VFDs – also commonly referred to as Adjustable or Variable Speed Drives – improve the efficiency of motor-driven equipment and allow for accurate continuous control over a wide range of operating speeds. The consistently growing VFD market helps to support highly intelligent and sophisticated machines designed by engineers; machinery utilizing high performance VFD motors are often bundled with cables from overseas manufacturers to complement the “complete package”.
VFDs have become increasingly prevalent in industrial applications where frequency is used to adjust the speed of the motor. Given the time-critical mission to ensure uninterrupted operations, cable performance has never been more in the spotlight.
Frequency is an electrical term describing power pulses of voltage and current over time, and these power pulses are called frequency cycles – there is one positive and one negative pulse of voltage in a single frequency cycle. The external source power for electrical operating equipment may go through a factory’s transformer to either step up (increase) or step down (decrease) the voltage, but the frequency will remain constant at 50 or 60 hertz, depending on individual countries’ regulations.
There are 4 main components of a drive system: source power, a VFD, the cable, and the motor, and ancillary components (e.g. encoder feedback devices, tachometers, sensors, relays) might be included for increased performance.
The primary role of the drive is to transport the power pulses that control the motor’s start-up, operating speed, and stopping functions. By increasing the frequency of the drive, the motor’s speed will increase as well. Conversely, decreasing the frequency will cause the motor to slow down. To adjust the speed of the motor, the VFD performs these three steps:
The conversion of power from Alternating Current (AC) to Direct Current (DC) and then back to AC is not a clean transition, as power distortions created during conversion can result in unwanted additional voltage and current, causing overheating and high-voltage stress in the motor. Applications using sensitive clock or timing functions can also become disordered due to the damage to motor and power supply cables.
Ideally, the motor should anticipate a power pulse and regulate the correct amount of current provided to sustain the speed increase. However, where non-linear power (a change in voltage without the same change in current) is necessary, the current does not properly support the motor’s requirements, this contributes to high-voltage stress and generates excessive heat.
To illustrate with an example, when inversion occurs, the voltage must rise from zero to 650 volts, then back to zero approximately 20,000 times per second. During this process, the nominal voltage can overshoot from 650 volts to 2,000 volts or more. A longer length of power supply cable typically experiences greater and more intense voltage spikes than a shorter cable length.
During initial motor start-up, there may be an inrush of current, causing the motor and power supply cable to act as a large capacitor that must be charged up to its normal operating level. When the motor is first energized, there can be a draw of up to six times its full load power requirements. Hence, it is critical that the installed cable is of adequate American Wire Gauge (AWG) size to avoid any significant voltage drop.
component in the VFD system.
The cable itself can be the most critical component to the VFD system. Since the drive has a self-diagnosis program, a shorted motor can be easily detected as the equipment will just drop offline when a voltage spike happens. When a cable’s insulation is punctured from a voltage spike, the current will travel into the braid shield. This creates an extreme amount of heat and causes the braid to burn until a large enough hole has been created, at which point the cable insulation will heal itself. This process repeats at different locations along the cable length until total failure has occurred.
There are many methods of diminishing unwanted electrical conditions in the components of a VFD system. Changing the pulse rate or switching the inverter to a slower frequency can eliminate some harmonics, as do adding filters, reactors, and isolation transformers to the drive, although it can cause additional voltage drops from the power supply. Since VFD motors are double insulated, any nicks in the insulation windings can be avoided, and as long as the power supply cable has been manufactured to prevent failure due to power distortions, the system should be equipped to handle the type of power that a VFD generates.
Propelled by strong momentum in the water and wastewater industry, the low-voltage and medium-voltage VFD markets have seen healthy growth, with widespread installation of VFDs globally. Strong government support and wide adoption of VFDs across industries in Asia Pacific have also fuelled its demand.
Consuming 28% of the world’s electricity, an average low-voltage (LV) motor can operate for a minimum of 10 years, with some operating up to 15 to 20 years. Every 1% efficiency gain translates to over USD 100K electricity savings during the span of its lifetime3!
Given that energy efficiency requirements differ geographically, the use of the right VFDs further amplify future electricity savings by combating motor load inefficiencies and mitigating costs incurred.
