Civil Engineering Reference
In-Depth Information
Extended motor life.
Abrupt starts and stops put a motor under stress from high
torque and electrical current surges. The electrical current drawn at motor start
without a VFD is usually six to ten times the full-load current. The VFD's soft
starting capability brings motors to full load gradually, resulting in less wear or
stress on motor bearings, shafts, windings, and insulation. This ability can also
reduce voltage sags that could affect other equipment on the electrical system.
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Increased capacity of standby generators.
VFDs require lower starting current,
enabling standby generators to handle more load.
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Improved control and equipment reliability.
VFDs can be used to vary chemical
feed rates and process equipment motor speed, allowing for more efficient process
control. Wear and maintenance needs are reduced by eliminating unnecessary
starts and stops.
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Motor Selection for VFD Operation
VFDs work on both induction and wound-rotor motors.* VFDs are commonly installed
on existing motors when the motor is still in good condition. The suitability of the
existing motor should be checked for operation with a VFD. For example, if harmonics
are present, increased heating may damage the winding insulation on motors not rated
for such conditions.
Motor cooling is also a critical factor. Power requirements of positive displacement
pumps or blowers are directly proportional to speed. In these applications, as well as
others, motors can overheat at speeds less than 50 percent of full nominal speed.
Cooling is less of a problem for centrifugal applications, where power is proportional
to the cube of speed.
Motors are now available with inverter duty ratings. These motors are essentially
premium efficiency models with higher insulation ratings, underrated to protect against
harmonic current heating. Models with inverter duty ratings, as well as energy-efficient
models, perform better with VFDs than standard motors.
Potential Problems with VFDs
A VFD can cause certain side effects, all of which can be controlled with proper
planning and design. The following paragraphs discuss some commonly encountered
problems.
Harmonic Distortion
VFDs can contribute to high harmonic currents in the power
supply. These currents result in the need for electrical equipment, such as conventional
power transformers, to be derated (oversized). Otherwise, the transformer or circuit
breaker that appears to have adequate capacity to handle the 50-Hz load could overload
from high harmonic currents. Therefore, transformers must be evaluated before VFDs
are added.
Selecting a method to control harmonics is part art and part science. Although small
VFDs may not produce significant harmonic currents, larger units can create substantial
problems if steps are not taken to mitigate harmonics. For design purposes, these
* This section and the following section draw from EPRI (1997); see Reference 7.
