AutomationDirect presents – 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).

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