Starting Time Calculation of an Induction Motor

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The starting time of an induction motor is the time it takes for the motor to reach its rated speed. The starting time is affected by a number of factors, including the motor's load, the voltage applied to the motor, and the motor's design.

The load on the motor is the amount of work that the motor has to do. The heavier the load, the longer it will take the motor to reach its rated speed.

The voltage applied to the motor is the amount of electrical power that is available to the motor. The higher the voltage, the faster the motor will reach its rated speed.

The motor's design also affects the starting time. Some motors are designed to start faster than others.

The starting time of an induction motor can be calculated using the following formula:


Ts = (2 * pi * n * J) / (9.55 * Tm):
  • Ts = starting time in seconds
  • n = motor's rated speed in revolutions per minute
  • J = motor's moment of inertia in kilogram-meters squared
  • Tm = motor's maximum torque in newton-meters

For example, a motor with a rated speed of 1,500 revolutions per minute, a moment of inertia of 10 kilogram-meters squared, and a maximum torque of 100 newton-meters would have a starting time of 0.24 seconds.

The starting time of an induction motor is an important factor to consider when designing a motor system. The starting time should be long enough to allow the motor to reach its rated speed without overloading the motor or the electrical system.

Factors Affecting Starting Time

The following factors affect the starting time of an induction motor:

  • Motor load: The heavier the load, the longer it will take the motor to reach its rated speed.
  • Motor voltage: Lower the voltage, Higher the starting time
  • Motor design: Some motors are designed to start faster than others. Higher starting torque will help to overcome the initial inertia.
  • Motor LRWT: LRWT values are crucial to select a motor against the load. it decide the suitability of the motor.
  • Use of the proper starters: This is very important factor for a successful start of a motor. That is why starting time may varied in case of DOL / Star Delta / Soft Starter or VFD starting.  

LRWT - Locked Rotor Withstand Time

Methods for selecting the correct motor:

1. Understand the application first. Take the Load data like Load BKW and GD2 Value. GD2 = 4 x Moment of Inertia

2. Know about the starting method i.e DOL / Star Delta / Soft Start / VFD start

3. Take the nearest higher Motor KW than the load. Take the Datasheet and the note down the LRWT value.

4. Calculate the starting time of the Motor and check whether the starting time is much more than LRWT HOT time of the motor or not. If it is far more than the same then select next higher size motor.

5. Apply the same process until you find a motor which starting time will be lesser than the LRWT Hot of the motor. This mean your selected motor is suitable to give One Hot start.


However, in some cases Motor can be suitable even the starting time is higher than LRWT Hot time. Hence, it is always advisable to take the concern from the Motor manufacturer only.


Conclusion

The starting time of an induction motor is an important factor to consider when designing a motor system. The starting time should be long enough to allow the motor to reach its rated speed without overloading the motor or the electrical system. There are a number of methods that can be used to reduce the starting time of an induction motor, such as reducing the motor load, increasing the motor voltage, using a soft starter, using a star-delta starter, or using a VFD.

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