Motor efficiency improvement
Making sure the motors operating on your plant floor or in end products are extremely dependable is the name of the game for mechanical engineers. Here's a look at why the connection of variable frequency drives is often the optimal choice for motor operation enhancement.
To some degree, a motor operates more efficiently—certainly with greater dependability—when teamed with a variable frequency drive (VFD).
One of the primary reasons for this is that motors operated by VFDs are not adversely affected by "inrush" current, as are their line-started counterparts. An "inrush" is a high current, typically six to seven times full load, which causes more stress on the end turns of the motor than the program-controlled ramp-up of a VFD. If you installed plexiglass in the motor's endbell, you could actually see physical movement of the end turns in a motor that was line started. Upon startup, the VFD acts much like a soft-starting device, resulting in lower mechanical insulation stresses, thereby lengthening the insulation life of the motor.
Another factor contributing to a VFD's ability to improve motor efficiency are the lower operating frequencies typical of such drives. Motors operating at these lower frequencies typically experience less iron loss due to infrequent polarity reversals. Any time a particle is magnetically charged in one polarity (for example, "north"), and then must reverse its polarity (for this example,"south"), energy is expended. Think of this phenomenon as "magnetic friction." At lower frequencies, this polarity reversal occurs less frequently, thus "magnetic friction" is reduced. The lower iron losses allow the motor to run cooler.
Unfortunately, the laws of physics operate in the same manner at higher frequencies; therefore, motors typically run a little warmer above 60 Hz. The PWM (pulse-width modulation) waveform, carrier frequency, motor type, and load type contribute to how hot the motor gets at sub-base speeds. Consider a variable torque load like centrifugal fans and pumps. The amount of torque required to run these applications is proportional to the square of the speed and the horsepower and the cube of the speed. Therefore, a fan running at half speed would require only one-quarter of the torque in order to run the application properly, hence the amount of torque producing current is lower—allowing a cooler running motor. On the other side of the coin, a conveyor application (constant torque) running at slower speeds will tend to increase heat in the motor.
When the discussion surrounding VFDs comes to bottom-line efficiency, engineers should realize that the real issue is no longer component (or even the motor/drive system) efficiency. It is really more a matter of improving the efficiency of the process powered by the drive/motor system. Consider a fan or pump application, for example, where—at lower frequencies—the HVAC system's airflow or the pump's fluid flow may be more appropriately matched to the requirements of the user. Reducing motor (and thus fan and pump) speed, versus using dampers or valves, utilizes power more efficiently, thus improving system efficiency. Optimal speeds are maintained by VFDs to make machinery or conveyors operate more efficiently.
Flow sensors can also be used to monitor the system and feed back information into the drive via PID operations to adapt the system to changing conditions, thus making them even more efficient.
Other benefits of using a VFD are:
Reduced noise on the power line: Using across-the-line starters draw enormous amounts of current (5 to 7 times FLA [full load amperage]) when activated. This causes undesirable noise to appear on the power lines, which may affect other more sensitive components.
Controllable acceleration, deceleration, torque, and rotation direction.
Possible elimination of mechanical components usually required for non-VFD controlled systems (gearheads and reducers).
AutomationDirect offers Marathon's performance-matched microMax motors. Ranging fromexcellent torque-to-inertia, and smooth, low-speed performance. Motor estimated life before maintenance is up to 100,000 hours. microMax motors can replace permanent magnet dc systems, since no brushes or commutator changes are needed. The motors also offer dual mounting options, C-face rigid baseand C-face round body, cooler running and lighter weight design. Top-mounted conduit boxes allow for easy wiring and installation. All microMax motors carry a 3-year warranty and meet all UL, CSA, and NEMA requirements. The motors range in price from $109 to $789.
Black Max motors are designed for inverter or vector applications where a constant torque speed range of up to a 1000:1 is required. Available fromiced from $149 to $2,369. Blue Max 2000 vector-duty motors are available from 40 to 100 hp and are designed for inverter or vector applications. They feature the Class H Max Guard insulation system and offer constant horsepower operation to 1.5 times base rpm, as well as constant torque operation from 0 to base speed on vector drive, including TEFC (on V/Hz drives, TEFC motors are limited to 20:1 constant torque). Blue Max motors range in price from $2,999 to $5,069.
Blue Chip XRI inverter-duty motors are available from 40 to 100 hp and meet NEMA premium efficiencies. They feature 10:1 variable torque and 20:1 constant torque on VFD with 1.0 service factor. Blue Chip motors offer Class F insulation and continuous duty at 40
All Marathon motors carry a 3-year warranty and meet all UL, CSA and NEMA requirements.
Sensorless vector ac drives
AutomationDirect offers the Durapulse series of sensorless vector ac drives. Durapulse drives are available in 1 to 100 hp models and offer sensorless vector control, autotuning, 150% starting torque, and 150% rated current for one minute. Automatic torque and slip compensation, internal dynamic braking circuit for models under 20 hp and programmable jog speed are also featured. Durapulse drives provide three analog inputs (0-10 V, -10 to +10 V dc or 4-20 mA), 16 preset speeds, 11 programmable digital inputs, four programmable outputs (three digital and one relay) and one analog output. The PWM output of the drive is controlled by a 16-bit microprocessor with an output frequency from 0.1 to 400 Hz. Open serial communications are available with a standard RS-485 Modbus serial interface, which offers communications up to 38.4 K. An Ethernet option, available through the use of AutomationDirect's GS-EDRV Ethernet module, allows access to Modbus TCP/IP or a built-in Web server. The Ethernet option can be combined with AutomationDirect's KEPDirect EBC I/O server to connect the drives to any OPC client. Durapulse drives are UL and CE listed and come with a two-year replacement warranty. Prices range from $289 to $3,999.
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