Could you get your motor matched in your power drive system?

In our clarification of the requirements of the power drive system, we will work our way through the drive train, starting from your application and moving towards the grid supply. After our zoom on your application, we need to ensure that your motor will be compatible to operate reliably in the drive train of your critical application. You can go through the clarifications below or click straight through to your preferred subjects:

Considerations when reusing your existing motor:

If you retrofit the drive train, you might consider reusing your existing motor. In this case, you should get a clear picture of your motors’ actual health (i.e., partial discharge tests, status of the bearings, repair history, …).

Let us consider the case where your existing motor will be powered by a variable-frequency drive (voltage source inverter) instead of the mains supply. This change in the motor’s operation will set further requirements for the variable-frequency drive. You should pass these on to the supplier in a smart way.
Let’s have a short closer look. While the diagram above shows the drive as a simple conversion from fixed voltage and frequency to variable voltage and frequency, the reality is quite different.

The AC voltage from the power grid is rectified and, via intelligent high-frequency switching, recombined to a nearly sinusoidal output voltage. Depending on the quality of this output voltage, the reliability of your motor might be endangered:

• Your motor will heat up more and be noisier.
• Your motor must withstand high voltage peaks and high voltage steepness over time.
• Further investigation regarding mitigating damaging bearing currents due to the high common-mode voltages is highly recommended.
• The high-frequency switching will also induce high-frequency interference and leakage currents, which will need to be dealt with.
• Torque pulsations or cogging can excite resonances with consequential motor, gear, and machine damage.

In our chapter “variable speed drive,” you can learn more about the power quality delivered to the motor by the diverse types of variable frequency drives.
Driving your machine at variable speed creates some interesting advantages, such as energy savings, flexibility for production, gently ramping up to speed without mechanical shock or high starting currents, etc. But please always consult the machine manufacturer about the expected machine behaviour when operating at variable speed versus fixed speed.

Solutions when reusing your existing motor:

We do not want to scare you away with all the potential issues above.
We are confident that specifying the right requirements, their validation method, and setting the right expectations towards your solution provider will make these risks acceptable.
First things first, what is the actual health status of your motor?
Look for services that can predict the remaining lifetime of your stator’s insulation system. Check all the documentation and history on your existing motor.
You will need to require insights and reassurances from your variable frequency drive manufacturer. Some VFD manufacturers might impose in their offer supplemental output filters. So that you know, these supplemental output filters can be costly and might also introduce performance restrictions on the power drive system. Your VFD manufacturer should also provide specifications on the preferred motor output cable and clear installation instructions (EMC (*)). Preventing damaging bearing currents in your motor and machine might require a thorough investigation.
Some types of variable frequency drives, such as medium-voltage multi-level cascaded output drives, on the other hand, do not impose any voltage stress or supplementary heat losses to the motor thanks to their superior sine wave output. These drives also allow for the reuse of existing motor cables, which can significantly reduce costs, especially in cases involving long motor cables.
Look for more valuable data on this in the chapter MV-topologies.
Conclusion:
To gain confidence in the VFD manufacturer, you must ask the right questions to your manufacturer candidates. Even more important is the evaluation of their answers. You’re welcome to contact us as independent subject matter experts.

Opting for a new motor

In general, the cost of a new motor versus the price of the variable frequency drive is not that high. This new motor will be defined for inverter duty according to industry standards (withstand high voltage rise, insulated bearings, etc.). Manufacturers of Medium Voltage motors and Medium Voltage drives might be better positioned to take full responsibility for the right match. To prevent issues with compatibility and reliability in the field, specifying the correct drive and motor for your application is paramount. We would be more than happy to advise you on choosing the right Medium Voltage motor and drive combination for your machine.

 Match your supply-drive-motor to your application.

Consider one manufacturer taking responsibility for the complete power drive system (match of supply-drive-motor). Of course, the correct engineering of the power drive system requires your application input data, such as load torque versus speed and required transient over-torques, if any. Your motor’s torque versus speed needs to be matched to your application data. The manufacturer or solution provider should make the engineering proof available, preferably in graphic format, presenting torque versus speed.
As there is no such thing as “one drive fits all applications”, your vendor should offer a multitude of drives tailor-made to different applications and specific requirements. The vendor should be able to provide you with references of successful power drive systems similar to your application. Give preference to reputable 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 nearby robust service and support network.

This system approach, based on the manufacturer’s engineering experience, is an important advantage, especially in critical applications. Integrating these key elements into your requirements definition can prove to be extremely valuable. If you need any help with this, just contact us for a free consultation.

In all the cases above, we are ready to help. Contact us for a free consultation, no strings attached.
Further down the road, we can facilitate informed decision-making by offering you our independent advice and support for any of the following tasks:

• Set up the project plan presentation.
• Define and manage the requirements of the power drive system PDS (transformer, inverter, VFD, and motor).
• Evaluate the composition of the power drive system.
• Set up and coordinate internal and external pre-bid meetings.
• Set up the invitation to bid with Engineering, Procurement, and Contracting (EPC) companies and/or vendors.
• Screen the preferred manufacturers of transformer, inverter, and motor.
• Verify the offers and exceptions regarding your specs of your PDS.
• Assist you in selecting the vendor.
• Support in order clarification.
• Develop witnessed and non-witnessed Factory Acceptance Tests (FAT) requirements.
• Define the Site Acceptance Tests (SAT) requirements.
• Validate On-site the SAT for your power drive system PDS.

(*) The term “Electro Magnetic Compatibility” (EMC) according to the definition of the EMC Directive:
“The ability of a device to operate satisfactorily in an electromagnetic environment without itself causing electromagnetic disturbances that would be unacceptable for devices operating in this environment”.