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
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
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
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