The use of power electronics to supply induction motors
is widely spreading out due to significant advantages in terms of speed
and torque control of electrical machines. The most common are
ASD (Adjustable Speed Drives) systems,
lead by PWM (Pulse Width Modulation)
Unfortunately, it has been proved that this kind of
power supply can cause overstress on electrical insulation in comparison
to the sinusoidal 50/60 Hz voltage supply [1-13], thus decreasing
drastically life and reliability of controlled motors.
The reliability of winding insulation, as it occurs for
other insulation systems, can be investigated by Partial Discharge (PD)
Actually, this research is focused on:
on-line and off-line investigation of voltage distribution along
development of UHF non-intrusive sensors, e.g. antennas, for partial
discharge detection in motor stator winding insulation;
reduction of radiated commutation noise signal of the inverter by
high-pass filtering the signal picked-up by the sensor;
of defects through patterns relevant to multiple partial discharge
development of a non-intrusive diagnostic system for on-line motors.
Figure 1 shows the off-line testing apparatus, TechSquare,
TechImp. It is composed of a HV squarewave generator controlled in
amplitude, frequency, rise time and duty cycle. The voltage waveforms are
acquired by the PDSolver system developed by
TechImp. The motor
under testing is usually supplied with open star, one phase connected to
the inverter and the other two grounded. Increasing the supply voltage, it is
possible to reach the partial discharge inception voltage (PDIV) of the
Fig. 1: TechSquare with PD measurement
Figure 2 shows an example of PD signal (2A) and pattern
(2B) acquired with this configuration, respectively.
Fig. 2: PD signal (2A) and PD pattern (2B)
measured with PDSolver system
On-line measurements on motors supplied by electronic
power converters is another aspect of the research. An inverter bench is
used for motor supply with the purpose of understanding the differences
between on-line and off-line configurations, e.g. EM disturbs (sparks,
mobile phones, etc.) and voltage distribution along motor winding.
 W. Yin,
“Failure Mechanism of Winding Insulation in Inverter-Fed Motors”, IEEE
Electr. Insul. Mag., Vol. 13, n. 6, pp.18-23, November 1997.
 G. C.
Stone, R. G. van Heeswijk and R. Bartnikas, “Investigation of the Effect
of Repetitive Voltage Surges on Epoxy Insulation”, IEEE Trans. on Energy
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the Failure Mechanism of Insulation Subjected to Repetitive Fast Voltage
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Voltage Electric Machines Fed by Pulse-Controlled Inverters”, IEEE Electr.
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Montanari and A. Contin, “Aging Investigation of Turn Insulation under
Fast Repetitive Pulse Voltage with or without Partial Discharges”,
invited paper, Int. Conf. Coil Winding, Insulation and Electrical
Manufacturing (CWIEME’99), pp. 88-96, Berlin, Germany, June 1999.
 D. Fabiani and G.C. Montanari, “The Effect
of Voltage Distortion on Aging Acceleration of Insulation Systems under
Partial Discharge Activity”, IEEE El. Ins. Magazine, Vol. 17, No. 3, pp.
24-33, May 2001.
 D. Fabiani and G.C. Montanari, “On the
Degradation of Winding Insulation of Ac-motors Supplied by Adjustable
Speed Drives – an Overview”, Invited paper, 4th Int. Symp. on Advanced
Electromechanical Motion Systems, pp. 321-326, Bologna, Italy, June 2001
and published on ELECTROMOTION, Vol. 8, No. 2, pp. 89-95, June 2001.
 D. Fabiani, G.C. Montanari and A. Contin,
“Aging Acceleration of Insulating Materials for Electrical Machine
Windings Supplied by PWM in the Presence and in the Absence of Partial
Discharges” IEEE ICSD, pp. 283-286, Eindhoven, The Netherlands, June 2001.
 D. Fabiani, G.C. Montanari and A. Contin,
“The Effect of Fast Repetitive Pulses on the Degradation of Turn
Insulation of Induction Motors”, IEEE SDEMPED, pp. 413-418, Grado (GO),
Italy, September 2001.
 A. Cavallini, M. Conti, D. Fabiani and G.C.
Montanari, “Evaluation of Corona-resistant Magnet Wires through Partial
Discharge and Space Charge Measurements”, IEEE EIC/EMCWA, pp. 11-16,
Cincinnati, OH (USA), October 2001.
 D. Fabiani, Accelerated Degradation of
AC Motor Insulation due to Voltage Waveforms Generated by Adjustable Speed
Drives, Ph.D. Thesis, Bologna, 2002, published by Gedit Edizioni,
Bologna, May 2003.
 D. Casadei, A. Cavallini, D. Fabiani, C.
Rossi, G. Serra and G.C. Montanari, “The influence of power-electronic
waveforms on partial discharge inception in low voltage rotating
machines”, Invited paper, Int. Conf. Coil Winding, Insulation and
Electrical Manufacturing (CWIEME’03), pp. 39-44, Berlin, Germany, June
 D. Fabiani, G.C. Montanari, A. Cavallini
and G. Mazzanti, “Relation between Space Charge Accumulation and Partial
Discharge Activity in Enameled Wires under PWM-like Voltage Waveforms”, to
be published on IEEE Trans. on Dielectrics and Electrical Insulation.