Integrated drive systems Webcast: Your questions answered

Siemens experts offer additional insights into how variable frequency drives (VFDs) provide greater system efficiency.

07/27/2015


At the July 14 CFE Media Webcast, "Integrated Drive Systems Ensures Reliability for Your Application," Dave Hernandez, Senior Product Engineer, R&D Projects Group, and Sumit Singhal, Technical Head of Integrated Drive Systems for Large Drives for Siemens Industry, Inc., presented on how electric motor systems with variable frequency drives (VFDs) are becoming more common in large industrial systems because VFDs provide higher system efficiency, greater operational flexibility and improved reliability with reduced carbon footprint.

There were many more questions on the topic than could be included in the Webcast itself, so Hernandez and Singhal have provided their responses to other viewer questions. To review the original Webcast on demand, click here.

Dave Hernandez, Senior Product Engineer, R&D Projects Group, Siemens Industry. Courtesy: SiemensSumit Singhal, Technical Head of Integrated Drive Systems for Large Drives, Siemens Industry. Courtesy: Siemens

Q: What is the importance of variable speed drive (VSD) location?

A: Drive location is very important. The further away the drive is located from the motor it's feeding, the longer the supply cable must be. Long cabling can inflict "reflective wave phenomena" on a motor which is where the input voltage to the motor can double due to cable impedance being higher than the motor winding impedance. This overvoltage condition can inflict damage on the motor insulation. This problem is typically mitigated with load filters.

Q: Talk about overhauling loads, sending current back to the line, and when to use a line or load reactor.

A: Line reactors help protect the VFD from damaging harmonic currents it back-generates. Load filters help protect the motor from "reflective wave phenomena" when there is long cabling from the VFD to the motor. This is where the input voltage to the motor can double due to cable impedance being higher than the motor winding impedance. This overvoltage condition can inflict damage on the motor insulation.

Q: Could you discuss the different control modes and how they work? Could you discuss the relationships between torque and amps as well as the relationship between speed and voltage? Could you also discuss the relationship between those relationships?

A: The more torque required the higher the current. For ANEMA motors, speed and voltage don't carry the same relationship as seen in DC machines. Voltage supplied to the motor determines the strength of the electromagnetic field the motor can generate; the higher the voltage the stronger the field. The field strength will be the driver for torque generation; the stronger the field, the higher the torque capability. Therefore, voltage is needed to generate torque; and current is the effect of that torque being put to use. Speed is an effect of frequency input into the motor; the higher the frequency, the faster the speed. As your motor becomes overloaded, it starts to relinquish speed to generate more torque, this is known as slip.

Q: What is best application for drives?

A: When operating at lower then AC motor nominal speed offers energy savings or required by the process. Variable torque applications (centrifugal pumps, fans, compressors) offer highest energy saving benefits

Q: Why VFDs are becoming more popular even though it acts as pollution in electrical system?

A: Because the basic need to control a motor-driven load effectively is alive and well. There are always trade-offs when using electrical equipment. For drives, it is accepting the harmonics it introduces to your system, while it gives you the ability to control the speed of your drive train.

Q: Discuss long lead length limitations and associated voltage spikes and line side harmonic distortions caused by VFDs.

A: Long cabling can inflict "reflective wave phenomena" on a motor where the input voltage to the motor can double due to cable impedance being higher than the motor winding impedance. This overvoltage condition can inflict damage on the motor insulation. This problem is typically mitigated with load filters. Line reactors help protect the VFD from damaging harmonic currents it back-generates.

Q: Can one VFD drive three motors?

A: Sure, as long as it is sized for the current draw of all three motors. The driving factor for sizing a drive is the current draw of the motor. The higher, the more power electronic cells are needed, making the drive larger. Typically, this would be allocated on a bus system in which the VFD would feed the bus where multiple motors are connected too.

Q: How do you minimize harmonics and how are total harmonics measured?

A: Line reactors help protect the VFD from damaging harmonic currents it back-generates. THD (total harmonic distortion) is very difficult to measure however the voltage drop on line reactors can be observed to approximate distortion levels.

Q: Talk about the use of line reactors and load filters?

A: Line reactors help protect the VFD from damaging harmonic currents it back-generates. Load filters help protect the motor from "reflective wave phenomena" when there is long cabling from the VFD to the motor. This is where the input voltage to the motor can double due to cable impedance being higher than the motor winding impedance. This overvoltage condition can inflict damage on the motor insulation.

Q: At what horsepower should a medium-voltage drive be considered?

A: A NEMA motor tends to jump in voltage class when they begin to approach upwards of 700HP and up. Although it is not uncommon for low voltage drives to run higher HP motors; the current ratings become a significant limitation.

Q: Explain the differences between overload, over current, and drive current and how they relate to the constant or variable torque sizing of your drive.

A: Overload is a condition in which the motor load supersedes full-load typically referred to as service factor greater than 1. Over current is a type of protection scheme in which CTs are rated to detect upon electrical fault and disconnect to protect the motor. Drive current is the current the motor draws from the drive during rated load operation aka full-load current. All of these have a direct impact on the VFD primarily because it must be sized to handle all current characteristics of the motor.

Q: Is there a way to extend the life of an existing non-inverter duty motor when adding a drive?

A: Yes. If there is appropriate filtering available to ensure the winding will be free of added electrical stresses; the drive can be used as a soft starter which will alleviate the motor from thermal stress during start-up. Motors have a finite number of starts before their windings become thermally worn so drives may help extend insulation life in this case.

Q: If the speed never changes, why is a drive better than a soft starter for larger motors?

A: Because the drive can control the motor starting current so that it is ramped up slowly without inflicting thermal repercussions to the motor winding. Critical thermal factors such as starting duty, acceleration time and load inertia become a non-factor under VFD start-up. A soft starter only limits the starting current to a certain level which forces the motor to accelerate its load with less of what it needs. This causes thermal wear, more stringent starting duty, and longer acceleration times.

Q: What are the key factors that determine which drive to use with which motor? What is commonly the biggest issue with wrongly spec'd motors?

A: Electrical Output (Filtering) — Not all motors can handle overvoltage spikes an unfiltered drive can output; these spikes can damage motor winding insulation. For unfiltered drives, motor insulation must be upgraded to handle overvoltage. For standard motor insulation, drives must have a filtered output so as to not damage insulation.

Q: Method to protect motor from stray currents generated by VFDs?

A: Harmonic filtering may be installed to mitigate the amount of stray currents the motor may see. This can significantly decrease motor heating and thus prolonging the thermal life of motor insulation.

Q: Discuss VFD application on deep well motors and water plants?

A: Loads such as these would commonly be centrifugal pumps. They utilize Constant Torque applications in which the VFD must have the capability of providing Constant V/Hz to the motor so it can maintain rated torque throughout the entire speed range of operation.

Q: What advantages does low voltage motor has over high voltage motors?

A: Low voltage motors do not require large spacing between their winding coils and therefore can be wound tighter with less space infringement. This aspect allows for higher power generation compared the same size motor with a higher voltage class. However, the limiting factor for a LV, high HP motor is the high current draw it will inflict on the power system. Another large adverse effect would be additional I2R electrical losses resulting in heating.



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