Motor Starting Studies
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.
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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
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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.
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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
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Direct on line
-
With compensation
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Auto – transformers
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Soft Starters
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Start – Delta Start
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Performance curves supplied from manufacturer
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Variable frequency drives
View Typical Plots of Motor Starting Studies (with
assumed worst case load torque characteristic)