Medium Voltage VFD FAQ

1. What is a medium voltage VFD and why is it important to choose the right one?

A medium voltage VFD (Variable Frequency Drive) is used to control the speed of motors operating above 1,000 volts. Selecting the correct VFD is crucial for motor performance, system reliability, and energy efficiency. Choosing the wrong VFD can lead to motor failures, power quality issues, and costly downtime. Look for variable speed drive manufacturers that are able to provide you with references of successful power drive systems similar to your application. Prioritize reputable Medium voltage VFD manufacturers with a proven reliability record, at least 10 years of drive-application experience, and multiple similar field-installed units. A large installed base in your area often translates into a robust nearby service and support network. Combine this with 24/7 remote support, and you can significantly increase your machine availability.

2. How does the application influence VFD selection?

Different applications have unique demands. For example, a mining application with a heavy-duty crusher will have different requirements than a simple pump application. Factors like speed response, regeneration needs, motor nameplate data, and the surrounding environment all influence the choice of VFD.

3. Why is an isolation transformer recommended for a medium voltage VFD?

An isolation transformer enhances system protection by separating motor grounds from the main power distribution system. This safeguard helps prevent damage to both the motor and the VFD from voltage transients, lightning and other electrical disturbances. It also offers flexibility in matching the input voltage from the power system to the required motor voltage. The converter duty transformer also reduces the maximum value of short-circuit power in case of short-circuit fault in the drive.
The intelligent combination of secondaries with a  dedicated phase shift, cancels harmonics (the fifth and the seventh in case of a 3 winding transformer) on the primary side and thereby limits the line harmonic distortions and voltage ripple .

4. What power quality considerations are important when choosing a VFD?

Input power quality generally refers to the stability and cleanliness of the electrical supply. VFDs can introduce harmonics (electrical distortions) into the power system, potentially affecting other equipment. A good VFD should have a high displacement power factor (over 95%) and limit total harmonic current distortion to less than 5%, as per IEEE 519 standards.
For output power quality of the drive to the motor, please check FAQ 5.

5. How does the VFD’s output waveform affect the motor?

The waveform generated by the VFD determines the smoothness of the voltage supplied to the motor. A waveform with more steps (e.g., a 5/9 waveform compared to a 2-step waveform) results in less stress on the motor insulation, extending motor lifespan and improving reliability. Drives based on the modular multi-level or the multi-level cascaded H Bridges cells topology deliver a superior sine wave output voltage and current, thereby eliminating harmonic heating and insulation stress on the motor and reducing significantly the torque pulsations (<1%). For applications that require long motor leads, multi-level cascaded output medium voltage drives provide all the benefits common to all VFDs, while reducing the risks of insulation breakdown from reflected wave voltage spikes. Multi-level cascaded output drives provide a reliable option to retrofit systems to variable frequency control without introducing additional voltage stress on motors with standard insulation designed for 50 or 60Hz line operation.

6. What factors should I consider when evaluating VFD manufacturers?

• Experience: A manufacturer with a long history of producing medium voltage VFDs is more likely to offer reliable and well-supported products.
• Installed Base: A substantial number of installations in similar applications to yours indicate a proven track record.
• Breadth of Product Line: A wide range of VFDs tailored to specific applications suggests expertise and flexibility.
• Long-Term Support: Ensure the manufacturer will be able to provide support, parts, and service for the lifespan of the VFD (typically 20-30 years).

7. What aspects of field service are critical for medium voltage VFDs?

• 24/7 Support: Rapid response time is crucial in mission-critical applications to minimize downtime.
• Qualified Technicians: Technicians should be highly skilled and specialized in medium voltage VFDs.
• Remote Diagnostics: The ability to remotely troubleshoot and diagnose issues can save time and costs.
• Training: The manufacturer should offer training for your personnel to handle basic troubleshooting.
• Availability of spare parts: Are the right spare-parts to repair the drive available on-site?

8. How are remote diagnostics beneficial for medium voltage VFDs?

Remote diagnostics allow technicians to connect to the VFD system from a remote location, enabling them to:

• Analyze real-time data to identify and diagnose issues.
• Troubleshoot problems without the need for an immediate on-site visit.
• Provide faster support and potentially resolve issues remotely, minimizing downtime.
• Monitor system performance and anticipate potential problems.

9. How reliable are medium-voltage AC drives?

Medium-voltage AC drives are often used in applications that require a high level of reliability, e.g. feed water pumps, draft fans, and compressors in power stations. In the past, reliability was a major concern for users of these types of drives. However, recent advancements in technology have significantly increased the reliability of medium-voltage AC drives. One of the biggest improvements has been in the power semiconductor technology used in these drives. The Mean Time Between Failure (MTBF) is now very high for these drives and users should expect an availability of 99.9% and more. Downtime per year is a more intuitive way of understanding the availability.

10. What factors impact the cost of a medium-voltage AC drive?

The cost of a medium-voltage AC drive is determined by a variety of factors, such as the size and complexity of the drive system, the level of customization required, and the availability of skilled labor. Other factors that impact the price include floor space required, installation and commissioning costs, training required, energy consumption, maintenance, spare parts, after-sales support, and disposal. The initial purchase price of a medium-voltage AC drive is only one part of the total cost of ownership.
It is good practice to base the project scope requirements and purchase decisions on life cycle cost and not only on capital cost and/or schedule considerations. To obtain management support, you will need to collect at least following data to back this up:

• The cost of unavailability per unit of time (€ or $ per minute, hour or day).
• The daily revenue at stake.
• The possible savings over total life time of the machine.

Users should consider all of these factors when evaluating the cost of a medium-voltage AC drive.

11. What is the use of a rigorous specification of your drive system?

A rigorous specification ensures that your drive aligns with the specific requirements of your application and significantly:

• reduces your downtime
• enhances safety
• reduces your operation cost thanks to a high degree of efficiency
• mitigates financial risks throughout the drive’s lifecycle.

We can support your decision-making process by providing expert, independent advice and assistance on AC or DC-drive applications for the following tasks:

• Develop the project plan presentation.
• Define and manage the requirements for the Power Drive System (PDS).
• Evaluate the composition and components of the Power Drive System.
• Organize and coordinate internal and external pre-bid meetings.
• Prepare and issue invitations to bid in collaboration with Engineering, Procurement, and Contracting compagnies and/or vendors.
• Assess preferred manufacturers for transformers, inverters, and motors.
• Review offers and verify compliance with your PDS specifications.
• Assist in the selection of vendors for transformers, variable frequency drives, and motors.
• Clarify any outstanding issues related to your PDS.
• Define requirements and test plans for both witnessed and non-witnessed Factory Acceptance Tests (FAT).
• Establish requirements for Site Acceptance Tests (SAT).
• Validate the on-site execution of Site Acceptance Tests for the components of your Power Drive System (PDS).

Medium Voltage VFD FAQ

1. What is a medium voltage VFD and why is it important to choose the right one?

A medium voltage VFD (Variable Frequency Drive) is used to control the speed of motors operating above 1,000 volts. Selecting the correct VFD is crucial for motor performance, system reliability, and energy efficiency. Choosing the wrong VFD can lead to motor failures, power quality issues, and costly downtime. Look for variable speed drive manufacturers that are able to provide you with references of successful power drive systems similar to your application. Prioritize reputable Medium voltage VFD manufacturers with a proven reliability record, at least 10 years of drive-application experience, and multiple similar field-installed units. A large installed base in your area often translates into a robust nearby service and support network. Combine this with 24/7 remote support, and you can significantly increase your machine availability.

2. How does the application influence VFD selection?

Different applications have unique demands. For example, a mining application with a heavy-duty crusher will have different requirements than a simple pump application. Factors like speed response, regeneration needs, motor nameplate data, and the surrounding environment all influence the choice of VFD.

3. Why is an isolation transformer recommended for a medium voltage VFD?

An isolation transformer enhances system protection by separating motor grounds from the main power distribution system. This safeguard helps prevent damage to both the motor and the VFD from voltage transients, lightning and other electrical disturbances. It also offers flexibility in matching the input voltage from the power system to the required motor voltage. The converter duty transformer also reduces the maximun value of short-circuit power in case of short-circuit fault in the drive.
The intelligent combination of secondaries with a  dedicated phase shift, cancels harmonics (the fifth and the seventh in case of a 3 winding transformer) on the primary side and thereby limits the line harmonic distortions and voltage ripple .

4. What power quality considerations are important when choosing a VFD?

Input power quality generally refers to the stability and cleanliness of the electrical supply. VFDs can introduce harmonics (electrical distortions) into the power system, potentially affecting other equipment. A good VFD should have a high displacement power factor (over 95%) and limit total harmonic current distortion to less than 5%, as per IEEE 519 standards.
For output power quality of the drive to the motor, please check FAQ 5.

5. How does the VFD’s output waveform affect the motor?

The waveform generated by the VFD determines the smoothness of the voltage supplied to the motor. A waveform with more steps (e.g., a 5/9 waveform compared to a 2-step waveform) results in less stress on the motor insulation, extending motor lifespan and improving reliability. Drives based on the modular multi-level or the multi-level cascaded H Bridges cells topology deliver a superior sine wave output voltage and current, thereby eliminating harmonic heating and insulation stress on the motor and reducing significantly the torque pulsations (<1%). For applications that require long motor leads, multi-level cascaded output medium voltage drives provide all the benefits common to all VFDs, while reducing the risks of insulation breakdown from reflected wave voltage spikes. Multi-level cascaded output drives provide a reliable option to retrofit systems to variable frequency control without introducing additional voltage stress on motors with standard insulation designed for 50 or 60Hz line operation.

6. What factors should I consider when evaluating VFD manufacturers?

• Experience: A manufacturer with a long history of producing medium voltage VFDs is more likely to offer reliable and well-supported products.
• Installed Base: A substantial number of installations in similar applications to yours indicate a proven track record.
• Breadth of Product Line: A wide range of VFDs tailored to specific applications suggests expertise and flexibility.
• Long-Term Support: Ensure the manufacturer will be able to provide support, parts, and service for the lifespan of the VFD (typically 20-30 years).

7. What aspects of field service are critical for medium voltage VFDs?

• 24/7 Support: Rapid response time is crucial in mission-critical applications to minimize downtime.
• Qualified Technicians: Technicians should be highly skilled and specialized in medium voltage VFDs.
• Remote Diagnostics: The ability to remotely troubleshoot and diagnose issues can save time and costs.
• Training: The manufacturer should offer training for your personnel to handle basic troubleshooting.
• Availability of spare parts: Are the right spare-parts to repair the drive available on-site?

8. How are remote diagnostics beneficial for medium voltage VFDs?

Remote diagnostics allow technicians to connect to the VFD system from a remote location, enabling them to:

• Analyze real-time data to identify and diagnose issues.
• Troubleshoot problems without the need for an immediate on-site visit.
• Provide faster support and potentially resolve issues remotely, minimizing downtime.
• Monitor system performance and anticipate potential problems.

9. How reliable are medium-voltage AC drives?

Medium-voltage AC drives are often used in applications that require a high level of reliability, e.g. feed water pumps, draft fans, and compressors in power stations. In the past, reliability was a major concern for users of these types of drives. However, recent advancements in technology have significantly increased the reliability of medium-voltage AC drives. One of the biggest improvements has been in the power semiconductor technology used in these drives. The Mean Time Between Failure (MTBF) is now very high for these drives and users should expect an availability of 99.9% and more. Downtime per year is a more intuitive way of understanding the availability.

10. What factors impact the cost of a medium-voltage AC drive?

The cost of a medium-voltage AC drive is determined by a variety of factors, such as the size and complexity of the drive system, the level of customization required, and the availability of skilled labor. Other factors that impact the price include floor space required, installation and commissioning costs, training required, energy consumption, maintenance, spare parts, after-sales support, and disposal. The initial purchase price of a medium-voltage AC drive is only one part of the total cost of ownership.
It is good practice to base the project scope requirements and purchase decisions on life cycle cost and not only on capital cost and/or schedule considerations. To obtain management support, you will need to collect at least following data to back this up:

• The cost of unavailability per unit of time (€ or $ per minute, hour or day).
• The daily revenue at stake.
• The possible savings over total life time of the machine.

Users should consider all of these factors when evaluating the cost of a medium-voltage AC drive.

We can support your decision-making process by providing expert, independent advice and assistance on AC or DC-drive applications for the following tasks:

• Develop the project plan presentation.
• Define and manage the requirements for the Power Drive System (PDS).
• Evaluate the composition and components of the Power Drive System.
• Organize and coordinate internal and external pre-bid meetings.
• Prepare and issue invitations to bid in collaboration with Engineering, Procurement, and Contracting compagnies and/or vendors.
• Assess preferred manufacturers for transformers, inverters, and motors.
• Review offers and verify compliance with your PDS specifications.
• Assist in the selection of vendors for transformers, variable frequency drives, and motors.
• Clarify any outstanding issues related to your PDS.
• Define requirements and test plans for both witnessed and non-witnessed Factory Acceptance Tests (FAT).
• Establish requirements for Site Acceptance Tests (SAT).
• Validate the on-site execution of Site Acceptance Tests for the components of your Power Drive System (PDS).