Wednesday, 15 October 2025
Line Reactors vs. Transient Filters: Understanding Their Roles in VFD Systems
Variable Frequency Drives (VFDs) have become an essential part of modern power and ...
Reactors & Filters
AC Line/Load Reactors
Rex Power Magnetics AC Line and Load Reactors are designed to improve power quality, protect equipment, and extend the life of electrical systems. Installed on the line or load side of variable frequency drives (VFDs), motor controllers, and other sensitive equipment, these reactors limit harmonic distortion, reduce voltage spikes, and mitigate current surges.
Each reactor is engineered for precise inductance, low losses, and superior thermal performance. Constructed using high-grade core steel, copper or aluminum windings, and robust insulation systems, Rex reactors provide reliable operation under continuous and transient conditions. Their dry-type construction and durable enclosures make them suitable for harsh industrial environments.
Available in a wide range of current ratings and voltage classes, Rex AC Line and Load Reactors can be tailored to specific system requirements. All units are designed and tested to meet applicable CSA and UL standards, ensuring safe, efficient, and long-lasting performance in demanding applications.
| Capacity | 2A to 1,250 A Three Phase |
| Voltage Class | 600V Max |
| Cooling | Air Cooled (ANN / AN) |
| Frequency | 60Hz (50/60Hz Optional) |
| Conductors | Copper (Cu) Windings |
| Insulation System | 220°C Insulation System (150 °C Temp Rise Max) |
| Sound Level | Below 40dBA |
| Overload Capability | 150% of rated current for 1 min. |
| Saturation Level | Not less than 250% of rated current (min 50% of rated inductance at 350% rated current) |
| Enclosure Type | Type 1 Indoor |
| Warranty | 12 Months (See Warranty and Limitations) |
| Quality System | ISO 9001:2015 Quality Management System |
| Certifications |
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| Reference Standards |
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| Rating HP |
Voltage= 208 Volts | Voltage= 240 Volt | ||||
| Maximum Current (Amps) |
Impedance | Maximum Current (Amps) |
Impedance | |||
| 3% | 5% | 3% | 5% | |||
| 1 1.5 2 |
4 8 8 |
3PR-0004C3L 3PR-0008C3L 3PR-0008C3L |
3PR-0004C5L 3PR-0008C5L 3PR-0008C5L |
4 8 8 |
3PR-0004C3L 3PR-0008C3L 3PR-0008C3L |
3PR-0004C5L 3PR-0008C5L 3PR-0008C5L |
| 3 5 7.5 |
11 17 27 |
3PR-0011C3L 3PR-0017C3L 3PR-0027C3L |
3PR-0011C5L 3PR-0017C5L 3PR-0027C5L |
11 17 27 |
3PR-0011C3L 3PR-0017C3L 3PR-0027C3L |
3PR-0011C5L 3PR-0017C5L 3PR-0027C5L |
| 10 15 20 |
34 45 60 |
3PR-0034C3L 3PR-0045C3L 3PR-0060C3L |
3PR-0034C5L 3PR-0045C5L 3PR-0060C5L |
27 45 60 |
3PR-0027C3L 3PR-0045C3L 3PR-0060C3L |
3PR-0027C5L 3PR-0045C5L 3PR-0060C5L |
| 25 30 40 |
80 100 130 |
3PR-0080C3L 3PR-0100C3L 3PR-0130C3L |
3PR-0080C5L 3PR-0100C5L 3PR-0130C5L |
80 80 100 |
3PR-0080C3L 3PR-0080C3L 3PR-0100C3L |
3PR-0080C5L 3PR-0080C5L 3PR-0100C5L |
| 50 60 75 |
160 160 200 |
3PR-0160C3L 3PR-0160C3L 3PR-0200C3L |
3PR-0160C5L 3PR-0160C5L 3PR-0200C5L |
130 160 200 |
3PR-0130C3L 3PR-0160C3L 3PR-0200C3L |
3PR-0130C5L 3PR-0160C5L 3PR-0200C5L |
| 100 125 150 |
255 320 410 |
3PR-0255C3L 3PR-0320C3L 3PR-0410C3L |
3PR-0255C5L 3PR-0320C5L 3PR-0410C5L |
255 320 410 |
3PR-0255C3L 3PR-0320C3L 3PR-0410C3L |
3PR-0255C5L 3PR-0320C5L 3PR-0410C5L |
| 200 250 300 |
500 600 750 |
3PR-0500C3L 3PR-0600C3L 3PR-0750C3 |
3PR-0500C5L 3PR-0600C5L 3PR-0750C5 |
500 600 750 |
3PR-0500C3L 3PR-0600C3L 3PR-0750C3 |
3PR-0500C5L 3PR-0600C5L 3PR-0750C5 |
| 350 400 500 |
1000 1000 1250 |
3PR-1000C3 3PR-1000C3 3PR-1250C3 |
3PR-1000C5 3PR-1000C5 3PR-1250C5 |
1000 1000 1250 |
3PR-1000C3 3PR-1000C3 3PR-1250C3 |
3PR-1000C5 3PR-1000C5 3PR-1250C5 |
Product Catalogs
Line Reactor Catalog (English)
Instruction Sheets & Manuals
Calculate the inductance of a line or load reactor from the system voltage, percent impedance, rated current, and frequency.
V
%
A
Hz
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Inductance is calculated from the entered percent impedance referenced to the system voltage at the stated frequency, assuming a balanced linear system. Results do not account for saturation, tolerance, temperature, or harmonic effects. Rex Power Magnetics makes no representations as to the accuracy of the information used by or generated by this calculator. The results are provided for general reference only and should be independently verified by a qualified professional before being relied upon. Rex Power Magnetics assumes no responsibility for the results generated by the calculator or for any errors or omissions in connection with its use, and under no circumstances shall Rex Power Magnetics be held liable for any damages arising from the use of, or results generated by, the calculator.
Faqs
- General FAQs
- Technical Specification FAQs
- Installation & Application FAQs
- Performance & Product Selection FAQs
- Protection, Troubleshooting & Ordering FAQs
An AC line/load reactor is an inductive device installed in series with either the input (line side) or output (load side) of variable frequency drives (VFDs) or other power electronic equipment. It limits current surges, reduces harmonic distortion, and protects sensitive components from transients. Rex Power Magnetics AC reactors improve power quality and extend equipment life by mitigating voltage spikes, reducing current distortion, and filtering unwanted noise from the electrical system.
Reactors protect both the drive and the motor from electrical stress. They dampen voltage transients, filter switching harmonics, and reduce the effect of power line disturbances on drive operation. When installed on the line side, they prevent nuisance tripping and extend VFD lifespan. On the load side, they reduce cable stress, protect motor insulation, and minimize electromagnetic interference. Rex Power Magnetics reactors help ensure stable and reliable operation in harmonic-rich environments.
A 3PR reactor is a three-limb stack of electrical steel laminations with one copper coil per limb, the assembly dipped in polyester resin and clamped in a steel frame. The detail that separates it from a transformer core is the air gap cut into each limb. A transformer wants maximum permeability so that a small magnetizing current produces the working flux. A series reactor wants the opposite: a defined, stable inductance that does not depend on the steel’s non-linear permeability. The gap places most of the magnetic reluctance in air, where permeability is constant, so the inductance is set by turns and gap length rather than by where the steel sits on its magnetizing curve.
This is what makes the published saturation specification possible. Because the gap dominates, the core does not begin to saturate below 250% of rated current, and the unit retains at least 50% of rated inductance at 350%. Without the gap, a reactor of the same size would saturate near rated current and offer almost no impedance to the current peaks a drive rectifier produces. The gap also fixes the ratio of inductance to physical size, which is why a 5% unit is larger than a 3% unit at the same current. The Knowledge Hub article on air core vs. iron core inductors places the gapped iron core between the two extremes it describes.
Often yes, but for a narrower set of reasons. A DC bus choke sits between the rectifier and the bus capacitors and smooths the charging current from the DC side. For harmonic reduction it is roughly equivalent to a 3% to 5% AC line reactor, and a drive that already has one gains little further reduction in current distortion from adding a 3PR-C3L on the input. If the only reason for the reactor is the drive’s contribution to facility harmonics, the choke has already done the work.
What the DC choke cannot do is protect the rectifier diodes from what arrives on the AC side. Utility capacitor-bank switching, lightning-induced surges and notching from adjacent SCR loads all reach the diodes before the choke has any effect. A 3PR line reactor on the AC side limits the rate of rise of those events and reduces nuisance overvoltage trips and diode failures, which is why drive manufacturers continue to recommend an input reactor on drives with a choke.
The practical rule: on a drive with a DC choke, specify a 3% unit for transient protection unless a harmonic study calls for more. On a drive without one, the 3% vs. 5% decision in the Reactors & Filters category FAQ applies; the Power Quality page covers the facility-level question.
A three-phase 3PR can be used on single-phase AC by connecting two of the three coils in series with the two supply conductors and leaving the third coil unconnected. Each coil then carries the full single-phase current, so the reactor must be selected on that current, and the impedance is the sum of the two coils, twice the per-phase value. A 3PR-0017C3L wired this way on a 240 V single-phase drive input presents 6% referenced to the per-coil rating, not 3%, and the selection should be confirmed with Rex. The unused coil is left open, never shorted. For volume single-phase applications, Rex builds a dedicated single-phase reactor, which is smaller than a three-limb unit with one idle coil. Rex designs DC reactors to order when the ripple frequency, DC current and required inductance are stated; the Get a Quote form is the starting point, and the article on custom vs. standard products explains what a non-catalogue design involves.
The Rex 3PR AC Line/Load Reactor is available in open-style or enclosed configurations for indoor use. Open units are designed for installation inside a ventilated panel that provides Type 1 protection, while enclosed units come in a steel enclosure for stand-alone mounting.
Larger enclosed models may be supplied with Type 3R protection. See the Enclosure Drawings for dimensions and available configurations.
A 3PR reactor should not be connected in series with a capacitor bank to form a tuned or detuned filter. A tuned filter needs an inductance that stays within a close tolerance through the harmonic current it absorbs, and it must survive the capacitor’s switching inrush without saturating. The 3PR is designed as a drive reactor: its inductance is specified at rated current for 3% or 5% impedance, and its iron core is guaranteed only to 250% of rated current. Capacitor inrush can be many times that, and a saturating core in a resonant branch detunes the filter and can overheat it. The Rex Air Core Reactor in the Specialty Products category is the product for filter and capacitor duty because its inductance is linear at any current.
Where a drive with a 3PR line reactor and a fixed capacitor bank share a transformer, the concern is parallel resonance between the bank and the source inductance at a frequency near a drive harmonic. The reactor does not cause the resonance, and it reduces the harmonic current the drive injects into it, but it does not remove it. A harmonic study should confirm that the bank is detuned or that the resonant frequency falls away from the 5th and 7th. The article on IEEE 519 harmonics describes this interaction.
Each reactor uses high-grade electrical steel laminations and precision-wound copper or aluminum conductors. The core and coils are dipped in polyester resin to improve mechanical rigidity, insulation integrity, and moisture resistance. This coating reduces vibration and noise while protecting against environmental contaminants. Reactors are air-cooled and designed for continuous-duty service, with thermal and mechanical bracing that ensures stability under short-circuit and harmonic loading conditions.
AC reactors are rated by their inductance (in millihenries) or percentage impedance (%Z) at a given current. Common ratings are 3% or 5% impedance, which determine the degree of harmonic reduction and transient suppression. Rex Power Magnetics engineers select core materials and conductor sizes to maintain linear performance under full-load current without core saturation. Custom ratings and tolerances can be provided for unique system conditions or harmonic profiles.
The inductance is fixed by construction; what changes with system voltage is the percent impedance it represents. A 3PR-0080C5L built for 480 V has a reactance of 0.173 Ω, which is 13.9 V at 80 A, or 5% of 277 V. Applied at 600 V the drop is still 13.9 V, but the line-to-neutral voltage is now 346 V, so the effective impedance is 4.0%. The reactor is thermally and dielectrically fine because both voltages are inside its 600 V class, but a specification that called for 5% is not met.
The reverse case gives more impedance than ordered. A 600 V, 5% unit at 80 A has a reactance of 0.216 Ω; on a 480 V system it drops 17.3 V against 277 V, or 6.25%. Harmonic reduction improves, but the larger drop may pull the drive’s DC bus down at full load. Neither direction risks the core: a series reactor’s flux is set by the current through it, not by system voltage, so the saturation margin is unchanged.
The part number does not carry the voltage, so the system voltage must be stated on the order and checked on the nameplate at receipt. The Part Number Builder records the voltage alongside the part number.
Basic impulse level is a medium-voltage concept. It defines the lightning impulse a winding can withstand and is specified for equipment above 1,000 V, where insulation must be coordinated with surge arresters. Low-voltage dry-type equipment rated to 600 V, including the 3PR reactor, is instead qualified by an applied-potential test at power frequency under the conventions Rex takes from NEMA ST-20 and its CSA and UL certification files. The conventional test for 600 V class equipment applies twice rated voltage plus 1,000 V, on the order of 2,200 V, between all winding terminals connected together and the core and frame for one minute. The winding-to-core insulation of the 3PR reactor is built for that test, and the resin dip adds moisture and tracking resistance along the creepage paths.
Turn-to-turn stress is a separate matter on the load side, where the reactor sees the drive’s PWM edges. The first turns of each coil absorb a disproportionate share of a fast-rising pulse, exactly as a motor winding does, and Rex insulates the 3PR for that duty within the 220°C system; where overshoot needs damping, the Motor Guarding Transient Filter is used.
The BIL ratings article explains where impulse levels do apply; test values for a shipped unit appear on its report, available through Product Documentation.
Within the working range it changes very little, which is the point of the gapped design. Below the 250% saturation threshold the inductance is set by the air gap and is nearly independent of current; the steel contributes a droop of a few percent between light and full load. Temperature affects the winding resistance, which rises about 0.4% per °C for copper, but not the inductance. The reactive impedance of a 3PR-0130C3L at 150°C rise is essentially the same as at room temperature, while its resistive drop and losses are higher.
For a drive reactor the tolerance matters far less than for a tuned filter, because the drive does not care whether it sees 4.7% or 5.3%. A specification should state the percent impedance at rated current, voltage and frequency and, if a tolerance is needed, a symmetrical band that reflects the application rather than an instrument-grade figure; Rex confirms the achievable tolerance at quotation. A specification that calls for a fixed millihenry value from a drive manual should say so, and Rex will build to that value instead of to a nominal percent.
Two conditions do shift the effective impedance: operation on a different voltage than ordered, and operation at 50 Hz. Both change the percent figure without changing the inductance. The article on impedance concepts and calculations covers the arithmetic.
Each reactor uses high-grade electrical steel laminations and precision-wound copper or aluminum conductors. The core and coils are dipped in polyester resin to improve mechanical rigidity, insulation integrity, and moisture resistance. This coating reduces vibration and noise while protecting against environmental contaminants. Reactors are air-cooled and designed for continuous-duty service, with thermal and mechanical bracing that ensures stability under short-circuit and harmonic loading conditions.
AC reactors are rated by their inductance (in millihenries) or percentage impedance (%Z) at a given current. Common ratings are 3% or 5% impedance, which determine the degree of harmonic reduction and transient suppression. Rex Power Magnetics engineers select core materials and conductor sizes to maintain linear performance under full-load current without core saturation. Custom ratings and tolerances can be provided for unique system conditions or harmonic profiles.
Each coil of a 3PR reactor has two terminals, and the six are marked in pairs by phase, with an input designation on one side of the assembly and an output designation on the other. Electrically the reactor is symmetrical: a coil has no polarity, and the unit performs identically whichever end faces the source. The markings keep the three phases on the correct limbs and keep incoming conductors on one side and outgoing conductors on the other.
Three wiring rules do matter. Each phase must pass through its own coil; landing two phases on one coil or bypassing a coil leaves a phase unbuffered. Phase sequence through the reactor should match the rest of the panel so that A, B and C remain consistent at the drive terminals, although the reactor itself is indifferent to rotation. On a load-side installation the input side connects to the drive output terminals and the motor cable leaves from the output side, keeping the unfiltered PWM run short.
Conductor sizing follows the circuit ampacity and the terminal type on the unit, from box terminals on small ratings to compression lugs or bus pads on large ones. The Contact page reaches Rex engineering for a terminal drawing, and the 3-phase wiring diagrams article shows the conventions used on the associated transformers.
The 3PR is cooled by natural air circulation over the coils and core, listed as ANN/AN in the specification. The design assumes the coils are vertical, with the core limbs upright and the coil windows open top and bottom, so that air warmed by the winding rises through the coil and draws cooler air in below. Mounting the reactor with its base horizontal on a floor, shelf or back-plate bracket preserves this convection path. Mounting the unit on its side, with the coils horizontal, reduces the chimney effect and should be reviewed with Rex; on small ratings it is often acceptable at a modest derate, on large units it is not.
Clearance is about airflow and heat. Leave free space above and below the coils, at least the height of the coil window, and do not place a heat-sensitive component directly above the reactor. The surface of a 150°C-rise unit at full load is well above the temperature rating of ordinary control wiring, so conductors not part of the reactor circuit should be routed clear of the coils. A panel fan helps if the airflow passes through the coil windows.
Enclosed units carry ventilation openings sized for the rating, which must not be blocked. The article on thermal management in dry-type equipment explains natural-air cooling classes, and the Enclosure Drawings page shows opening positions on the standard sizes.
Both connections work electrically, with cautions. Two reactors of the same rating in series add their impedances. Two 3PR-0100C3L units built for 480 V, each 0.083 Ω, connected in series give 0.166 Ω, or 6% at 100 A. This is occasionally done to reach a drive manufacturer’s specified inductance from stock parts, and it is acceptable as long as both units are rated for the full circuit current. The combined voltage drop is that of a 6% reactor and must be checked against the drive’s bus margin.
Two identical reactors in parallel share the current and halve the impedance. Two 3PR-0100C5L units in parallel carry 200 A at 5%, the same as a single 3PR-0200C5L. The caution is current sharing: the division depends on closely matched inductance, equal lead lengths and equal connection resistances, and any mismatch loads one unit above its rating. Rex would normally recommend the single larger reactor, which is also smaller and lighter than two units and has one set of terminals; paralleling is a field expedient rather than a design choice.
Mixed ratings should be avoided because the current or voltage division becomes unequal. Where a non-standard impedance is genuinely required, a custom 3PR at the exact value is the correct order, and the Product Configurator accepts a millihenry or non-standard percent value. The principles match those in the article on paralleling.
A generator is a much weaker source than a utility. Its subtransient and transient reactances are typically several times the impedance a utility feeder presents at the same rating, and its voltage regulator responds to load changes in tens of milliseconds rather than instantaneously. A six-pulse drive on such a source draws its peaky rectifier current through the generator’s reactance, which distorts the terminal voltage, confuses the regulator’s sensing and can cause the set to hunt. A 3PR line reactor helps by widening the drive’s conduction angle and lowering the current peaks, which reduces voltage distortion at the generator terminals and steadies the regulator.
The counter-consideration is voltage. The reactor’s reactive drop adds to the generator’s already substantial regulation, and on a set that is close to fully loaded the drive can trip on DC bus undervoltage at full torque. On generator-fed drives the usual specification is therefore 3% rather than 5%, sized on the drive’s rated input current, with the total source-plus-reactor impedance checked against the drive’s undervoltage margin. Where several drives share one generator, one reactor per drive is preferred, because a common reactor gives no decoupling between drives.
The Temporary Power and Mining pages describe the equipment Rex builds for these fleets, frequently ordered with Type 3R enclosures and 50/60 Hz ratings. The power quality in industrial facilities article covers distorted loads on weak sources.
Reactors limit harmonic distortion, reduce motor noise, and improve current waveform quality. They also help achieve compliance with IEEE 519 harmonic distortion limits. Line reactors reduce the risk of nuisance drive tripping due to voltage spikes or sags, while load reactors protect motor insulation from reflected wave voltages. The result is smoother operation, lower maintenance, and extended equipment life for both VFDs and motors.
The effective impedance a reactor presents is proportional to the current through it. At 208 V the selection table lists 50 HP at 160 A and 30 HP at 100 A. A 3PR-0160C5L built for 480 V has a reactance of 0.087 Ω; at 160 A it drops 13.9 V, or 5%, but at the 100 A the 30 HP motor actually draws it drops only 8.7 V, about 3.1%. Sizing the reactor to the drive therefore delivers roughly the performance of a 3% unit, not a 5% unit.
Two considerations pull in opposite directions. Sizing to the actual load current, with a 3PR-0100C5L, delivers the full 5% at the operating point and a smaller reactor. Sizing to the drive rating protects against a future motor change and never risks overloading the reactor. An undersized reactor is a thermal problem; an oversized one is only a performance shortfall.
The recommended practice is to size a line reactor on the drive’s rated input current if the drive may ever be loaded to its rating, and on the motor’s full-load current if the drive is permanently dedicated to that motor, then choose 5% where a 3% effective value would be too little. A load-side reactor is sized on motor current in either case. The Product Configurator accepts both figures, and the sizing and selection article describes the same trade-off for transformers.
It can, and the drive should be tuned with the reactor in the circuit. During an autotune the drive measures stator resistance, leakage inductance and, in a rotating tune, the motor’s magnetizing characteristics. With a 3PR load reactor in place, the drive measures the reactor’s resistance and inductance as part of the stator, which is the correct result as long as the reactor stays in place. If the reactor is added after the tune, the motor model is wrong by the reactor’s impedance, and sensorless vector drives may show poor low-speed torque or unstable current regulation until the tune is repeated.
Sensorless vector control is the mode most sensitive to the reactor, because the drive estimates rotor flux and speed from terminal voltage and current, and a 5% series inductance introduces an error the tune must capture. Most drive manufacturers accept an output reactor in sensorless vector mode up to a stated impedance, commonly 3% to 5%, and prohibit output reactors on high-bandwidth closed-loop modes. A 3% unit is the safer choice for vector applications. Volts-per-hertz control is indifferent to the reactor beyond the voltage drop.
The motor thermal model uses measured current, which the reactor does not change, so overload protection is unaffected. Where the drive manual states a maximum output inductance in millihenries, the Inductance Calculator converts the 3PR’s percent impedance for comparison.
The reactor has losses of its own, but it also lowers losses elsewhere, and the net can go either way. The added losses are the reactor’s copper and core loss, typically a fraction of a percent of drive throughput for a 3PR-C3L and somewhat more for a C5L. The reduced losses come from the change in current shape. A six-pulse drive without series impedance draws a current whose RMS value can be 20% to 40% above the fundamental because of harmonic content, and every upstream element dissipates I²R loss on that RMS current. A 3% reactor cuts the harmonic content substantially, which lowers I²R loss in the whole upstream path and eddy-current loss in the transformer, where harmonics are penalized by the square of their order.
For a drive close to a large supply transformer on short feeders, the upstream savings are small and the reactor is a minor net loss. For a drive at the end of a long feeder, or on a transformer shared with many drives, the reduction in RMS current and transformer harmonic loss can offset the reactor’s losses, and it allows the transformer to be specified at a lower K-factor. The article on understanding losses covers the harmonic loss component, and the Data Centers page describes facilities where the transformer loading side of this calculation dominates.
The two positions solve different problems. A line-side reactor deals with what comes from the supply and what the drive sends back to it: transients, inrush and harmonics. A load-side reactor deals with what the drive sends to the motor: dV/dt, cable-charging current and reflected-wave overshoot. Neither substitutes for the other, so the question is whether both problems are present.
Both are present in a typical municipal pumping station, where drives are fed from a utility with capacitor switching, share a transformer with other drives, and feed remote motors; in a mining conveyor lineup on a weak source with long motor leads; and often in an older plant retrofitting drives onto existing motors and cable. The specification is then a 3PR-C3L or C5L on the input and a 3PR-C3L on the output, or a Motor Guarding Transient Filter on the output if the cable is long enough to need damping. The two need not match; 5% on the line and 3% on the load is a common pairing.
A drive close to its motor on a clean supply needs neither. The cumulative drop of two reactors, up to 8% at full load, should be checked against both the drive’s undervoltage margin and the motor’s voltage tolerance. The Power Quality page describes the system view Rex takes of these installations.
Yes. Rex Power Magnetics specializes in custom line and load reactors tailored to specific drive, voltage, or harmonic requirements. Options include custom inductance values, current ratings, terminal configurations, and enclosure types. Engineering teams perform system-level analysis to optimize performance for harmonic mitigation, voltage waveform improvement, and equipment protection. Each custom reactor is fully tested and certified to ensure compliance and long-term operational reliability.
No, and a simple calculation shows how misleading the linear assumption would be. On a 480 V system with 30 kA available, the source impedance is 277 / 30,000 = 0.0092 Ω per phase. A 3PR-0100C5L has a reactance of 0.139 Ω at 480 V. If the reactor stayed linear, the fault current at the drive would be 277 / (0.0092 + 0.139), about 1.9 kA, a reduction of more than fifteen to one. That is not what happens. The 3PR’s iron core is guaranteed linear only to 250% of rated current, retains half its inductance at 350%, and at a bolted fault of tens of times rated current is fully saturated. The fault current at the drive is therefore very close to the 30 kA available upstream.
The reactor must therefore be ignored in the short-circuit study, in the panel’s short-circuit current rating under UL 508A or CSA C22.2 No. 286, and in the selection of the drive’s semiconductor fuses, which are sized exactly as they would be without it. The reactor needs no overcurrent device of its own; the branch-circuit protection ahead of the drive covers it.
Where a genuine reduction in fault current is required, the correct product is a current-limiting Air Core Reactor, whose inductance holds through the fault. The fuse sizing and short-circuit testing articles give the background.
Yes, as an option specified at order. The standard catalogue reactor has no sensor, because the 220°C insulation system and 150°C rise limit leave a wide margin in a correctly sized application. In sealed panels, in high ambient, or where the reactor is critical to a process, Rex can fit a normally closed thermostat or a resistance temperature sensor to the winding of a 3PR reactor with leads brought to a terminal block. The trip setting is chosen below the insulation limit so the contact opens before the winding reaches a damaging temperature.
The sensor also gives a maintenance reference: a winding temperature that rises over months at constant load signals a loosening connection, blocked ventilation or increased harmonic current. The maintenance checklist article describes how such trends are used, and the Product Configurator records the sensor option.
Start with the current, since most overheating is simply overload. Clamp all three phases with a true-RMS meter at full process load and compare with the nameplate; a reactor selected from the horsepower column rather than the actual current, or a drive later reprogrammed for a higher overload, are common causes. An unbalance between phases points to a supply problem or a coil wired incorrectly. On a load-side unit, ask whether the carrier frequency has been raised since installation, because ripple current and core loss rise with it.
Second, check the connections: thermal cycling loosens terminals, and a single high-resistance joint heats one coil end far above the rest, which a thermal image at load shows immediately.
Third, check the environment: a failed panel fan, blocked ventilation, a reactor relocated onto its side, or a panel ambient above the 40°C basis of the rating.
Fourth, confirm the rating matches the system: a 3PR ordered for 600 V and installed on 480 V has 25% more effective impedance than intended, and a 60 Hz unit on 50 Hz runs at higher flux. Only then suspect a winding fault: compare the three phase resistances and the insulation resistance to frame with the test report. The transformer failures article follows the same logic; a unit that fails these checks should be referred to Rex through the Contact page with its nameplate details.
There are two methods, one under load and one at shutdown. Under load, measure the voltage across each coil, input terminal to output terminal of the same phase, with a true-RMS meter, and the current through that phase at the same time. Voltage divided by current gives the coil impedance in ohms; dividing by the line-to-neutral voltage and multiplying by rated current gives the percent impedance at rating. For a 3PR-0080C5L on 480 V carrying 60 A, the expected coil voltage is 60 × 0.173 = 10.4 V. The reading includes resistive and harmonic voltage, so a result a few percent above the calculated figure is normal. On a load-side reactor the meter must be rated for PWM waveforms.
At shutdown, an LCR meter across each coil gives the inductance directly, to be compared with the value from the Inductance Calculator for the nameplate voltage, current and percent impedance. The three coils should agree closely, and a winding resistance measurement compared against the factory test report completes the check.
A measured impedance much lower than expected under load suggests saturation, meaning current well above rating or the wrong voltage class; a correct inductance at shutdown but a high coil voltage under load indicates a large harmonic or ripple component. The commissioning checks article outlines the general approach, and the test report is available through Product Documentation.
A retrofit reactor is ordered against measured conditions, not the original drawings, and five checks avoid most surprises.
1. Measure the drive’s actual input current (line reactor) or the motor’s running current (load reactor) at full process load with a true-RMS meter, and select from the current rather than the horsepower; a 40 HP drive running a 25 HP load at 208 V is a 3PR-0080 selection, not a 3PR-0130.
2. Confirm the system voltage at the drive terminals, since the part number does not carry it.
3. Look at the drive lineup: whether there is a bypass contactor and where the reactor must sit relative to it, and whether the drive already has a DC choke.
4. Measure the available space and check the panel’s cooling, because a reactor adds a continuous heat source; an enclosed unit outside the panel is often simpler than finding room inside. Dimensions for the selected voltage and impedance are in the Line Reactor catalogue.
5. On a load-side retrofit, record the motor cable length and type and the drive’s carrier frequency, which decide between a load reactor and a Motor Guarding Transient Filter, and plan to re-run the drive’s autotune after installation.