The starting current of most ac motors is several times normal full load current when starting them directly on line at full rated voltage. The starting torque varies directly as the square of the applied voltage. Excessive starting current results in drop in terminal voltage and may result in the following:

• Failure of motor starting due to low starting torques.
• Unnecessary operation of under voltage relays.
• Stalling of other running motors connected to the network.
• Voltage dips at the power sources and consequent flicker in the lighting system.

Motor starting studies can help in the selection of best method of starting, the proper motor design, and the proper system design for minimizing the impact of the motor starting.

Analysis of motor starting methods can be performed by both static and dynamic simulation techniques as follows. These techniques have their own conveniences, advantages and drawbacks. We believe mainly in transient (dynamic) motor starting studies that reproduce observed (measured) motor starting conditions.

•         Load flow type solution with the perceived starting impedance of the motor modeled as part of network modeling

•         Short circuit method type of calculations considering pre-fault short circuit conditions and using voltage drop calculations considering motor starting currents. Alternatively

•         Where accurate dynamic model of the motor electric circuit and load – torque characteristics are available, dynamic model of the motor can be used in traditional transient stability algorithm to assess the impact of the motor starting.

•         In the absence of accurate model information, transient stability studies can be used where the observed (measured) starting current can be used as nodal injection at the motor bus as a dynamic event and the system response to this dynamic event can be evaluated.

The various methods described above can take into account all types of motor starting such as

•         Direct on line

•         With compensation

•         Auto – transformers

•         Soft Starters

•         Start – Delta Start

•         Performance curves supplied from manufacturer

•         Variable frequency drives