VFD efficiency: Three best practices
The use of variable frequency drives (VFDs) to control the speed of an AC induction motor has many benefits including improved process control, energy savings, higher reliability and reduced wear and tear. AC motors are very common in manufacturing and processing plants. When a motor is run at half its maximum speed, it consumes significantly less energy than it does at full speed.
For centrifugal loads (fans and some pumps) the power at half speed can be as little as 1/8th the base speed power. When using a VFD, motor speed can be changed almost instantaneously to address load and process changes (temperature, pressure, force, etc.). An added benefit is its ability to increase the precision of process control with the ability to control motor speeds to within 0.1% tolerance.
A VFD can provide a "soft start" capability for a motor (that is, the motor can be ramped up to desired speed instead of being turned on at full RPMs), decreasing the mechanical stresses associated with full-voltage startups. This results in lower maintenance costs and a longer motor life. In cyclic loads, the VFD also helps to avoid motor overheating.
VFDs have advanced over the years in both functionality and high speed switching technology. The output waveform is not a perfect sine wave, which can present some challenges that can be overcome by following some best practices upon installation.
Here are three best practices when looking to improve VFD efficiency:
1. Specify/install an input line reactor
Transient voltages on the AC power lines can cause inrush currents to a VFD drive, resulting in an overvoltage condition of the DC bus. These transient voltage conditions are often caused by utility capacitor switching and will cause VFDs to shut down without warning. The addition of a line reactor will limit the magnitude of inrush current. This current prevents trips and component failures and reduces the amount of potential downtime.
A line reactor will also reduce input line distortion, which is caused by the nonlinear characteristics of drives. The line reactor will limit the inrush current to the rectifier, rounding the waveform, reducing the peak currents, and lowering the harmonic current distortion. High-peak currents may cause distortion of the voltage waveform. The reduction of those peak currents also reduces total harmonic voltage distortion and mitigates harmonics sent back on to the line.
2. Specify/install shaft grounding rings
Due to the high-speed switching frequencies in pulse width modulated (PWM) inverters, variable frequency drives induce shaft currents in AC motors. The switching frequencies of insulated-gate bipolar transistors (IGBT) used in these drives produce voltages on the motor shaft during normal operation through parasitic capacitance between the stator and rotor and discharge through the bearings and can cause pitting, fluting, and premature motor failure.
Shaft grounding rings for AC motors divert harmful shaft voltages to ground and extend bearing life. Most motor manufacturers stock standard motors with grounding rings installed internally. They can also be added externally in the field or installed internally by a motor repair facility.
3. Install an output filter for long motor lead lengths over 100 feet
The inverter section of a drive does not produce sinusoidal voltage, but rather a series of voltage pulses created from the DC bus. These pulses travel down the motor cables to the motor. The pulses are then reflected back to the drive. The reflection is dependent on the rise time of the drive output voltage, cable characteristics, cable length, and motor impedance. If the voltage reflection is combined with another subsequent pulse, peak voltages can be at a destructive level.
One IGBT drive output can have reflected wave, transient voltage stresses of up to twice the DC bus. Research has indicated that the fast switching capability of the IGBTs, along with an excessive lead length between motor and VFD, will contribute to reduced motor life. To reduce problems, use an output filter such as:
- Line reactors at the inverter output (typically protects to about 500 feet)
- dv/dt filter (RLC-Resistance, Inductance, Capacitance) at the inverter output (typically protects to about 2000 feet)
- Sine filter at the inverter output (not distance limited)
- Snubber circuit at motor (not distance limited).
These devices reduce the rise (dv/dt) and reduce the voltage level seen at the motor terminals. VFD-rated cable is also recommended.
These best practices are good rule-of-thumb recommendations and will help prevent premature motor failure and improve system reliability. See the VFD manufacturer’s installation manual and the motor manufacturer’s guidelines for more specific details.
-Gary Jacott has spent 30 years as an electrical engineer, including 20 years specializing in the industrial automation niche. A graduate of the University of Minnesota, Gary currently works with Motion Industries‘ Process Pumps & Equipment division in Omaha, Neb.