A start relay (also called a starting relay or motor start relay) is an electromechanical or electronic component used in single-phase electric motors to temporarily engage the motor's start winding during startup. It plays a critical role in ensuring motors achieve sufficient torque to start while protecting them from damage caused by prolonged current surges.

1. Primary Function

The start relay enables single-phase induction motors to generate the rotating magnetic field required for startup. Unlike three-phase motors, single-phase motors lack inherent rotational force, so they rely on a secondary "start winding" to initiate rotation. The relay:

• Connects the start winding circuit when power is applied

• Disconnects the start winding once the motor reaches ~75% of its rated speed

• Prevents overheating by limiting start-winding activation time

2. Types of Start Relays

a. Electromechanical Relays

• Current Relay:

• Uses the motor's inrush current to energize a coil, closing contacts to engage the start winding.

• Deactivates when current drops as the motor approaches operating speed.

• Common in refrigerators and small compressors.

• Potential Relay:

• Activated by back electromotive force (back EMF) generated by the motor.

• More precise disengagement timing; often used in HVAC systems and large compressors.

b. Solid-State Relays (SSRs)

• Semiconductor-based (e.g., thyristors) with no moving parts.

• Faster switching, longer lifespan, and quieter operation.

• Common in modern appliances like washing machines.

c. Hot-Wire Relay

• Older design using a resistive wire that expands with heat to break the circuit.

• Rarely used today due to reliability issues.

3. Key Components

• Coil: Generates a magnetic field when energized.

• Contacts: Close or open to control the start winding circuit.

• Spring Mechanism: Returns contacts to default position after deactivation.

• Terminals: Connect to the motor's power supply, start winding, and run winding.

4. Working Principle

1. Power On: Current flows through the relay coil, creating a magnetic field.

2. Contact Closure: The magnetic field pulls contacts closed, completing the start winding circuit.

3. Motor Acceleration: The motor gains speed, reducing current draw (for current relays) or generating back EMF (for potential relays).

4. Disengagement: The relay opens the contacts, disconnecting the start winding. The motor continues running on the main (run) winding.

5. Applications

• Refrigeration Systems: Compressors in refrigerators/freezers.

• HVAC Equipment: Air conditioners, heat pumps, and fans.

• Power Tools: Table saws, compressors.

• Appliances: Washing machines, pool pumps.

6. Failure Symptoms

• Motor hums but doesn't start.

• Intermittent operation or delayed startup.

• Overheating due to a stuck-closed relay.

• Burnt smell or visible damage to relay components.

7. Testing & Replacement

1. Visual Inspection: Check for burnt marks, cracked housing, or loose connections.

2. Multimeter Test:

• Measure continuity across terminals when power is applied.

• Verify resistance values match manufacturer specifications.

3. Replacement: Match relay type (current/potential), voltage rating, and terminal configuration.

8. Advantages of Start Relays

• Prevents motor burnout by limiting start-winding activation.

• Reduces energy consumption during operation.

• Extends motor lifespan by minimizing heat generation.

9. Limitations

• Electromechanical relays wear out over time due to arcing contacts.

• Incorrect relay selection can cause premature failure or poor motor performance.

10. Comparison with Run Relays

11. Industry Standards

• UL 508: Safety certification for industrial control equipment.

• IEC 61810: Specifications for electromechanical relays.

• NEMA ICS 2: Standards for motor starters and controllers.

12. Future Trends

• IoT Integration: Relays with sensors for predictive maintenance.

• Wide-Bandgap Semiconductors: More efficient solid-state relays using SiC/GaN.

• Miniaturization: Compact designs for portable devices.

Conclusion

Start relays are indispensable for single-phase motor operation, balancing startup torque requirements with long-term reliability. As motor technology evolves, solid-state and smart relays are increasingly replacing traditional electromechanical designs, offering enhanced efficiency and diagnostic capabilities.