PTQ Q4 2026 Issue

Staged batch restart for motor groups If many motors restart simulta- neously after voltage recovery, the superimposed inrush cur- rent can cause a second severe voltage dip, potentially leading to complete restart failure. To avoid this, motors are restarted in staged batches with differ- ent delay settings. The need for staged restart arises when the total start- ing current of motors that trip simultaneously significantly affects the bus voltage. A practical engineering criterion is that if the expected voltage drop exceeds 15% of rated voltage, staged restart should be applied. For a rigorous analysis, refer to IEEE 3002.7- 2018 IEEE Recommended Practice for Conducting Motor- Starting Studies and Analysis of Industrial and Commercial Power System. The procedure is as follows: u Sort motors by process importance, load characteris- tics, and starting current.

400V/230V,3Φ,50Hz

Restart status signal sent by MPR

L1:1

L1 L2 L3 N

L2:1

L3:1

DCS

-KA1

-MPR

33 32

-X01

L2 L3

-MPR

-TA1

-KA2

PMAC801A

MPR Unit

1 3

5 12 14

96 98

-QF

11 95

-KA3

2 4 6

1L1 3L2 5L3

-KM

2T1 4T2 6T3

U V W

-X02

M 3~

-Motor

Figure 2 Main circuit diagram of the motor. The main circuit includes circuit breaker, contactor KM, MPR, and motor. The MPR integrates protection, measurement, control, and restart functions

v Calculate the capacity and number of motors in the first batch based on the maximum starting current and the bus voltage recovery level. w Start the first batch immediately after voltage returns. x Monitor the bus voltage and total current after the first batch starts. Recalculate the allowable capacity for the next batch. y Assign increasing delay times to subsequent batches. For example, if a motor takes five seconds to start, the first batch restarts at 0.2 seconds, the second batch at 5.2 seconds, the third at 10.2 seconds, and so on, following the same interval. This method prevents excessive voltage drop and has become the most practical approach for motor group restart in petrochemical plants. System design and field application Figure 2 shows the main circuit. The MPR is connected in series with the contactor KM. Figure 3 shows the control circuit. The MPR provides a combined output contact (ter- minals 29-30) for restart. When a voltage sag occurs, KM releases. The MPR closes its restart output for 500 ms. If power returns within 500 ms, KM recloses immediately. For motors on a normal bus, if power does not return within 500 ms, the restart output opens. The MPR then waits for power restoration within a longer window (e.g., three seconds). If power returns within three seconds to a voltage above the threshold, the MPR issues a closing

command (possibly delayed for batching). If the interrup- tion exceeds three seconds, the MPR treats it as an external fault and inhibits restart. For motors on an emergency bus with diesel generator backup, the restart delay includes generator start-up and transfer switch time. A typical setting is: allow power loss up to 15 seconds; if voltage recovers to ≥85% of rated volt - age within 15 seconds, the MPR performs delayed batch restart; beyond 15 seconds, no restart is performed. Avoid dual-pump operation In typical petrochemical processes, pumps A and B are in mutual standby. The distributed control system (DCS) mon- itors contactor auxiliary contacts to determine motor status. After a voltage sag, both pumps may trip. To avoid starting both simultaneously, the MPR provides a status signal con- tact (terminals 32-33) to the DCS. During the allowable restart time window, this status contact remains closed (indicating the motor was running before the sag), locking the standby pump. Only after the allowed time expires does the contact open, allowing the DCS to start the standby unit if needed. Energy storage component inside the MPR The MPR shares power with the control circuit and does not require a dedicated auxiliary power supply. To maintain

PTQ Q4 2026