Checking and correcting rotor unbalance

Electrical Apparatus, Jan 2001 by Nailen, Richard L

Both mechanical and electrical inertia influence the amount of shift. In a piezoelectric accelerometer, neither has much influence.

A variation is the velocimeter or piezoelectric velocity transducer. It is an accelerometer containing an integrating amplifier converting its output to a measure of velocity rather than acceleration. At low frequency, its signal-tonoise ratio is better than a standard accelerometer (although not as good as the direct prod velocity pickup to be described later). The velocimeter also exhibits both electronic and natural frequency generated phase shift.

A second type of transducer used in balancing equipment is the seismic velocity pickup (Figures 6, 7, and 8). It's called seismic because it responds to external vibration in the same way structures do under the influence of an earthquake moving the ground on which they stand.

The outer case of this device contains a wound coil, inside which is a permanent magnet suspended on springs within a bath of damping fluid. When the case is vibrated, the magnet tends to remain stationary under the influence of its own inertia. The result is relative motion between magnet and coil, generating coil voltage (Figure 9).

That voltage output is directly proportional to peak velocity of the vibratory motion. However, the fundamental natural frequency (NF) of the spring system-typically 750 to 900 cycles per minute-masks that effect at low balancing machine speeds.

For any RPM below NF, much of the magnet movement will be in the same direction as the coil movement, resulting in low coil voltage. Above NF, magnet and coil will move largely in opposite directions, for much higher voltage.

Among the advantages of the seismic velocity transducer are relatively high voltage output for a given unbalance, no need for a separate power supply, and suitability to balancing machines exhibiting large support system movement. Offsetting those are: unreliability at low speed, eventual leakage and calibration change associated with the damping fluid, large phase shift, and relatively heavy weight, which leads to reduced sensitivity.

The transducer is mounted in the balancer so that its case (with the coil) picks up the vibration directly. The magnet is entirely free to move under the influence of inertia subject only to the restraining influence of the damping system.

Because they can tolerate large vibratory movement, seismic velocity transducers are common in soft bearing balancers using a strobe light for phase indication. Balancing speed, however, should not be less than 300 RPM (some suppliers say 450), and preferably at least 750. Below that, large phase shift in the output causes trouble with computerized balancing programs. Accelerating heavy workpieces to higher speed may not only be dangerous; it also takes time, as well as demanding greater accelerating and braking power.

A simple change converts this device to a direct prod velocity pickup (Figures 10 and 11). Instead of being freefloating, its magnet is attached directly to the source of vibration and moves with it. (Some damping is retained.) The case and coil are fixed to a solid, non-vibrating part of the structure. This type of transducer is widely used by most balancing machine manufacturers.