I have a noise problem associated with a ac drive/motor (75HP 460v wye start/delta run) system. This system (motor/variable frequency drive) runs the blower to a concert hall. The low level lines (microphone lines) of which there are 200+ pick up to varying degrees the trash (noise) produced by the ac drive.
We have tried a variety of solutions, including line reactors, variable frequency drive isolation transformer, moving the ac drive to an alternate location (closer to the motor).
We still have the noise...
We have established that the noise is not coming from the power input side of the audio system. We "believe"/"think" that the noise is broadcasted into the low level lines (mics/intercom)...Thses are shielde-ballanced lines, but they still see the trash....
We theorised that the ac drive/motor current (50A) was radiating noise into mic lines that were roughly 10-15 feet away, but which ran paralell for 100+ feet. We then decided to test our theory by powering the ac drive/motor via a 460v 3PH generator.
...SUCCESS!....but not really....
The audio system was quiet with the generator setup. We reasoned that the ac drive/motor power should be fed via an alternate location...The parking garage next door, (3x125 feet of wire run), and away from all mic lines ... We hooked it up... The noise was back...
The "GROUND" issue...
From the beginning, the grounding concept was discussed... The concert hall building is one giant ground loop in my opinion. At every junction box the contractors tied the ground, "the little green wire", to the side of the box. It is certainly concievable that in the maze of electrical systems throughout the building, a "neutral" might be tied to ground (this would be additional to the neutral/ground connection at the service entrance). That could well be the magic wire (if it exists).
The ac drive motor is grounded, and is a likely source of at least part of the problem.
When the ac drive motor power was sourced from the parking garage the ground was left in it's orriginal location,-- tied to the building steel... The ground circuit does have a "copper path" back to the service entrance, but it is by no means isolated.
The audio system ground is (or at least appears to be), an isolated ground.
This e-mail was prompted by a 2002 discussion about floating 3PH systems...
Is it safe (for humans/hardware) to float the motor?... Should I tie the ac drive to the motor, as it is at present, and disconnect those units from earth?... Keep in mind that we communicate to the variable frequency drive via RS485 (232?)...
As I recall, I measured 3-5 amps from ac drive/motor to ground....
The ac drive is in an odd location, and certainly not optimal for a low noise installation. My current plan is to move the ac drive very close to the motor (<20ft.), tye the motor to the ac drive, and run a low impeadance ground (#2 welding cable) back to the parking garage earth stake.
It looks like the noise is EMF noise on output line and you must use ac drive cable from a recommended ac drive manufacturer. It does sound like switching harmonics are radiating from the variable frequency drive wiring. If so, reducing the wire length fron the variable frequency drive to the motor may help. You might also check into a different variable frequency drive or a different means of speed control for the blower. Some variable frequency drives with very fast switching devices generate harmonics well into the RF region. You might also check the shielding and actual balance of the input cables. If the noise goes away when you disconnect the long cables, you are probably going to have a hard time getting rid of this without rerouting them far away from the source. Something else you might consider is not using long cable runs, replacing them with a wireless setup. The cost on these has come down quite a bit. more...
