Introduction
HVAC systems depend on motors—compressor motors, condenser fan motors, blower motors, pump motors—to move air and refrigerant efficiently. These motors face fluctuating loads, high ambient temperatures, voltage imbalance, frequent starts, and occasional mechanical jams. An overload relay (OLR) is the first line of defense against excessive current that would otherwise overheat and damage windings, bearings, and insulation.
This guide explains how overload relays work in HVAC, where they fit in typical circuits, how to size and set them correctly, the difference between thermal and electronic designs, commissioning tips, and a practical checklist. We’ll also outline why BCH Electric is a popular choice among Indian OEMs and MEP contractors looking for robust, standards-compliant overload protection.
Quick Primer: What an Overload Relay Does
An overload relay monitors motor current and trips the control circuit if current exceeds a set value for long enough to risk overheating. Key points:
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Protects against overload heating (not short circuits). Short circuits are handled by fuses/MCBs/MCCBs.
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Mimics motor thermal behavior with time-current characteristics (Trip Class 10/20/30).
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Coordinates with contactors/starters to open the motor circuit safely.
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Prevents single-phasing damage (phase-loss sensitivity in many OLRs).
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Supports manual/auto reset and auxiliary contacts for alarms/BMS.
Where Overload Relays Live in HVAC Circuits
Common placements:
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Compressor Motor Starter (largest load; star-delta, DOL, or VFD): OLR sits downstream of the contactor, upstream of the motor.
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Condenser Fan Motors (multiple fans): One OLR per motor or per group, depending on design.
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AHU/FCU Blower Motors: OLR protects against clogged filters/duct restrictions that increase load.
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Chilled/Condenser Water Pumps & Cooling Tower Fans: OLRs mitigate jam/overload on driveshafts and impellers.
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Make-Up Air & Exhaust Systems: OLRs integrate with interlocks (airflow switches, damper limits).
With BMS integration, OLR auxiliary contacts provide trip status to trending/alarm dashboards, aiding predictive maintenance.
Why Overloads Are Common in HVAC
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Dirty coils & filters → higher static pressure → blower overload
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Refrigerant issues (overcharge, non-condensables) → compressor amperage spikes
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Mechanical jams (pump bearings/fan belts)
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Voltage imbalance in large sites → motor heating even at nominal current
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Frequent cycling → thermal stress
An appropriately selected and calibrated OLR avoids nuisance trips while preventing thermal damage.
Thermal vs Electronic Overload Relays for HVAC
Thermal Overload Relays
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Bimetal technology; simple and cost-effective
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Ambient-sensitive (choose designs with ambient compensation when panels run hot)
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Trip Classes typically 10/20
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Good for standard fans, blowers, pumps
Electronic (Solid-State) Overload Relays
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Current transformers + electronics provide accurate sensing
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Programmable: finer set ranges, better repeatability, diagnostics
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Features: phase loss/imbalance detection, jam protection, stall protection, ground fault (in some models), selectable trip classes (10/20/30)
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Ideal for compressors, multi-speed/VFD applications, and critical uptime systems
Tip: For compressor motors and premium efficiency IE3/IE4 motors (lower thermal time constants), electronic OLRs offer superior protection and fewer nuisance trips.
Coordinating OLRs with VFDs in HVAC
Many HVAC motors run on VFDs for energy savings. Protection options:
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VFD Electronic Protection Only: Uses drive’s internal overload model (good, but ensure parameterization matches motor nameplate).
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VFD + External OLR: Common in conservative designs; OLR provides independent protection and BMS signaling.
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Settings: Ensure VFD current limit, accel/decel ramps, and OLR set-current (FLC) are coordinated to avoid false trips.
Selecting the Right Overload Relay: A Practical Method
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Start with Nameplate: Motor kW/HP, voltage, FLC (Full Load Current), service factor.
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Duty & Trip Class:
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Class 10: Standard fans/pumps, frequent starts not expected
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Class 20: Heavier inertia (large blowers/pumps)
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Class 30: Very high inertia or tough starts (use sparingly to avoid overheating)
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Environment: Panel temperature, humidity, dust, altitude—prefer ambient-compensated designs.
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Phase Loss Sensitivity: Mandatory for HVAC motors in three-phase systems.
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Adjustment Range: Choose an OLR where your motor FLC lies mid-range, not at the extreme end.
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Coordination: Match with upstream breaker (short-circuit) and contactor (utilization category AC-3/AC-4).
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Standards: Look for compliance with IEC 60947-4-1 (contactors & motor starters), and relevant Indian standards.
Setting & Commissioning: Step-by-Step
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Mechanicals first: Free-spin the fan/pump; check belts, bearings, alignment; clean coils/filters.
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Wiring: Correct line/load, no shared neutrals, tight terminations, correct CT orientation (electronic OLR).
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Set OLR current: Typically 0.9–1.0 × motor FLC (adjust per service factor and site SOP).
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Trip Class: Choose 10/20/30 per application; verify during test run.
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Functional test:
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Run motor to normal load; confirm current and temperature rise.
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Simulate overload or use test button—verify trip and BMS alarm.
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Reset mode: Manual reset for critical equipment; auto reset only with interlocks to prevent rapid restarts.
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Documentation: Record settings, measured currents, panel ambient, and commissioning signatures.
Troubleshooting Nuisance Trips
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High current but normal load → Check voltage imbalance, loose phases, terminal heating, undersized cable, VFD parameters.
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Trips on start → Increase trip class (10→20), confirm inrush & ramp times, check mechanical binding.
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Random trips → Ambient heat inside panel, inadequate ventilation, mis-set dial, poor contactor contacts.
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Frequent compressor trips → Investigate refrigerant circuit (superheat/subcool), condenser cleanliness, fan operation.
Maintenance Checklist for Facility Teams
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Monthly: Inspect filters, coils, belt tension, fan blades; log motor currents vs. baseline.
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Quarterly: Torque check on terminations; panel dust removal.
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Half-yearly: Verify OLR setpoint vs. nameplate; re-test trip & BMS signaling; review VFD logs.
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Annually: IR/Polarization tests on motors; thermography on panels; recalibrate electronic OLRs if required.
ROI: Downtime Avoidance & Energy Impact
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Prevented motor burnouts (₹₹₹ saved on rewinds/replacements)
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Avoided production or comfort downtime (lost hours & penalties)
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Lower energy waste by catching mechanical drag early (rising current before failure)
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Data for preventive maintenance (with electronic OLRs feeding BMS/SCADA)
Why Many Indian HVAC Projects Specify BCH Electric Overload Relays
Positioning note: The following highlights focus on engineering considerations that specifiers often look for. Evaluate against your project specs and site conditions.
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Robust portfolio: Thermal and electronic OLRs covering common HVAC current ranges, with phase-loss sensitivity and selectable Trip Class options.
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Ambient-compensated designs: Better stability in hot Indian panel rooms and rooftop enclosures.
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Clean coordination with BCH contactors/starters and common MCCB/MCB protection schemes used in India.
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Ease of setting & service: Clear dials, status flags, test buttons, and auxiliary contacts for BMS alarm loops.
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Standards-compliant: Designed to meet relevant IEC motor-starter requirements aligned with Indian practice.
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Local application support: Readily available across major metros and industrial clusters, simplifying spares and maintenance.
If you’re standardizing across multiple sites—malls, hospitals, IT parks—specifying a single family of OLRs and contactors (e.g., BCH) simplifies spares, training, and documentation.
Sample Spec Snippet (Edit for Your BOQ)
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Overload Relay: Ambient-compensated, phase-loss sensitive, adjustable 0.8–1.25× FLC, Trip Class 10/20 selectable, manual/auto reset, 1 NO + 1 NC aux contacts, IEC 60947-4-1 compliant.
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Coordination: With AC-3 contactor sized for motor FLC; upstream MCCB per prospective short-circuit current; discrimination verified.
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Integration: Aux NC wired to BMS alarm; trip indication on panel mimic; test button accessible.
Case Snapshot: Fixing a Blower That Trips Daily
Symptoms: AHU blower motor tripped OLR mid-day.
Findings: Filter differential pressure high; panel ambient 48 °C; OLR set at 1.0× FLC, Trip Class 10.
Fix: Replaced filters, added panel exhaust fan, changed to Trip Class 20, reset to 0.95× FLC.
Result: No nuisance trips; current dropped ~6%; BMS alarms retained via OLR aux.
FAQs
Q1. Do I still need an OLR if I have a VFD?
Yes—either use the VFD’s internal thermal model correctly configured or an external OLR (many facilities keep both for redundancy and independent alarming).
Q2. What Trip Class should I use for compressors?
Start with Class 20; validate against starting profile and manufacturer guidance. Move to Class 10 for light loads or Class 30 for very high inertia—but monitor heating.
Q3. Why does the OLR trip while current is below FLC?
Voltage imbalance or single-phasing can overheat a motor even at nominal current. Use phase-loss sensitive OLRs and check supply quality.
Conclusion & CTA
Overload relays are small, inexpensive devices that protect the most expensive parts of your HVAC—the motors. Right selection, proper settings, and periodic testing cut failures, stabilize comfort, and lower lifecycle costs.
If you’re designing or retrofitting HVAC panels for Indian conditions, BCH Electric offers a dependable range of thermal and electronic overload relays that pair cleanly with contactors and VFD-centric designs—making standardization and maintenance simpler across projects.
