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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 11, Gear Vibration

Section 5, Minimizing the Effect of Magnetic Field on Velocity-Type Pickups

For synchronous speed motors and generators (exactly 3600 rpm, 60 Hz current, or exactly 3000 cpm, 50 Hz), the magnetic field is easily picked up with electromagnetic-type vibration pickups, giving a false reading of the vibration. For nonsynchronous units that are running at speeds slightly lower than synchronous speeds (slightly below 3600 rpm for 60 Hz current, and slightly below 3000 rpm for 50 Hz current), the magnetic field picked up by the pickup will be equal to the current frequency, but the 1 x rpm vibration frequency of the rotor will be slightly less. This causes the instrument to indicate a beat, when in fact the vibration itself may not have a beat.

When the instrument is tuned to the rotor speed, it is almost tuned to the magnetic field frequency, picking up a good portion of it. This can be minimized by either using a magnetic shield for the pickup, a non-electromagnetic pickup or by using the following procedure:

1. Put pickup against vibration measuring point on the machine. Read phase and amplitude. (Phase and amplitude are usually oscillating.)

2. Back the pickup away from that point so that it just misses touching the machine (it could pick up the magnetic field, but not the vibration).

3. If there is a sizeable amplitude or phase reading (strobelight usually reveals a slowly rotating phase mark), back the pickup further away until there is almost no reading from the magnetic field. Keep backing away until the amplitude meter reads almost zero and the strobelight flashes very erratically.

4. Measure the approximate distance between the pickup and the measuring point. Use either a length of pipe or wood to "extend the machine" to the pickup, thus enabling the stick or pipe to transfer the vibration to the pickup. Simply extending the pickup probe is usually not a good idea as the long length could make it too subject to a possible resonance.

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