and exceeds those requirements.

Why are VFDs gaining so much popularity lately?

Adding VFDs allows electrical control over rotational speed and motor torque. Adding a Drive allows to conserve energy by using just the right amount of energy needed instead of wasting energy by running at full speed all the time. VFDs are used in a wide variety of industries to:

  • coordinate speeds and accelerations of multiple motors working together in one machine.
  • limit acceleration, speed, start up currents, torque or inrush currents, e.g. in elevators or when handling fragile goods.
  • conserve energy, e.g. by reducing the speed of ventilation fans.
  • controlling air or water flow with electrical pumps or fans.

Does a VFD to motor cable connection really require a special cable design?

Yes, because a VFD creates electrical issues that are unique to the VFD application. In simple words the problems are mainly that you don’t know exactly how much voltage and amperage is really applied to the cable in your application. There are three main electrical issues:

  • the cable can cause a capacitive charging current in particular on long cable lengths
  • VFDs are known to have “reflected wave phenomena” resulting in Voltages at the motor end up to 2 times the nominal voltage. A low impedance cable can be a contributing factor to the strength of these waves. A cable with an impedance value closer to the Motor impedance can reduce the reflected wave phenomena. This is especially true for small Drives using AWG16 to AWG12 conductors.
  • The potential Corona Discharge on insufficient PVC insulation can lead to premature cable failures by creating micro arcs weakening the insulation. (see Corona Discharge)

All this makes the VFD application unique by creating electrical issues not present in standard power transmission in 3-phase systems. For these reasons the VFD cable must have a more rugged construction than a standard power cable. A well constructed cable with good Electrical Data can help reduce the negative effects in VFD applications.

What type of motor is typically used with VFD’s?

In VFD applications it is common to use electric motors that were originally designed for fixed-speed mains voltage operation. This is due to the fact that pre-existing machinery is often upgraded later to be used with a VFD device. In most cases it is not necessary to replace the motor although motors that are designed specifically for VFD operation would offer better performance and higher reliability. Today, the most common motor type used with VFD is a three-phase AC induction motor. However, variable-frequency motors (inverter rated motors) are becoming more popular.

What is ideal capacitance for a cable?

There is no ideal specific value but generally it is desired for a Drive cable to have a low capacitance value. Rule of thumb is: Lower capacitance is better as it reduces the cable charging current. Cables with low capacitance therefore allow longer cable runs from the Drive to the motor.

Is semi-conductive layer required with XLPE?

A semi-conductive layer is used often with PVC insulated Drive cables to improve the insulation and claims to improve the Corona Inception Voltage Level (CIV level). XLPE insulation has much better insulation capabilities than PVC and is used for that reason e.g. in medium voltage applications. In addition, it ages much slower than PVC and maintains a higher CIV, therefore XLPE does not need a semi conductive layer.

Can a cable be designed to improve the VFD function or lifetime?

Yes, a cable can be designed to meet several requirements: Very important is a low cable capacitance to reduce the cable charging current. In addition, it is important to address the possible Corona Discharge and insulate with thick insulation walls to accommodate the high fluctuation in voltage caused by the VFD itself.
Cables with impedance values close to the Motor impedance can reduce the reflected wave phenomena.

Why is flexibility of the cable so important?

Drive cables are being routed in different factory and machine environments. The cable construction must allow easy routing in order to make the installation fast and easy. At the same time the cable must be tough enough to withstand the harsh environments often to be found in industrial applications. DRIVEFLEX® is the first flexible XLPE insulated Drive cable in its class.

Why is it an advantage to be able to strip the jacket easily?

Often times Drive cables are installed in the field. Installing in the field is more difficult because there may be limited access to the proper tooling or machining, compared to installations in a well equipped shop. DRIVEFLEX® is made to strip easily without having to cut longitudinal through the jacket. This saves time and money.

What ratings or approvals should a Drive cable have?

As Drive cable could be routed through cable tray, therefore the cable should have TC-ER listing verified by a third party NTRL such as UL. DRIVEFLEX® cable has also been verified by UL for use as flexible VFD Servo cable as well as Wind Turbine Tray Cable. TC-ER listing allows installing the cable without conduit in accordance with NEC 336.10 (7). In addition all these UL listings help the inspection process because the cable has already been 3rd party verified for the intended use.

What stranding is used with DRIVEFLEX® cable?

We pride ourselves in providing flexible cables. In order to maintain flexibility we are using class K stranding on all DRIVEFLEX® cables up to AWG 2. Class K stranding is made of AWG 30 wires and individual wires are being added to make up the conductor size. For example, AWG 12 (65/30) means that the #12 conductor is made of 65 strands with AWG 30 wire. The flexible stranding is one of the many design features to make DRIVEFLEX® the first flexible Drive cable in its class.

Why should I use a Drive cable?

Many installations still use THHN wire pulled through conduit. This type of wiring is not recommended and some Drive manufacturers go as far as to void the warranty on the Drive. The problems with THHN in conduits are:

  • THHN wall thickness is not strong enough to withstand the voltage spikes created by VFD applications.
  • The high capacitance of THHN wires leads to high cable charging current, which in turn reduces the usable current on the motor end.
  • THHN insulation is made with PVC/Nylon. PVC has a lower Corona Inception Voltage (CIV) level than XLPE. If the environment is wet or damp, the CIV level of PVC is further reduced and leads to Corona discharge.
  • The impedance mismatch of THHN wire to the Motor impedance amplifies the reflected wave phenomena leading to overvoltage spikes in the cable.
  • THHN is a thermoplastic insulator and can deform, or even drip in case of fire or high temperature stress. A thermo-set insulation such as XLPE will remain as a safe insulator much longer under thermal stress. THHN wiring in conduit does not provide the proper shielding to contain noise or a proper ground path, therefore causing EMC related problems with the Drive installation.
  • Drive cable such as DRIVEFLEX® provides a dual layer shield consisting of 100% coverage foil-shield and 80% braid-shield therefore effectively shielding against high and low frequencies.
  • Modern Drive cable such as DRIVEFLEX® provides proper ratings to install the cable without the use of conduit saving time and money.

What does it mean that XLPE in DRIVEFLEX® is thermo-set material?

Thermo-set insulation does not melt under high temperatures. Thermoplastic material such as PVC can deform, melt or drip under excessive heat. Thermo-set material, such as XLPE has a higher temperature stability and won’t deform or melt. Therefore thermo-set insulation materials are much safer for use on VFDs.