Rotor dynamic balance is an extremely important process in the motor production and manufacturing process, which is directly related to whether the noise and vibration performance of the motor are up to standard.  Due to the defects in the electrical design of the motor, the noise and vibration will also exceed the standard, interwoven with the mechanical imbalance, resulting in extremely complex noise and vibration problems of the motor.  Today, Ms participated in the discussion on the unbalanced problem of the motor rotor itself, that is, regardless of electromagnetic factors, to explore the mechanical causes of noise and vibration problems.

The motor rotor itself belongs to a typical rotating body.  In the manufacturing process, processing error or uncertainty of manual operation causes uneven mass distribution ( i.e. eccentricity ), resulting in imbalance.  If the force generated by unbalance is not corrected, it will not only cause damage to the bearing and motor, but also cause cracking of the machine foundation and cracking of the weld seams of the structural parts.

By means of weighting or de - weighting, reducing or even eliminating this unbalanced factor is called rotor balance.  When the unbalanced mass moment exists in the radial plane of the center of mass and there is no force moment, it is directly weighted or reduced by the radial plane of the center of mass, which is called static balance correction.  If the center of mass of the rotor is exactly on the axis, the center of gravity of the rotor on the cross section of the axis on both sides of the center of mass is not on the axis, and the two centrifugal forces are equal in magnitude and opposite in direction when rotating, forming a pair of force couples, this force couple moment causes the two end bearings to generate a periodically changing dynamic reaction force, so it must be weighted or reduced in two planes at both ends of the center of mass, which is called dynamic-static balance correction.  In general, the centrifugal force when the motor rotor rotates is a resultant force and a couple, so the often-said rotor dynamic balance should actually be the correction operation of the mixed dynamic and static imbalance.

Rotor dynamic balancing method

The centrifugal force caused by unbalance depends on the rotation speed and weight of the rotor.  In order to find the corresponding relationship between unbalanced mass and amplitude, the trial weight can be used to change the vibration amplitude and to know how much amplitude change the unit unbalanced weight causes.

Assuming that A is the original amplitude ( μm ), G is the rotor weight ( kg ), R is the balance radius ( mm ), and the rotation speed is n(r/min ), the trial weight is estimated as follows:

The weighting position mainly depends on the accumulation of experience to determine the bearing of the trial weight, and generally the unbalanced weight is 15 - 45 degrees ahead of the vibration measuring point.

Balance Accuracy and Its Calculation

Balance accuracy, also called balance quality, is an index to measure the degree of rotor balance in mm / S.  If the unbalance rate ( unit g.mm/kg ) or eccentricity ( unit μm ) of the rotor after balancing is E, the balancing accuracy G ( unit: mm/s ) is:

G = ωe/1000

Where ω is the rotor angular velocity ( rad/s ) and the relationship with the rotational speed n is ω=2πn/60

If the balance radius is R ( unit mm ), the formula for calculating the allowable residual unbalanced mass m ( unit g ) is derived as follows:

2mω2R=Wω2e

m=We/2R

Where w is the total mass of the rotor in kg

If there is a motor rotor, balance accuracy G6.3, rotor speed n=800r/min, rotor mass 0.142kg, rotor diameter 17mm, then:

R=17/2=8.5(mm)

e=1000G/ω=30000G/πn=75.2(μm)

Allowed unbalance m = we / 2r = 0.63 ( g )

At present, various motor manufacturers use dynamic balancing equipment of different specifications and models.  According to the measuring principle, there are hard support dynamic balancing machine and soft support dynamic balancing machine.  According to the type, there are horizontal and vertical dynamic balancing machines.  According to the application, there are universal and special dynamic balancing machines.

In the actual balancing process, we can find that some rotors do not find problems in the balancing process, or the rotors that have been determined to be balanced by dynamic balancing will have vibration or noise caused by the problems of the rotors themselves in actual operation, especially for flexible rotors.  In addition, if the difference between the motor speed and the balancing equipment speed is large, the balancing effect will not be consistent with the actual operation effect.  It is based on the analysis and solution of many similar problems in the past, and based on continuous experience accumulation and theoretical innovation, a wide variety of improved and special dynamic balancing equipment have been derived.