Inductive motors require high starting currents in the range of about six times the normal operating current.
The low voltage occuring with the high starting currents can prevent the motor from accelerating quickly to operating speed.
To avoid momentary overloads on the generators and distribution systems, all large electric motors should be applied/started with a time lag (stepwise).
Here are some reasons for giving a time lag while trying to restart a large electric motor especially coupled to loads (pump/compressor):
1). Residual Voltage Decay
When a motor is shut down, residual voltage remains in the motor windings and cables.
This residual voltage can cause a high inrush current when the motor is restarted, potentially damaging the motor or associated equipment.
A time lag allows the residual voltage to decay, reducing the risk of high inrush currents.
2). Motor Bearing Cooling
When a motor is running, its bearings heat up due to friction.If the motor is restarted too quickly, the bearings may not have cooled sufficiently, potentially leading to premature wear or failure.
A time lag allows the bearings to cool, reducing the risk of damage.
3). Pump Pressure Equalization
When for example, a pump coupled to the electric motor is shut down, pressure remains in the pump and associated piping.
If the motor is restarted too quickly, the pump may experience a pressure surge, potentially causing damage to the pump or associated equipment.
A time lag allows the pressure to equalize, reducing the risk of damage.
4). Electrical System Stabilization
When a motor is shut down, the electrical system may experience a transient voltage disturbance.
If the motor is restarted too quickly, the electrical system may not have stabilized, potentially causing damage to the motor or associated equipment.
A time lag allows the electrical system to stabilize, reducing the risk of damage thereby reducing the normal service life if the equipment.
Some examples of these large motors in the process plant are Boiler FD fan motors, Exhauster fan motors, BFW motors, Cooling water transfer motors, Electric driven air compressors.
Typically, a time lag of 30 minutes to 1 hour is recommended before restarting these large electric motors coupled to a pump/compressor.
However, the exact time lag required may vary depending on the specific motor (coupled pump,compressor specifications) and electrical system design.