High vibration in an Ex "d" motor with terminal box on the side

[krisys]

One pump motor is having high vibration. The installation is located in a large gas processing plant. The installation is installed in explosion hazardous area. The motor is explosion proof, enclosure type Ex "d".

The pump vendor has told that the high vibration is due to the large terminal box, mounted on the side of the motor. According to him, probably, if the terminal box is mounted on the top, the vibration might be eliminated.

The motor in question is 800 kW, 3.3 kV, 2950 rpm. It is highly de-rated for the high ambient temperature. Also the motor has Class "F" insulation, with temperature rise limited to Class "B". Hence the motor is relatively large in dimension.

I appreciate, if you could share your experience.

 

[1gibson]

Tell the pump vendor that the motor is part of their package, and they need to resolve it with the motor vendor, or add bracing from the box to the baseplate. Then ask when someone will be coming to site to fix it.

 

[krisys]

1gibson, thanks for your initial view and you are right if it were to be a new installation. But the installation is sufficiently old enough for the pump vendor to wash his hands off. So we have to take the ownership and resolve the issue.

Nevertheless, did you come across such high vibration problem arising out of the large terminal box mounted on the side of the motor? Can I get some reference or write ups on this issue?

 

[ScottyUK]

The terminal box isn't a source of mechanical energy, so it isn't the source of the vibration. Something rotating is causing the vibration because it is out of balance. Improve the balance of the rotor and whatever it is connected to and the problem may just go away.

The terminal box might be able to cause the vibration if it is sufficiently heavy and the rest of the machine is so poorly braced that the box is able to distort the main frame of the machine and alter the bearing alignment. This is just about credible with an Ex'd' motor with a heavy box catilevered out to one side, although it's a poorly designed machine if this wasn't taken in to account during design.

Is it possible that the cable installation is contributing to the problem with a large unsupported mass of cable hanging from the glands, or worse could the cable be under tension due to thermal contraction? Either of these scenarios could impose forces far larger than the mass of the terminal box alone.