The national standard for the measurement, evaluation, and limits of motor vibration is GB10068. This standard applies to three-phase AC and DC motors with a rated output power of 50MW or less and a rated speed of 600-3600 r/min. For vertical motors and flange-mounted motors, this standard only applies to motors that can be measured in a freely suspended state. However, this standard does not apply to motors installed at the operating site, three-phase commutator motors, vertical hydro generators, permanent magnet generators, and series-wound DC motors. Ms. [ Name] would like to share some information on this topic.
Special emphasis on vibration measurement
●Specifications for measured values. Vibration measured values include vibration displacement (double amplitude) at the motor bearing, effective value of vibration velocity, and vibration acceleration.
● Measurement Method. For rigid installations, the motor should be securely fastened to the device, avoiding additional vibrations caused by uneven tightening force or uneven mounting surfaces. For V1 vertical motors, the motor should be mounted on a dedicated stand; for B5 horizontal motors, smaller motors can be placed directly on a sponge pad, while larger motors should be placed on a suitable V-bracket, with sponge or rubber padding placed between the bracket and the motor to reduce additional vibrations.
● Half-key installation. Fully insert the appropriate half-key into the keyway. When using a "full-height, half-length" half-key, position it axially in the center of the keyway. Then, secure the half-key to the shaft using a special nylon or steel sleeve. Ensure a secure fit to prevent it from being thrown out during high-speed rotation, which could cause a safety accident.
●Motor operating status. Unless otherwise specified, the motor should be operated under no-load conditions with no output. (1) For AC motors, the rated frequency and rated voltage should be applied. (2) For DC motors, the rated armature voltage and appropriate excitation current should be applied to bring the motor to the rated speed. (3) For multi-speed motors, the motor should be operated and measured at each speed separately. (4) For speed-regulating motors, the motor should be measured over the entire speed range or at the speed where the maximum vibration value is found through trial testing. (5) For generators, the motor can be operated under no-load conditions at the rated speed; if it cannot be operated under no-load conditions, it should be driven by other power sources to bring the speed to the rated value for no-load operation.
● Measurement point location. Use the 6-point method: Measure from the shaft extension end and observe the following points: ① Right side of the bearing outer cover → ② Upper side of the bearing outer cover → ③ Upper side of the bearing outer cover on the shaft → ④ Top of the machine base near the non-side → ⑤ Right side of the machine base near the non-side → ⑥ At the non-side. If the measurement cannot be taken at this end of the motor due to the presence of a fan and shroud, but the motor is allowed to reverse, the value at point 6 can be measured again at point 1 after reversing the motor.
Vibration measurement result determination
●Generally, the maximum value among the measured data is taken as the vibration value of the motor and compared with the standard value.
● Induction motors, especially 2P motors, are particularly prone to beat frequencies of vibration velocity at twice the slip frequency. In this case, the effective value of the vibration velocity Vt can be determined by equation (1):
In formula (1)
Vmax — the effective value of the maximum vibration velocity;
Vmin — the effective value of the minimum vibration velocity.
Vibration limits
● Vibration intensity limits. Vibration values are the broadband root mean values of vibration velocity, displacement, and acceleration measured within a specified frequency range. The maximum value of these three measurements is used to evaluate the intensity of the vibration.
●Limits for vibration velocity of AC motors at twice the grid frequency. A 2P AC motor may generate electromagnetic vibration at twice the grid frequency. Rigid mounting of the motor is required to accurately assess this vibration component.
● Axial vibration. Axial vibration of a bearing is related to its function and structure. For thrust bearings, axial vibration is related to thrust fluctuations, which can damage the metal materials of sliding bearings or the parts of rolling bearings. The evaluation method for axial vibration of these bearings should be the same as that for radial vibration.
Relationship between vibration amplitude and effective velocity value
Since the amplitude of vibration depends only on the magnitude of the oscillation of the vibrating particle, while the vibration velocity depends not only on the magnitude of the oscillation but also on the frequency of the oscillation, it is difficult to convert between the two using a fixed formula. However, when the axis vibrates in a circular trajectory, theoretical calculations suggest the following relationship between the two.
The vibration velocity is calculated as shown in equation (2), and equation (3) can be derived from equation (2).
In equations (2) and (3):
S---Vibration amplitude , in mm;
n --- Motor speed , in r/min;
vt --- Effective value of vibration velocity, in mm/s;
ω--- Angular frequency, unit rad, ω is calculated according to formula (4).
