Do not choose one fixed delay for all eight motors. The robust design is a PLC state machine that advances on measured proof that the previous motor has finished accelerating, with a timeout as backup.
For every motor collect FLA, locked-rotor current/code, permitted starts per hour, load torque/inertia, starting method and the manufacturer's acceleration/current curve.
A useful transformer-only screening equation is:
start voltage drop % ≈ (motor locked-rotor A / transformer secondary full-load A) × transformer impedance %
https://www.se.com/ca/en/faqs/FA102209/ uses that relation and suggests roughly 10–12% as a typical desirable maximum at starting. It is only a screening calculation: the final study must include upstream source impedance, cables, already-running motors, power factor and contactor-coil ride-through. ABB notes that a direct-on-line IE3 motor can draw about 6–13× rated current during acceleration, so using only nameplate FLA is unsafe.
An example commissioning rule—not a universal setting—is: RUN auxiliary contact true, phase current below 1.25× FLA for 2 s, and MCC bus voltage above 0.95 pu for 2 s. The maximum starting timeout must remain below the motor's hot locked-rotor withstand time and coordinate with the overload relay.
Use one state per motor and never allow overlapping starts:
IDLE -> START_M1 -> VERIFY_M1 -> START_M2 -> VERIFY_M2 ... -> COMPLETE
VERIFY_Mi:
if RunFB[i] AND Current[i] < StableLimit[i]
AND BusVoltage > Vmin for StableTime[i]:
advance to START_M(i+1)
if StartTimer[i] > MaxStartTime[i]:
stop sequence; latch START_FAIL_i
if overload, phase-loss, undervoltage or E-stop:
go to FAULT
Also enforce minimum off-time and maximum starts/hour, make restart after power loss a deliberate operator choice, and record which permissive blocked each start. If process conditions permit, start the largest motor while the transformer is least loaded; otherwise rank by process priority and validate the worst combination.
This can be implemented in IEC 61131-3 Structured Text or Sequential Function Chart in OpenPLC; hardwired safety and motor protection must remain independent of ordinary PLC logic.
If the voltage study fails, sequencing alone is not enough. Use a soft starter, VFD, wye-delta/autotransformer starter, larger transformer or dedicated feeder. I would not use a pneumatic sequencer for an MCC: it makes diagnostics, interlocking and recovery less deterministic.
Before commissioning, have a qualified power engineer run short-circuit, motor-starting, protection-coordination and arc-flash studies, then tune the thresholds from recorded current and bus-voltage traces.
Do not choose one fixed delay for all eight motors. The robust design is a PLC state machine that advances on measured proof that the previous motor has finished accelerating, with a timeout as backup.
1. Screen the transformer and bus first1. Screen the transformer and bus first
For every motor collect FLA, locked-rotor current/code, permitted starts per hour, load torque/inertia, starting method and the manufacturer's acceleration/current curve.
A useful transformer-only screening equation is:
start voltage drop % ≈ (motor locked-rotor A / transformer secondary full-load A) × transformer impedance %https://www.se.com/ca/en/faqs/FA102209/ uses that relation and suggests roughly 10–12% as a typical desirable maximum at starting. It is only a screening calculation: the final study must include upstream source impedance, cables, already-running motors, power factor and contactor-coil ride-through. ABB notes that a direct-on-line IE3 motor can draw about 6–13× rated current during acceleration, so using only nameplate FLA is unsafe.
2. Derive the delay from acceleration, not guesswork2. Derive the delay from acceleration, not guesswork
For an approximate check:
t_acc ≈ J_total × (ω_final - ω_initial) / (T_motor_avg - T_load_avg)Use the motor/load curve or a measured start trace whenever possible. Then set:
next-start enable = acceleration complete + current-stable time + engineering marginAn example commissioning rule—not a universal setting—is: RUN auxiliary contact true, phase current below 1.25× FLA for 2 s, and MCC bus voltage above 0.95 pu for 2 s. The maximum starting timeout must remain below the motor's hot locked-rotor withstand time and coordinate with the overload relay.
3. PLC sequence3. PLC sequence
Use one state per motor and never allow overlapping starts:
IDLE -> START_M1 -> VERIFY_M1 -> START_M2 -> VERIFY_M2 ... -> COMPLETE VERIFY_Mi: if RunFB[i] AND Current[i] < StableLimit[i] AND BusVoltage > Vmin for StableTime[i]: advance to START_M(i+1) if StartTimer[i] > MaxStartTime[i]: stop sequence; latch START_FAIL_i if overload, phase-loss, undervoltage or E-stop: go to FAULTAlso enforce minimum off-time and maximum starts/hour, make restart after power loss a deliberate operator choice, and record which permissive blocked each start. If process conditions permit, start the largest motor while the transformer is least loaded; otherwise rank by process priority and validate the worst combination.
This can be implemented in IEC 61131-3 Structured Text or Sequential Function Chart in OpenPLC; hardwired safety and motor protection must remain independent of ordinary PLC logic.
If the voltage study fails, sequencing alone is not enough. Use a soft starter, VFD, wye-delta/autotransformer starter, larger transformer or dedicated feeder. I would not use a pneumatic sequencer for an MCC: it makes diagnostics, interlocking and recovery less deterministic.
Before commissioning, have a qualified power engineer run short-circuit, motor-starting, protection-coordination and arc-flash studies, then tune the thresholds from recorded current and bus-voltage traces.