Additional information about VFDs, VSDs, and motors

Presenters provided more details about “Energy efficiency: Variable frequency drives (VFDs), variable speed drives (VSDs), and motors.” Several questions from the June 11, 2015, webcast are addressed here.


Presenters provided more details about “Energy efficiency: Variable frequency drives (VFDs), variable speed drives (VSDs), and motors.” John Yoon, PE, LEED AP, senior staff electrical engineer at McGuire Engineers Inc., Chicago; and Kenneth L. Lovorn, PE, president of Lovorn Engineering Associates, Pittsburgh, responded to questions that were not answered in the live webcast about “Energy efficiency: Variable frequency drives (VFDs), variable speed drives (VSDs), and motors.”

Question: What specific information do mechanical engineers need to communicate to the design electrical engineers?

John Yoon: Motor horsepower and voltage, load brake horsepower, variable frequency drive (VFD) type (6- or 12-pulse), if a bypass starter is being specified, motor and drive location (inside/outside, distance between controls and motor), who performs the IEEE 519 study, etc.

Question: Will operating the disconnect between the VFD and the ac motor while the VFD is running cause damage to the VFD, disconnect switch, or motor?

Yoon: That’s more of a manufacturer question based, on the design of the VFD inverter section.

Question: As a rule of thumb, at what cable run length do you become concerned about harmonics or standing wave voltages?

Yoon: Worst-case manufacturer’s recommendation that I’ve seen on insulated-gate bipolar transistor (IGBT) drives with high carrier frequencies is 20 ft.

Question: Does running a 60 Hz motor via a VFD higher than 60 Hz damage or shorten the life of the motor?

Yoon: In general, overspeed beyond 60 Hz base speed results in greater heat and reduced torque. Heat reduces the life expectancy of the motor’s insulation system. For extreme situations, the magnitude and duration of overspeed capacity without mechanical damage depends on the NEMA motor design type. The motor manufacturer typically will define a maximum safe operating speed for a particular design. The maximum safe operating speed is typically much greater than the synchronous speed. Overspeed capacity typical range from 10% to 25% of maximum safe speed and allowable duration is typically only 2 min.

Question: Why aren't motor lead filters included in VFDs when built at the factory?

Yoon: There are two reasons: cost and the level of mitigation varies depending on the application. In some applications, it may not be required depending on source voltage, motor lead distance, carrier frequency, etc.

Question: Is a surge suppressor advisable in parallel to the variable speed drive (VSD)?

Yoon: Yes. While the rectifier section of a VFD may have some level of integral protection depending on the manufacturer, they are power electronics devices that have operational voltage limits and should be protected by a surge protection device (SPD)/transient voltage surge suppression (TVSS).

Question: Can you use one harmonic filter for four VFDs, for example in an air-handling unit (AHU) with a fan wall?

Yoon: Not advisable. You want to match the line reactor to the individual VFD. The harmonic characteristic of the load change depending on what each individual VFD is doing (on/off/part load).

Question: Does a 12- or 18-pulse drive have the ability to operate with reduced pulse output (i.e. 6-pulse) if one of its internal circuits fails?

Ken Lovorn: This is dependent on the manufacturer’s internal configuration. It would be possible, electrically, if the manufacturer chose to design it that way.

Question: Does the VFD have an influence on protection coordination?

Yoon: Yes. Considerations include: is braking of the load required (i.e. will the braking methodology push current back on the source somewhat like a synchronous motor), what type of internal/external protection it has to comply with NEC Article 430, and fault current withstand rating of the drive.

Question: How much does the efficiency of a motor drop at 50% turndown?

NEMA MG 1 defines full load efficiencies for various motor types but does not specify minimum part load efficiencies. The efficiency at a particular speed also depends on the load’s torque requirements  But in general, at half speed, you may take a 10% to 20% motor efficiency hit. This is negligible compared to the horsepower savings associated with operating at half speed. If very lower operating speeds are desired, permanent magnet ac (PMAC) motors can maintain higher levels of efficiency compared to standard ac induction motors.

Question: On a downhill regenerative conveyor application where a VFD slows down a fully loaded conveyor to a full stop, for how long it can continue to apply negative full load torque to keep the conveyor stalled? Will the motor overheat?

Yoon: There are too many variables here. Your ability to do this will vary depending on the drive (does it have braking resistors, dc bus tie, or will it push current back on to the ac power source), NEMA motor design type, and control methodology (open/closed loop). Braking/regenerative applications will always have the potential for motor overheating, but you also have to be concerned with the drive. As such, you would have to consult with the equipment manufacturer. The primary question to ask for a particular motor/drive combination is what the duty cycle limit is for a braking application.

Lovorn: To our knowledge, braking of a load is possible, within the horsepower ratings of a particular VFD, but there are no VFDs that are designed to operate at 0 rpm with an applied load.

Question: I read an article that stated the affinity laws square the function instead of cubing the function to estimate energy savings. In your experience, will squaring the function give you a more accurate real world scenario?

Yoon: In our example, we were looking at flow, not pressure. You will note that the graph for centrifugal loads had two curves on it. The relationship of speed to pressure/head is the square. Speed to horsepower is the cubed and flow. Overall, flow is directly proportional to speed. An excellent reference for helping understand this relationship is the ASHRAE Handbook—HVAC Systems and Equipment, Affinity Laws for Centrifugal Pumps, section 43 (2008 edition) or section 44 (2012 edition).

Question: Can multiple fan motors (such as for a condenser) be run on a single VFD? If so, how do you size it and what concerns are there?

Yoon: Yes, but:

  • All of the motors have to be the same
  • You cannot mix and match horsepower ratings
  • You still need to protect each individual motor in accordance with NEC 430.

Also, remember that VFDs are typically current rated and not horsepower devices. As such, sizing is based on the total connected full load current represented by the motors. The primary concern is motor-drive impedence mismatch. This potential impedance mismatch can cause reflected wave issue resulting in damage to the motor insulation system.

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