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Increase VFD System Performance & Reliability
by John A. Houdek - MTE Corporation
Reactors have been used for many years to solve problems in variable speed drive installations. About ten years ago the use of line reactors started to become more common as they helped to solve typical problems on the input (line side) of variable frequency drives (VFD) and SCR controllers. They often have been used as low cost substitutes for 1:1 isolation transformers. The typical problems that line reactors solved were drive nuisance tripping, voltage notch reduction (for SCR controllers) and harmonic attenuation. They were called "line reactors" because they were always used on the "line side" or input of a variable speed drive. Attempts to use "line reactors" on the output side of a drive tended to fail because the line reactors typically overheated due to the harmonic content of the output waveform.
In 1989 the industry experienced the introduction of Harmonic Compensated Reactors which now offered a product that was suitable for use on either the input or output of a variable speed drive. Harmonic compensation meant the reactor was designed to handle the harmonic spectrum and high frequency carrier waves which are typical on the output side of a variable speed drive. Not only are the frequencies higher, but the RMS current is also higher whenever harmonics are present. (Example: 100%fundamental current+100% harmonic current = 141% RMS current; via Pythagorean theorem). Harmonic compensated reactors would not only handle these conditions from a thermal perspective, but they also offered full performance and inductance in the presence of even severe harmonics. Therefore Harmonic Compensation offered an assurance of both safety and performance.
Now that reactors could be used on the output of a VFD, many more application problems could be solved. The most typical problems included motor temperature rise, motor noise, motor efficiency, and VFD short circuit protection.
Motor Temperature Reduction
Motors operated on a VFD tend to run warmer than when they are operated on pure 60hz, such as in an across-the-line stator application. The reason is that the output waveform of the VFD is not pure 60hz,, but rather it contains harmonics which are currents flowing at higher frequencies. The higher frequencies cause additional watts loss and heat to be dissipated by the iron of the motor, while the higher currents cause additional watts loss and heat to be dissipated by the copper windings of the motor. Typically the larger horsepower motors (lower inductance motors) will experience the greatest heating when operated on a VFD.
Reactors installed on the output of a VFD will reduce the motor operating temperature by actually reducing the harmonic content in the output waveform. A five percent impedance, harmonic compensated reactor will typically reduce the motor temperature by 20 degrees Celsius or more. If we consider that the typical motor insulation system has a "Ten Degree C Half Life" (Continual operation at 10 degrees C above rated temperature results in one half expected motor life), then we can see that motor life in VFD applications can easily be doubled. Harmonic compensated reactors are actually designed for the harmonic currents and frequencies whereas the motor is not.
Because the carrier frequency and harmonic spectrum of many Pulse Width Modulated (PWM) drives is in the human audible range, we can actually hear the higher frequencies in motors which are being operated by these drives. A five percent impedance harmonic compensated reactor will virtually eliminate the higher order harmonics (11th & up) and will substantially reduce the lower order harmonics (5th & 7th). By reducing these harmonics, the presence of higher frequencies is diminished and thus the audible noise is reduced. Depending on motor size, load, speed, and construction the audible noise can typically be reduced from 3 - 6 dB when a five percent impedance harmonic compensated reactor is installed on the output of a PWM drive. Because we humans hear logarithmically, every 3dB cuts the noise in half to our ears. This means the motor is quieter and the remaining noise will not travel as far.
Because harmonic currents and frequencies cause additional watts loss in both the copper windings and the iron of a motor, the actual mechanical ability of the motor is reduced. These watts are expended as heat instead of as mechanical power. When a harmonic compensated reactor is added to the VFD output, harmonics are reduced, causing motor watts loss to be reduced. The motor is able to deliver more power to the load at greater efficiency. Utility tests conducted on VFD's with and without output reactors have documented efficiency increases of as much as eight percent (at 75% load) when the harmonic compensated reactors were used. Even greater efficiency improvements are realized as the load is increased.
Short Circuit Protection
When a short circuit is experienced at the motor, very often VFD transistors are damaged. Although VFD's typically have over correct protection built-in, the short circuit current can be very severe and its rise time can be so rapid that damage can occur before the drive circuitry can properly react. A harmonic compensated reactor (3% impedance is typically sufficient) will provide current limiting to safer values, and will also slow down the short circuit current rise time. The drive is allowed more time to react and to safely shut the system down. You still have to repair the motor but you save the drive transistors.
Output reactors solve other problems on the load side of VFD's in specialized applications also. Some of these include: Motor protection in IGBT drive installations with long lead lengths between the drive and motor, Drive tripping when a second motor is switched onto the drive output while another motor is already running, and Drive tripping due to current surges from either a rapid increase or decrease in the load.
Whether you are using reactors on the input or output of a VFD the best performance results will always be achieved using reactors which are fully compensated for the harmonics which are present. This assures maximum inductance and thus best attenuation of harmonics with maximum current surge protection. Harmonic compensated reactors have proven to be the most cost effective solution to a wide variety of typical VFD application problems. Use them on the input or output of your drives to improve the total system performance and reliability.
impedance for typical applications:
3% -Current surge protection
3% -Voltage transient protection
3% -Drive nuisance tripping
3% -Voltage notch reduction (SCR's)
3% -Capacitor switching spike protection
3% -Motor short circuit protection
3% -Multiple motor applications
5% -Harmonic reduction
5% -Motor temperature reduction
5% -Motor noise reduction
5% -Motor efficiency improvement
5% -IGBT w/ long lead lengths
For further assistance, contact your Galco
Aparently the commertial units have anti harmonic stuff built into them.
Rex Line and Load reactors
Rex Manufacturing Line and Load Reactors are Today's Solution to SCR Drive - Inverter Application Problems
Inductors placed at the input and output of electrical equipment, can provide protection and improve performance. Line Reactors absorb many power line disturbances which could damage or shut down your inverters, variable speed controllers or other voltage sensitive equipment
Rex Line Reactor designs conform to UL, CSA, and IEC international standards.
Three phase AC line reactors when used as input or output filters on inverter electronic speed drive applications provide several significant benefits.
When applied as Line reactors placed ahead of an AC inverter drive, the reactor absorbs many power line disturbances which could damage or shut down the inverter. Additionally, the Line reactor protects sensitive surrounding equipment from transients associated with quick switching of IGBT devices in the drive.
When applied as Load reactors placed on the output of the AC inverter drive, the reactor provides two principal benefits. The load reactor will slope the edges of PWM waveforms applied to long cables and conductors, thereby reducing the dv/dt and stress due to uneven voltage distribution. However, load reactors used alone are only partially effective in reducing the peak voltage appearing at the end of long lines. For conductor lengths (between the drive and motor) in excess of 50 feet, the Rex Motor Guarding Transient Filter is recommended.
Long lines, particularly long cables, have capacitive effects producing charging currents in the order of 10 to 20 amperes which can cause spurious protective trips in small or low power drives. Reactors reduce cable charging current, producing higher reliability of operation and freedom from nuisance tripping.
Rex Motor Guarding Transient Filters, incorporate reactors, resistors, and capacitors. When these devices are placed on the output of adjustable frequency drives they protect the motor windings from the damaging voltage spikes associated with the fast switching effects of IGBT's and long lines and cables.
"Long lines, particularly long cables, have capacitive effects producing charging currents in the order of 10 to 20 amperes which can cause spurious protective trips in small or low power drives."
The voltage naturally rises towards the end of a lightly loaded line.
Conversely, the voltage naturally falls towards the end of a heavily loaded line.
If the lines are short, or are short in relation to their gauge, then there is usually no noticeable affect.
The easiest solution for an HSM-er is to use a heavier than normal wire gauge, particularly for heavy machine loads, but not necessarily for lighting and plug loads or for light machine loads.
Applying Line/Load Reactors
Input to Inverter/Drive:
On the input of an electronic motor speed controller, line reactors protect sensitive electronic equipment from electrical noise created by the drive or inverter (notching, pulsed distortion, harmonics). They also protect the controller from surges or spikes on the incoming power lines, as well as reduce harmonic distortion. They help to meet the requirements of IEEE 519.
Output of Inverter/Drive:
In long motor lead applications use reactors (IGBT protected) between the inverter and motor to reduce dv/dt and motor terminal peak voltage. The use of a load (output) reactor also protects the controller from a surge current caused by a rapid change in the load, and even from a short circuit at the load.
Our reactors also reduce operating temperature and audible noise in motor loads. Harmonic compensation and IGBT protection of all Guard-AC reactors allows standard units to be used here with confidence. They improve the waveform integrity, thus enhancing motor performance and system efficiency.
Multiple drives or inverters on a common power line require one reactor per controller. Individual reactors provide filtering between each controller (reduce crosstalk) and also provide optimum surge protection for each unit. A single reactor serving several controllers does not provide adequate protection, filtering or harmonic reduction when the system is partially loaded.
When more than one motor is controlled by a single drive, a single reactor can typically be used between the controller and the motors, as illustrated. The reactor should be sized based on the total motor/load horsepower.
Nice piece of research Chevy.
I looked into this EMF filter/reactor business about 10 years ago but haven't since. My Hunk-O'-Pipe load reactors resulted from these researches but I've never devoted time to tune them for best effect. All I know is they can be fiddled with and a fully loaded motor temp does go down as the understandability of AM stations goes up. Looks like I'm going to have to borrow my oscilloscope back and tinker with my load side Pi filters.
One thing that we machinists should keep in mind is that unless we are running CNC machine to full capacity our machine spindle motors seldom see a full load cut lasting longer than a few minutes. For that reason many of these heating, di/dt, and load conductor capacitance issues are more often of interest than real concern.
Your new material gives me much to think about.
the guy writing the article works for MTE . . . who pretty much sells only line/load reactors.
I don't deny that what he says might be true, but at my last check, we have installed over 800 VFDs / Vector drives since 1995 between 2HP and 150HP and we have never used an output load reactor between the drive and the motor (with the exception of when the motors were servo motors - and then only when the motors are really expensive and on the end of motor leads in excess of 100 feet and connected to 480V drives with a 720VDC bus)
Motor leads for up to 8 motors at a time share a common 4" metal conduit from the cabinet to a pull box on the machine covering a distance between 100 ft to 300 ft. Motor leads are individual THHN wires all the way to a local disconnect near the motor.
800+ motors running on VFDs . . . some for over a decade. 99% of them are still the original installed motors and they run 24x7 with RMS loading of between 10 and 70 percent of rated current.
The MTE guy does love me however, I put his line reactors between each drive and the line - big plants have crappy power - we don't want to make it any worse.
Unless you have a really crappy motor, load reactors are over rated.
Slightly OT, but Forrest's reply made me think about the AM radio static issue. Others have commented on this and I was expecting the worst. I found that static is only present when the motor is coming up to speed (10 seconds) and then disappears. Am I just lucky, or what did I do to escape this problem??
Using a Teco/Westinghouse TM100-15 (15 hp)on a 5HP lathe motor. Power cord is 10ga rubber protected cord about five feet long.
Hmm. "Hunk-O-Pipe" reactors. Interesting.
Any way to elaborate on the fabrication of
"I found that static is only present when the motor is coming up to speed (10 seconds) and then disappears."
A VFD is simulating a sine wave by a series of pulses, which can be very narrow when starting to very wide when running.
Very narrow pulses have very high harmonic content, surely well into the 530-1710 kHz region, the standard (radio) broadcast band.
Very wide pulses have less such harmonic content.
Radio frequency interference can be minimized by the installation of an RFI filter.
Here is the line & load reactor that I bought for my 25 Hp VFD. I spoke to the tech rep and he said get one as close as possible for the amperage of the loads you will be switching. They will cover up to 200% overload for 3+ minutes. Dont get a way oversize one or it won't give you much protection.
Here the manufactures web page with info. on it: