7.Vibration7.Vibration
Severe vibration may adversely affect the windings , bearings and coupling mechanism thereby causing them breakdown . When motor is running with load , the values of vibration must be no more than those listed in 5.2C. In addition , figure 21 also serves as a basis for deciding whether the motor is serviceable due to vibration . In case the values of vibration have gone beyond desired values , please try to find out where the trouble lies and see whether remedial measures may be adopted .
(Remarks : In case the main vibration reaches values up to 2,000 rpm , even if the motor actually rotates at 1,000 rpm , the vibration should be considered as 2,000 rpm rotor .)
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The list below is for your information :
| Vibrations | Description | Cause | Remedial measures |
| Vibrations caused by distortion of main magnetic flux | Vibration frequency=2f. Nothing to do with load. Proportional to V^2. |
Natural vibration frequency of multiple-nod distorted stator approximates 2f. | Stablize stator core. Check and see whether stiffness of foundation is sufficient. |
| Vibration caused by unbalanced main magnetic flux | Vibration frequency=2f or f/pxm (m=1.2);yielding a noise by 2sf. Stator shakes in certain directions. Severe vibration not proportional to voltage and having nothing to do with load. |
Distorted rotor gives non-uniform air gap
Perimeterwise-dis-tributed windings are not even Unbalanced rotor gives vibration of great
magnitude . Natural vibration frequencies of foundation stand, stator and rotor approximates the frequency of power source . |
Repair the rotor to obtain uniform air gaps Adjust windings to
obtain balanced flux. Reduce bearing gaps. Install voltage balancing line. Test the stiffness of foundation. |
| Vibration caused by mutual action forces between currents of stator and rotor | Vibration frequency=2f. Large magnitude at starting or with load. |
Unbalanced windings (broken wire or unbalanced resistance in secondary circuit . | Balance the windings. |
| Vibration caused by pulsating torque | Vibration frequency=2f. Vibrating force acting toward the perimeter of rotor. |
Unbalanced voltage source. Unbalanced windings. |
Adjust the voltage and windings. |
| Vibration caused by unbalanced weight | Vibration frequency= n. | Unbalanced residuum. Unevenly accumulated dust. Dried insulation. Eccentric deformation by heat. Worn vanes |
Dynamic balancing. Cleaning and repairing. Replace worn vane and cutter. |
| Vibration caused by bent shaft | Vibration frequency = n. | Bending caused by external force. Deformation by heat. |
Straighten or replace bent shaft. |
| Vibration caused by cylindrically deformed shaft | Vibration frequencies=2n,3n | Elliptic or triangular shaft section . | Repair deformed shaft. |
| Vibration caused by defective rolling bearings | Vibration frequency uncertain . If vibration is caused by defective rolling surface , frequency=number of balls in the bearing x N (nature number) |
Depression caused by external vibration during
transport or none running periods. Damaged by over load or worn out through normal use. |
Replace the bearing. |
| Vibration caused by ill-installed rolling bearing | vibration = n,or uncertain. Larger axial vibration magnitude. |
Shaft not at right angle with rolling surface. Distorted bracket. |
Repair the shaft. Knockdown for reassembling. |
| Vibration caused by characteristics of bearings | Frequency uncertain . Larger axial vibration . Nothing to do with rpm . |
Non-linear characteristics. Resonant brackets. Excessive gaps. |
Pre-compression Reduce gaps Replace bearings Modify fittings, change grease. |
Oil-whip |
Frequency = n/2. Occurs when speed is 2 times of dangerous speed or above . |
Self-excited vibration caused by oil film. | Grease of low viscosity. Reduce the width of bearings. Enlarge bearing gaps. Check the diameter at the neck of shaft. |
| Vibrations caused by distortion coupling devices of driven machine | Frequency = n. Vibration disappears when uncoupled. |
Mal-alignment between shafts. Insufficient straightness of shaft. |
Adjust the coupling Re-alignment of coupling device. |
| Vibration caused by improper installation (resonance with installation system) | Vibration frequency equals n, 2n; f, 2f. | Motor base and foundation are not properly coupled
with driven machine. Resonance between vibration system of both sides. |
Adjust the installation system . Change natural frequency of the system. |
f: frequency s: slip p: pole pairs m: integer n: rpm
If the motor vibrates severely during running , cut off the power supply and see whether the vibration is mechanical or electrical . Next , vary the load when the motor is running or let it run without load for determining the cause of vibration .
Measure and alter the natural frequency.
Measuring natural frequency by strike test.
A synchronous vibration meter be attached on one side of the upper part of motor . Then collide the motor at the other side of the meter , as illustrated in figure 22. The test should be conducted both axially and laterally. Natural frequencies obtained by this test should be within the ranges below :
In 50Hz Area : Below 2000cpm, 3600-4800cpm , and above 7400cpm.
In 60Hz Area : Below 3000cpm, 4400-6000cpm , and above 8000cpm.
If any natural frequency falls beyond the above ranges , an adjustment of base cotter may be considered , or additional grout be poured into or around the base to rise up the natural frequency . The reason why we recommend grout instead of concrete is that the latter shrinks.
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