8/6/2019 Electric Motor Vibration
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FIGURE 2: One of WEGs computerised balancing machines
ELECTRIC MOTOR
VIBRATION
In recent years vibration monitoring has become part of
themaintenance procedure of many firms. On one hand vibration
analysis is a powerful tool for assessing machinery
condition
and preventing unexpected failures, on the other hand,
determining if a motor vibration is within the limits set by
standards is not as simple as it may seem. If compliance
with
standards or benchmarking is the objective of a vibration
analysis test, it should be carried out in accordance with
the
international standards. IEC 34-14 (AS 1359-50 and
AS2625) deals with vibration tests and limits for
rotatingelectrical machinery. According to IEC a motor vibration
must
be measured at rated voltage, rated frequency, with a
virtually
sinusoidal wave form, uncoupled from the prime mover and
driven load. The keyway must be filled with a half key - either
a
full length rectangular key of half height or a half length key
of
full height centred axially in the keyway is acceptable.
Vibration
must be measured in 6 points, which covers the three
dimensions the machine may vibrate in (see figure 1). The
machine must also be tested in a state of free suspension so
that all external excitations are eliminated. This condition
is
achieved by suspending the machine on a spring or by
mounting in on an elastic base. The three grades of
balancing
and respective maximum vibration level are listed in table 1
Technical Bulletin 02/99
by Alex Settimi SohlerTechnical Managerfor WEG Australia
FIGURE 1: Measuring points
WEG MOTORS ARE
DYNAMICALLY BALANCED
FOR LOW VIBRATION
DIDYO
U
KNOW
www.weg.com.br
4
6
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TABLE 1: Maximum vibration levels as per IEC 34-14. Machines
with
shaft height greater than 630mm the limit values should be
subject to prior
agreement between manufacturer and purchaser.
To ensure compliance with these vibration levels, motors
must
be properly balanced. In fact, vibration is created by
forces
arising from uneven mass distribution of a rotating body.
The
balancing process aims at improving the mass distribution,
hence minimising unbalanced forces, which can be detrimental
to the equipment. Nevertheless, in the real world, no matter
how good the balancing process is there will always be a
residual unbalance, hence the limits in table 1.
Motor manufacturers usually balance their motors to grade N.
Grade R and S are available on request. Table 2 gives some
guidance on the application of the different balancing
grades.
BALANCING TYPE OF MACHINE
NORMAL(N) General application machines such as pumps,
fans,compressors, crushers, etcREDUCED
(R)Precision machines such as drilling and precision milling
machines, lathes, etcSPECIAL
(S)High precision machines such as grinding and balancing
machines.
TABLE 2: Application of grades of balancing
Grade Rated Speed(rpm)
56 < 132 132 H 225 225 H 400 400 H 630
N
(normal)600 n 1800
1800 n 36001.8
1.81.8
2.82.8
4.52.8
4.5R
(reduced)600 n 1800
1800 n 36000.71
1.121.12
1.81.8
2.8S
(special)600 n 1800
1800 n 36000.48
0.710.71
1.121.12
1.8
Maximum r.m.s. Values of Vibration Velocity (mm/s)
Machines Measured in a State of Free Suspension
Motors can be either statically or dynamically balanced.
Moreover
balancing can be done in 1, 2 or more planes. Since the rotor is
a
dynamic component static balancing is not the best method. On
the
other hand, dynamic balancing machines require substantial
investment. Some companies dynamically balance their motors,
but
only a few carry this out at motor rated speed. WEG motors are
all
dynamically balanced by computerised machines in two planes
at
or close to motor rated speed, even the large 16 ton rotors.
Although
WEG guarantees compliance with grade N, due to the strict
quality
control and consistent manufacturing process, most WEG
motors
comply with grade R or better. This ensures
reliability in operation
enhanced bearing and equipment life
minimum mechanical stress
WEG is ahead of the standards to ensure your company also
keeps
ahead of the competition.
