How Do You Start Multiple 6.3 kV Motors on a Weak Supply?

After limiting acceleration current, reducing shaft load, and selecting a transfer method that matches the motor type, multiple 6.3 kV, 50 Hz motors can start without forcing a weak supply beyond its voltage-disturbance limit. The number that matters is not switchgear current alone: it is the source voltage depression produced by starting current, followed by motor heating, acceleration time, and transfer torque.

Starting Current, Voltage Depression, and Heating

A weak power system has enough steady-state capacity to run the motors but insufficient short-circuit strength to tolerate a large starting-current step. Motor current flowing through transformer and system impedance causes a voltage drop at the motor bus. That drop can slow acceleration, disturb other loads, and create visible voltage flicker.

A transformer impedance of 6% was proposed as an estimating assumption. That number alone cannot determine the bus voltage during starting. The calculation also needs transformer rating, upstream source impedance, cable and reactor impedance, existing load, motor current versus speed, and the allowable bus-voltage disturbance.

For a three-phase motor, calculate apparent power at each operating point as:

kVA = sqrt(3) × V_LL × I_line / 1000

Use the actual line current from the VFD study or motor-starting model. Starting duration matters because current produces winding and rotor heating while the motor is accelerating. A reported VFD-assisted start took about one minute while remaining below motor full-load current; another large-motor application required a couple of minutes or more because acceleration was intentionally kept low. Neither duration is a universal limit. Read the permissible start time, starts-per-hour restriction, and thermal capacity from the motor data and protection model.

Quantity	Controlling limit	Where to obtain it
Motor current versus speed	VFD, transformer, cable, and motor thermal capacity	VFD trend, motor-starting study, and motor data
Bus voltage during acceleration	Motor torque and connected-load disturbance	Power-system model and bus voltage recorder
Acceleration time	Motor thermal model and process requirements	Speed trend, protection relay, and driven-equipment data
Transfer mismatch	VFD current, electromagnetic torque, and shaft torque	Synchronizing device, VFD diagnostics, and transient study
Source impedance	Starting voltage drop and flicker	Transformer nameplate, utility data, and short-circuit study

Starting and Transfer Approaches

The supported approaches divide into reduced-voltage line starting, VFD-assisted acceleration with closed transition, VFD-assisted acceleration with open transition, and synchronous-motor overspeed transfer. Their differences appear at the transfer event, not merely during acceleration.

Approach	Starting-load requirement	Transfer characteristic	Primary engineering concern
Reduced-voltage or autotransformer starting	Reduced load is normally required because motor torque falls with applied voltage	Connection changes can create a voltage and torque transient	Whether reduced torque still accelerates the load before thermal capacity is exhausted
VFD closed transition	No load or very light load may be necessary when the VFD is much smaller than the motor	VFD and utility are momentarily connected to the motor together	Voltage magnitude, frequency, phase-angle matching, circulating current, and VFD protection
VFD open transition	Reduced load and low acceleration remain necessary	The VFD disconnects before the utility closes	Residual motor voltage, phase drift, dead time, speed loss, and transient torque
Synchronous-motor overspeed transfer	Motor starts with no load or substantially reduced load	Motor is taken slightly above synchronous speed, disconnected, and allowed to coast to the synchronizing point	Excitation state, coast-down rate, synchronizer performance, and breaker timing

An example concept used a 400 hp VFD to start a 1500 hp motor and then a 600 hp motor. Such ratios make the driven-load torque decisive. The VFD can supply only the torque associated with its available current; it does not acquire the continuous output capability of the larger motor merely because the motor is unloaded.

Recommended Decision Path

For several motors on a weak bus, a shared VFD starter is a practical candidate when each machine can start unloaded or at substantially reduced load and long acceleration is acceptable. Select the transition architecture only after classifying the motors and driven loads.

1. Identify each motor as induction or synchronous. Record rated current, allowable starting time, thermal limits, inertia, load torque versus speed, and permitted starts per period from the motor and driven-equipment documentation.

2. Establish the maximum permissible voltage disturbance at the 6.3 kV bus and at other affected buses. Use the system study rather than switchgear rating as the acceptance criterion.

3. Calculate the VFD current needed to overcome accelerating torque: T_accel = T_motor - T_load. Combine the torque-speed curves with total reflected inertia to obtain acceleration time.

4. For synchronous motors with continuous excitation and suitable process mechanics, evaluate open-transition overspeed and coast-down synchronization first. This avoids intentional parallel operation of the VFD and utility.

5. For a closed-transition proposal, require a VFD explicitly engineered for synchronous transfer. Verify its synchronization, current control, output filtering, isolation, interlocking, and self-protection functions as one coordinated system.

6. For open-transition induction-motor transfer, simulate residual-voltage decay and phase movement during the breaker interval. Select the closing point from the transient study and synchronizing logic rather than speed alone.

7. Compare the selected VFD scheme against reduced-voltage or autotransformer starting using the same source model, motor model, acceleration load, and voltage-disturbance criterion.

Closed-Transition Synchronization

Closed transition temporarily places two energized sources on the same motor terminals. Even with a filtered sinusoidal VFD output, a difference in voltage magnitude, frequency, or phase angle creates current through the combined source and connection impedance. This is current and torque, not logic. A waveform that looks sinusoidal does not prove that the sources are synchronized.

The synchronization function operates on generator-like variables: voltage difference, frequency difference, phase-angle difference, and the predicted breaker closing instant. Because a medium-voltage breaker takes time to close, the controller must compensate for measured breaker operating time and phase movement during that interval. The permitted synchronization window and timing values must come from the VFD transfer design, breaker test data, and transient analysis.

The VFD must tolerate and interrupt abnormal transfer current without exposing its power stage or output filter to utility-source energy beyond their ratings. Interlocks must prevent a closed transition when synchronization quality, VFD readiness, breaker status, protection status, or control power is invalid. A general-purpose VFD plus external voltage-matching logic is not equivalent to a designed synchronous-transfer system.

Open-Transition and Synchronous-Motor Transfer

Open transition removes the direct source-paralleling interval, but it does not remove transfer torque. Once disconnected from the VFD, the motor carries a residual internal voltage whose magnitude and phase evolve with speed, excitation, rotor flux, load torque, and transition time. Closing the utility breaker at a poor electrical angle can produce high current and shaft torque.

A demonstrated synchronous-motor arrangement maintained excitation throughout starting, accelerated the motor slightly above synchronous speed, opened the VFD connection, and allowed the rotor to coast down. A synchronization device then connected the motor to the 6 kV busbar at the synchronizing point. This was an open transition.

That sequence does not transfer automatically to induction motors. An induction motor has no separately controlled rotor excitation, and its residual terminal voltage decays and changes phase after disconnection. For either motor type, measure the actual breaker interval and model the motor during that interval. Mechanical systems with rapidly changing load torque or little speed margin may lose the intended closing point before the breaker completes its operation.

Switchgear and Interlocking Requirements

Switchgear is part of the solution, but it cannot correct inadequate accelerating torque, excessive voltage depression, or a poorly controlled transfer. Specify it from the final topology and study results.

Function	Required design check
Source isolation	Define which VFD-output and utility breakers or contactors may be closed simultaneously for each transfer mode
Mechanical and electrical interlocking	Block prohibited states independently of normal sequence software
Short-circuit duty	Rate equipment for the calculated available fault current at its installation point
Switching duty	Confirm suitability for the expected motor-starting and transfer operations
Protection zones	Coordinate motor, VFD, transformer, bus, cable, and transfer-bus protection
Synchronizing supervision	Permit closing only inside the approved voltage, frequency, phase, and timing window
Failed-transfer response	Trip to a defined isolated state and inhibit automatic retries that exceed motor thermal capacity
Maintenance isolation	Provide a verifiable isolated state for the VFD output, filter, transformer, and motor feeder

The transfer sequence must account for auxiliary-contact disagreement, a breaker that fails to open, a breaker that fails to close, loss of excitation where applicable, VFD trip, and loss of synchronizing inputs. Capture these states in the event record so a failed start can be diagnosed without repeating a thermally stressful test.

Commissioning and Verification

1. Verify phase sequence, voltage-transformer scaling, current-transformer polarity, breaker status feedback, interlocks, and trip paths with the motor isolated.

2. Measure actual opening and closing times for every transfer device. Enter or validate compensation values only through the approved transfer-control procedure.

3. Run the motor uncoupled or at the lowest process load permitted by the machine. Trend VFD output current, motor current, speed, bus voltage, output frequency, and motor thermal-model utilization.

4. Confirm that acceleration remains below the VFD current limit and inside the motor thermal envelope. A longer ramp is acceptable only while available motor torque remains above load torque and cooling assumptions remain valid.

5. Capture voltage on both sides of the transfer point, phase-angle difference, frequency difference, transfer command, breaker auxiliary contacts, motor current, and shaft speed through the switching event.

6. For closed transition, verify the measured transfer lies inside the engineered synchronization window and that no VFD protection or filter limit is approached.

7. For open transition, verify residual-voltage behavior, speed loss, current peak, and torque response against the transient study.

8. Repeat the test at progressively higher permitted load only after reviewing bus depression, acceleration time, motor heating, and transfer records from the preceding test.

9. Test rejected synchronization, failed breaker operation, VFD trip, and loss of sensing. Confirm that the system isolates the motor and records the initiating condition.

Acceptance requires more than a successful breaker close. The start must remain within the power-system voltage criterion, VFD and motor current limits, motor thermal capacity, switchgear duty, and driven-train torque limits.

Frequently Asked Questions