Locked Rotor Relays

Locked Rotor Relays (or Functions) protect electric motors from damage by detecting when the rotor stops turning (locks) during startup, a dangerous condition where current stays extremely high, potentially melting windings. These relays monitor high starting current, allowing a brief delay for normal startup but tripping the motor if current remains excessive for too long, preventing overheating and catastrophic failure. They are often part of advanced motor protection relays, offering adjustable settings for current and time to match specific motor needs, protecting against issues like jammed impellers or bearing failures

This video provides a simple explanation of locked rotor protection:

How They Work

• Monitors Current: The relay watches the motor's current, especially during the high-current startup period (locked rotor current, often 5-8 times normal).
• Time Delay: A set time delay (e.g., 5-10 seconds) prevents nuisance trips from normal starting inrush.
• Trip Action: If the high current persists beyond the set time, indicating a locked rotor, the relay trips the motor's power supply.

You can watch this video to learn more about the locked rotor current:

 Key Features & Benefits

• Protects Against Mechanical Issues: Detects problems like jammed pumps, bad bearings, or shaft misalignment.
• Prevents Winding Failure: Stops excessive heat buildup that can melt rotor bars or damage insulation.
• Adjustable Settings: Electronic versions allow tuning of current thresholds and time delays for optimal protection.
• Integrated Protection: Often combined with overload (Function 49) and phase fault (Function 50) protection in one device (Type 49/50/51 relay)

Common Applications

• Industrial motors in pumps, fans, conveyors, and compressors.
• Low-voltage motor control centers (MCCs).

Examples

• Schneider Electric Tesys T
• Siemens 3RV
• Eaton PXR
• GE Multilin 269

A locked rotor relay is a protection device that safeguards electric motors from damage by detecting when the motor's rotor fails to rotate or is otherwise mechanically locked while power is applied. The relay monitors for high current that persists beyond the normal startup period and then disconnects power to prevent overheating and burnout.

Function and Operation

When an electric motor starts, it draws a temporary, high "inrush" or locked rotor current, typically several times its normal running current, because there is no back electromotive force (EMF) generated by a spinning rotor to oppose the applied voltage. As the motor accelerates, the back EMF increases, impedance rises, and the current naturally drops to the normal running level.