Force from the legs of an electric motor due to torque

[Stefan]

I need to perform stress analysis for the stand for an industrial sized electric motor like pic related. The motor stands on four legs. I need to know the force that the legs of the motor exert on the stand due to its torque. I have all the dimensions of the motor including weight and torque, but I'm not quite sure how to go about solving for the force specifically at the legs. What should I do?

 

[russ_watters]

You should be able to draw a simple diagram of the geometry (even using a manufacturer's drawing) and calculate the forces from it. Try it!

 

[Dr.D]

You will discover immediately that this problem is statically indeterminate. Even so, you will be able to evaluate certain force sums (for pairs of forces) which is a starting point for your stress analysis if you then invoke symmetry.

 

[Tom.G]

Without the pump plumbing, wouldn't the reaction torque be the rated stalled-rotor motor torque?

 

[Dr.D]

Depending on the motor type, it is likely that the stall torque (stall -> zero speed) is less than the max torque the motor can deliver.

Tom.G makes a good point about the pump plumbing which will surely impose additional loads of one sort or another on the system.

 

[jrmichler]

Your figure is of a face mounted motor directly bolted to a centrifugal pump. The only reaction torque is during startup and shutdown, and it is shared between the motor and pump. The loads on the motor and pump feet are from the piping, coupling misalignment, pump impeller unbalance, thermal expansion, etc. I once saw a misaligned 60 hp pump vibrate so bad that it broke both pipes within a day after initial startup even though the pump and motor was solidly bolted to a solid concrete base.

If, however, the motor was mounted separately and the only connection to the pump was a flexible shaft coupling, the motor feet would be subject to the actual motor torque. The motor torque is easily calculated from the equation relating torque, power, and RPM.

Also, be advised that the pump feet are not necessarily in the same plane as the motor feet. If the installer bolts the motor/pump assembly to a flat surface, the forces involved can get very high. This is a case where practical realities are greater than simple theory (motor reaction torque) predicts.

 

[Dr.D]

The loads on the motor and pump feet are from the piping, coupling misalignment, pump impeller unbalance, thermal expansion, etc.

I believe that we need to add the weight of the components to this list.