The effects of electric motor failure may be very severe. In case of high speed motors one of most common problems is extensive vibration and its effects. Here we look into the problem to see if it's always related to motor only.

1. Is the motor itself the only source?
Jan Marek Lipiński
Zakład Maszyn Elektrycznych EMIT S.A.

2. Electric motors are widely used in various industries ie.:
 Power generation,
 Mining,
 chemical or
 paper industry.
Common applications:
 high-speed pumps,
 compressors.
2

3. As the centrifugal force increases with the square of the rotational speed, in two-
pole motors with the rotational speed of ns=3,000 RPM (50Hz)in comparison with
four-pole motors with the rotational speed of ns=1,500 RPM (50Hz), provided
identical balance quality “e”, the centrifugal force of unbalance is four times higher.
Therefore, the issue of excessive vibration predominantly emerges in two-pole
motors.
3

4. The basic source of vibration in rotating machines is the centrifugal force of the
rotational movement of the rotor mass whose centre of gravity does not coincide
with its axis of rotation. The displacement “e” of the rotor's centre of gravity from
the axis of rotation decreases as the accuracy of rotor balancing improves. The
centrifugal force of the rotational movement can be expressed in the formula:
Fcf=m2e
Fcf – centrifugal force
m – weight of the rotor
– angular velocity =n/30
n – rotational speed
e – displacment (distance) of the rotor centre of gravity and its axis of rotation
4

5. It is rather obvious that in the case of all motor rotors static balancing is not
used, instead solely dynamic balancing in at least two planes is used which
eliminates all instances of unbalance.
The inaccurate balancing of the rotor is characterised by the occurrence in the
frequency spectrum of the component corresponding to the rotational speed
of the rotor, called the first rotational.
5

6. To protect motors from excessive vibration, aside from the need to balance rotors
accurately, the vibration resonance must be avoided. The vibration resonance occurs
when the frequency of proper vibration of the motor rotor as well as of the entire motor
on the foundation frame is too close to the operating rotational speed of the motor.
The resonance is not the source of vibration but instead it is a vibration amplifier that
can multiply the vibration rate several fold. The multiplication factor depends on the
position of the rotational speed of the motor on the background of the resonance curve
and on the attenuation coefficient.
In order to avoid vibration resonance, normative requirements have been defined both
for the motor rotor designs and for mounting motors in the place of operation. The
method and precision of coupling (alignment) of the motors with the driven machines
have a substantial effect on the vibration level of the motors in the operation sites.
6

7.  mounting of the motor in the operating site,
 method and accuracy of coupling (alignment) with the driven machine,
 design and manufacturing of the motor (rotor in particular).
7

8.  Elastic foundation:
In the case of an elastic mounting, in accordance with EN 60034-14, the proper
vibration frequency of the mounting system (in this case the motor with the driven
machine and the frame) should be lower in all directions than 1/3 of the frequency
corresponding to the motor operating speed.
 Rigid foundation:
In the case of rigid mounting, the vibration velocity on the motor feet or on the frame
near the feet measured horizontally and vertically should not exceed 25% of the
maximum vibration velocity measured on the nearest bearing.
 Moreover, the foundation should be passive, appropriately isolated from the effect of
neighbouring stations - the motor vibration velocity in the rest state should not
exceed 25% of the vibration of the operating motor.
8

9. In the case of rigid mounting (more common), the natural frequency of vibration of the
motor and mounting system should, in practice, differ by at least 20% from the
operating speed. As it turns out, often (especially when motors are mounted on high
frames) this condition is not met, largely in the Horizontal (H) direction, and vibration in
this direction is considerably more severe than in other directions: Vertical (V) and Axial
(A).
9

10.  Motor: 630 kW, 6000V, 3000rpm
 Application: compressor of the refrigerating unit in coal mine
 Mounting: together with the compressor on a frame about 0.9m high
The highest vibration occurred on the motor on drive end in the Vertical
(V) direction. On non drive end the motor vibration in V direction were 4
times lower. On drive end the frame under the motor did not feature the
stiffening that was used on the end of the frame on non drive end.
10

11. Measurements show clearly that the
first rotational dominated in the
vibration velocity spectrum.
11

12. Impact impulse test in the Vertical (V)
direction was performed on drive end
after the motor was stopped. The test
shows that the natural frequency of
vibration in this area coincides with the
motor rotational frequency. It proves
that the high vibration level is caused by
the resonance.
Problem was resolved by increasing
the natural vibration frequency to
over 60Hz by stiffening the frame under
the motor on DE side.
12

13. The method of coupling consists in the appropriate selection of the coupling type
geared to the nature of the motor load and the design of the bearing assembly of
the motor and the driven machine.
The accuracy of the alignment depends on the type of the coupling and the
rotational speed, it has a considerable effect on vibration level.
It is crucial to check if the motor uniformly rests on all feet, i.e. the so-called „soft
foot” must be checked, before commencing the alignment.
13

14. The lack of accuracy in the alignment
and the „soft foot” increase the
vibration level. This manifests itself
with the appearance on the vibration
velocity spectrum of the second and
the third rotational component
aside from the first rotational which is
illustrated by the vibration velocity
spectrum.
14

15. To avoid the vibration resonance, the design of the motor specifies the
requirements for the rotor so that the natural frequencies of its vibration differ
by an appropriate amount from the frequency of rotor rotation. The natural
frequency of rotor vibration transformed into rotational speed is called the critical
speed.
Electric motors rotors can be divided into rigid and flexible rotors.
The motors with rigid rotors can be powered with inverters with wide-range
downward frequency control.
15

16.  The rotor is rigid if its first critical speed exceeds the maximum motor rotational
speed
 Due to various widths of resonance curves, the first critical speed should, in
practice, exceed the maximum motor rotational speed by at least 20.
 Therefore, the first critical speed of the rotors of two-pole motors (driven at
50Hz) should exceed 3,600 RPM which is difficult, and, in many cases, impossible
to achieve due to the special design of two-pole motors.
 The critical speed depends on the rigidity of the rotor supported on the bearings
and also on the rotor weight. In general, this can be expressed with the following
formula:
 n cr – critical speed
 k – rotor rigidity
 m – rotor weight
m
k
ncr 
16

17.  To increase the critical velocity, the rotor rigidity would have to be increased,
because the lowering of the rotor weight is out of the question.
 Unfortunately, in the case of two-pole motors, in comparison with, e.g. four-pole
motors, the diameters of shafts used must be smaller and
 higher distances between bearings are required as the outreach of the stator
winding faces is larger.
 the “k” parameter in this formula is under the square root sign so the double
increase in rigidity increases the critical speed by just 41%
 In the case of various types and variants, sizes and powers of two-pole motors,
the first critical speeds vary.
17

18. Example vibration chart obtained using
the motor overrun method (when the
critical speed reaches nearly 3,600 RPM)
shows that the resonance curve is not
excessively wide and barely overlaps
with the operating speed, therefore the
vibration amplification is not high. At
3,000 RPM the vibration level is 0.70
mm/s, but at the critical speed of nearly
3,600 RPM the resonance amplifies
vibration up to 5.74 mm/s
In many cases critical speed exceeds
3,600 RPM, but resonance curve is wide
and is overlapping with the motor
operating speed. Here we can see that
at the operating point of 3,000 RPM the
vibration amplification occurs however
it is significantly lower in comparison to
critical speed.
18

19.  In the case of flexible two-pole motor designs which can be found with high rotor
weights and high shaft lengths, the first critical speed occurs under 3,000 RPM
(motors driven at 50 Hz)
 The motor must be designed so that the critical speed is lower than the rotational
speed by at least 20
 In example below at 3,000 RPM the vibration level is 0.63 mm/s. At critical speed of
2,080 RPM the resonance amplifies the vibration level to 3.36 mm/s. The motor with
such wide resonance curve is not suited for operation with inverter power supply and
downward rotational speed adjustment towards the critical speed region.
 The case shown below is extreme. The majority of motors with flexible rotors are
characterised by much narrower resonance curves and can be powered with inverters
with frequency control within a limited range.
19

20. A trouble-free motor operation is extremely important. One of the most common
problems is vibration. As it has been shown many factors influence motor
operation. It is essential to take all of those factors into consideration while
project is in design stage.
For more information about motors visit us at www.emit-motor.eu
and follow us on LinkedIn
20