electromagnetic vibration in conduit

[MichaelJ]

I understand what sets up the electromagnetic fields. But why does some times a conduit will vibrate when an electric motor starts up ( in this case a A/C compressor) and other conduits not. and is there any why to prevent this? These units are in a church with the conduits running above the ceiling in the auditorium, electrical panels are in mech. room off to side at from and conduits feed over ceiling to units in the rest of building. some of the conduits are pretty loud when a/c unit start and other are quite. units are 208/3ph 5 ton, not sure of the min cir amps or breaker size or wire size. could it be under sized wire? no other problems other than noise at start-up.

 

[GoldDigger]

I understand what sets up the electromagnetic fields. But why does some times a conduit will vibrate when an electric motor starts up ( in this case a A/C compressor) and other conduits not. and is there any why to prevent this? These units are in a church with the conduits running above the ceiling in the auditorium, electrical panels are in mech. room off to side at from and conduits feed over ceiling to units in the rest of building. some of the conduits are pretty loud when a/c unit start and other are quite. units are 208/3ph 5 ton, not sure of the min cir amps or breaker size or wire size. could it be under sized wire? no other problems other than noise at start-up.

The wires inside the conduit are carrying current which generates a magnetic field. That field causes a force between the wires which makes the wires move toward or away from each other. During each cycle whenever the current goes through zero the wires spring back.
If the motor starting current is 10 times the normal running current, the force between the wires will be 100 times as great. Pretty easy to make the conductors rattle.

To prevent it you have to use a cable instead of loose wire, tie or lace the wires together before pulling them, fill the conduit with foam (not a serious suggestion) or use a VFD or soft starter to reduce the current. If you can reduce the starting surge by a factor of two, you will reduce the force by a factor of 4.
A conduit with a greater fill percentage or with other wires among the current carrying wires may rattle less.
Using heavier gauge wire (beyond what is needed for ampacity), if it will fit into the conduit, may also reduce the motion.