Checking and correcting rotor unbalance

Electrical Apparatus, Jan 2001 by Nailen, Richard L

One reason is its high voltage output per ounce-inch of unbalance at any speed (50% to 100% more than the seismic type, and far above accelerometer capability). No phase shift exists, because the attachment of the "prod" makes the magnet motion follow exactly in step with the vibratory force. Linearity is absolute from near zero to over 10,000 RPM. Because only the magnet and prod move, little mass is added to the supporting system. Like the seismic transducer, the direct prod unit requires no separate power supply.

Unlike some of the other transducers, this type is unsuited to field balancing because of the obvious difficulty in mounting. In a balancing machine, the allowable movement of the work support system cannot exceed the permissible travel of the magnet with the transducer case, which can be a limitation for soft bearing support systems.

A fourth type of balancing machine transducer is the piezoelectric force link (Figure 12). Like the direct prod pickup, it directly couples workpiece motion to the unmoving machine base, but through a piezoelectric crystal rather than electromagnetically. The resulting output voltage, proportional to the force exerted on the crystal, increases as the square of workpiece RPM.

The limited motion possible in the crystal linkage makes this device workable only in hard bearing balancers. In some machines, force link pickups are functionally interchangeable with the direct prod type.

Pros and cons? Much the same as for accelerometers. Force links exhibit little phase shift because of the high natural frequency of the crystal assembly. Computerized readouts compensate for this through their software. Linearity is excellent, as is high speed response. But the force link, like the accelerometer, requires accessory electronics, is quite fragile under overload, and produces such low voltage at low speed that balancing becomes questionable below about 1,000 RPM. Good results are not to be expected for lightweight rotors at low speed.

Clearly, then, the results to be expected from balancing machine use are dependent upon transducer characteristics. Attempting to balance outside the working ranges established by those characteristics will be risky. And replacing existing sensors in a balancer by another type should not be undertaken except after thorough consultation with the machine supplier.