How Does the MagnaDrive ASD Compare with Electronic Variable Frequency Drives?
Comparison Overview The MagnaDrive is unique; it is much more than a speed control device. It offers productivity and equipment life benefits to motor systems unavailable through any other means.
The electronic Variable Frequency Drive (VFD) is the most utilized speed control technology currently established in the market - commanding more than 80% of the installed base. The MagnaDrive ASD offers many advantages over VFDs. While there may be some cases where the goals of the designer are best met with electronic technology, in most cases the MagnaDrive should be the speed control method of choice.
The following information is an overview of how the MagnaDrive technology compares with VFDs.
Electrical Fluting All VFDs can cause a current to pass through bearings. At the contact surfaces the process is similar to electric arc welding - resulting in the formation of fluting (corrugation) in the bearing raceways and rollers. Bearings damaged by these circulating shaft currents become noisy and vibration increases because the smooth raceway surfaces are damaged. Unlike VFDs, the MagnaDrive ASD is a mechanical device and therefore does not cause electrical fluting.
Energy Efficiency MagnaDrive ASDs are the most energy efficient speed control option available to the designer today. At average load speeds above 90% of the rated motor speed, MagnaDrive is more energy efficient than a VFD when all system power consumption is considered. The MagnaDrive ASD has approximately the same energy efficieincy as VFDs when the average load speed is between 80% and 90% of rated motor speed. VFDs may show greater efficiency when the average load speed is below 80% of the rated motor speed, however this occurs when power demands are reduced. Therefore, as long as at least 50% of the duty cycle operates above 80% of the rated motor speed, a MagnaDrive ASD is greater than 5% more efficient than a VFD.
Additionally, the efficiency data offered by VFD manufacturers do not take into account the energy required to cool sensitive electronic components, or other auxiliary power needs including harmonic filtration, power factor control, the cost of air conditioning and lighting the equipment room, etc. A VFD with a nameplate efficiency of 95% could have a system efficiency of 85% or lower when all power consumption is included.
In contrast to VFDs, a MagnaDrive ASD operates regardless of power quality in a wide range of environmental conditions so there are no additional sources of energy consumption to consider.
Harmonic Distortion of Power As VFDs manipulate power, they generate significant harmonic distortion that can stray beyond the facility and contaminate the utility power grid. Stray current spikes can damage bearings and increase the likelihood of mechanical equipment failure. In some cases, the VFD is its own worst enemy. The harmonics created by the VFD can contribute to its own early failure.
Electronic filters must be included with any VFD design. While they capture some of the harmonics, many harmful harmonics escape these filters and corrode nearby power quality. Additionally, these destructive harmonics are intensified as the motor is placed farther from the drive. Many VFD manufacturers limit the distance between the VFD and the motor to 300 feet. Most VFD installations also include isolation transformers, transient voltage surge suppression, power factor correction, line reactors, electromagnetic interference (EMI) filters, and other auxiliary devices in an attempt to clean up the power.
The MagnaDrive is a simple mechanical device. It has no impact on the power of the system surrounding the motor. Therefore, there are no harmonic distortion issues associated with the MagnaDrive technology.
Motor Type - Inverter Duty Motors Some VFDs require an inverter duty motor that is more expensive than a standard duty motor. Additionally, there can be long lead items, requiring significant advance ordering time, or tying up capital in spare motors. The MagnaDrive does not influence motor type designation. Usually, standard duty motors are less expensive and readily available.
Power Quality Requirements The sensitivity of VFDs limits them to locations where power supply quality is very good. VFD life is quickly compromised in areas where power quality is less than optimal. The MagnaDrive operates independent of the load power source; it operates whenever the motor turns. The MagnaDrive performs well in outlying areas or international locations where power quality is a concern.
Independent industrial site testing of the MagnaDrive with a "glitch generator" (a device which simulates short term power outages) demonstrated that load characteristics when utilizing a MagnaDrive ASD are not measurably changed during these events. The same "glitches" (a simulation of 2 power outages within 3 seconds) applied to VFDs rendered them useless in every case, requiring service and/or replacement before the VFD was operable again.
In areas where engine generators are used as a primary power source or for reliability measures, the use of electronic VFDs should be discouraged. The damaging harmonics referred to earlier may significantly shorten the life of engine generation equipment. The MagnaDrive does not affect engine generator power sources.
Total Cost of Acquisition The catalog price of the electronic drive is merely a beginning. A number of other cost factors come into play when utilizing a VFD.
First, there are auxiliary equipment needs. Each drive is accompanied with harmonic filters. Power factor correction devices are also standard. Power cables are often increased in size. The entire system has a necessary complexity and sensitivity.
Second, inverter duty motors required by VFDs can be very costly. List prices of inverter duty motors can be 50% to 100% more than the cost of a standard duty motor of equal quality and efficiency.
Third, cabinetry and location are important. A few drives with their accompanying power conditioning devices can quickly fill an electrical room with sensitive gear. Often each panel is cooled and the entire space must have conditioned air for occupancy. The drives must be placed within the recommended distance from the motors, which could adversely impact the efficiency of the floor plan.
Fourth, installation labor can be significant. VFDs must be installed by electricians and instrumentation technicians. Often the installation cost of a VFD exceeds the cost of the equipment itself.
Unlike the electronic VFD, the MagnaDrive ASD is a simple, mechanical device. The only auxiliary equipment needed is an actuator that can be driven by an existing controller. Electricians are not needed during installation.
Long Term Cost of Maintenance and Capital Replacement Electronic VFDs are very complex. Often plant staff is unable to understand the operation of the drives, or repair them. In most cases, when drives fail, expensive factory trained technicians are brought in. Additionally, factory support is relatively brief. It is difficult and expensive to get spare parts for a five-year- old VFD. Consequently, this encourages the purchase of spare parts and electronic boards when the drives are new "just in case," tying up precious capital.
Many VFD manufacturers claim a ten-year life for their drives. However, actual field data suggest that VFDs last far less than the ten-year life described by the manufacturer.
The MagnaDrive ASD is a simple mechanical device. Most maintenance personnel quickly understand its principle of operation and routine maintenance can be performed by a mechanic. Due to the reduced system vibration inherent in the MagnaDrive "disconnected connection," overall system maintenance is greatly reduced. Because the drive is a high quality, durable piece of rotating equipment, it is expected to last for decades.