CALCULATION
Overview
Challenge
A motor starting study is used to evaluate the behavior of the power system and motor-driven loads during transient conditions associated with motor starting and supply restoration after a disturbance.
It allows to:
- determine whether a motor can start successfully
- assess voltage dips in the network
- verify the impact of motor starting on other loads
- evaluate the possibility of motor reacceleration after voltage recovery
What it means
Motor starting is one of the most severe transient conditions in an industrial power system.
At that moment, the motor draws elevated current, which may lead to:
- significant voltage dip
- slow motor acceleration
- disturbance of other connected equipment
- unsuccessful motor reacceleration after a fault-related outage
In practical terms, this study shows whether the motor can start reliably and whether the system can maintain stable operation during starting or voltage recovery.
Applications
- design of power systems with large motor loads
- selection of motor starting method
- verification of motor reacceleration after short supply interruptions
- assessment of load stability during motor starting
- modernization of existing power system configurations
- reduction of process shutdown risks
Conditions Considered
The study may include:
- direct-on-line starting
- soft starter starting
- variable frequency drive starting
- individual and group motor starting
- motor reacceleration after restoration of supply
- normal, maintenance, and post-fault network configurations
Parameters Analyzed
- motor starting currents
- time-dependent voltage variation at network buses
- motor acceleration time
- frequency and other operating parameter variation
- impact of motor starting on parallel loads
- successful or unsuccessful motor reacceleration
When the Study is Required
- when connecting new large motors
- when selecting a starting method and starting equipment
- during design of facilities with high motor loading
- when investigating failed motor starts or motor trips
- when verifying reacceleration after voltage dips or short interruptions
- when modifying the power system configuration
What the Study Provides
A motor starting study allows to:
- determine whether the selected starting method is acceptable
- identify unacceptable voltage dips
- assess the impact of starting on process load stability
- verify the possibility of group motor reacceleration
- determine the need for changes in system parameters or equipment
- develop measures to improve starting reliability and system recovery
Deliverables
The final report includes:
- description of input data and study scenarios
- time-domain plots of voltage, current, and speed
- assessment of starting and reacceleration performance
- conclusions and recommendations for technical solutions
Customer Value
- reduced risk of failed motor starts and equipment stoppage
- improved reliability of technological processes
- correct selection of starting method and equipment parameters
- reduced impact of motor starting on other consumers
- justification of design and operational decisions
Why it matters
Motor starting and reacceleration issues often appear not in simplified design assumptions, but in real operation:
- the motor does not reach rated speed
- voltage dips exceed acceptable limits
- adjacent equipment trips
- after a short disturbance, the system does not recover automatically
This study allows such scenarios to be identified in advance and helps ensure reliable operation under actual site conditions.
Input data
- Single-line diagrams
- Utility system parameters
- Transformer, cable, and overhead line parameters
- Motor parameters
- Driven equipment and shaft load data
- Parameters of soft starters, variable frequency drives, and other starting equipment
- Power system operating conditions
- Equipment status in normal, maintenance, and post-fault configurations
Results
- Assessment of suitability of direct-on-line, soft starter, or VFD starting methods
- Evaluation of voltage dips during motor starting
- Assessment of motor acceleration time
- Verification of impact on other connected loads
- Evaluation of motor reacceleration capability after supply restoration
- Identification of causes of unsuccessful starting or reacceleration
- Recommendations for changes in system configuration, equipment parameters, and control logic