Model Series: S74D Series (Motor Protection Relays)

Technical Specifications:

• Rated Current (In): 1 A to 10 A (selectable via DIP switches)
• Voltage Range: 100V AC to 690V AC (3-phase)
• Undervoltage Setting (V00): 270V to 440V (adjustable, default 380V)
• Overvoltage Setting (F0V0): 400V to 500V (adjustable, default 440V)
• Frequency Range: 45 Hz to 65 Hz
• Output Contacts: 2 × Changeover Relay (8A / 250V AC) + 1 × Alarm Contact (NO, 5A)
• Accuracy: ±1% of setting value
• Response Time: <50 ms (undervoltage) / <100 ms (overvoltage)
• Operating Temperature: -25°C to +60°C
• Protection Rating: IP20 (front panel: IP40 with cover)
• Mounting: DIN rail 35 mm (EN 60715)
• Power Supply: 100–240V AC/DC (auxiliary supply)
• Dimensions: 90 mm × 70 mm × 62 mm
• Weight: 280 g
• Standards Compliance: IEC 60255-1, IEC 60947-6-2, UL 508

Functional Features:

• Adjustable undervoltage (V00) trip — motor disconnects when voltage drops below setpoint
• Adjustable overvoltage (F0V0) trip — motor disconnects when voltage exceeds setpoint
• Phase failure / phase reversal detection
• Thermal motor protection simulation (I²t curve)
• Lockout / restart inhibition timer (adjustable 0–60 s)
• Event log memory — stores last 10 fault events with timestamps
• Front-panel LED indicators: Power, Run, Fault, Alarm

Structural Characteristics:

• Modular DIN rail housing with snap-in mounting
• Front cover with transparent window for LED visibility
• Terminal blocks: Screw terminals (2.5 mm² max) for all connections
• DIP switches on front panel for parameter setting

Working Principle:
The relay continuously monitors three-phase voltage via internal voltage dividers. The ADC (16-bit) samples voltage at 2 kHz. The MCU compares instantaneous values against programmed thresholds (V00 for undervoltage, F0V0 for overvoltage). If a fault condition persists beyond the time delay setting, the MCU de-energizes the output relay, disconnecting the motor contactor. The I²t thermal model calculates accumulated heat from overcurrent to simulate motor winding temperature.

Advantages & Highlights:

• Dual voltage protection (under + over) in a single device — eliminates need for separate relays
• Event logging enables root-cause analysis without external recorder
• Wide voltage range (100–690V) — one relay for all motor sizes
• DIP-switch programming — no PC software required for basic setup

Applicable Industries:

• Industrial motor protection (pumps, compressors, fans, conveyors)
• Commercial HVAC (chillers, AHUs, cooling towers)
• Water treatment plants
• Mining and heavy industry

Installation Requirements:

• Mount on DIN rail TS35 in control panel
• Connect voltage sensing leads to L1, L2, L3 (phase-to-phase)
• Connect output relay contacts in series with motor contactor coil
• Use auxiliary supply (100–240V AC/DC) for relay power — separate from motor voltage

Usage Precautions:

• Set undervoltage threshold 10% above minimum motor operating voltage
• Set overvoltage threshold 5% below motor insulation rating
• Do not exceed 10A rated current through sensing circuit
• Reset faults only after verifying root cause — use lockout timer for safety