Your questions answered: Curbing VFD harmonics in HVAC applications

How and why are variable frequency drives (VFDs) used in HVAC applications and building automation systems? And what are the best practices to mitigate the effects of VFD harmonics on system performance?

Larry Gardner, product manager, HVAC Drive Products, Yaskawa America Inc., responds to questions not addressed during the Dec. 14, 2018, webcast Curbing VFD harmonics in HVAC applications.

Question: Would contribution of fault current from motors increase as a result of allowing reverse current to flow from motor to variable frequency drive (VFD)?

Answer: The drive still maintains its overload, ground fault, phase loss, and overcurrent protections. The overcurrent protection will shut down drive operation if more than ~200% of the drive heavy duty rated current is reached.

Question: How does the Matrix system relate to leading power factor (PF)?

Answer: The result is significantly lower reactive power than trap filters.

Question: With the Matrix switching directly from the line how are the high switching frequency harmonics handled? Input filters?

Answer: Each and every Matrix drive has an input filter circuit. The circuit isolates the line from the transients that occur during switching. This filter is also what converts the regenerative pulse-width modulation (PWM) waveform into a sine wave.

Question: Does a poor PF in a building show up as kWh charges or kW demand charges? If the PF is poor the kW is still the same as shown on the power triangle, i.e. we do not pay for kVA only kW. Is this correct?

Answer: Poor PF means you are not using current or power efficiently; therefore, you will need more power (kW) to run the same load. You cannot have the same application provide two different PFs and the same kW. If you have lots of reactive power (high kVA versus kW), then those currents will cause losses in other equipment, usually the upstream transformer. The additional heating leads to limiting of the transformer’s available power.

Question: Do HVAC Matrix drives contribute additional energy to short circuit and arc flash calculations compared to traditional VFD applications?

Answer: The Z1000 Matrix is like any PWM drive and will electrically isolate the motor from the power supply (a short circuit at the motor will not reflect back to the input of the drive). The maximum allowable current will be approximately 200% in either direction and is protected by both software and hardware current limits in the drive. Also, fuse protection (input fusing) is built into the Z1000U platform. The short-circuit rating is 100 kAIC for the Z1000U.

Question: Does HVAC Matrix technology represent a single source or patented technology solution?

Answer: Yaskawa is the only drive manufacturer offering the Matrix design; however, it is not considered a single-source for harmonic mitigation.

Question: If the distribution system is small and cannot accommodate the regenerated power, what will happen to the regenerated power on overhauling loads?

Answer: A Matrix drive’s regenerative power will first begin supplying power to other loads on the same system. If there are no other loads to supply with the added power, then it will return to the source. I do not believe it is possible to overload the utility in this manner. However, hypothetically speaking, if the regen has nowhere to go, the line will continue to build until its voltage reaches a trip point within the drive, where the drive will stop and prevent further operation.

Question: What kind of circuit protection is required on Matrix Drives, say due to insulated gate bipolar transistor (IGBT) failures

Answer: All Matrix drives have embedded semiconductor fusing for device protection. There is no additional requirement for device protection. When it comes to branch (upstream) protection, the built-in fusing of the Matrix drive allows you to use whatever NFPA 70: National Electrical Code (NEC) and local code requires. This usually means a simple molded case circuit breaker (MCCB).

Question: are the harmonics an electronically commutated motor (ECM) produces similar to a VFD with no harmonic mitigation?

Answer: ECMs typically produce iTHD up to 120%, which is significantly higher than VFDs; however, they are also generally smaller horsepower motors.

Question: Is the Matrix drive UL listed for use with variable speed smoke control system?

Answer: Not at this time.

Question: I have sine wave and dv/dt harmonic filters along VFD cables used for my drive, yet the harmonics still causes issues with communication signal. What do you suggest doing to help correct this issue?

Answer: There are two forms of harmonics. Low frequency (those defined by IEEE 519) and high frequency harmonics (noise disrupting signals). Your corrective measures are battling high-frequency harmonics. Specifically, they are battling output radiated harmonics that affect local electrical equipment. Make sure you have proper grounding in place or they will not be nearly as effective as you desire them to be.

Also, VFD cable can sometime increase capacitive coupling to ground, i.e., they can increase the ground currents. If you have a bad grounding scheme, then these grounding currents could find a better path to ground using your communication leads. Again, a good solid ground is key. For VFDs, this means a stranded ground conductor, not a solid ground conductor. Solid ground conductors limit the amount of high frequency currents they can process.

Additionally, there are conducted currents drawn from the line due to the drive’s switching. These currents affect equipment that share the same power supply. Corrective measures include ferrite cores on input and ground leads, and input RF/EMC filtering.

Question: Matrix drive has integrated input fusing providing 100 kA SCCR?

Answer: Yes.

Question: What are available sizes of the Matrix VFD?

Answer: HVAC Matrix units start at 7.5 hp. 208/240 V models go up to 100 hp, 480 V up to 350 hp.

Question: In Eco-mode, what is the Matrix drive’s efficiency under full load?

Answer: By locking the IGBTs in a fixed position the drive is eliminating is IGBT switching losses, thus, increasing the drive’s efficiency to nearly 99%.

Question: Can the Matrix VFD work with single-phase ac input?

Answer: The Matrix is not rated for single phase.

Question: How does cost of Matrix VFD compare to adding passive harmonic filter for each VFD?

Answer: Please contact Yaskawa sales for information.

Question: Any energy savings difference between conventional and Matrix drives?

Answer: Yes; the Matrix is generally more efficient due to the elimination of the rectification and dc bus components.

Question: Do Matrix drives reduce the voltage to the motor?

Answer: The Matrix drive has three output voltage modes. One for maximizing harmonics, one for maximizing output voltage, and an Eco-Mode to provide full line voltage directly to the motor at line frequency.

Question: Could you provide a link for the harmonic calculation software?

Answer: Please see the Yaskawa information here.

Question: Can Matrix drives be used for group motor applications?

Answer: Multiple induction motors can be run by any Yaskawa drive, including the Matrix drive. However, I would not recommend running multiple permanent magnet motors with any drive.

Question: How do you measure harmonics?

Answer: With a power analyzer. There are expensive ones and more economical ones. Your selection depends on your accuracy requirements.

Question: Does harmonics impacts the lifecycle of a motor?

Answer: No, these currents are isolated from the motor by the drive. However, if harmonics are present in the system in significant quantity, it can cause extreme voltage distortion that can impact the performance of a motor running on across-the-line power.

Question: Do you have any of the same problems with ECMs?

Answer: ECMs typically produce iTHD up to 120%, which is significantly higher than VFDs; however, they are also generally smaller horsepower motors.

Question: What is the THDI for the Matrix drive at 25 Hz?

Answer: A 75 hp Matrix drive operating a variable torque load at 40% speed (24 Hz) will have input current harmonics of less than 7% iTHD. The quality of the pre-existing input power supply will affect drive performance.

Question: Can you comment on effectiveness of passive harmonic filters?

Answer: Passive filters reduce harmonics. However, they reduce efficiency, create large amounts of reactive power (high kVA versus kW ratios), boost drive’s dc bus (causes faults and failures), and are configured with less-reliable components leading to premature failures.

Question: Do ECMs produce harmonic distortion? If yes, how do they compare with VFDs?

Answer: ECMs typically produce iTHD up to 120%, which is significantly higher than VFDs; however, they are also generally smaller horsepower motors.

Question: Authorities having jurisdiction (AHJs) are typically concerned with regenerative power being put back into the grid. How do you estimate the amount of regenerated power?

Answer: Regenerated power will never go back onto the grid while there are other loads to draw power. Those loads will absorb the additional power from regeneration. In the vary rare event that power is returned to the grid: regenerated power = shaft power – motor losses – drive losses.

Question: Does IEEE have any intentions of developing a standard to define the acceptable performance of a drive at the VFD terminals to simplify practical application by the industry

Answer: Not to my knowledge.

Question: What is the approximate cost premium for a Matrix drive versus a conventional PWM drive?

Answer: Please contact Yaskawa sales for pricing.

Question: Why do HVAC systems need VFDs? What are the minimum hp needed?

Answer: VFDs save money by their ability to reduce motor speed to match the system need and reducing wear and tear on mechanical parts. Yaskawa offers VFDs starting at fractional horsepower models.

Question: Is the industry also applying the Matrix technology to uninterruptible power supply (UPS) systems? Is it applicable and beneficial for other applications?

Answer: No.

Question: Is this technology of the Matrix VFD provided only by Yaskawa or do other manufacturers also provide this same technology?

Answer: Yaskawa is the only drive manufacturer offering the Matrix design; however, it is not considered a single-source for harmonics mitigation.

Question: Have you ever had a problem with too many Harmonic filters causing a problem with overall PF?

Answer: No, only when dealing with generators should you use caution with passive/trap filters. The Matrix input filter is not a trap filter. It is not intended to improve harmonics. Harmonic mitigation is done naturally as the drive runs the motor. The input filter of the Matrix drive is for isolation from the line and to clean the input PWM waveform during regeneration.

Question: Does Yaskawa publish recommended upstream breaker sizing recommendations for each type of drive you offer?

Answer: Yaskawa generally recommends an MCCB between 2 and 2.5 times the drive rated current. However; since the drive has input fusing, we do not limit branch protection. Size per the recommendations of NEC and local codes.

Question: How does the Matrix do with distorted waveforms?

Answer: The Matrix drive will still operate with a distorted input voltage. However, its harmonics performance will be reduced since it does not have a clean waveform from which to generate its output.

Question: how can harmonics be dealt with when using ECM’s?

Answer: Off topic.

Question: Does the Matrix Drive also has a Integrated EMC Filter

Answer: Yaskawa’s Z1000U HVAC drive comes standard with an embedded EMC filter.

Question: How are individual harmonic currents from multiple VFDs added. Is there a diversity factor to account for the fact that the angle of each harmonic order current will not be the same due to varying reactances ?

Answer: Very complicated question. You generally assume the worst, that they occur at the same time. Each harmonic is added to the others and then summed to determine the total harmonic distortion. Only with a field measurement will you be able to see your exact harmonic contribution of the entire system while in operation.

Question: What do you recommend as the maximum distance from the VFD to the motor to avoid using/specifying load reactors, assuming the right cable type is used.

Answer: The cable distance from drive to motor varies based on cable characteristics, motor characteristics, grounding practices, etc. Generally speaking, an ouptut reactor should not be needed until 300 feet of motor lead length is observed.

Question: Does the Matrix add fault current to a system bolted fault?

Answer: The Z1000 Matrix is like any PWM drive and will electrically isolate the motor from the power supply (a short circuit at the motor will not reflect back to the input of the drive). The maximum allowable current will be approximately 200% in either direction and is protected by both software and hardware current limits in the drive. Also, fuse protection (input fusing) is built into the Z1000U platform. The short circuit rating is 100 kAIC for the Z1000U.

Question: It looks like the Matrix VFD is indicating there is no need for line side reactors or PF correction capacitors. Is that correct?

Answer: That is correct. Yaskawa recommends no added reactors or any other additional components be added before the Matrix drive.

Question: How can you dimension your VFD to meet Industry requirements? What happen to the VFD is overheating the phase conductors?

Answer: The drive has overload and overcurrent protections will not allow currents in excess of its ouptut current rating. The output current will dictate the input current draw. With a balanced impedance system there should be no reason conductor heating will occur if properly sized for the drive.

Question: What are harmonic concerns on a dc input (stand alone battery powered systems) VFD to ac motors?

Answer: I don’t see how a battery powered dc system will affect the Matrix drive. The Matrix drive only uses ac input power. These devices would be on two different grids.

Question: What is the frequency and pulse width of the switching?

Answer: Varies with carrier frequency and frequency referrence (motor speed).

Question: Do you recommend a redundant drive or bypass for critical applications? What is the cost difference?

Answer: Redundant drive packages are becoming more popular because of their ability to vary speed even if one of the drives should fail. In some systems, switching over to 60 Hz operation can cause problems.

Question: How is the reliability history of Matrix drives as compared to other existing solutions?

Answer: Because fewer components and less complexity in design mean higher reliability, the Matrix drive is more reliable than a multi-pulse or active front end system. Moreover, the Matrix drive is made with Yaskawa quality, which stands on its own merits.

Question: Will Matrix drives help with generator issues with regard to high harmonic load percentages?

Answer: Matrix drive’s reduced harmonic content means a smaller generator would be needed to supply the required power. Conventional drives need a generator 3 to 5 times the drive size to combat the harmonic limitations of the generator. For Matrix drives, a generator twice the size of the Matrix drive is sufficient. However, please be sure the regenerative loading is within the means of the generator. Generators can only handle so much regen power through losses. For variable torque loads regen is not an issue.

Question: How long of a feed from drive to motor do you typically recommend a filter? 50 or 100 ft?

Answer: Generally, the shorter the better. The drive can operate up to 300 ft without need for additional filters (reactor, dv/dt, etc.). However, every attempt should be made to limit motor lead length.

Question: Will the Matrix be made for constant torque applications?

Answer: Yes. There is a separate market offering for industrial applications, the U1000 Industrial Matrix drive.

Question: What is the probability of the catastrophic failure of the Matrix VFD?

Answer: Mean time between failure (MTBF) is well in excess of 28 years for Yaskawa drives.

Question: Does the Matrix VFD eliminate motor shaft grounding mitigation equipment additions?

Answer: The Matrix significantly reduces motor bearing current problems. The decision to eliminate motor shaft grounding should be made on a case-by-case basis.

Question: For hospitals where imaging equipment is sensitive to harmonics, is there a cost-effective means of measuring and correcting issues after initial installation rather than guessing up front?

Answer: The Z1000U HVAC Matrix drive limits current harmonics to less than 5% iTHD at the drive terminals. It also comes with a built-in ECM filter to mitigate high frequency harmonics.

Question: Are you saying that harmonic distortion of individual items cannot reinforce each other so that the sum is the same as the singular maximum? I am referring to the punchbowl analogy.

Answer: Total current harmonics (amperes) will go up. However, the percentage of harmonics compared to the overall load will not. Example: existing system 5 A iTHD out of 100 A load (5% iTHD). Adding another of the exact same load means 10 A iTHD out of 200 A. So, current (amp) harmonics increases but percent harmonics stays the same (10/200 = 5%).

Question: Can we get a general idea of how the physical size and cost of the Matrix drive versus an equivalent non-Matrix/standard Yaskawa VFD of similar hp with internal dc reactor and harmonic mitigation?

Answer: The Matrix is generally larger than the equivalent hp conventional VFD. Example: Matrix 15 hp 480 V model is ~19 H x 10 W x 14 D. The equivalent Z1000 is ~18 H x 5 W x 9 D.

Question: How VFD can improve a PF? I have seen the PF on VFD output to the motor at about 0.8. At the same time PF at the VFD input was 1.0.

Answer: All VFDs isolate power draw from the motor using switching (IGBT) circuits. The isolation allows the input and output to act independent from each other.

Answer: What are the smallest Matrix drives available?

Answer: They are available at 7.5 hp. A normal duty is the smallest Matrix drive, although they are technically capable of operating a motor 1/10th its size. Harmonic performance will suffer, though, with motors smaller than the drive’s rating.

Question: Have you experienced nuisance tripping of circuit breakers due to VFD operations? What was the fix?

Answer: No, not with Matrix drives. I have seen this occur with conventional drives due to their inrush current at power up. The cause is almost always a low instantaneous trip setting on the breaker.

Question: Is there a single phase VFD for fractional hp motor?

Answer: Yaskawa offers V1000 and J1000 VFDs in fractional hp sizes with single phase ratings.

Question: what size of motors that using VFD will not be efficient anymore?

Answer: It is always more efficient to use a drive to operate a motor at reduced speed/loads. It is only economical to use a motor across line power if you expect to run at full speed and full load continuously.

Question: When is the Matrix drive not a good choice? is there a hp size of the motor where PWM 16 or 12 pulses are a better application?

Answer: The Matrix should perform better than multi-pulse packages for harmonics, efficiency, and PF at the full range of horsepowers.

Question: Compared with standard VFDs, what are the harmonics of the two without using harmonic filters?

Answer: At rated power operation:

• Matrix drive: < 5% iTHD
• Conventional drive, no impedance: ~85% iTHD
• Conventional drive with impedance internal or before the drive: ~35% iTHD

Question: what is the cost differential between a Matrix VFD versus a 18 PWM VFD for the same size load?

Answer: Please contact Yaskawa sales for pricing.

Question: Do Matrix drives work on non-inverter duty motors?

Answer: Yes, but the question is for how long. These motors are not inverter duty rated for a reason. They do not have the ability to withstand the dv/dt of the output waveform for a prolonged period of time. Ideally, we recommend to replace the motor or add a sine wave filter to correct the waveform.

Question: Is the Matrix design differs from other manufacturers’ AFE?

Answer: Yes, completely different design.

Question: How does your Matrix VFD perform at 100% speed?

Answer: Really well.

Question: When a linear load is added to a variable-speed drive (VSD) load, the total I thd reduces, why does the V thd increase?

Answer: Current distortion causes voltage harmonics. A higher % iTHD doesn’t mean a higher % vTHD. Only with higher current (Amp) harmonics should you see the voltage affected.

Question: What types of events or equipment in a facility could cause a voltage dip, that would result in an overcurrent fault on the VFD?

Answer: Utility power outage or a large load on the system turning on across-the-line, such as a furnace or large compressor.

Question: In a typical drive regeneration is through dc common bus. In Matrix drives there are no dc common bus, can you explain how regeneration works in Matrix drives

Answer: An AFE takes the ac motor power puts it onto the dc bus. Then, it creates a waveform back on to the line to pull power out of the dc bus. The Matrix drive is different that, instead of using a dc bus, the Matrix drive directly converts ac motor voltage to ac line voltage. It’s the same process as for motoring, only in reverse.

Question: What are distortion limits for hospitals?

Answer: Hospital spec-dependent.

Question: Is the Matrix VFD based on V/f control or vector control?

Answer: V/f control for induction motors; open loop vector control for permanent magnet motors.

Question: How confident should we be in theoretical calculated harmonics against testing actual harmonics on site? Should we be confident in a theoretical exercise without physical testing in the as-built condition?

Answer: Harmonic estimation software is only that, an estimate. The actual test results will vary based on quality of power supply, distortion created by other loads, and cleanliness of the installation.

Question: How are background harmonic currents and voltages added to the additional harmonic currents and voltages due to new VFD installations?

Answer: Using a Fourier analysis.

Question: How do you compare reliability of Matrix VFD versus other technologies (multi-pulse, active front-end)?

Answer: The Matrix should have a higher reliability due to its significantly reduced component count.

Question: Does the Matrix drive have any operational limitations?

Answer: See yaskawa.com for operational specifications.

Question: There was a comparison between the configurations of VFD and other components and impact on true PF and other characteristics. Would it be possible to have some thoughts on the cost as part of the comparison?

Answer: Please contact Yaskawa sales.

Question: Is there an efficiency loss at max capacity? Thus, what is Matrix output when called for 60 Hz would output be 98% of 60 Hz? Does it vary for the drive and size?

Answer: The Matrix drive has a built-in input filter for power isolation and regen waveform mitigation. It also has a built-in RF/EMC filter for high-frequency noise.

Question: Can you explain a little more on how harmonics affect the PF? Also, explain the trade-off of limiting the harmonics versus the PF.

Answer: The Matrix doesn’t limit PF; PF comes along for the ride. The drive does its power conversion. The manner in which that is accomplished will dictate PF. PF is a function of distortion (harmonics) and displacement. By limiting distortion at any speed/load we limit harmonics and improve PF.

Question: Would it be possible to have as part of the comparison, a cost effectiveness analysis?

Answer: Please contact Yaskawa sales for pricing.

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