A line reactor goes on the input of the drive, between the supply and the drive. A load reactor goes on the output, between the drive and the motor. It is the same kind of three phase inductor. Where you put it decides the job. On the input it cuts harmonics and shields the drive from line spikes. On the output it slows the voltage edges and cuts the voltage spikes at the end of a long motor cable.
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Side by side
| Line reactor (drive input) | Load reactor (drive output) | |
|---|---|---|
| Wired to | Drive input terminals R/L1, S/L2, T/L3 | Drive output terminals U/T1, V/T2, W/T3, then on to the motor |
| Main job | Cuts line current harmonics, soaks up line spikes, cuts nuisance trips | Slows dv/dt, cuts peak voltage at the motor, cuts cable charging current |
| When the drive maker calls for it | TECO A510 and F510 manuals: install an AC reactor on the input when the supply is 600 kVA or more. It also improves power factor | Long motor cables. Rex says output reactors work on the high frequency effects of long cables |
| Peak voltage at the motor on a long cable | No effect | About 200% of applied voltage with no reactor, typically under 150% with a reactor alone (Rex) |
| Usual impedance | 2% or 3%; 4% to 5% for heavy transients or tighter harmonic limits (Rex) | 3% or 5% |
| Where to mount | Ahead of the drive, inside the panel | Right next to the drive output (Rex note) |
| Size it by | Drive input current at your voltage | Drive output current (or the motor current) at your voltage |
| Lines on this site | Block LR3 and Rex 3PR | Rex 3PR |
Sources: TECO Westinghouse A510 and F510 instruction manuals, section 3.15 (AC line reactor) and table 3.14.1 (cable length vs carrier frequency). Rex Power Magnetics line reactor catalog, input and output reactor sections. Block LR3 product data.
Worked example: 40 HP, 480 V drive, 250 ft of motor cable
| Step | Check | Result |
|---|---|---|
| Supply size | Plant transformer is 1,000 kVA. TECO rule: 600 kVA or more | Add a line reactor on the input |
| Line reactor pick | Rex 480 V table: 40 HP is a 60 A reactor. Block cross puts LR3 48-3/63 on 40 HP | Rex 3PR-0060C3H (3%) or Block LR3 48-3/63 (3%) |
| Carrier frequency | 250 ft is in the 166 to 328 ft band of the TECO table | Set parameter 11-01 to 5 kHz max |
| Load reactor | Long cable, motor peak voltage is the risk | Rex 3PR-0060C3H on the output, mounted next to the drive |
| Peak at the motor | Rex figures for a long cable | About 200% without, typically under 150% with the reactor. Under 125% needs a Rex Motor Guarding filter |
| Panel heat | Each reactor adds watts to the panel | Add the reactor loss to the heat load calculator |
Math notes: impedance % is the voltage drop across the reactor at rated current as a share of the supply voltage (Rex). A 5% reactor on a 400 V line drops about 11.6 V per phase at full current (Block LR3 40-5/63 data). Send the drive model, voltage and cable length and we confirm the part.
Reactor part numbers on this site
| Part number | Brand | Use | Rating |
|---|---|---|---|
| LR3 48-3/63 | Block | Line (input) | 63 A, 3% at 480 V, 40 HP |
| LR3 48-5/63 | Block | Line (input) | 63 A, 5% at 480 V, 40 HP |
| LR3 48-3/125 | Block | Line (input) | 125 A, 3% at 480 V, 100 HP |
| 3PR-0027C3H | Rex | Line or load | 27 A, 3%, 480/600 V, 15 to 20 HP at 480 V |
| 3PR-0060C3H | Rex | Line or load | 60 A, 3%, 480/600 V, 40 HP at 480 V |
| 3PR-0060C5H | Rex | Line or load | 60 A, 5%, 480/600 V, 40 HP at 480 V |
| 3PR-0080C3H | Rex | Line or load | 80 A, 3%, 480/600 V, 50 to 60 HP at 480 V |
| 3PR-0034C3L | Rex | Line or load | 34 A, 3%, 208/240 V, 10 HP at 208 V |
All Block LR3 reactors are in the Block collection and all Rex 3PR reactors are in the Rex Power Magnetics collection. Replacing a Rockwell 1321, MTE or TCI reactor? Use the line reactor cross reference.
HP ratings are from the Rex selection tables and the Block USA cross reference shown on each product page.
Common questions
Can one reactor do both jobs?
Not at the same time. If you need both, buy two. Rex rates its 3PR reactors for the input or the output of a drive. Block lists the LR3 as a line, commutating or power factor reactor, so we put it on the input.
3% or 5%?
Rex says most line reactor jobs need 2% or 3%. Go to 4% or 5% when the line has heavy spikes or you need cleaner line current. The trade is voltage drop: a 5% reactor drops about 5% of the voltage at full current.
My drive has a DC choke. Do I still need a line reactor?
Maybe. A DC choke and an AC line reactor both cut line harmonics, and Rex shows they stack. TECO still calls for an input AC reactor when the supply is 600 kVA or more.
How long can the motor cable be before I need a load reactor?
TECO says to lower the carrier frequency once the cable passes 25 m (82 ft), and caps it at 2 kHz past 329 ft. Long runs also make the voltage spikes at the motor that a load reactor cuts. Send the cable length and motor insulation rating and we size it with you. More in long motor leads on a VFD.
Where does the load reactor go?
Right at the drive output, not at the motor. Rex calls this out in its catalog. Leave room for heat and add its watts to the panel heat load.
Get it sized and quoted
Send the drive model or HP, the supply voltage, the plant transformer kVA and the motor cable length. We send back the Block or Rex part with lead time.
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Related: Long motor leads on a VFD · Line reactor cross reference · TECO AC drives · Block · Rex Power Magnetics · Heat load calculator · All cross references
TECO Westinghouse, Block and Rex Power Magnetics data are from each maker's published manual or catalog. Rockwell Automation, MTE and TCI are trademarks of their respective owners. Applied Control Inc. is not affiliated with them. Check the current manufacturer datasheet before you specify.