PTQ Q4 2026 Issue

The proposed method uses three sequential stages: arming (ready), discharge (blocking), and action (trigger). The arming condition ensures the motor is in stable normal operation before enabling the restart function. All of the follow- ing must be true simultaneously: • Undervoltage restart control is enabled in the MPR. • Actual motor current >10% of rated current (motor is running). • Bus voltage > preset restart voltage threshold (e.g., 85% of rated voltage). • Contactor is closed (normal operating position). When these conditions hold for a short, fixed delay, the restart logic becomes armed and enters standby. The restart function is imme- diately discharged (blocked) if any of the following occurs:

Power loss restart control enabled

In > 10% rated current

Power loss restart ready (Restart arming)

0.5s

Un > recovery voltage Contractor status = 1

Motor protection trip Motor trip command recieved

Un < 50% rated voltage In < 10% rated current Contractor status = 0

0.5s

The delay time in delay restart

Start motor

Power loss time < the instant restart setting

Un > recovery voltage setting

Power loss restart ready Un < 50% rated voltage In < 10% rated current Contractor status = 0

The power loss time in delay restart

Power loss time > the delay restart setting Un > recovery voltage setting

• The MPR issues a trip com- mand due to overload, phase loss, stall, earth fault, or other electrical fault. • A manual or remote trip com- mand is received. This prevents restart after an intentional stop or fault trip. The device starts timing the power-loss duration only when all three of the following conditions are met: • Motor current <10% of rated current (motor no longer draws power). • Bus voltage <50% of rated voltage (sag or interruption). • Contactor is open (has dropped out). The dual criteria (voltage and current) reliably distin- guish a voltage sag from a normal open command: during a normal stop, the current drops to zero while the voltage remains healthy, whereas during a sag, both voltage and current drop together. After the arming condition has been satisfied and the action condition is triggered, the MPR measures the power- loss duration. When the bus voltage recovers above the restart voltage threshold, then adhere to the following options: • If power-loss time < instant restart setting, then restart immediately. • If power-loss time is between instant restart setting and delayed restart setting, then restart after a preset delay. • If power-loss time > delayed restart setting, then cancel restart (no output). The undervoltage restart function can work together with undervoltage protection (see Figure 1 ). If the power loss exceeds the allowed time, the MPR can issue an undervolt- age trip and start a standby motor.

Do not start motor (Restart discharging)

Figure 1 Logic diagram of undervoltage restart

only criterion and cannot distinguish a voltage sag from a normal open command, leading to misoperation (restart after manual stop). Additional wiring and an external power supply add failure points. Most such relays lack communi- cation, protection functions, and internal energy storage, making them unsuitable for high-reliability plants. Modern low-voltage MPRs have replaced traditional thermal relays in many petrochemical plants. They provide complete protection (overload, phase loss, stall, undervolt- age, earth fault) and can embed undervoltage restart logic without external add-ons. Most qualified MPRs carry CE certification, meeting European and international standards and can be applied in global markets, including Europe, Asia, and Africa. Advantages when using MPRs include simple wiring, fewer components in the motor drawer, good ventilation, high reliability, and standard communication interfaces (such as Modbus, Profibus) for plant-wide monitoring. Undervoltage restart logic based on MPR When motors start from standstill, they draw a large inrush current that can depress the bus voltage and even cause undervoltage protection to trip. However, a motor that was running and loses power continues to rotate due to inertia. Restarting such a motor before it stops completely draws much less current and does not severely impact the grid. Therefore, the logic must differentiate between a brief sag (motor still spinning) and a long interruption (motor stopped).

PTQ Q4 2026