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Motors on modern industrial systems are becoming increasingly larger. Some are considered large even in comparison to the total capacity of large industrial power systems. Starting large motors, especially across the-line, can cause severe disturbances to the motor and any locally connected load, and to buses electrically remote from the point of motor starting.

Ideally, a motor-starting study should be conducted before a large motor is purchased, such that the motor can be installed with confidence that the motor’s life and applications performance will be satisfactory, and the remainder of the power distribution system will not be adversely affected.

It may also be performed anytime a change in the power supply is implemented. A starting voltage requirement and preferred locked-rotor current should be stated as part of the motor specification.

A motor starting study is performed to determine the voltages, currents, and starting times involved when starting large motors or a group of motors, either sequentially or simultaneously. Motor starting studies are carried out to help ensure that:

1. Motor(s) will start with appropriate/acceptable voltage drop
2. Voltage drop at time of start will not disrupt other loads
3. Motor feeder(s) will accelerate within acceptable start up times
4. An accurate evaluation of motor/load speed-torque characteristics and accelerating time is made
5. An accurate evaluation of thermal damage characteristic of motors is made
6. Ther motor will not experience nuisance tripping on the start
7. In the event of direct on line (DOL) start is not possible, that the type and size of starter/drive required to start the motor is known.
8. Motor protective devices are sized/set properly

Starting large motors can cause disturbances to the motor and other loads on other buses. In the worst cases, the starting motor may stall and be unable to start the driven load. In general, a motor starting study should be made if the motor horsepower rating exceeds approximately 30% of the base kVA rating of the supplying transformer. Likewise, if a motor is to be started from an isolated generator, a motor starting should be performed if the motor horsepower rating exceeds 10% to 15% (typical) of the generator kVA rating, depending on the generator characteristics.

Reference Standard: IEEE Std 3002.7, IEEE Std 399

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