# Moment of Inertia

• posted

I want to know about what is the GD square ( Gyrational Torque) of the motor. I am aware that higher kW motors for e.g 5MW motors load torque curves are checked by overlapping with the load curves i.e, for e.g pumps to check weather the selected motor will able to start the load or not. in this scenario i want to know very particularly about the meaning of GD square of the motor and a Pump and also the Wk^2 (Moment of inertia J ) of the motor / load ----- Finally how i have to cross check the values with motor and with load.

How those values are calculated - can anyone suggest a formula to arrive those values for a motor please.

I think nobody in the world doesn't have answer for What is GD^2 of a Motor Means and Load what is the use of the same.

If anybody knows the answer with proper reference let me know

regards

• posted

I want to know about what is the GD square ( Gyrational Torque) of the motor. I am aware that higher kW motors for e.g 5MW motors load torque curves are checked by overlapping with the load curves i.e, for e.g pumps to check weather the selected motor will able to start the load or not. in this scenario i want to know very particularly about the meaning of GD square of the motor and a Pump and also the Wk^2 (Moment of inertia J ) of the motor / load ----- Finally how i have to cross check the values with motor and with load.

How those values are calculated - can anyone suggest a formula to arrive those values for a motor please.

I think nobody in the world doesn't have answer for What is GD^2 of a Motor Means and Load what is the use of the same.

If anybody knows the answer with proper reference let me know

regards

• posted

For many simple loads such as centrifugal pumps and fans, the load torque follows a function of speed squared. Starting such loads is usually not much of a problem, especially if the discharge is shut while starting. But for some loads, positive displacement pumps / compressors, conveyors and such, the torque needed is not dependent on speed.

The difference between motor torque and load torque is what is available for acceleration of the motor/load combination. There are different classes of induction motors that have different torque versus speed characteristics. If you pick a motor that has locked-rotor torque that is near or less than running torque, it will still work fine for some applications such as centrifugal pumps. But it may not be able to create enough torque to move some loads such as a compressor or conveyor system. This means it stalls when trying to start and either trips a protection relay or burns up.

A simple home air-conditioning compressor is a good example. Most are piston type compressors. If they are shut off and then you attempt to restart them immediately, the motor cannot develop enough torque at 0 RPM to overcome the discharge pressure. To avoid damage, many units have a cycle timer that prevents restarting for several minutes. This gives the discharge pressure time to bleed down through the capillary or expansion valve. With lower discharge pressure, the compressor doesn't need as much torque to start spinning and the motor can once again start the load.

For small items like that, the moment of inertia is trivial and doesn't enter into things. For large equipment (>500 hp) and for equipment that uses a stepped-up gearing to drive the load, the moment of inertia of the combined equipment can be a problem. When sizing a motor to a load, it is wasteful and expensive to chose a motor that is much larger than what is needed to drive the load under normal rated conditions. But this can mean that the available torque for acceleration is only a bit larger than the torque of the load. If the moment of inertia is high, then the excess torque cannot accelerate the load up to full speed very quickly. If it takes several seconds to accelerate up to speed, the high starting currents last longer and more heat is generated in the motor. Too long and the motor is damaged or you have trouble with frequent tripping of protective relays.

'Gyrational torque' is a completely separate problem and doesn't come up with large, stationary motor installations. Gyrational torque is the torque developed when you try to change the orientation of a spinning axis. Look up gyroscopes and gyroscopic action. If you have a portable motor/load spinning at high speed, say with the shaft pointing north-south, and you try to change the orientation to something like up-down, gyroscopic action will generate a torque that will try to twist the axis to east-west. Depending on how fast you try to change the orientation, how fast the shaft is spinning, and the rotating element's moment of inertia, the amount of torque created in the east-west can be considerable.

In a free gymbal, this torque will simply twist the machine axis in a direction you didn't expect. In a limited gymbal type arrangement, the torque can produce a side force on the bearings holding the shaft. It can be several times the normal forces and cause bearing failure in some cases.

But in stationary equipment, its a non-issue. Only in portable or moving equipment and marine applications does it come up much. Most land transport and aircraft systems don't carry >500 hp equipment with large moment-of-inerta (except jet engines I suppose, but those aren't motor-driven :-)

One limited application of this that I've seen is in ship-board stabilizers. Basically, a two large spinning rotors to create two large gyroscopes spinning in opposite directions with their shafts oriented up-down. In heavy seas, as the ship tends to roll port-starboard, the top end of one shaft develops a gyrational torque to move forward and the other develops a torque to move aft (owing to them spinning in opposite directions). By restraining the fore-aft motion of the two units, another gyrational torque is created in the starboard-port direction that counters the roll of the ship. (one naval project I know of used exactly the same equipment in reverse in a test basin to force the test hull to roll port-starboard in the test basin in order to test some interesting things inside the hull). The journal bearings of such rotors have to be sized for forces several times larger than simply the weight of the rotor (depending on roll rates, you can get bearing loads on the order of 8x to 10x the rotor's weight).

A simple google of gyrational torque found several hundred hits, you should look there for more info.

daestrom

• posted

Very interesting !

• posted

Anal probe boy !!!

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