Motor Starting Time

 

Motor Starting Time

It is the time required to accelerate the motor and the load from standstill to rated speed.

                                       Motor rated speed x ( Load GD ² + Motor GD ² )

Starting time ∝  ...........................................................................................................

                                           Accelerating torque 

1)    Starting time is directly proportional to load GD²

§  So higher the load GD² (in case of fans), higher is the starting time

2)    It is inversely proportional to accelerating torque

§  Accelerating torque is (average motor torque – load torque)

§  Higher load torque reduces accelerating torque & thus increases the starting time

§  Higher average torque, i.e. higher starting torque and pull out torque increases the accelerating torque.

3)    Starting time is directly proportional to motor rated speed

§  Higher the rated speed (in case of 2P motors), higher is the starting time

 

·        Thermal withstand time of motor

It is the time the motor can withstand without any damage, when it draws the starting current while accelerating with full load, from standstill to motor rated speed. This is declared separately for motor running under hot condition and cold condition.

It depends on:

1)    Method of starting – DOL / Star-Delta / VFD

2)    Insulation class & limiting temperature rise

3)    Motor starting current in amperes

·        Number of consecutive starts

This indicates the frequency of safe starts. During start, the motor is subjected to heavy inrush of current in stator winding which causes rapid temperature rise in the windings. The life of motor insulation reduces when the temperature of the windings rises high. The life of a motor largely depends on the life of winding insulation. If there are too many starts with insufficient cool down time, the temperature rise causes problems. At start, there is also very high energy dissipation in the rotor causing the rotor bar temperature to rise dramatically. If the bars get too hot, the rotor will be damaged. If the motor is hot, the amount of energy that can be safely dissipated is reduced, therefore the starts must also be reduced. More than one consecutive starts are frequently required at the time of trials and commissioning of motor.

Frequency of safe and permissible consecutive starts depends on :

1)    Method of starting

2)    Time per start

3)    Thermal withstand time of motor

4)    Insulation class & limit of temperature rise

5)    If starting is with load or without load