Variable Frequency Drives (VFDs) have revolutionized industrial motor control by enabling energy savings, reducing wear, and enhancing the flexibility of motor operations. However, they are not always the right choice for every application. Understanding when and where not to use a VFD is crucial to ensuring optimal system performance and avoiding unnecessary costs. In this article, we explore various situations where a VFD may not be beneficial, highlighting key considerations and alternatives.
1. Constant Load Applications
Why VFDs Aren’t Suitable:
VFDs are designed to control motors that experience variable loads. If a motor operates under constant load conditions without needing speed variation, the benefits of using a VFD are minimal. For instance, applications that require fixed-speed operation, like certain fans or pumps, may not benefit from the variable speed control that a VFD offers. Using a VFD in such cases can result in unnecessary complexity, cost, and energy consumption.
Alternatives:
In constant load situations, traditional motor starters such as Direct Online (DOL) or Star-Delta starters are more suitable, as they provide simple, cost-effective motor control.
2. Motors Operating at Very High or Low Speeds
Why VFDs Aren’t Suitable:
VFDs are less efficient when used in applications that require very high or very low speeds. At extreme speeds, the performance of VFDs tends to degrade, leading to reduced efficiency and potential damage to the motor. For instance, motors that need to operate at speeds far beyond their rated capacity or at very low speeds (below 20Hz) could experience inadequate cooling and reduced motor life.
Alternatives:
For applications requiring high-speed operation, consider using motors specifically rated for high-speed applications. In the case of low-speed operations, using a motor with a higher service factor or one designed for low-speed operation might be a better choice.
3. Old Motors or Incompatible Motor Designs
Why VFDs Aren’t Suitable:
Older motors or those not designed to be used with VFDs may not perform well when controlled by a VFD. These motors may lack the insulation needed to handle the high-frequency switching that a VFD uses, potentially leading to insulation breakdown and motor failure. VFDs generate high peak voltages that can exceed the motor’s capacity, especially in motors not rated for such loads.
Alternatives:
For old motors, consider upgrading to newer, VFD-rated motors that are designed with better insulation to withstand the electrical stresses caused by VFD operation. If upgrading the motor is not feasible, using traditional motor control methods may be more reliable.
4. Long Cable Lengths Between VFD and Motor
Why VFDs Aren’t Suitable:
When the cable length between the VFD and the motor is too long, the peak voltages produced by the VFD’s Pulse Width Modulation (PWM) can cause insulation breakdown and motor failure. This is particularly problematic for long motor lead applications, where high voltage spikes may exceed the motor’s voltage rating, resulting in costly failures.
Alternatives:
In such scenarios, using VFDs with built-in soft PWM technology, or installing dV/dt filters or sine wave filters, can mitigate the peak voltage issue. However, these solutions can be costly, and in some cases, it may be better to limit cable length or use more conventional motor control methods.
5. Resistive Loads (e.g., Welders)
Why VFDs Aren’t Suitable:
VFDs are not designed to handle resistive loads, such as welders, because the current drawn by such loads is non-linear and does not require speed variation. The power modulation provided by a VFD, which works on variable torque loads, is ineffective and inefficient for resistive loads.
Alternatives:
For resistive loads, using a Rotary Phase Converter (RPC) is a better choice. RPCs can handle the demands of resistive loads more effectively than VFDs.
6. High Inrush Current Applications
Why VFDs Aren’t Suitable:
Although VFDs are effective in controlling motor starting sequences and reducing inrush currents, in applications where motors require significant inrush current (such as large centrifugal pumps), VFDs may not always provide the necessary starting torque or be the most efficient solution. If the motor requires a large initial surge of power, a VFD may struggle to meet those needs, causing performance issues.
Alternatives:
In such cases, Soft Starters, which reduce the initial inrush current gradually, may be a more appropriate solution. They offer a simpler and more cost-effective approach to managing the starting process of motors with high inrush demands.
7. Small Motors Under 1HP
Why VFDs Aren’t Suitable:
For small motors (typically under 1 horsepower), the cost of installing a VFD may outweigh the energy savings and benefits. VFDs have high initial costs, and the complexity involved in their installation may not justify the relatively small energy savings for smaller motors.
Alternatives:
In small motor applications, direct motor control methods such as simple on/off switches or basic starters are usually more economical and practical.
8. Applications with High Ambient Temperatures or Harsh Environments
Why VFDs Aren’t Suitable:
VFDs require a stable and controlled environment for optimal operation. In environments with high ambient temperatures, excessive dust, or moisture, the VFD’s components may overheat or suffer from contamination, leading to operational failures or reduced lifespan.
Alternatives:
In harsh environments, it may be better to opt for motor control solutions that are better protected from environmental stresses, such as enclosed motor starters or VFDs with higher environmental protection ratings (IP ratings).
9. Constant Torque Applications
Why VFDs Aren’t Suitable:
While VFDs are well-suited for variable torque applications (like fans and pumps), they may not be the best option for constant torque applications unless they are oversized. Applications such as conveyors, crushers, or mixers that require constant torque may require a larger VFD to handle the added current and maintain performance.
Alternatives:
For constant torque loads, other motor control methods like soft starters or traditional constant speed controls may be more efficient and cost-effective.
10. VFDs with Non-Inverter Duty Rated Motors
Why VFDs Aren’t Suitable:
Non-inverter duty rated motors are not designed to withstand the electrical stresses produced by VFDs. Using a VFD with these motors can result in premature motor failure, as the motor windings and bearings are not insulated to handle the high-frequency switching that a VFD generates.
Alternatives:
If using a VFD, always ensure that the motor is rated for inverter duty and is equipped with the proper insulation to handle the electrical conditions produced by the VFD.
For more information, please refer to our article “What is the best solution to reduce harmonics?” to better understand motor control technologies and their applications.
