Variable Frequency Drives (VFDs) serve an important purpose of ensuring the speed of your AC induction motor is at a desirable rate. Thus, the use of VFDs help improve the performance of a motor, as well as prolonging its lifespan. It is instrumental in minimizing the risks of premature wear and tear of AC induction motors, along with the enhancement of the process control and energy efficiency.
It is possible to change the speed of the motor at a quick rate through a VFD. A VFD is beneficial in addressing any changes in temperature, force, and load. As a result, the process control becomes more precise while making sure the motor speed remains at a low tolerance level.
By utilizing VFDs, the speed level of the motor upon starting it does not instantly reach the maximum. Instead, this component slowly builds up the motor’s speed until it is at the desired rate. Avoiding the inrush eliminates mechanical stress, overheating, and early failure, which also means reduced maintenance costs.
To further enhance the benefits you can expect from the use of VFDs, certain techniques may help achieve this goal. Increase VFD efficiency through these strategies that also aid in resolving some challenges that may come along.
Use an Output Filter
Make use of an output filter when there is more than 100 feet lead length between the VFD and the motor.
The DC bus produces voltage pulses that originate from the motor cables and towards the motor, before these return to the drive. Various factors impact the reflection of the pulses to the drive. Factors such as the length of the cable, motor impedance, quality of cable, and the drive output voltage’s rise time.During the event that the voltage reflection combines with a subsequent pulse, this may result to a detrimental level of peak voltages that impacts the motor.
During the event that the voltage reflection combines with a subsequent pulse, this may lead to a harmful level of peak voltages that can affect the motor.
This scenario can shorten the life of your motors, which is why it helps to install an output filter when there is a large length of more than 100 feet between the motor and the VFD. Certain devices minimize the voltage level and the rise observed at the motor terminals. These include a VFD-rated cable, sine filter placed at the inverter output, and a snubber circuit installed at the motor.
For maximum protection to a length of at least 500 feet, you may install line reactors at the inverter output. A dv/dt filter (Resistance, Inductance, Capacitance) placed at the inverter output can offer protection at about 2000 feet.
Use an Input Line Reactor
Consider using an input line reactor to minimize distortion.
There is a tendency for the DC bus to undergo an overvoltage issue when there are inrush currents to a VFD due to transient voltages present on the AC power lines. Utility capacitor switching is the common culprit behind the presence of transient voltage. This condition can instantly shut down VFDs and impact its efficiency over time.
This problem is why it helps to install a line reactor that reduces the impact of the inrush current. Also, this technique helps reduce risks of component failures while maintaining the VFD’s excellent condition. There will be fewer chances of unexpected downtime, which also prevents unnecessary maintenance expenses.
It is also worth noting that input line distortion becomes the least of your concern by using a line reactor. Moreover, this also helps reduce the harmonic current distortion, peak currents, and the inrush current towards the rectifier. Since high peak currents distort the voltage waveform, a line reactor effectively addresses this problem.
Use a Shaft Grounding Ring
VFDs tend to generate shaft currents in an AC motor when Pulse Width Modulate inverters are exposed to high-speed switching frequencies. At normal operation, voltages on the motor shaft may be present because of the switching frequencies of IGBTs used in the drives. Several problems such as fluting, premature failure of the motor, and pitting may occur when there is a parasitic capacitance existing between the rotor and the stator.
A practical way to address this issue is by using shaft grounding rings for your AC motor. These grounding rings help prolong the lifespan of the bearing by diverting excessive shaft voltages to ground.
In fact, several motor manufacturers feature standard motors equipped with grounding rings to enhance their performance further. It is also beneficial to add grounding rings in the field externally, as well as install them internally by specialists.
Through these techniques, you will be able to enhance the efficiency of your VFDs while promoting the overall performance of your motors.