Motor protection delivers reliable, efficient operation

04/15/2013


Benefits of using a VFD

  • Energy savings
  • Reduces peak energy demand
  • Reduces power when not required
  • Fully adjustable speed (pumps, conveyors, and fans)
  • Controlled starting, stopping, and acceleration
  • Dynamic torque control
  • Provides smooth motion for applications such as elevators and escalators
  • Maintains speed of equipment, making drives ideal for manufacturing equipment and industrial equipment such as mixers, grinders, and crushers
  • Versatility
  • Self-diagnostics and communications
  • Advanced overload protection
  • PLC-like functionality and software programming
  • Digital inputs/outputs (DI/DO)
  • Analog inputs/outputs (AI/AO)
  • Relay outputs  

VFDs offer the greatest energy savings for fans and pumps. The adjustable flow method changes the flow curve and drastically reduces power requirements. Centrifugal equipment (fans, pumps, and compressors) follow a general set of speed affinity laws. The affinity laws define the relationship between speed and a set of variables:

  • Flow
  • Pressure
  • Power

Based on the affinity laws, flow changes linearly with speed while pressure is proportional to the square of speed. The power required is proportional to the cube of the speed. The latter is most important, because if the motor speed drops, the power drops by the cube. 

Figure 4. Affinity Laws – Flow/Speed. Courtesy: Eaton

Figure 4. Affinity Laws – Flow/Speed 

Figure 5. Affinity Laws – Pressure/Speed. Courtesy: Eaton

Figure 5. Affinity Laws – Pressure/Speed 

Figure 6. Affinity Laws – Power/Speed. Courtesy: Eaton

Figure 6. Affinity Laws – Power/Speed

In this example, the motor is operated at 80% of the rated speed. This value can be inserted into the affinity laws formula to calculate the power at this new speed:

Therefore, the power required to operate the fan at 80% speed is half the rated power.

Selecting the correct equipment

Choosing a soft starter or a variable frequency drive often depends on your application. Soft starters are smaller and less expensive when compared with VFDs in larger horsepower applications. Larger VFDs take up more space and are usually more expensive than soft starters.

Figure 7: Flow/pressure relationship. Courtesy: Eaton

That being said, while a VFD is often more expensive up front, it can provide energy savings of up to 50%, thereby producing more cost savings over the life of the equipment.

Speed control is another advantage of a VFD, because it offers consistent acceleration time throughout the entire operating cycle of the motor, not just during start-up. VFDs can also provide more robust functionality than soft starters offer, including digital diagnostic information.

It is important to note that a VFD can initially cost two to three times more than a soft starter. Therefore, if constant acceleration and torque control is not necessary, and your application requires current limiting only during start-up, a soft starter may be a better solution from a cost standpoint. 

Robert Fenton is a product line manager at Eaton. 


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