Flexible, Reliable VFD Cables Save Energy, Provide Precise Motor Drive Control

Aug. 19, 2010
A Variable Frequency Drive (VFD) regulates the speed of a 3-phase AC electric motor by controlling the frequency and voltage of the power it delivers to the motor. These devices, also known as Adjustable Speed Drives or Variable Speed Drives, are becoming prevalent in a wide range of applications throughout industry – in motion control, conveying, indexing and ventilation systems, for example, as well as processing equipment, machining and other work areas.

A Variable Frequency Drive (VFD) regulates the speed of a 3-phase AC electric motor by controlling the frequency and voltage of the power it delivers to the motor. These devices, also known as Adjustable Speed Drives or Variable Speed Drives, are becoming prevalent in a wide range of applications throughout industry – in motion control, conveying, indexing and ventilation systems, for example, as well as processing equipment, machining and other work areas.

VFDs provide many benefits, principal among them the ability to save a substantial amount of energy during motor operations, making them an attractive “green” engineering solution and an economical choice.  Additional advantages of VFD systems include their ability to:
•    Regulate motor speed more precisely and efficiently
•    Maintain torque levels to match the needs of the load
•    Reduce mechanical stress on 3-phase induction motors by providing “soft start” capability, which is far less stressful on the motor system than abrupt start-ups
•    Improve overall process control, and improve an electrical system’s power factor

Since VFDs allow the speed of a motor to be varied electrically rather than by mechanical means, there is greater flexibility of operation.  Lacking a VFD, industrial motors run continuously at full speed regardless of varying demands, which means that a great deal of electric power is wasted.    

One important factor in designing and implementing a robust and reliable VFD system is selecting the right kind of flexible motor drive cables for the application.  This article will discuss some of the environmental risks related to VFD cabling and provide practical cable selection guidelines.  

Environmental Risks to VFD Cables

To transmit power efficiently and effectively from the drive system to the AC motor, VFD cables must offer a robust construction and be capable of providing consistently reliable electrical performance.  VFDs typically operate in extremely harsh environments in which ambient and operating conditions can not only damage the cable, but also damage the motor and drive it connects.  These environmental risks include:

Voltage spikes. The very fast voltage rise times associated with IGBT technology contribute to precise motor speed control, but can also lead to voltage spikes that may damage cables of poor quality or ones that are not properly insulated. 

Corona discharge.  Occurring between conductors of the cable, corona discharge can produce extremely high temperatures capable of causing damage to cables, as well as to motors, bearings, drives and related equipment – which can lead to drive system shutdown and costly production downtime.  If the cable insulation is a thermoplastic material such as PVC, this phenomenon can cause premature cable burn-out or a short circuit due to a gradual, localized melting of the insulation. For this reason alone, thermoset rather than thermoplastic insulations should be used for VFD applications.

Acoustical motor noise, motor heating. This can occur when currents induced by pulse width modulated switching flow in improperly grounded motor shafts, resulting in damaged bearings.

Radiated noise. Noise radiated from a VFD cable is proportional to the amount of varying electric current within it. As cable lengths grow, so does the magnitude of reflected voltage. This transient over voltage, combined with the high amplitudes of current associated with VFDs, creates a significant source of radiated noise. Noise can be controlled by shielding the VFD cable. Unshielded cables connected between a VFD and a motor can radiate noise in excess of 80V to unshielded communication wires/cables, and in excess of 10V to shielded instrumentation cables. Moreover, the use of unshielded cables in conduits is not recommended, as the conduit is an uncontrolled path to ground for the noise it captures.

Common mode noise. Any equipment in the vicinity of the conduit or conduit hangers may be subject to an injection of captured, common-mode noise. Therefore, unshielded cables in conduit are also not recommended for connecting VFDs to motors. Common mode noise can capacitatively couple from unshielded motor leads in a conduit to ground via conduit ground straps, supports or other adjacent, unintentional grounding paths.

Common-mode ground current is particularly troublesome because digital systems are susceptible to the high-frequency noise generated by VFDs. Signals susceptible to common-mode noise include those from proximity sensors, and signals from thermocouples or encoders, as well as low-level communication signals in general. Because this type of noise takes the path of least resistance, it finds unpredictable grounding paths that become intermittent as humidity, temperature, and load change over time. One way to control common-mode noise is to provide a known path to ground for noise captured at the motor’s frame. A properly designed cable ground/shield system, can provide the noise with an easier way to get back to the drive.

Look for Robust Cable Design

In evaluating and specifying VFD system cables, it’s important to ensure that the cables are built ‘tough enough’ to stand up to the environmental and operating conditions, and maintain the life of other components in the system. Selecting an appropriate VFD cable, designed specifically for the load and the task, can improve overall drive system longevity and reliability by mitigating the impact of reflected waves and withstanding the other environmental risks to which it is exposed.  

Additional design factors to consider in specifying VFD cables include:

•    Insulation Material.  Special attention should be paid to the cable’s insulation system material and electrical impedance. Robust thermoset insulation materials, such as those based on XLPE (cross-linked polyethylene), are recommended because of the proven electrical benefits and high temperature stability they exhibit.

Thicker, industrial-grade XLPE insulation systems are proven to provide more stable electrical performance and low capacitance, resulting in increased efficiency of power transfer and longer cable run capability, reduced likelihood of corona discharge, reduced magnitude of standing waves, and reduced peak motor terminal voltage (for extended motor life).

•    Shield/Ground Systems.  Shielding systems, including copper tape, combination foil/braid, and continuous armoring types, are the most appropriate for VFD applications because of the low impedance path they provide for common-mode noise to return to the drive.

In addition, when VFD cables are installed in close proximity to low-level communications cables and other susceptible devices, shielded instrumentation cable should be used.  It would also be prudent to limit the run length of VFD cable parallel to instrumentation cables to 10 ft or less to reduce the likelihood of radiated noise issues.

There are many different types, sizes and designs of VFD cables available today, suitable for use in a broad range of industrial motor drive applications, including those requiring direct burial or installation in wet and/or exceptionally hazardous environments, such as mines.  VFD cables are also available with an additional signal pair for brake or other applications

Cable selection will depend on the specific application requirements (e.g., relative to motor size in HP, speed and torque demands of the load, peak current, environmental conditions, etc.)  The cables selected should, of course, fully comply with all applicable industry safety standards. VFD cables are typically available in gages ranging from 16 to 4/0 AWG, in traditional or symmetrical designs, and in voltages ranging from 600V up to 2000V. 

To sum up:  A cable should never be the weak link in a VFD system. That’s why it is so important to invest the time to select just the right cable for each application.  Flexible variable speed VFD motor drive cables have proven over time to be extremely reliable, lower in cost and easier to install than traditional metal clad cables or lead wire in conduit.  

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