17-11-2012, 05:12 PM
Harmonic Loss and Torque Analysis of High-Speed Induction Motors
1Harmonic Loss and Torque.pdf (Size: 1.15 MB / Downloads: 34)
Abstract
In this paper, we investigate the harmonic losses and
torque of high-speed induction motors from both results of measurements
and calculations. The calculation method of harmonic
core losses and torque has been developed from the viewpoint of
practical and useful application to rotating machines. Thismethod
is based on the combination of 2-D and 1-D finite-element methods
with approximated core loss modeling, which requires only few
material constants obtained by Epstein frame tests. The frequency
and the flux density dependence of the core loss are modeled by
the 1-D analysis along the thickness direction of electrical steel
sheets. Furthermore, the proposed method can decompose the
total loss and torque into harmonic components because of its
simple modeling. This decomposition reveals the main loss factors
and the decrease in the torque of the high-speed induction motors
due to the harmonic core losses. The validity of the calculation is
confirmed by measurements. In addition, useful information for
the design of high-speed induction motors is obtained by using the
proposed method.
INTRODUCTION
IN RECENT years, induction motors have often been used
as high-speed motors driven by inverters. The maximum
rotational speed in these motors is greater than 10 000 r/min
in several applications. In this case, not only the inverter carrier
harmonics but also the stator and rotor-slot harmonics cause
considerable losses in the conductors and the cores in the
motors. In addition, the harmonic fields often produce a braking
torque, which also decreases the efficiency of the motors.
Therefore, the estimation of these harmonic losses and torque
must be indispensable in the design of high-speed induction
motors.
APPLICATION TO HIGH-SPEED INDUCTION MOTORS
Motor Specification and FEM
The proposed method is applied to high-speed induction
motors in order to confirm the validity by measurements.
The specifications of the analyzed motor are listed in Table II.
Themotor is a four-pole aluminum-cage inductionmotor driven
by a pulsewidth-modulated (PWM) inverter using insulatedgate
bipolar transistors. The carrier frequency of the inverter
is 5 kHz. The maximum rotational speed of the motor is
10 000 r/min. The air-gap width is 0.55 mm. The numbers of
stator and rotor slots are 48 and 40, respectively. The rotor
skew pitch is one stator-slot pitch. The laminated core without
shrink fitting is fixed by bolts. 50A1300 is applied for the
stator and rotor cores because of the cost of the electrical steel
sheet.
Validity of Proposed Method
The measured and calculated total loss excluding mechanical
loss when the fundamental voltage and frequency are 190 V and
333 Hz, respectively, is shown in Fig. 5. In the measurement,
the total loss is estimated by subtracting the output from the
input. The output and the input are measured by a torque meter
and a digital power meter, respectively. The mechanical loss is
estimated from the variation in the no-load loss with the primary
voltage. The calculated result of the conventional method
expressed by (1) is also shown. It is observed that the result
of the proposed method agrees well with the measurement,
whereas the result of the conventional method considerably
overestimates the loss. The calculated components of the total
electrical loss is shown in Fig. 6. The figure indicates that the
conventional method considerably overestimates the core loss.
CONCLUSION
Harmonic losses and torque values of high-speed induction
motors are investigated by using a calculation method, which
has been developed from the viewpoint of engineering. The
validity of the approximations employed for the calculation
method is discussed on the basis of experimental validations, as
well as field distributions in induction motors. Because of the
simple modeling of core loss, the proposed calculation method
can approximately decompose the loss and torque into harmonic
components. These decompositions have revealed that
the efficiency of the high-speed induction motor is considerably
deteriorated by the slot harmonic core losses because of both
the decrease in the torque and the increase in the loss. From
this viewpoint, the air gap of the motor is enlarged as nearly
twice of that of the conventional motor.