1. Basic requirements for electric motors
Electric motors, both AC and DC motors, come in many shapes and sizes.
Some electric motors are standardized for generalpurpose applications. Other
electric motors are intended for specific tasks. Very different applications
and requirements have resulted in the manufacture of many different types and
topologies. The customer's demand for safety, comfort, economy, clean environment
and quality is another reason for the explosive growth in the variety of electric
motors. The basic technical and economic requirements to electric motors can
be classified as follows:
• General requirements:
 low cost
 simple construction
 simple manufacture
 high efficiency and power factor
 low EMI and RFI level
 long service life
 high reliability
• Requirements depending on application and operating conditions:
 repairability, essential for medium and large power motors
 extended speed range and energy efficiency for motors for electric vehicles
 low noise, essential for public life and consumer electronics
 minimum size and mass at desired performance for airborne apparatus, handpieces
and power hand tools
 resistance to vibration and shocks, essential for transport and agricultural
drives and also for airborne equipment
 resistance to environmental effects and radiation essential for electromechanical
drives operating in nuclear reactors, space vehicles, underwater vehicles and
in tropics
 explosion safety, essential for mine drives
 low amount of gas escape, essential for electromechanical drives in stalled
in vacuum equipment
• Additional requirements for motors used in servo drives and automatic control
systems:
 fast response
 temperatureindependent response
 high torque at high speed
 high overload capacity
 stability of performance
2. Reliability
Reliability of an electrical machine is the probability that the machine will
perform adequately for the length of time intended and under the operating
environment encountered. Reliability of machines intended for a long service
life is to be ensured on a reasonable level with due regard to economic factors,
i.e., the best level of reliability is that obtained at minimum cost. The quantitative
estimate of machine reliability is made by using probability and mathematical
statistics methods.
According to reliability theory, all pieces of equipment are classified as
repairable or nonrepairable, i.e., those that can be repaired upon failure
and those that cannot. A failure (mechanical, thermal, electric, magnetic or
performance degradation) is an event involving a full or partial loss of serviceability.
Electrical machines, depending on their applications, may fall into either
of these classes.
The theory of reliability usually treats failures as random events. There
fore, all the quantitative characteristics are of a probabilistic nature.
Failure density, f(t). This is the unconditional probability ?tf(t) of failure
in a given time interval ?t.
Probability of troublefree operation (reliability), P(t). This is the probability
that the troublefree running time of a machine, until failure, is longer than
or equal to the specified time interval ?t = t2  t1, i.e.
(eq.
1)
Statistical estimation of the nonfailure probability is made, provided that
the machines that fail are neither repaired nor replaced by new units, by using
the formula
(eq. 2)
FIG. 1. Characteristic of failure rate λ(t) ("bathtub" curve).
Table 1. Failure rate λ(t) and MTBF of electromagnetic and electronic components
per hour
Table 2. Weibull database
The values of the shape parameter ß and MTTF for selected machines, devices
and components are given in Table 2. While the MTFB (number of hours that pass
before a component, assembly, or system fails) is a basic mea sure of reliability
for repairable items, the MTTF (mean time expected until the first failure
of a piece of equipment) is a basic measure of reliability for nonrepairable
items.
3. Failures of electric motors
FIG. 2. Per unit MTFB tmean of small DC commutator motors as a function of
per unit angular speed Ω (solid line) and temperature v (dash line).
The service expectation of electrical machines depends on the type, applications
and operating conditions. For large and medium power machines it is usually
over 20 years, for general purpose small DC and AC machines up to 10 years,
and for very small DC brush motors as for toys about 100 hours.
Electrical machines with movable parts are less reliable than semiconductor
devices or static converters as, e.g., transformers ( Table 1). It is evident
from experience that most failures occur due to trouble with the mechanical
parts and windings of electrical motors or changes in their material characteristics.
Given below are the parts most likely to fail:
• Bearings:
 contamination and scoring by foreign matter or dirt
 premature fatigue due to excessive loads
 loss in hardness, reduction of bearing capacity and deformation of balls
and rings due to overheating
 brinelling when loads exceed the elastic limit of the ring material
 excessive wear of balls, ring, and cages due to lubricant failure
 broken spacer or ring
 babbitt fatigue, wiping, creep and thermal ratcheting
 porosity and blisters in babbitted bearings
• Speed reduction gearboxes, if builtin:
 cracked teeth
 worn teeth
• Sliding contact:
 poor contact between commutator and brushes due to wear of brushes or insufficient
pressure of brush stud on brush
 mechanical damage to brush holders
 damage to commutator caused by brushes
• Windings:
 broken turns or leads as result of burning due to overloads, mechanical
strain due to temperature fluctuations or electrical corrosion action, especially
at high humidity
 disrupted soldered connections
 earth or turntoturn fault of insulation as a result of poor electric strength,
especially at severe thermal conditions and high humidity
 transient voltage surges or spikes
• Magnetic system:
 changes in PM performance due to high temperature, shocks, vibrations, strain
and hazardous gases
 changes in characteristics of laminations as a result of short circuits
between them, electrical corrosion, etc.
In the case of DC PM brush motors the most vulnerable parts are the commutator
and brushes. In brushless types of motors the most vulnerable parts are bearings.
The condition of bearings and sliding contacts greatly depends on the speed
of rotation of the rotor. Wear of most parts, commutator and brushes in particular,
increases with the rotational speed, hence, the reliability of these parts
and of the motor as a whole reduces.
FIG. 2 illustrates the MTBF tmean of small DC brush motors as a function
of angular speed O =2pn (solid line) at which they were tested. The rated speed
and the time within which they were tested are assumed as unity.
It means that the service life that can be guaranteed by the manufacturer
depends on the motor rated speed. For fractional horsepower commutator motors
the guaranteed service life is about 3000 h for the rated speed n = 2500 rpm
and 200 ... 600 h for the rated speed n = 9000 rpm.
A higher reliability of electric motors can be obtained by eliminating the
sliding contact. The guaranteed service life of small brushless motors is minimum
10,000 h at 12,500 rpm. Some PM brushless motors can run for almost 200,000
h without a failure .
Reliability of motor insulation greatly depends on ambient temperature, relative
humidity and temperature of the motor itself. Experiments with a number of
DC and AC fractional horsepower motors have shown that failures due to excessive
humidity and high or low temperature account for 70 to 100% of the total number
of failures resulting from operating the motors under unspecified conditions.
FIG. 2 shows the MTBF tmean of fractional horsepower DC commutator motors as
a function of ambient temperature Ω (dash line) at which they were tested.
The rated ambient temperature and the time corresponding to this temperature
are assumed as unity. In addition to probability of failures, motors operating
at temperatures beyond the specified limits worsen their performance characteristics.
In selecting electrical motors for a particular drive system, their thermal
conditions must be given serious consideration. Small motors are commonly manufactured
as totally enclosed machines. When mounted in the equipment, the frame can
be joined to a metal panel or any other element to improve the conditions of
heat transfer. When installing small machines in enclosed units or compartments
accommodating other heatemitting components, it is extremely essential to
calculate correctly the surrounding temperature at which the machine is expected
to operate. An increase in the ambient temperature causes an increase in the
absolute temperature of the motor, which influences its reliability and characteristics.
Even very small motors are themselves intensive heat sources. In thermal calculations,
it is important to take into account their operating conditions such as duty
cycle, noload or fullload running, frequency of startings and reversals.
Reliability of electric motors may reduce over the course of service due to
vibrations, shocks, and too low atmospheric pressure.
To increase the reliability of electric motors the following measures should
be taken:
• optimum electromagnetic design
• effective cooling system
• robust mechanical design
• application of good quality materials
• increase of heat resistance, particularly mechanical and electrical proper
ties of insulation
• quality assurance in manufacturing
• use machines under operating conditions as specified by the manufacturer
Reliability of electrical motors is associated with their mechanical and electrical
endurance. This may be determined by the service life of a motor from the beginning
of its operation until its depreciation. The mechanical and electrical endurance
is usually a criterion for evaluating repairable motors.
4. Calculation of reliability of small PM brushless motors
Probability of troublefree operation of a small PM machine with builtin
speed reduction gearbox can be calculated using the following equation
P(t)= Pb(t)Pg(t)Pw(t) (eq. 17)
where Pb(t), Pg(t) and Pw(t) are probabilities of troublefree operation of
bearings, gears and winding, respectively. It means that the most vulnerable
parts of small PM brushless motors are their bearings, gears (if any) and the
armature winding. Under normal operating conditions PMs have practically no
influence on the reliability of a motor.
Probability of troublefree operation of bearings is
Pb(t)= l , i=1 Pbi(t) (eq. 18)
where Pbi(t) is probability of troublefree operation of the ith bearing and
l is the total number of bearings. The life of an individual bearing is defined
as the number of revolutions (or hours at some given speed), which the bearing
runs before the first evidence of fatigue develops in the material of either
ring or any rolling element.
The dynamic specific load capacity of a bearing
Cb = Qb(ntt) 0.3 (eq. 19)
is a function of the equivalent load Qb in kG, speed n in rpm and time tt
of troublefree operation in hours also called "rating life." The
equivalent load Qb may be the actual, or the permissible, load on the bearing
and depends on radial and axial loads. Under normal conditions, a bearing installed
in an electric motor should work on average 77,000 h or MTFB = 77 000/(365
× 24)=8.8 years. The failure rate is ? =1/77 000 = 1.3 × 105 1/h.
Table 3. Classes of insulation, permissible service temperatures ?max and
temperature coefficients at of time of service expectation
5. Vibration and noise
Sounds are set up by oscillating bodies of solids, liquids or gases. The oscillation
is characterized by its frequency and amplitude. According to frequency, oscillations
are classified as
• low frequency oscillations, f< 5Hz
• infrasound, 5 <f< 20 Hz
• audible sounds, 20 = f = 16, 000 Hz or even up to 20, 000 Hz
• ultrasound, 16, 000 <f< 106 Hz.
In engineering practice, low frequency oscillations of solids, below 1 kHz,
are called vibration.
Noise is an audible sound or mixture of sound that has an unpleasant effect
on human beings, disturbs their ability to think, and does not convey any useful
information [98].
Part of the vibrational energy within the audible range is transformed into
sound energy. There is airborne noise radiating directly from the vibration
source and structureborne noise transmitted to the surroundings via mechanical
connections, couplings, base plates, supports, etc. Vibration and noise produced
by electrical machines can be divided into three categories:
• electromagnetic vibration and noise associated with parasitic effects due
to higher space and time harmonics, phase unbalance, and magnetostrictive expansion
of the core laminations
• mechanical vibration and noise associated with the mechanical assembly,
in particular bearings
• aerodynamic vibration and noise associated with flow of ventilating air
through or over the motor.
5.1 Generation and radiation of sound
[...]
FIG. 3. Radiation factor curves for a cylindrical radiator and r =0, 1, 2,...
6.
FIG. 4. Magnetomechanicalacoustic system.
5.2 Mechanical model
FIG. 4 shows how the electrical energy is converted into acoustic energy
in an electrical machine. The input current interacts with the magnetic field
producing highfrequency forces which act on the inner stator core surface.
These forces excite the stator core and frame in the corresponding frequency
range and generate mechanical vibration. As a result of vibration, the surface
of the stator yoke and frame displaces with frequencies corresponding to the
frequencies of forces. The surrounding medium (air) is excited to vibrate too
and generates acoustic noise.
By only considering the pure circumferential vibration modes of the stator
core, the deflection ?d of the stator core is an inverse function of the fourth
power of the force order r, i.e.,
?d ? 1 r4 (eq. 42)
The stator and frame assembly as a mechanical system is characterized by a
distributed mass M, damping D and stiffness K. The electromagnetic force waves
excite the mechanical system to generate vibration, the amplitude of which
is a function of the magnitude and frequency of those forces.
The mechanical system can be simply described by a lumped parameter model
with N degrees of freedom in the following matrix form:
[M]{¨ q} +[D]{ ? q} +[K]{q} = {F(t)} (eq. 43)
...where q is an (N, 1) vector
expressing the displacement of N degrees of freedom, {F(t)} is the force
vector applying to the degrees of freedom, [M] is the mass matrix, [D] is the
damping matrix and [K] is the stiffness matrix.
Theoretically, this equation can be solved using a structural FEM package.
In practice, there are problems with predictions of [D] matrix for laminated
materials, physical properties of materials, accuracy in calculation of forces
and proper selection of force components.
5.3 Electromagnetic vibration and noise
Electromagnetic vibration and noise is caused by generation of electromagnetic
fields. The slots, distribution of windings in slots, air gap permeance fluctuations,
rotor eccentricity and phase unbalance give rise to mechanical deformations
and vibration. MMF space harmonics, saturation harmonics, slot harmonics and
eccentricity harmonics produce parasitic higher harmonic forces and torques.
Especially radial force waves in AC machines, which act both on the stator
and rotor, produce deformation of the magnetic circuit.
If the frequency of the radial force is close to or equal to any of the natural
frequencies of the machine, resonance occurs. The effects are dangerous deformation,
vibration and increased noise.
Magnetostrictive noise in most electrical machines can be neglected due to
low sound intensity and low frequency.
In inverterfed motors parasitic oscillating torques are produced due to higher
time harmonics. These parasitic torques are, in general, greater than oscillating
torques produced by space harmonics. Moreover, the voltage ripple of the rectifier
is transmitted through the intermediate circuit to the inverter and produces
another kind of oscillating torque.
For the stator with frame under assumption of identical vibration of the core
and frame, the amplitude of the radial displacement due to vibration given
by eqn (eq. 26) can be found as Ar = pD1inLiPr Kr 1
[1  (f/fr)2] 2 +(2?Df/fr)2 m (eq. 44)
where Pr is the amplitude of the radial force pressure of electromagnetic
origin (5.114), Kr is the lumped stiffness of the stator core (yoke), f is
the frequency of the excitation radial force density wave of the given mode,
fr is the natural frequency of a particular mode and ?D is the internal damping
ratio of the stator (obtained from measurements). According to:
2p?D =2.76
× 105 f +0.062 (eq. 45)
The singlemode radiated acoustic power as a function of the radiation factor
s(r) given by eqn (eq. 37) and radial vibration displacement Ar given by eqn
(eq. 44) is expressed by eqns (eq. 35) and (eq. 36) in which v = ?Ar, i.e.,
P = ?c(?Ar) 2 s(r)S (eq. 46)
...where for a frame with its dimensions Df and Lf the radiation cylindrical
surface:
S = pDf Lf.
5.4 Mechanical vibration and noise
Mechanical vibration and noise is mainly due to bearings, their defects, journal
ovality, sliding contacts, bent shaft, joints, rotor unbalance, etc. The rotor
should be precisely balanced as it can significantly reduce the vibration.
The rotor unbalance causes rotor dynamic vibration and eccentricity which in
turn results in noise emission from the stator, rotor, and rotor support structure.
Both sleeve and rolling bearings are used in PM electrical machines. The sound
pressure level of sleeve bearings is lower than that of rolling bearings.
The vibration and noise produced by sleeve bearings depends on the roughness
of sliding surfaces, lubrication, stability and whirling of the oil film in
the bearing, manufacture process, quality and installation. The exciting forces
are produced at frequencies f = n due to rotor unbalance and/or eccentricity
and f = Ngn due to axial grooves where n is the speed of the rotor in rev/s
and Ng is the number of grooves.
The noise of rolling bearings depends on the accuracy of bearing parts, mechanical
resonance frequency of the outer ring, running speed, lubrication conditions,
tolerances, alignment, load, temperature and presence of foreign materials.
The frequency of noise due to unbalance and eccentricity is f = n and frequencies
of noise due to other reasons are f ? din/(di +do) where di is the diameter
of the inner contact surface and do is the diameter of the outer contact surface.
5.5 Aerodynamic noise
The basic source of noise of an aerodynamic nature is the fan. Any obstacle
placed in the air stream produces a noise. In nonsealed motors, the noise
of the internal fan is emitted by the vent holes. In totally enclosed motors,
the noise of the external fan predominates.
The acoustic power of turbulent noise produced by a fan is:
P = kf ?c3
_vbl c
_6 D2 bl (eq. 47)
where kf is a coefficient dependent on the fan shape, ? is the specific density
of the cooling medium, c is the velocity of sound, vbl is the circumferential
speed of the blade wheel and Dbl is the diameter of the blade wheel. The level
of aerodynamic noise level due to the fan is calculated by dividing eqn (eq.
47) by P0 =1012 W and putting the result into eqn (eq. 40). For example, for
kf =1, ? =1.188 kg/m3, c = 344 m/s, vbl = 15 m/s and Dbl =0.2 m the aerodynamic
noise level is Lw =75.9 dB.
According to the spectral distribution of the fan noise, there is broadband
noise (100 to 10,000 Hz) and siren noise (tonal noise). The siren effect is
a pure tone being produced as a result of the interaction between fan blades,
rotor slots or rotor axial ventilation ducts and stationary obstacles. The
frequency of the siren noise is:
fs = finblnbl (eq. 48)
...where ? are numbers corresponding to harmonic numbers, Nbl is the number
of fan blades and nbl is the circumferential speed of the fan in rev/s. Siren
noise can be eliminated by increasing the distance between the fan or impeller
and the stationary obstacle.
5.6 DC commutator motors
In DC commutator motors, noise of mechanical and aerodynamic origin is much
higher than that of electromagnetic origin. The commutator and brushes are
one of the major sources of noise. This noise depends on the following factors:
• design of brushgear and rigidity of its mounting
• backlash between the brush holder and brush, and between the commutator
and brush
• brush pressure
• materials of the brush and commutator, especially the friction coefficient
between them
• size, tolerances and commutator unbalance
• deflection of the commutator sliding surface during rotation
• condition of the commutator and brushes, especially coarseness of the sliding
surfaces
• current load
• operating temperature
• environmental effects such as dust, ambient temperature, humidity
5.7 PM synchronous motors
In large synchronous motors the vibration and noise of mechanical origin can
exceed those of electromagnetic origin. Noise produced by small synchronous
motors is mainly produced by electromagnetic effects. Mechanical natural frequencies
of small motors are very high so they are poor sound generators.
The predominant amplitude of the SWL in synchronous and PM brushless machines
is due to the radial force produced by interaction of the rotor poles and slotted
structure of the stator. The frequency and order of this force is:
fr =2µ?fr =2µ?p ± s1 (eq. 49)
where µ? = integer(s1/p). If the frequency of this force is close to the natural
frequency of the order r = 2, a large amplitude of the SWL is produced.
For both the induction and PM synchronous motors fed from solid state converters
the most significant sound levels occur at the modulation frequency of the
inverter, i.e., at the frequency of the major current harmonic with lesser
contributions at multiples of that frequency. Important causes of sound generation
are torque pulsations.
5.8 Reduction of noise
Electromagnetic, mechanical and aerodynamic noise can be reduced by proper
motor design and maintenance.
The electromagnetic noise can be reduced from the design point of view by
proper selection of the number of slots and poles, i.e., suppressing the parasitic
radial forces and torque pulsations, lengthtodiameter ratio, electromagnetic
loadings, skewing of the slots, keeping the same impedances of phase windings,
designing a thick stator core (yoke). The noise of electromagnetic origin is
high, if circumferential orders r of radial stator deformations given by eqn
(eq. 49) are low (r = 1 and r = 2). On the other hand, minimization of radial
forces due to interaction of the rotor poles and stator slot openings does
not guarantee that other noise producing harmonics of the magnetic field are
suppressed.
Proper maintenance, i.e., feeding the motor with balanced voltage system,
elimination of time harmonics in the inverter output voltage and selection
of proper modulation frequency of the inverter has also a significant effect
on the electromagnetic noise reduction.
The noise of mechanical origin can be reduced, from the design point of view,
by predicting the mechanical natural frequencies, proper selection of materials,
components and bearings, proper assembly, foundation, etc., and from a maintenance
point of view by proper lubrication of bearings, monitoring their looseness,
rotor eccentricity, commutator and brush wear, joints, couplings and rotor
mechanical balance.
The aerodynamic noise can be reduced, from the design point of view, by proper
selection of the number of fan blades, rotor slots and ventilation ducts and
dimensions of ventilation ducts to suppress the siren effect as well as optimization
of air inlets and outlets, fan cover, etc., and from a maintenance point of
view by keeping the ventilation ducts and fan clean.
cont. to part 2 >>
