Leroy Somer AVR Replacement Guide: R450, R438, R448 & R120 Voltage Regulators

Q: Can I replace an older Leroy Somer R448 or R438 AVR directly with an R450?

Yes. The R450 is the official backward-compatible successor to both the R448 and R438 in AREP and PMG brushless alternators. The power supply auxiliary winding inputs (X1, X2, Z1, Z2), exciter field outputs (E+, E-), voltage sensing terminals (0-110V-220V-380V), and CT droop connections translate directly. When upgrading, verify that the rotary switch selection for LAM and Under-Frequency roll-off matches your alternator frame size and engine speed (50 Hz / 1500 RPM or 60 Hz / 1800 RPM).

Q: What is the key functional difference between Leroy Somer R120 and R450 AVRs?

The R120 is a dedicated, compact regulator designed exclusively for self-excited (SHUNT) alternators (such as the TAL 040, TAL 042, and TAL 044 frames), providing up to 4.0 A continuous excitation with 2-line sensing. The R450 is a multi-system regulator designed for AREP auxiliary winding, PMG, or SHUNT excitation, capable of delivering 6.0 A continuous and 10 A (max 9.5 A) for 10 seconds with short-circuit ceiling capability (2.9x threshold) and advanced overload delay timing.

Q: How does the AREP auxiliary winding excitation system operate?

AREP (Auxiliary Regulated Excitation Principle) utilizes two independent auxiliary windings inserted in the alternator stator slots: the X1-X2 winding delivers a voltage proportional to the alternator main terminal voltage, while the Z1-Z2 winding delivers a current proportional to the alternator load current. This combination supplies the AVR with intrinsic power boost during sudden motor starting and sustains 300% short-circuit fault current for 10 seconds to trip downstream protective breakers without requiring an external PMG.

Q: How do I calibrate the P3 (Excitation Threshold / I_EXC) potentiometer on the R450?

Run the generator at nominal speed and apply full rated electrical load. Measure the DC excitation current at terminals E+ and E-. Adjust potentiometer P3 clockwise until the green LED remains steadily illuminated. The P3 threshold establishes the continuous excitation limit (adjustable between 1.0 A and 5.5 A). If an overload exceeds 110% of this threshold, an internal 90-second accumulation counter starts (1 count/second); after 90 seconds, the AVR automatically reduces excitation to protect the exciter rotor.

Q: What happens if voltage sensing is lost on an R450 AVR?

The R450 includes an integrated loss-of-sensing safety circuit. If the sensing voltage wires (terminals 0-110V/220V/380V) break or disconnect during operation, the regulator detects the fault and collapses the excitation output within 1 second on AREP/SHUNT configurations (or within 10 seconds on PMG systems), reducing exciter field current to below 1.0 A to prevent dangerous overvoltage.

Q: Are POPULACE compatible Leroy Somer AVRs interchangeable with original units?

Yes. POPULACE AVR-R450, AVR-R438, AVR-R448, and AVR-R120 are engineered as 100% compatible drop-in replacements. They feature identical mounting dimensions, Faston/screw terminal designations, factory-matched potentiometer ranges, heavy-duty semiconductors, and moisture-proof tropicalized potting, backed by a 13-month factory warranty. Each unit undergoes full dynamic bench testing prior to dispatch.

November 3, 2026 Chen, Grace

Maintaining stable terminal voltage and rapid transient recovery under fluctuating electrical loads is essential for diesel generator sets powering industrial, commercial, marine, and critical standby facilities. In brushless alternators equipped with Leroy Somer excitation systems, the Automatic Voltage Regulator (AVR) controls excitation power to hold steady-state voltage regulation within ±0.5% to ±1.0%.

Over decades of alternator production, excitation architectures evolved from traditional self-excited Shunt topologies to dual auxiliary winding systems (AREP) and permanent magnet generators (PMG). When maintaining, retrofitting, or repairing generator sets, field engineers and service technicians frequently face the task of replacing legacy or damaged AVR boards such as the R438 or R448 with the universal R450, or servicing compact shunt units like the R120.

This engineering guide provides a technical comparison of Leroy Somer AVR topologies, terminal conversion mappings, potentiometer tuning workflows, and static multimeter diagnostics based on verified technical manual data.

1. Technical Selection & Comparison Matrix

Selecting the correct replacement AVR requires verifying the excitation system, sensing voltage tap, continuous output current, and short-circuit withstand rating. The matrix below summarizes key parameters for the R450 AVR, R438 AVR, R448 AVR, and R120 AVR.

Engineering Parameter	Leroy Somer R120	Leroy Somer R438	Leroy Somer R448	Leroy Somer R450
Excitation Topology	SHUNT (Self-Excited)	AREP / PMG	AREP / PMG	AREP / PMG / SHUNT
Steady-State Regulation	±1.0%	±0.5%	±0.5%	±0.5%
AC Sensing Input	277V 1-ph / 415V 3-ph (±10%)	95–140V / 170–260V / 340–528V	95–140V / 170–260V / 340–528V	95–140V (110V) / 170–260V (220V) / 340–528V (380V)
Power Supply Input	277 VAC rms (±10%), 50/60 Hz	AREP (X1, X2, Z1, Z2) or PMG	AREP (X1, X2, Z1, Z2) or PMG	Max 150 V, 50/60 Hz (AREP/PMG/Shunt)
Continuous DC Output	4.0 A DC (into ≥15 Ω field)	4.5 A DC (into ≥10 Ω field)	6.0 A DC (into ≥10 Ω field)	6.0 A DC (threshold 1.0–5.5 A adjustable)
10s Overload Output	6.0 A DC	8.0 A DC	10.0 A DC	10.0 A DC (Max 9.5 A, ceiling 2.9× threshold)
Short-Circuit Sustain	None (voltage collapses)	Sustained via AREP/PMG	300% for 10 s (drops to ~1 A)	300% for 10 s (trips to 0.5–0.7 A after 10 s)
Over-Excitation Protection	Setpoint: 60 VDC (10–85 VDC)	Internal electronic trip	Trips to ~1 A after 10 s >10 A	P3 threshold, 110% rating, 90 s accumulation
Loss of Sensing Trip	Integrated detection	Integrated circuit	Integrated circuit	AREP/Shunt: 1 s; PMG: 10 s (current <1 A)
Parallel Operation (Droop)	Not standard	Optional CT input	Module R724 / R725A + CT	P4 Droop (0–8% @ 400V, CT 1A/5A secondary)
Alternator Frame Match	TAL 040, TAL 042, TAL 044	LSA 42.2, 43.2, 44.2	LSA 46.2, 47.2, 49.1	LSA 46.2, 47.2, 49.1, 50.2

For an overview of generator parts cross-referencing across different brands, refer to our Generator Parts Cross Reference Guide.

2. Excitation Topologies: SHUNT vs AREP vs PMG

Understanding the electrical supply architecture is essential when selecting and wiring replacement AVRs on Leroy Somer alternators.

+-----------------------------------------------------------------------------------+
|               LEROY SOMER EXCITATION POWER TOPOLOGY ARCHITECTURE                  |
+-----------------------------------------------------------------------------------+
|                                                                                   |
|  1. SHUNT (R120 / R450 Shunt Mode)                                                |
|     Main Stator (Terminals) ----[Power & Sensing]----> AVR ----> Exciter Stator   |
|     (Simple, economical; excitation collapses during short-circuit or heavy DOL)  |
|                                                                                   |
|  2. AREP (R438 / R448 / R450 AREP Mode)                                           |
|     Aux Windings:                                                                 |
|       X1 - X2 (Voltage Proportional) ---+                                         |
|       Z1 - Z2 (Current Proportional) ---+--> [Power Supply]                       |
|     Main Stator ---------------------------> [Sensing 0-110/220/380V] --> AVR     |
|                                                                          |        |
|                                                                   Exciter Stator  |
|     (Provides 300% short-circuit current for 10s without external PMG pilot)      |
|                                                                                   |
|  3. PMG (R438 / R448 / R450 PMG Mode)                                             |
|     Shaft PMG Rotor/Stator (Independent AC) -> [Power Supply]                     |
|     Main Stator -----------------------------> [Sensing Input] -------> AVR       |
|                                                                          |        |
|                                                                   Exciter Stator  |
|     (Complete electrical isolation between power source and load transients)      |
|                                                                                   |
+-----------------------------------------------------------------------------------+

1. SHUNT Excitation (Self-Excited)

In self-excited setups utilizing the R120 AVR, operating power and voltage sensing are derived directly from the main stator terminals. This provides a compact, cost-effective solution for small standby and prime generators with steady electrical loads. However, if a severe short-circuit or large motor inrush occurs, stator voltage dips sharply, starving the regulator of power and collapsing field excitation.

2. AREP Excitation (Auxiliary Regulated Excitation Principle)

Patented by Leroy Somer, AREP incorporates two dedicated auxiliary windings embedded in the main stator slots:

X1 - X2 Winding (Voltage-Proportional): Delivers power proportional to the alternator output voltage.
Z1 - Z2 Winding (Current-Proportional): Delivers power proportional to load current, functioning as an internal booster during heavy load steps or short-circuits.

Because power is generated internally by magnetic flux rather than terminal voltage, AREP-equipped regulators like the R438, R448, and R450 sustain 300% short-circuit fault current for up to 10 seconds, enabling selective downstream circuit breaker tripping without requiring an external pilot generator.

3. PMG Excitation (Permanent Magnet Generator)

A permanent magnet generator mounted on the non-drive end of the shaft supplies independent AC power to the AVR. PMG isolation guarantees that severe waveform distortion from non-linear VFD or thyristor loads cannot interfere with excitation power rails.

3. Terminal Conversion & Replacement: Upgrading to R450

The R450 AVR serves as the primary drop-in replacement for obsolete R448 and R438 boards. Below is the verified terminal conversion table.

Function	Legacy R448 Terminals	Legacy R438 Terminals	Modern R450 Terminals	Connection Notes
Exciter Field Output (+)	`E+` / `F+`	`E+` / `F+`	`E+`	Connect to exciter stator field positive
Exciter Field Output (-)	`E-` / `F-`	`E-` / `F-`	`E-`	Connect to exciter stator field negative
AREP Aux Winding 1	`X1`, `X2`	`X1`, `X2`	`X1`, `X2`	Voltage auxiliary supply (or PMG lines)
AREP Aux Winding 2	`Z1`, `Z2`	`Z1`, `Z2`	`Z1`, `Z2`	Current auxiliary booster supply
Voltage Sensing Common	`0 V`	`0 V`	`0 V`	Neutral or reference sensing line
Voltage Sensing 110V	`110 V`	`110 V`	`110 V`	Sensing range: 95–140 VAC
Voltage Sensing 220V	`220 V`	`220 V`	`220 V`	Sensing range: 170–260 VAC
Voltage Sensing 380V	`380 V`	`380 V`	`380 V`	Sensing range: 340–528 VAC
Quadrature Droop CT	Ext. Module	Optional CT	`S1`, `S2`	Connect 1A or 5A secondary paralleling CT (<2.5VA)
External Potentiometer	`ST1` / `ST2`	`ST1` / `ST2`	`ST1`, `ST2`	Remove jumper to connect 1 kΩ / 1W external trimmer

For general AVR sensing and wiring fundamentals, consult our detailed Generator AVR Wiring and Sensing Guide.

+-----------------------------------------------------------------------------------+
|                         R450 TERMINAL STRIP LAYOUT                                |
+-----------------------------------------------------------------------------------+
|                                                                                   |
|   [ 0V ] [ 110V ] [ 220V ] [ 380V ]      [ E- ] [ E+ ]      [ S1 ] [ S2 ]         |
|   |------------ SENSING ----------|      |-- FIELD --|      |-- CT DROOP --|      |
|                                                                                   |
|   [ X1 ] [ Z1 ] [ X2 ] [ Z2 ]            [ ST1 ] [ ST2 ]    [ F1 / F2 FUSES ]     |
|   |------- POWER SUPPLY -----|           |- EXT VOLT -|     [ 10A 250V Fuses ]    |
|                                                                                   |
|   +---------------------------------------------------------------------------+   |
|   | ROTARY SWITCH: LAM & U/F Knee Selection (50 Hz / 60 Hz)                   |   |
|   | POTENTIOMETERS: P1 (VOLT)  P2 (STAB)  P3 (I_EXC)  P4 (DROOP)              |   |
|   | LED INDICATORS: Green (Normal/Steady)  Red (Overload/Trip)                |   |
|   +---------------------------------------------------------------------------+   |
|                                                                                   |
+-----------------------------------------------------------------------------------+

4. Potentiometer Calibration & Commissioning Procedure (R450)

When commissioning a newly installed R450 AVR, calibrate potentiometers P1 through P4 in sequence under controlled operating conditions:

+-----------------------------------------------------------------------------------+
|                        R450 POTENTIOMETER COMMISSIONING SEQUENCE                  |
+-----------------------------------------------------------------------------------+
|                                                                                   |
|   STEP 1: Initial Preset                                                          |
|   - P1 (VOLT) counter-clockwise (lowest voltage)                                  |
|   - P2 (STAB) mid-position                                                        |
|   - P3 (I_EXC) clockwise (maximum threshold for initial start)                    |
|   - P4 (DROOP) counter-clockwise (zero droop for single unit)                     |
|                                                                                   |
|   STEP 2: Voltage Adjustment (P1)                                                 |
|   - Start engine, reach rated speed (1500 RPM @ 50 Hz / 1800 RPM @ 60 Hz)         |
|   - Slowly adjust P1 clockwise until target voltage (e.g., 400V L-L) is reached   |
|                                                                                   |
|   STEP 3: Stability Tuning (P2)                                                   |
|   - Turn P2 counter-clockwise until voltage shows slight hunting or oscillation   |
|   - Rotate P2 slightly clockwise past the point of stability (critically damped)  |
|                                                                                   |
|   STEP 4: Excitation Ceiling & Overload Calibration (P3)                          |
|   - Apply full rated load to the generator                                        |
|   - Measure excitation current at E+/E- terminals                                 |
|   - Adjust P3 clockwise until Green LED remains steady at 110% rated excitation   |
|                                                                                   |
|   STEP 5: Quadrature Droop Calibration (P4 - Paralleled Sets Only)                |
|   - Connect 1A or 5A secondary CT across S1/S2                                    |
|   - Apply rated inductive load (PF 0.8) and adjust P4 for 2%–4% voltage droop     |
|                                                                                   |
+-----------------------------------------------------------------------------------+

Protection Thresholds & Delay Timings (P3)

Continuous Threshold Range: Adjustable between 1.0 A and 5.5 A DC.
Over-Excitation Timing: When excitation exceeds 110% of the P3 setting, the internal counter accumulates 1 count per second for 90 seconds. If the overload condition clears before 90 seconds, the counter counts down at the same rate. After 90 seconds, the AVR clamps excitation to the rated limit.
Short-Circuit Trip (10s): During a severe short-circuit, the regulator allows up to 2.9× the threshold current (capped at 9.0 A ± 0.5 A, maximum 9.5 A). If the fault persists for 10 seconds, the AVR automatically cuts excitation current to 0.5–0.7 A, protecting exciter windings from thermal burnout.
Post-Short-Circuit Voltage Limiting: On AREP alternators, voltage is limited to 70% of nominal voltage after a cleared short-circuit until normal steady-state is re-established.

5. Bench Testing & Multimeter Static Checks (R120 & R450)

Before installing a replacement AVR or condemning a suspect board, perform static bench tests using a digital multimeter in diode and resistance modes with the AVR completely disconnected from power.

Static Diode Checks for R120 AVR

F1 to F2 (Diode Test): Connect the positive meter probe to terminal F1 and the negative probe to F2. The measured reading should be between 0.4 kΩ and 0.6 kΩ (forward diode drop). Reversing the probes should indicate Open Circuit (Infinity).
U to N1 (Resistance Test): Resistance between sensing terminal U and neutral N1 should measure between 200 kΩ and 260 kΩ. An open circuit indicates an internal sensing circuit failure.
N to F1 (Resistance Test): Resistance between terminal N and F1 should measure very low or near zero ohms.
Internal Protection Fuse: Verify continuity across the 4 A / 240 VAC ceramic fuse on the board.

Static Diagnostics for R450 AVR

Fuse Check: Verify continuity across both 10 A / 250 V power fuses (F1/X1 and F2/Z2). Blown fuses indicate exciter rotor diode failure, exciter stator short-circuit, or severe auxiliary winding overload.
Power Transistor / Thyristor Check: Check between X1/X2/Z1/Z2 and field outputs E+/E- for shorted internal semiconductors. A zero-ohm reading between power inputs and field output indicates semiconductor puncture.
LED Diagnostic Indicators:
- Green LED On: Normal operation; excitation current is below the continuous threshold.
- Green LED Flashing: Overexcitation timer is actively counting down (load reduced after transient overload).
- Green LED Off / Red LED On: AVR has entered shutdown mode (excitation collapsed to 0.5–0.7 A after a 10s short-circuit or sustained sensing loss).

6. Factory Compatibility & Quality Assurance

POPULACE manufactures precision-engineered compatible replacements for the full range of Leroy Somer voltage regulators, including the AVR R450, AVR R438, AVR R448, and AVR R120.

Every unit is manufactured under ISO9001 quality management, featuring:

100% Dynamic Bench Testing: Verified for voltage regulation accuracy, stability response, UFRO knee points, and short-circuit cutoff timing.
Industrial-Grade Solid-State Components: High-temperature thyristors, metal-film precision potentiometers, and silicone damping encapsulation.
13-Month Factory Warranty: Full repair or replacement warranty support.

To verify AVR compatibility for your specific generator set, email your alternator nameplate details and photo to [email protected] or submit an inquiry through our product pages.

Frequently Asked Questions

Can I replace an older Leroy Somer R448 or R438 AVR directly with an R450?

Yes. The R450 is the official backward-compatible successor to both the R448 and R438 in AREP and PMG brushless alternators. The power supply auxiliary winding inputs (X1, X2, Z1, Z2), exciter field outputs (E+, E-), voltage sensing terminals (0-110V-220V-380V), and CT droop connections translate directly. When upgrading, verify that the rotary switch selection for LAM and Under-Frequency roll-off matches your alternator frame size and engine speed (50 Hz / 1500 RPM or 60 Hz / 1800 RPM).

What is the key functional difference between Leroy Somer R120 and R450 AVRs?

The R120 is a dedicated, compact regulator designed exclusively for self-excited (SHUNT) alternators (such as the TAL 040, TAL 042, and TAL 044 frames), providing up to 4.0 A continuous excitation with 2-line sensing. The R450 is a multi-system regulator designed for AREP auxiliary winding, PMG, or SHUNT excitation, capable of delivering 6.0 A continuous and 10 A (max 9.5 A) for 10 seconds with short-circuit ceiling capability (2.9x threshold) and advanced overload delay timing.

How does the AREP auxiliary winding excitation system operate?

AREP (Auxiliary Regulated Excitation Principle) utilizes two independent auxiliary windings inserted in the alternator stator slots: the X1-X2 winding delivers a voltage proportional to the alternator main terminal voltage, while the Z1-Z2 winding delivers a current proportional to the alternator load current. This combination supplies the AVR with intrinsic power boost during sudden motor starting and sustains 300% short-circuit fault current for 10 seconds to trip downstream protective breakers without requiring an external PMG.

How do I calibrate the P3 (Excitation Threshold / I_EXC) potentiometer on the R450?

Run the generator at nominal speed and apply full rated electrical load. Measure the DC excitation current at terminals E+ and E-. Adjust potentiometer P3 clockwise until the green LED remains steadily illuminated. The P3 threshold establishes the continuous excitation limit (adjustable between 1.0 A and 5.5 A). If an overload exceeds 110% of this threshold, an internal 90-second accumulation counter starts (1 count/second); after 90 seconds, the AVR automatically reduces excitation to protect the exciter rotor.

What happens if voltage sensing is lost on an R450 AVR?

The R450 includes an integrated loss-of-sensing safety circuit. If the sensing voltage wires (terminals 0-110V/220V/380V) break or disconnect during operation, the regulator detects the fault and collapses the excitation output within 1 second on AREP/SHUNT configurations (or within 10 seconds on PMG systems), reducing exciter field current to below 1.0 A to prevent dangerous overvoltage.

Are POPULACE compatible Leroy Somer AVRs interchangeable with original units?