XXIV Symposium
Electromagnetic Phenomena in Nonlinear Circuits
June 28 - July 1, 2016 Helsinki, FINLAND
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Torsional Vibrations in Small Variable-Speed
Induction Motors
Stefan Schmuelling and Oliver Drubel
WILO SE, Nortkirchenstraße 100, 44263 Dortmund, Germany
e-mail: stefan.schmuelling@wilo.com, oliver.drubel@wilo.com
Abstract – Small variable speed 2-pole induction motors,
with an active power below 7.5kW, are available within a wide
range of applications. Those motors operate with low torsional
vibration amplitudes for various types of variable-frequency
drives. In the past some motors have shown high levels of
torsional vibrations on special frequency converters during
laboratory measurements. This has been overcome by
subsequent modifications of grid-connected induction motors.
Indeed it has been found, that the efficiency class does not
abandon the cause of vibration. Torsional vibration occurs on
special converters in laboratory environment. A much deeper
investigation shall prepare the combination with these special
converters. The root cause shall be eliminated for the motors
within the market niche of those special converters.
Measurements are performed on different types of induction
motors with delta and star connection. A simulation model
containing the variable-frequency drive and the induction
motor shows the interaction between the mechanics and the DC
link components.
Index Terms—Induction Motors, Numerical simulation,
Variable speed drives, Vibration measurement
I.
INTRODUCTION
Modern variable-speed drives are employed in many
industrial products. In order to increase the efficiency for
circulating pumps, old systems are replaced with modern
variable-speed induction motors [1, 2]. Typically, those
induction motors are fed by variable-frequency drives.
Compared to old pumps, new systems are able to run with
variable rotational speed, which allows running the impeller
at its optimal operational point. This point depends on the
condition of the hydraulic system, e.g. desired pressure and
flow rate.
Nevertheless, most induction motors run quite accurate
with variable-frequency drives. Compared to fixed-speed
pumps and other applications, system efficiency will
increase.
During laboratory measurements of induction motors
which are fed by a special variable-frequency drive, torsional
vibrations are observed independent upon the efficiency
class. The torsional vibration reached high amplitudes and is
visible without any sensor as severe pounding during noload operation. It is noticeable, that this issue preferentially
concerns induction motors. The vibration amplitude varies
with the switching frequency of the variable-frequency
drive. Thus, the oscillation might be excited by the inverter,
e.g. due to sub harmonic components within the output
voltage. A deep understanding of the whole system of the
electronic components, the electric motor and the
mechanical components is necessary to figure out the root of
oscillation.
The paper at hand analyzes this phenomenon of small
induction motors and explains the reason of torsional
vibration. Therefore, measurements and simulations are
performed. The simulation model contains not just the
induction motor, based on the arbitrary reference frame [3],
but also a variable-frequency drive with DC link. Hence, the
simulation results can be compared directly with the
measurements. The pulse width modulation (PWM) is based
on the space vector PWM (SVPWM) and 60° discontinuous
PWM (DPWM). Both methods are used to minimize the
current harmonics during operation. The DPWM is used for
high amplitudes of the fundamental voltage, while the
SVPWM is used for low amplitudes [4].
II.
MEASUREMENTS AND SIMULATION
Measurements are performed on different types of induction
motors. All motors have a rated power around 4kW. During
laboratory tests, the 3-phase motors are directly connected to
a variable-frequency drive, which is fed by a high quality
amplifier. The amplifier provides effectively sinusoidal
voltage supply without any disturbances, flicker or harmonic
components.
In order to verify the simulation model, the calculated and
measured current waveforms can be compared. Therefore, a
simulation is performed with 60° DPWM and a fundamental
frequency of 60Hz. Fig.1 shows the time-domain current
during no-load operation of a 4kW induction motor with a
frequency of 60 Hz. The waveform not only depends on the
Fig. 1: Comparison of measured and calculated phase current during
undisturbed no-load operation at 60Hz.
motor parameters, but also varies with the selected PWM
method and the switching frequency. The switching
frequency is set to 6 kHz (c.f. Fig. 1). Another method is to
compare the spectrum of the measured and calculated
waveform.
While reducing the speed of the induction motor, the
mechanical system starts to oscillate and becomes unstable.
As depicted in Fig. 2, the operation with 30 Hz is unstable.
The vibration measurement shows a high peak below 20 Hz.
This is visible within the measured current. The current has
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two characteristic peaks next to the fundamental frequency.
The amplitude of the sidebands is as high as the fundamental
wave. Those sidebands are visible within the voltage
Fig. 3: Unstable operation of the variable speed induction motor at 30Hz.
Fig. 2: Frequency domain of the measured vibration spectrum at the motor
housing, current and voltage waveform.
spectrum, but 100 times smaller as the fundamental wave.
Fig. 3 shows the simulated operation with an electrical
frequency of 30 Hz. Thus, the synchronous speed of the
motor is at 1800 rpm and the natural damping of torsional
vibration is reduced. The rotational velocity shows an
oscillation between 1400 rpm and 2400 rpm, while the
torque and the current oscillate in the same way. The
measurements show a similar behavior.
If the rotational velocity exceeds the synchronous speed of
1800 rpm, the induction motor starts to work as a generator.
Hence, the DC link voltage within the variable-frequency
drive increases. While braking, the induction machine works
as motor and consumes energy. The DC link voltage
decreases. Thus, the energy changes between the mechanical
system and the DC link capacitor. While the DC link
capacitor is fed by an uncontrolled 6-pulse rectifier, the
system gets supplied with electrical energy once every
oscillation period. The drive system has a resonance which
depends primarily on the size of the DC link capacitor and
the DC link choke, but also on the inertia of the induction
motor and its parameters, such as the size of the main and
leakage reactance.
The inertia of a typical induction motor with a mechanical
output power of 4kW is very small. By increasing the
efficiency of motors, it is necessary to increase the motor
size. Thus, the rotor inertia increases with more efficient
induction motors.
III. CONCLUSION
The paper at hand shows the origin of low frequency
torsional vibrations in high efficient induction machines fed
by variable-frequency drives. Therefore, measurements and
simulations are performed. Both, measurements and
simulations show that the rotational velocity is superimposed
with a low frequency oscillation. Thus, a beat is visible
within the current. The energy exchanges between the rotor
and the DC link capacitor of the variable-frequency drive.
Within the DC link it is visible as increasing voltage
amplitude, until the rotational velocity slows down.
The phenomenon is influenced by a lot of parameters, e.g.
the inertia of the motor or the parameters of the DC link
components.
IV. REFERENCES
[1]
[2]
[3]
[4]
A. T. de Almeida, F. J. T. E. Ferreira and J. A. C. Fong, "Standards for
Super-Premium Efficiency class for electric motors," Industrial &
Commercial Power Systems Technical Conference - Conference
Record 2009 IEEE, Calgary, AB, 2009, pp. 1-8.
doi: 10.1109/ICPS.2009.5463983
J. Malinowski, J. McCormick and K. Dunn, "Advances in construction
techniques of AC induction motors: preparation for super-premium
efficiency levels," in IEEE Transactions on Industry Applications, vol.
40, no. 6, pp. 1665-1670, Nov.-Dec. 2004.
Krause, P.C.; Wasynczuk, O.; Hildebrandt, M.S., "Reference frame
analysis of a slip energy recovery system," in Energy Conversion,
IEEE Transactions on , vol.3, no.2, pp.404-408, Jun 1988
A. M. Hava and E. Ün, "Performance Analysis of Reduced CommonMode Voltage PWM Methods and Comparison With Standard PWM
Methods for Three-Phase Voltage-Source Inverters," in IEEE
Transactions on Power Electronics, vol. 24, pp. 241-252, Jan. 2009
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Proceedings of EPNC 2016, June 28 - July 1, 2016 Helsinki, FINLAND