Regarding GD Square

GD square is the moment of inertia of the motor. Moment of inertia is to rotation as mass is to linear motion. The greater the moment of inertia, the more torque is required to increase the rotation speed.

The moment of inertia is proportional to the amount of mass times the distance from the axis of rotation squared. A moment of inertia of 2500 kgm2 would be equivalent, for example, to a flywheel having a mass of 100 kg at a distance of 5 meters from the axis of rotation.

You did not answer the question since what he asks for is the MOTOR GD^2 if the LOAD has 2500 kgm^2 of GD^2.

I would not be able to give a pertinent answer since the 2 values are NOT related to each other, at least in my humble opinion.

There are a lot of other parameters which should be given to make only an ESTIMATION of the rotor GD^2.

Nick is right, there is no way to even approximate the motor's GD² without a lot more information. If you don't care how long it takes to get the load up to speed the motor could be 1/100 the size of the load; or if cost means nothing then the motor could be 100 times bigger than the load and get it up to speed in a few seconds. This is where those pesky composite speed-torque curves come in real handy.

You are correct, I should have read it more closely. If the GD squared of the motor is significantly less than the load's GD squared, then it is not a factor. The rate that the motor can start up will be determined by the load GD squared. The motor's start up torque will determine how fast it will get up to speed.

Saying it is NOT a factor just because it is less than the load inertia is not something I would ever assume, it is ALWAYS a factor, just sometimes not as much of one as in other times.

Any decent motor mfr is going to be able to provide you with the inertia value of the rotor of their motor, either in the engineering spec sheet or by calling them. If you are trying to use an existing motor that has no info available, look for a currently available new motor of similar size, shape, power rating and shipping weight, then get the inertia values of that, it will be close enough.

Obviously it's the TOTAL GD squared (motor + load) that determines the excess torque required to increase motor speed. The motor needs to supply this amount of torque above that required to drive the load at a steady speed.

But to get total GD² seen by the motor the load GD² must be converted to load speed, i.e. multiplied by (load speed/motor speed)². Then add the motor GD². Can then calculate acceleration time etc if that's the idea.

That should help a lot, as a GD² 2500 kg.m² (if that's what OP means by kpm²) is a BIG load, likely to run at low speed.

I don't think the last bit is quite right. 100 kg at a distance of 5 meters gives M.r² 2500 kg.m². GD² = 4*M.r² = 10000 kg.m².

IMHO GD² causes confusion. All the formulas I know eg torque = MoI*angular acceleration are simpler using M.r². I've even seen it multiplied by g and quoted as N.m² which makes no sense scientifically.

You are correct, it should be 5 meters diameter.

Not enough information given, but see this:

http://www.kollmorgen.com/uploadedFiles/kollmorgencom/Service_and_Support/Knowledge_Center/White_Papers/KOL_MotorInertiaMismatch_Brief_08_12_10.pdf

The search below will yield many more references to follow up:

https://www.google.co.uk/search?q=matching+motor+inertia+to+load+inertia