Motors and Drives System Webcast: Your questions answered

Webcast presenter William R. Finley, Sr. answered additional questions about topics such as VFDs for cooling tower fans and whether a VFD can be used in fixed speed and variable load systems.


William R. Finley, Sr. Director of Technology, Process Industries and Drives for Siemens. Courtesy: Siemens

A recent Plant Engineering Webcast featured William R. Finley, Sr. Director of Technology, Process Industries and Drives for Siemens, on the topic: “Motor and Drive System Considerations to Ensure Maximum Performance, Increase Reliability and Extend Product Life.” Finley responded to Webcast attendee questions that were not able to be answered during the Webcast. Those responses appear below.

An archive of the full webcast can be found here.

Q: At what motor horsepower is it better to stay with 480V for drive and motor instead of 4160V?

A: It is not cut-and-dried since it is based on cost and functionality but in general, 800 to 1000 HP is a good place to start evaluating the difference in cost.

Q: Are VFDs really required for cooling tower fans? My understanding was that if all other variables are properly controlled for operation of a cooling tower, VFDs are not economically justified?

A: Not necessarily, many cooling towers can turn on and off fans based on needs so unless that is not enough sensitivity a VFD would not be required.

Q: Do you recommend lubrication with synthetics?

A: Siemens motors are set up with proper materials that won't be adversely affected by the oil but you really should advise the manufacturer to verify. When properly set up synthetics can extend the life of the lubrication.

Q: I need a couple of 3PH motor and driver to make a torque curve against a typical quarter turn actuator in different ranges precisely, so the motor will be in braking region and the currents are definitely high. Please advise me about motor and driver selection and the control loop if it's necessary.

A: A low voltage motor and drive would be selected. If the torque is constant then a speed independent cooling system may be warranted. If speed and holding torque are important then vector control should be considered.

Q: How to define the performance envelop of a selected motor and drives system?

A: Normally you would define the corner point as was discussed in the presentation and then define if constant torque or Variable torques is required. Also define if it is constant horsepower in the over speed.

Q: What are the criteria for using input and output filters, distances between drives and motors and determining factors for use of VFD cables?

A: Need to always coordinate with the drive manufacturer when applying filters. The application manual provides good information on maximum cable length but this will vary dependent on drive types.

Q: Can a VFD be used in fixed speed and variable load systems?

A: It can be used but may not add any value in a fixed speed application. Only adds additional losses.

Q: What's the starting motor inrush when VFD started?

A: The current will typically be around rated current plus or minus dependent on load requirement. No inrush will be seen.

Q: What about using ground shaft device?

A: It is common to use ground shaft devises but as discussed in the presentation be aware of the current path and determine if an insulated coupling is required. If device is arcing it is not allowed in a hazardous location.

Q: How about 60Hz motor run on 50 Hz?

A: It does matter which motor one uses on a VFD as long as it is in the speed range of the motor and you maintain a constant volts per hertz.

Q: How voltage overshoot is caused?

A: The rapid rise time will result in an overshoot. See the application manual presented in the presentation.

Q: Kindly explain voltage stress levels.

A: This is not a simple topic but I would recommend you review the NEMA application guide and the IEC standard 60034-25 & 60034-18-41.

Q: At what motor power level is it more cost advantageous to go from 480 VAC to 4160 VAC for motor, drive and overall installation costs?

A: It is not cut and dry since it is based on cost and functionality but in general 800 to 1000 HP is a good place to start evaluating the difference in cost.

Q: What is the best way to tell if you have common mode current or voltage before taking apart equipment and experiencing a problem?

A: One could measure the voltage from shaft to ground but it is not a perfect indicator. If it is over .38 volts it could be a problem.

Q: How do VFDs differ for permanent magnet motors vs. induction motors?

A: Most all the performance concerns are identical.

Q: What are my best resources for getting deeper knowledge on VFD control modes and their application? (Feedback vector, sensorless vector, Volts/Hz)

A: See the NEMA application manual referenced in the presentation. It is free.

Q: How much of an issue is shaft grounding for common industrial motor applications with "typical" TEFC, 1800 rpm, Class F insulation motors, particularly in sizes 100 hp, 75 hp, or less? This is for consideration of retrofit installations.

A: Not sure what is the concern is here. Shaft grounding is a problem in a hazardous area due to the arcing.

Q: Do we have to use special cable-3-ground conductor or shielded cables for 480V motor VFD application? We run motor at very high torque for very short time.

A: Not if the system is properly grounded but if this is unknown it won't hurt. Discuss with the drive manufacturer.

Q: I am getting information a push from vendors on "Voltage Optimization" equipment that is supposed to drive down electricity use on motors and am concerned about how lowering the input voltage will affect the VFD's and motors when lowering the voltage from 480 to say 460 or 450?

A: Lowering the voltage will normally hurt the energy conservation. Higher voltage will help but be careful equipment can handle the higher voltage.

Q: What type of AC motors you can connect to VFD systems?

A: All types as long as they are designed to be on the drive.

Q: Can we use VFD with an existing wound rotor motor? Concerns?

A: Yes but you may need to filter the output. Siemens has MV drives that require no special filtering.

Q: Earlier in this presentation you mentioned damage that may occur as a result of 1 running motor next to a resting motor on a shared base. Will this damage occur over a relatively short period of time? Do you have any suggestions for preventing the damage to the bearings if you cannot avoid the running condition and proximity of the motors to one another?

A: It will not happen in a matter of minutes but it will happen in a matter of hours. Use sleeve bearings or isolate them.

Q: Bill talked about a number of ways to mitigate shaft current. How do you know when you have the problem taken care of?

A: You can measure the shaft voltage to ground and make sure it is below .375 volts but it is not a perfect conclusion.

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