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Effects Of Load Gd2 In Motor


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#1 animna

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Posted 06 August 2008 - 01:56 AM

To anyone who could be of help,

What are the effects of the increased load GD2 on motors? Further, how is the allowable load GD2 of motor be computed?


animna




#2 marke

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Posted 06 August 2008 - 06:52 PM

Hello animna

Welcome to the forum.

When you start an induction motor, there are initially very high slip losses due to the difference in speed between the rotating magnetic field and the rotor speed.
(The slip loss is the product of the shaft torque and the slip speed.)

If the load inertia is high, the time to start is long and so the rotor dissipation is high.
Essentially, the rotor dissipation during start, is equal to the full speed kinetic energy of the load.

The maximum start time, and the maximum load inertia are two figures that are quoted and indicate the thermal capacity of the rotor.
This is not something that you can easily calculate, you need to have the manufacturers guidance on this and there is a very wide variation in capacity. (10 seconds to 50 seconds for Locked Rotor Time)

Best regards,
Mark.


#3 animna

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Posted 08 August 2008 - 03:08 AM

QUOTE (marke @ Aug 7 2008, 03:52 AM) <{POST_SNAPBACK}>
Hello animna

Welcome to the forum.

When you start an induction motor, there are initially very high slip losses due to the difference in speed between the rotating magnetic field and the rotor speed.
(The slip loss is the product of the shaft torque and the slip speed.)

If the load inertia is high, the time to start is long and so the rotor dissipation is high.
Essentially, the rotor dissipation during start, is equal to the full speed kinetic energy of the load.

The maximum start time, and the maximum load inertia are two figures that are quoted and indicate the thermal capacity of the rotor.
This is not something that you can easily calculate, you need to have the manufacturers guidance on this and there is a very wide variation in capacity. (10 seconds to 50 seconds for Locked Rotor Time)

Best regards,
Mark.


Dear Mr Mark,

Thanks for helping. However I would like to add further questions;

1) How is the cooling time of motor windings affected by the load GD2?(e.g load GD2 has been increased; I have consulted a motor manufacturer and was informed that for normal motors, cooling time of winding from full load is about 6 hrs)

2) Mechanically/ Electrically, can you please tell me how the load GD2 affects motor characteristic?

3) How is the load GD2 become a factor for the temperature rise of motors?

4) Finally (I hope it's not too abusive..hehehe), can you explain to me the GD2 in lay man's term?( I'm confused with these types of inertia, I hope you could help)..=)


Thanks in advance!!!!


Animna

#4 marke

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Posted 09 August 2008 - 09:16 AM

Hello animna

  1. The cooling time of the windings is not directly related to the inertia of the load, however as the inertia of the load is increased, the energy dissipated in the windings during start is increased. This will lift the winding temperature at the end of the start.
    The cooling time of the motor from full operating temperature is a function of the thermal resistance of the motor and the thermal mass of the motor. This is influenced by surface area, air velocity and mass.
    At the end of a start, the temperature will be higher with a high inertia load than with a low inertia load
  2. The load inertia can not affect the motor characteristics, but it will determine the start time and temperature rise of the motor.
  3. An increase in the load inertia causes and increase in the slip losses in the motor during acceleration. (longer acceleration time)
  4. The moment of inertia is a measure of the "inertia" of the load moving in a circular motion.
    In linear motion, we have the momentum of an object = mass x velocity. In circular motion, we have the moment of inertia = mass times rotational velocity.
Best regards,
Mark.

#5 honey

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Posted 18 March 2010 - 11:22 AM

Sir

please tell me how to calculate GD2 @ of H.T motor @6.6K.V having K.W 5600 and 996 RPM slip ring motor

Fan connected to this motor having gd2 of 70000 kgm2

fan speed is 980 rpm fan shaft shaft power 5086 kw
full load torque is 4981 kgm
staring torque 665 kgm

reg.

honey

#6 jOmega

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Posted 19 March 2010 - 03:59 PM

QUOTE (honey @ Mar 18 2010, 06:22 AM) <{POST_SNAPBACK}>
Sir

please tell me how to calculate GD2 @ of H.T motor @6.6K.V having K.W 5600 and 996 RPM slip ring motor

Fan connected to this motor having gd2 of 70000 kgm2

fan speed is 980 rpm fan shaft shaft power 5086 kw
full load torque is 4981 kgm
staring torque 665 kgm



If value of motor GD˛ is not on motor rating plate, then you will need to contact motor manufacturer for such information. Without knowing the mechanical construction details of the motor's ROTOR, the value is not possible to calculate.




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